Jay Fisher - World Class Knifemaker

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"Orion" obverse side view in 440C high chromium stainless steel blade, hand-engraved 304 stainless steel bolsters, Rio Grande Agate gemstone  handle, hand-carved leather sheath inlaid with frog skin
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Heat Treating/Cryogenic Processing of Knife Blade Steels

Improving the Service Properties of High Alloy Blade Steels

Welcome to the best page on heat treating and cryogenic processing of knife blade steels you will find on the internet!

Some Wisdom:

It's extremely important to know that the processing of the steel during heat treat is one of the largest successful or detrimental factors in blade steel performance. Along with blade shape and geometry, the knife blade's performance is a result of the knifemaker's understanding and employment of steel heat treating process. While people in this field often generalize the relative performance of steels based on anecdotal tales, poor testing, and popular gossip, most inferior blade steel performance is based on the geometry of the blade and the processing during heat treat. Many, many steels perform well, and properly processed high alloy steels are the very best steels we have access to in the modern world.

Knife blades warming up to room temperature after cryogenic treatment and aging:
Knife blades warming from -320 F to room temperature after cryogenic treatment
Deep Cryogenic Treatment and Aging of CPM154CM Hypereutectoid Stainless Steel

This Page

Thanks for being here. I created this page as a service to my community of knife enthusiasts, knife collectors, knife users, knife aficionados, and knife makers. I am certain that after you read this page, you will have a greater understanding of modern, high alloy steels used in the finest knife blades, and how steel is physically processed to achieve the very best knife blades ever made in the history of man. We are lucky to be alive in a time when this is possible, and when knowledge and research are available for free, for the advancement of mankind, in an instant.

What I want you to know and learn from this page is what ultra modern, high alloy and stainless steels are, what role they play in the world of fine knives, and how heat treating and processing works correctly in my professional field. It has taken many decades to achieve the level of understanding I have in this field, and there is always more to learn, and, God-willing, I'll continue this journey until I'm finished with this world.

I also want you to know what this particular part of knifemaking is not, and what misleading and mythical ideas are still prevalent, and how inferior and antiquated processes, ideas, steels, and techniques are being hyped as of some value other than appearance and tradition. I want my clients purchasing knives because they are the best they can possibly be, and that starts with the finest, most advanced metals and treatments that bring them to the pinnacle of their performance.

What kind of performance am I writing about? The performance of knives is cutting, cleanly, repeatedly, and continually. Simple enough; any piece of sharpened metal or other hard material will cut. The performance issue then becomes durability, longevity, strength of not only the design of the blade, but in the steel alloy itself, offering advanced chemistry and metallurgy, properly and scientifically treated, for the highest wear resistance, toughness, strength, and corrosion resistance. This is the working end of the knife, the cutting edge, and performance has to be built into the blade alloy and brought to its maximum physical state by processing, typically done by the knifemaker himself.

A knife is not just appearance, it is first, about performance, and that starts with an extremely finely made advanced technology blade. While the other parts of the knife are just as important, this page deals with heat treating and processing modern, high alloy tool and stainless steels, which far surpass traditional lower carbon, lower alloy blade steels by many orders of magnitude and in many distinctive characteristics.

Welcome to what is perhaps the best page about heat treating modern high alloy custom knife blade steels you will find on the internet, and thanks for taking the time to be here.

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A special note about this page:

When this page went public, some readers (other knifemakers) complained about generalizations on the page, that each statement was not indexed, bookmarked, and referenced. I didn't want to create a thesis-style research document, full of footnotes, endnotes, and references, I wanted an easier-to-read casual text.

If you are a person who doubts what you are reading here, please read every single reference below, and then enter the terms you are doubting into any good search engine (Google is nice) and please do your own research. Then, apply that research to make your own knives.

While I'm not a professional technical writer, I am a professional knifemaker, and doing my best to offer reasonable, specific information on how I do what I do and why I do it. Other knifemakers may post their own research and results on their own sites, backed with their own examples based on their own research and backed with their own successes.

Dear Mr. Fisher,
I just finished reading your new article about heat treating and cryogenic process. WOW, thank you very much for sharing such lots of information and knowledge. Reading it sure does brings back old memories of college times, as metallurgy is one thing I studied back in college. The way you describe it amazes me; you do it as like you are a lecturer. Very clear explanation, so easy to understand.
Thank you for sharing, and keep up the good work.

--Hendrik Rinaldi


Knifemaking starts with a saw:
28" bandsaw, laser guided, modified for knifemaker use
2 horsepower, three phase, 28" throat, laser guided, power table, regulated rate and feed band saw

Simple Steel?

"Over 80% of all metals in use are iron and steel alloys"

"Elements of Metallurgy and Engineering Alloys," ASM International, 2008

There is no way to make this simple. While media, movies, videogames, and television shows always tend to show armorers, bladesmiths, and knifemakers heating, forming, quenching, and using steels in highly visual and active procession, the reality of steel and its processing is much more technical. If you just want a brief overview of the process to make superior knife blades, here's a quick step guide:

  1. Cut, form, or forge blade using machines, heat, pressure, or tools.
  2. Harden blade using timed heat and quenching with air, water, brine, salts, and/or cryogenic means.
  3. Using heat, temper to specific hardness required.
  4. Finish blade surfaces.

And that's pretty much it! If you are not interested in the technical nature of blade making, you're done with this page.

But that's not really why you're here, is it? Due to the complexity of the process and material, there is no way for the sake of brevity to sum it up with the four steps above. Steel and its crystalline forms are quite complicated, and our understanding of them determines, as knifemakers, knife users, and knife owners, the choices and nature of the knife that interests us.

For example, as a continuation of the idea that any manufacturer or maker who will not tell you what steel they are using for their knife blades, it's clear that the the steel type is absolutely critical to the performance of the alloy and its function, place, and value as a knife. Without demonstrating or revealing even a basic understanding of steel (much less identifying the alloy), the manufacturer or maker of the modern knife is negligent in his service to his customer, or he simply caters to a customer who doesn't care. That's not my client, not my customer, and not my patron.

Another important issue is one of authenticity. Makers and manufacturers are claiming superior blade performance when there is none, and that somehow, a knife made of modern high alloy steel is somehow inferior to the plain carbon steel blade, which is an outright, easily verifiable lie. I do not want to be part of a profession that allows lies to stand for the sake of egos, history, or profit, and the best way to eliminate them is with knowledge and scientific facts.

Still another consideration is one of choice. There are a vast number of blade steels available in the market today; steel has reached an extremely high level of sophistication and the science will continue to grow. Understanding the nature of this very special material will offer a greater insight into how, why, and where these steels function as they do, and why premium steels are at the forefront of modern technology in nearly every field, not just knifemaking.

This page is for the knife enthusiast, the blade aficionado, and the client, collector, patron, or user who wishes to know why steel is what it is, and how a knifemaker can create a superior blade and why. Other knifemakers will also find it a useful resource, I'm certain. This page will also clarify why modern high alloy tool steels are so special and important to our trade and civilization. It will also clearly show why simple, low alloy carbon steels and hand-forging are crafts based in the romance and tradition of the past, and high alloy accurately-processed steel knife blades are the present and future superior performers and premium value.

I'm not here to discard hand-forging, which typically involves lower alloy steels by necessity. If you like a hand-forged or primitive knife, that's a personal preference and I'd be glad to refer you to several knifemakers who can make this kind of knife for you. But this is not the kind of knife I make, for a reason. The reason is that I use extremely high alloy hypereutectoid tool steels, and they cannot be hand-forged; they must be treated in highly controlled processes, more like a laboratory than a forge. These are the most modern, most refined, most advanced tool steels made, and these are the steels I prefer to make my knives with. More so, these are the steels my clients request, and they are who I make for.

This is not a required page for my profession and career; gratefully, I've been successful for decades without having this page on my site. But it's a significant reference and insight into the world of creating effective, superior, and valuable knife blades with some of the finest alloy steels on our planet. And I want to honor you who are reading this with as much viable information as reasonable, since your (and my) time is so very valuable. This is my service to the my field, trade, art, and people who have made it possible for me to do what I do. Improving the service properties of the highly specialized tool steel blades is critical to what I do, and after you finish reading this page, I guarantee you'll know more than most knife owners, most knife manufacturers, and most other knifemakers about this fascinating process.

Thanks for being here!

For complete transparency, please note that since my first knife made in the 1970s, until the present day, I've heat treated every single one to the best of my knowledge and ability. Know, also, that I've never had one failure, not one return, not one complaint about the hardness and wear resistance of a single knife blade I've made.

Heat treating is not mystical wisdom, not a mystery of scientific knowledge, and not an unobtainable goal: it is simply a process. It's hot, it's cold, it's timing, it's workflow. It's numbers, it's temperatures, it's logical, like any process. And like any process, understanding and repeatability is key for reliable results.

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Hi Jay,
Just had to say thanks a ton for all the great info on heat treating and cryogenic treating of knives. I'm a novice to knives but an analyst by trade, so I appreciate that level of detail to learn more about the process. The more I learn, the more I appreciate all that goes into the art and science of knifemaking.

--E. B.


"Ari B'Lilah" Tactical Combat Counterterrorism commemorative Knife, obverse side view in CPM154CM high Molybdenum Powder metal technology stainless steel blade, 304 stainless steel  bolsters, Sardius (Jasper) gemstone handle, tension-locking sheath of kydex, aluminum, stainless steel, 6AL4V titanium, ultimate belt loop extender
Mirror polished powder metal technology 154CM on this "Ari B'Lilah"

What is Steel?
Nature has no steel.

Steel is a tremendously important part of our lives. It's everywhere, from the nails and screws that hold our homes together to the vehicles we drive. From the flatware we eat our meals with to the handles of our doors. Steel is part of just about every device, machine, or object that requires some level of durability, and steel is definitely part of the machine or process that allows us to make every device or object we create.

"Steel is the world's most useful and the world's most used metal."

--Robert Raymond
"Out of the Fiery Furnace;
The Impact of Metals on the History of Mankind"
1984

Steel is, then, a creator's dream, and everything that involves steel in any way in our lives is created, because humanity created steel. Steel does not exist in nature; it is entirely man's realm. Though metals may by chance or God's hand in nature come together in rare and random circumstances, steel, as far as we know, came about only because man was tinkering with other metals: copper and tin, and then moved on to iron. Though meteoritic iron was used throughout ancient times, it's a modification of a rock, in essence, and not a direct creation of man.

There is natural iron, and the purest in antiquity comes from outer space.

About 2500 years ago, an ancient metalsmith created a dagger, with a wrought meteoritic iron blade and a gold hilt (or handle). It's one of the earliest known artifacts made of man-made iron. It was found in the Hattic royal tombs at Alaca Huyuk, near Hattusa in Turkey.

Iron trinkets were found with bronze works in ancient Egypt, and it is believed that they were accidental creations discovered when smelting copper and casting bronze, evidenced by findings in Israel in ancient copper smelters. Iron oxide (rust and the powdered rock hematite) was used as a flux, a cap to prevent oxidation of copper melt. Some ancient man skimmed off the iron he had used to protect the copper. But then, the iron formed stringy lumps and had to be discarded. Because the melting temperature of iron was so high, they could not do with iron what had been done with copper. For a true melt, the smith needed 1537°C (2800°F), so he had to settle for working the iron in a spongy mass called a "bloom" and by repeated hammering, the slag could be forced out. This is beaten iron, or wrought iron.

Near Delhi, India, there is a 13,000 pound tower, carefully constructed of pieces of forged iron. The iron is high in phosphorous, so because of this, has formed a passive protective film of oxide on the surface that inhibits corrosion. Uncounted hands have passed by, touching, petting, and stroking the iron and it is polished and oiled by the human tide. It is the largest ancient piece of manmade iron. It was made in about 300 AD, when Constantine established his Capital at Byzantium, when Galerius convinced Diocletian to persecute the Christians, and when the Ostrogoths were subjected by the nomadic Mongols sweeping in from Asia. Manmade iron alloys are very old.

Incidentally, wrought iron is a very specific type of iron, iron with less than .08% of carbon, and it's creation is described above. Other than in conservatory or historic reproduction practice, there is no wrought iron commercially available today! Does that surprise you? It should. What we see sold as wrought iron today is simply mild steel, or low carbon steel. This is the practice of using an old, valued, and traditional name to help sell the romance of the past. What would you think of the railing, table stands, and garden furniture if they sold it as "mild steel, painted black?" No, wrought iron sounds so much more... classic.

In the past, wrought iron was not as malleable and formable as cast and forged bronze, so it languished until the Mediterranean basin was invaded by fierce invaders from the sea, whose identity to this day remains mysterious, and the chaos collapsed the entire bronze age. Bronzes disappeared, and more and more, metalworkers turned to iron. Determined to create better iron, the Hittites of Anatolia, peoples of mysterious origin, created a material that was known in their language as "good iron." It was much more durable and superior to wrought iron, and then the Hittites themselves disappeared, prey to European tribes. They left behind the physical evidence of improved iron, and an iron culture that continued widespread.

In the second millennium BC, iron smiths worked furiously with the material and in order to do this, had to expose the iron to white-hot charcoal and carbon monoxide from the combustion. This they repeated over and over to keep the iron hot enough to forge. This exposure forced carbon into the iron, and, simply as a side effect of working with the iron, steel was born. It was harder, more durable, stronger, and tougher than iron.

Just .03% increase of carbon in the iron makes into a steel that is harder than bronze. That was the final blow for the bronze age, and the addition of carbon could be directed by exposure in the forge to just the tip of an iron shaft, turning it to case hardened steel (c. 1200 BC). During that same time, smiths stumbled onto the amazing discovery of the effects of quenching. They were probably just tired of hammering, and wanted to get their projects done, and didn't see the need to wait while their work cooled, so they quenched it in water. And now, a new property was in play.

‘As he finished speaking I handed him the bright wine. Three times I poured and gave it to him, and three times, foolishly, he drained it. When the wine had fuddled his wits I tried him with subtle words: “Cyclops, you asked my name, and I will tell it: give me afterwards a guest gift as you promised. My name is Nobody. Nobody, my father, mother, and friends call me.”

Those were my words, and this his cruel answer: “Then, my gift is this. I will eat Nobody last of all his company, and all the others before him”.

As he spoke, he reeled and toppled over on his back, his thick neck twisted to one side, and all-conquering sleep overpowered him. In his drunken slumber he vomited wine and pieces of human flesh. Then I thrust the stake into the depth of the ashes to heat it, and inspired my men with encouraging words, so none would hang back from fear. When the olivewood stake was glowing hot, and ready to catch fire despite its greenness, I drew it from the coals, then my men stood round me, and a god breathed courage into us. They held the sharpened olivewood stake, and thrust it into his eye, while I threw my weight on the end, and twisted it round and round, as a man bores the timbers of a ship with a drill that others twirl lower down with a strap held at both ends, and so keep the drill continuously moving. We took the red-hot stake and twisted it round and round like that in his eye, and the blood poured out despite the heat. His lids and brows were scorched by flame from the burning eyeball, and its roots crackled with fire. As a great axe or adze causes a vast hissing when the smith dips it in cool water to temper it, strengthening the iron, so his eye hissed against the olivewood stake. Then he screamed, terribly, and the rock echoed. Seized by terror we shrank back, as he wrenched the stake, wet with blood, from his eye. He flung it away in frenzy, and called to the Cyclopes, his neighbours who lived in caves on the windy heights. They heard his cry, and crowding in from every side they stood by the cave mouth and asked what was wrong: “Polyphemus, what terrible pain is this that makes you call through deathless night, and wake us? Is a mortal stealing your flocks, or trying to kill you by violence or treachery?”

Out of the cave came mighty Polyphemus’ voice: “Nobody, my friends, is trying to kill me by violence or treachery.”

--Homer, The Odyssey, Bk IX:360-412
--8th Century BC

The only thing Homer got wrong in his comparison is that quenching is not tempering, at least not during our current times and definition, in this vast history of mankind and metals.

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"Yarden" obverse side view in CPMS30V powder metal technology high vanadium tool steel blade, 304 stainless steel guard and pommel, Sambar Stag handle, hand-stamped heavy leather sheath
Powder metal technology CPMS30V blade in this "Yarden"

Steel: More Than Iron

"Steel is not the most popular metal because it's easy to produce and plentiful. Iron is not the most plentiful element and steel is more expensive and complicated to produce. For instance, copper may be found in nearly pure form in many locations on earth. Steel is such an important material because of its tremendous flexibility in metal working and heat treating to produce a wide variety of mechanical, physical, and chemical properties."

--American Society of Metals (ASM), International

Carbon

Steel, in its basic form, is iron with carbon. Carbon is the number one element in steel that affects its alloy properties. While I'll get deeper into this later, it's important to know that carbon is key. As little as a few hundredths of a percent of carbon in iron makes it steel, and the percentages top out in the standard steels type at about one percent. Remember, I'm mentioning standard steels, not high alloys, tool steels, stainless steels, or specialty steels. I'll start out simply, and we'll get to the really good stuff later!

Standard steels also contain varying amounts of other elements.
Manganese
acts as a deoxidizer since oxygen is harmful in the refining process, and in small amounts, reduces brittleness to improve forgeability, meaning less brittle, more easily forced around: hammered, bent, deformed: forged. Higher amounts increase hardenability, but not by much.
Silicon
is a deoxidizer also, readily attaching to oxygen in the pour. Silicon (sometimes called dirt) is harmful to the surface finish, is hard on cutting tools, and is regulated to low limits by steel manufacturers. It is helpful in creating more shock resistant steels.
Sulfur
is also known by some metalworkers as dirt, and is strictly regulated and considered an impurity in steels. It helps to make steel easier to machine, but little else.
Phosphorus
is a solution strengthener in steels, and will increase both the yield and tensile strength of steels, aid in machinability, and aid somewhat in corrosion resistance. Remember the ancient iron pillar loaded with phosphorus described in the previous topic on this page? Like any element, phosphorus must be limited, because it will also embrittle steel.

There are many other alloy elements in modern tool steels, but just for the carbon steel discussion, these are the important and prevalent players.

High alloy, stainless, or hardenable alloy steels also contain additional elements:

These are the best steels available today: high alloy steels and stainless steels. Unlike carbon steels, most of them cannot be hand-forged and must be machined (with power tools and by hand) and processed in a clean, specific, and highly controlled environment. The predominant additional alloy elements in these high performance steels are:

Chromium
is probably the most important alloy in these types of steels, next to carbon. It is the hardest element on the periodic table, it's incredibly corrosion resistant, it forms extremely hard carbide particles, it adds toughness and wear resistance, it stabilizes steels in wide ranging environments, and it works in concert with other alloy elements to increase all strength, corrosion resistance and wear resistance properties.
Tungsten
improves "hot hardness," or the ability to resist softening of the alloy when exposed to elevated temperatures, and forms hard, wear-resistant tungsten carbides
Vanadium
helps refine the carbide structure by creating initialization nuclei for carbide grains to precipitate on, and forms extremely hard vanadium carbides adding to wear resistance
Molybdenum
helps with deep hardening and hot strength (not particularly essential since knife blades are not thick and blocky with deep dimensions and are not exposed to extremely high temperatures) but moly also helps tremendously with toughness and corrosion resistance, particularly in stainless steels. Also contributes to forming extremely hard molybdenum carbides.
Cobalt
is a hot hardness alloy, typically used in high speed tool steels which may work at elevated temperatures. Not fairly common in knife blades, it has been on the increase in use in recent years. It also contributes to wear resistance by working with carbon, chromium, and tungsten to form critical carbides.
Nickel
is used, particularly with chromium, to increase toughness of the alloy, but is limited since it's an austenite stabilizer and can inhibit the formation of martensite to some extent.
Niobium
this element is used in stainless steels because it stabilizes the steel against intergranular corrosion in heat affected zones, strengthening the metal microstructure which improves toughness. It also contributes by improving wear resistance by creating tremendously hard niobium carbides. Unfortunately, there are solubility issues currently so the niobium content in steels is limited, but who knows what great new materials are on the horizon using this element?
Copernicium
is a laboratory-created element and an extremely volatile metallic compound, adding multi-species erasure capabilities to any knife blade, but since it only has a half-life of 29 seconds, you'll have to be really quick in dispatching your alien invaders. Of course, taking a time to build a handle on the copernicium alloy knife blade is impossible, so it's limited to skeletonized blades. (Just wrote this to see if you were paying attention... since only 75 actual atoms of copernicium have been detected as of the time of this writing, you can't have any for your custom blade.)

While I could go on and on in the periodic table of elements to detail each alloy, the important thing to know for at this time is steel's relationship with carbon, and how important carbon is. Carbon is the most important alloy in steel and you'll understand why as you continue to read.

This can be a lot to take in; don't bother trying to remember each specific alloy and its contribution. It's enough to know that the relationship of iron, carbon, and the alloy set is synergistic, with the performance of the whole being greater than the individual elements, in strength, hardness, wear resistance, heat resistance, corrosion resistance, and toughness, when properly processed.

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Mr. Fisher.
Thank you for your wonderful and well informed site about knives.
So far I have spent quite a few hours reading fascinating info way beyond of what I was looking for.
I have masters degree from mechanical engineering. In the course of my study I have also studied some [steel] metallurgy subjects. I work as an IT contractor for a large steelmaking corporation. I *very* much appreciate your very sensible, balanced and pragmatic info on the topic.
I just wanted to say how much I appreciate the info and wonderful advertisements on site - the pictures of your fantastic work.

Best regards
--Stanislav
from Slovakia


"Mercury Magnum" obverse side view: 440C stainless steel blade, 304 stainless steel bolsters, Red Leopard Skin Jasper gemstone handle, basket weave crossdraw leather sheath
Beautiful mirror polished and etched 440C high chromium martensitic stainless steel blade of this "Mercury Magnum"

Steel as a Crystal
Crystalline lattice structures, Bravais lattices

Steel and iron are crystals. This doesn't mean you can hold them up in the sunrise light beams and call the forest nymphs to do your bidding with a chanting spell; it simply means that they have a regular, repeating pattern of atomic arrangement. Like quartz crystals and other mineral, rock, and gem crystals, there is a uniformity of structure based on the bonding of molecules. While there are lots and lots of molecules in, say, a knife blade, for clarity it's best to look at the smallest piece of molecular arrangement, the lattice.

It's important to understand that nearly all iron has carbon, but it's not considered steel until the percentage is significant enough to affect phase changes. To get an idea of the variability of iron and steel in context:

  • Pure iron does not typically exist; all iron contains some carbon, along with small amounts of other elements. If iron has less than .002% carbon, it's considered pure iron. Pure iron is actually rare, and is not obtainable by smelting.
  • Steels are iron with from .1% carbon to 2.1% carbon, and some steels can be 1000 times harder than pure iron.
    • Mild steels typically contain from .105% to .3% carbon
    • Carbon steels (or medium carbon steels) contain .3% to .6% carbon
    • High carbon steels contain over .9% to 2.5% carbon
    • Ultra high carbon steels contain 2.5% to 3% carbon
  • Cast iron has 2.1% to 4% carbon and 1% to 3% silicon

From this, you can see that iron and steel have a similar makeup, but they are vastly different materials.

The lattices of metals are many, but let's start with the simplest, iron. As I mentioned before, it's important to understand that all iron contains some carbon. The carbon atom is only 1/30th the size of the iron atom. Iron only has two lattice arrangements, face-centered cubic and body-centered cubic. We all know what a cube looks like; it's like a box. In the box of your mind, use a red marker to put a red dot on each corner (8 total). These red dots are atoms of iron. Now with a blue marker, put a blue dot in the very center of each flat face (6 total). These are carbon atoms. The face-centered cubic lattice has 14 atoms in this arrangement, eight of iron and six of carbon. The body-centered cubic lattice has a different arrangement. It's still a cube, but in our box we put one red dot on each corner (8 total or iron) and one blue dot in the very center of the inside of the box (1 total of carbon). So the body-centered cubic has only nine atoms, eight of iron and one of carbon.

We humans are are all about heating things. We cook, we bake, we like blowing things up (heating up stuff to a point of massive instantaneous burning or oxidation). Our vehicles combust (burn) fuel, we heat our homes with natural gas (burning) or oil (burning) or electricity (produced by burning). Understanding steel, though, requires a different perspective. We must think of matter and elements cooling or freezing, for that is where the real magic takes place.

When we heat up our iron to become liquid it takes a lot of heat (2790°F/ 1530°C). It looses all of its crystalline structure, just like ice looses its crystalline form when heated to liquid water; everything moves around. It's not the liquefying of the iron that's unusual; it's when it cools, or solidifies into a crystal. As it cools, the atoms lose their energy and bond with nearby atoms to form the crystal lattices. The iron first forms a body-centered lattice with the the nine atoms. Remember, this is a solid, but it will transform into different crystalline structures while solid. Strangely, as the iron cools to 2550°F/1400°C, the lattices change from body-centered lattices to face-centered lattices (with the 14 atoms). Then, in more strangeness, at 1670°F/910°C, the face-centered lattices change again to body-centered lattices! It's fun for me to visualize this weird structural morphing taking place as the atoms bond, re-bond, move, lose energy, and transform their crystalline form and their molecular arrangement, and the iron and carbon move in form and structure. Remember, all of these crystalline changes happen while the material is solid. The temperatures at which this takes place are critical (important) for us to know, for the next considerations.

When iron becomes steel (with the addition of more carbon), these phase transformations are even more astounding, but the important point is that the molecular crystalline lattice is changeable in both iron and steel.

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Back side of liquid cooled, thermally regulated quenching blocks:
Quenching blocks of aluminum, liquid cooled, thermally monitored and regulated, pressure sensitive contact
Custom equipment for specialized steel knife blade processing

From Alpha to Omega: Steel Phases

It is helpful to understand that when steel cools (or is heated), it undergoes a group of changes in the crystalline structure, forming allotropes of iron. Allotropy is the phenomenon that allows a singular substance to have completely different forms; carbon can be both graphite and diamond. In steels, these changes have been called various things over time, and currently, in materials science, Greek system has been adopted that incorporates some of the older terms to describe these various structures and molecular forms. We also use terms based on the name of the person who discovered or researched the phase (austenite and martensite), and we also have names based on the appearance of the phase structure when viewed through a microscope (pearlite). We even name some of the structures for their physical properties (cementite). This confusing mish-mash of terms comes from many directions, from discovery, industry, research, and applications of steel in our modern world. It's important to know what these terms are, and how they interrelate, because steel metallurgy science crosses so many disciplines.

Please understand that steel and transformations that occur are well known, but there are some things that are not understood! Though we trust our current knowledge of steels (and other metals) we still have a lot to learn, and that's exciting! Here are the terms:

  1. Alpha-ferrite (Ferrite) (α-ferrite or α-Fe), also called alpha-iron or just plain ferrite, is a materials science term for pure iron. This, in itself, is a misnomer, since iron is never pure and the molecule always contains some carbon! α-ferrite is the body-centered lattice with eight iron atoms and one carbon atom in the very middle. It is very soft and ductile. The structure is highly magnetic, and is what gives cast iron and steel their magnetic properties. Mild steel consists mostly of ferrite. Ferrite does not absorb much carbon, and is therefore limited in strength properties.
  2. Beta-ferrite (β-ferrite or β-Fe), also called beta-iron, is actually an obsolete term! Okay, so much for scientific regularity of terminology. In any case, it was once thought that this was a different crystalline structure, but is now believed to be the heated, liquidus version of alpha-ferrite. This is the high temperature end of the alpha phase, the point at which paramagnetism occurs. Paramagnetism means there are unpaired electrons attracted to a magnetic field, but not strong magnetism of a solid crystalline structure. So this is the range of alpha-ferrite where it loses magnetic attraction.
  3. Gamma-ferrite (Austenite) (γ-ferrite or γ-Fe), also called austenite. Austenite is an important phase. When alpha-ferrite is heated above a critical temperature, it changes into austenite (or gamma-ferrite). This is the face-centered crystalline lattice structure which can contain up to 2% carbon in solution. It is non-magnetic. Austenite is created by heating steel above its critical eutectoid temperature and that temperature varies depending on the alloy components of the steel.
  4. Delta-ferrite (δ-ferrite or δ-Fe). This is the allotrope of high-temperature iron, formed on cooling low-carbon alloys but before being transformed to austenite. Delta-ferrite has body-centered structure and can be retained at room temperature. Delta ferrite is not good in knives. It can lead to precipitation of sigma-phase iron.
  5. Sigma phase (σ-Phase). This phase of crystalline structure in FeCr occurs in high chromium stainless steels, and is a complex tetragonal structure (rectangular, and body centered). While hard, it's also brittle and detrimental to stainless steels overall. It occurs due to prolonged heating and welding. Many details of forming of this phase are unknown. Since this phase typically occurs as a precipitate of delta-ferrite in high chromium austenitic stainless steels, it has little bearing currently on knife blade steels.
  6. Hexaferrum (ε-ferrite or ε-Fe), also called epsilon-iron. This is a hexagonal crystalline structure, created only at extremely high pressure (13 gigapascals, or over 1 million psi!) applied to alpha-ferrite. Okay, so this has no bearing on current metallurgy, for when the pressure is removed, the epsilon-iron reverts back to alpha-ferrite. But it may have some interest in geology, where immense pressures within the planet core create this structure. Our very earth is built on a core of hexaferrum, or epsilon-iron! I wonder if I can get some for a knife blade...
  7. Zeta-ferrite (ζ-ferrite or ζ-Fe), also known as light saber powder. This is a very special ferrite that is created in the intermolecular spaces of matter (air) created by the oscillations of an amplified voltage wave generator. It uses the electrostatic repulsion/attraction between particles as a balance point for rigidly forming a short energy beam from a specialized hand-held emitter. Zeta-ferrite is only made by the Massassi on the red gas giant planet Yavin, from Star Wars Episode IV: A New Hope. Okay, just a humorous diversion; there is no such thing, so don't write me asking me to make your light saber. I do congratulate you, however, on reading this far!
Other important terms

It's okay if you don't completely understand the term descriptions below; there is a lot to absorb. Steel phases and transformations can be quite detailed and complicated. Some of the descriptions are repetitive, since these phase structures all interrelate and are formed in interactions with each other.

  1. Cementite: this is iron carbide (Fe3C), an orthorhombic crystal structure. This is a very important component of steel, and is extremely hard and brittle. So hard is cementite that in it's pure form, it's considered a ceramic. It's iron with a lot of carbon, up to 6.7%! It's formed from austenite during cooling, or from martensite during tempering. In slow cooling of austenite, the cementite precipitates in bands or layers alternating with ferrite, and the layered structure is called pearlite (from the layering appearance of seashells), visible by microscope. In tempering (heating after forming martensite), the martensite is heated, the lattices of the martensite are deformed by displacive transformation. The saturated carbon in the ferrite precipitates into cementite. While cementite is hard, it's also a detriment to steels in large quantities, because it's brittle.
  2. Pearlite: This is a layered structure created by large scale movement of iron and carbon atoms that has a "pearly" appearance under the microscope. Pearlite is created by very slow cooling of austenite. Pearlite in the whole is not a phase, it's a combination of two phases: one layer is of the iron carbide cementite, and the other layer of ferrite (alpha-ferrite). The cooling of austenite transforms the carbon-laden face-centered cubic lattice structure to body-centered cubic. Since, in austenite, there is much more carbon than can be accommodated in the interstitial areas, the carbon precipitates into iron carbide lattices of cementite, which aggregate in layers. This forms the two layers: alpha-ferrite (body-centered cube) and cementite (the large orthorhombic crystal lattice consisting of 12 iron atoms and 4 carbon atoms). Since the structure looks like the nacre or seashell layers in a microscope, it was named pearlite.
  3. Bainite (bainitic ferrite): this is cementite and ferrite, formed in a plate-like structure. To form bainite, austenite is cooled more quickly than the rate which forms pearlite, but more slowly than the rate that forms martensite. At first glance, you might think that bainite is like pearlite, in that it's composed of alpha-ferrite and cementite, but this is not the case. In bainite, the alpha-ferrite is dislocation-rich, making the alpha-ferrite harder than it would ordinarily be. Also, there is no layering present as in pearlite. Bainite is an intermediate or pearlite and martensite, and has characteristics between the two, and forms as sheaves separated by austenite, cementite, martensite. There are also distinctions of upper bainite and lower bainite, but I won't go into them here. In bainite steels, there is an inherent hardness and toughness that does not require complete transformation to martensite, and these steels can be used in the as-quenched condition. Bainite steels are used in large steam turbine rotors, pressure vessels, and nuclear reactor components, as well as other commercial purposes. You might wonder why not just direct heat treating and composition towards bainite, and do away with the whole hardening and tempering process. After all, bainitic ferrite can be used "right out of the can" so to speak, with minimal processing. The limitation is that the cementite particles are coarse and reduce toughness in the steel. So for knife blades, which are very thin at the cutting edge, toughness must be improved. There is a way to do this, by adding silicon (dirt) to the alloy, which inhibits precipitation of the cementite, but there are other problems (like blocks of austenite that transform into martensite upon stress, creating brittleness) that make bainite unsuitable for our purposes of knife blades. Bainite does play a role in the overall structure of knife blade steels though, as it is formed by decomposition of retained austenite during the second temper. This increases the toughness of knife blade steels, so it's critical that the second temper is accomplished for increased toughness in some types of blade steels.
  4. Martensite: This is a big one, so much so that most of the high alloy tool steels I use and many tool steels used in premium material knives are classified as "martensitic stainless steels." Martensite is formed as a direct transformation from austenite. Heat the steel to its austenitizing temperature, and then cool it quickly, and martensite is formed. The key here is "cool it quickly." More on the distinctions of cooling later. Martensite is extremely hard, because carbon is trapped in the solid solution. While cementite and ferrite are formed in slow cooling and as the atoms diffuse into these two crystalline forms, the formation of martensite is sudden and drastic. The face-centered austenite lattice transforms into a body centered tetragonal ferrite, highly stressed, super saturated with carbon. This happens by a shear stresses and dislocations. Dislocations are simply areas where atoms are not in position in the crystal structure. Martensite is, clearly, the primary strengthening structure in tool steels, particularly knife steels. When it is formed, it creates both lath-like structures and plate like structures, microscopically visible. Martensite is deteriorated or destroyed by the application of heat, and in knifemaking, we call this tempering. Typically, knife and tool steels are processed for an overabundance of martensite, to leave as little austenite as possible, and then, in tempering, a controlled amount of martensite remains, giving a specific hardness and planned martensite volume with martensite's distinctive properties.
  5. Ledeburite: This requires mention because you may see this material listed on phase diagrams and in the discussion of cast irons. Ledeburite is a 4.3% carbon-containing eutectic mixture of austenite and cementite. Though technically, it is not steel, it is formed in phase transformations as a constituent of high alloy steels. It's also an equilibrium phase constituent, so is less important to knifemaking as we are not about equilibrium, but rapid phase changes in heat treating!
  6. Trootsite: this is a now rarely used term that originated when microscopy was not detailed enough to define specific structures. Trootsite referred not specifically to a material with a determinable hardness, which is standard when giving the -ite nomenclature, but referred instead to a transition pattern seen in fine pearlite (i.e. trootsitic structure). There is no delineation between trootsite and sorbite (below)
  7. Sorbite: this is a now rarely used term that originated when microscopy was not detailed enough to define specific structures. Sorbite referred not specifically to a material with a determinable hardness, which is standard when giving the -ite nomenclature, but referred instead to a transition pattern seen in unsegregated pearlite. (i.e. sorbitic structure). There is no delineation between sorbite and trootsite (above)

The predominate allotropes, constituents, and crystalline structures for our specific discussion of fine knife blade steels are :

  • Ferrite
  • Cementite
  • Pearlite
  • Austenite
  • Martensite

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Hello Mr. Fisher,
As the title of this email already says, each time I am visiting your website (daily :) ) I become even more and more impressed.
You are for sure the best knifemaker alive and not only for your gorgeous work but also for your vast knowledge.
Any visitor, no matter of his profession will definitely find in your website a reason to go further, to learn more and to improve reaching for perfection. I never tried to find a fault in your work as I am sure it would be a waste of time, the way you are judging things, the sack of knowledge behind each and every thing you make is enough to know that you are facing a very fine educated man and craftsman.

I simply adore your courage to face and combat the lies promoted by the huge "sharks" on the market, never seen this before and maybe I will never see it again; it requires arguments, self trust and motivation for the good of the customers. Once again thank you very much for all your efforts to share your vast knowledge with us! May God bless you for long and peaceful years in the Enchanted Spirits Studio! :) All the best,

--A.


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Phase Diagrams

An important part of understanding steel phase transformations is the phase diagram. While scientists and metallurgists diagram many things, the phase diagram illustrates precisely under what circumstances individual phases form.

The key to understanding heat treating is understanding eutectoid transformations, and our induction of these transformations. This is another reason I believe the knifemaker should do all his own heat treating, so he can precisely control the structures of his creations. This may not matter much if he's using plain carbon steels and hammering out his blades in an open air forge; rarely do bladesmiths who work this way use any method other than looking at a relative and generalized color of heated steel for their control. They look at the color of the heated steel and make a judgment. There have been attempts to substantiate this process by giving it a technical sounding name like thermo-optical emission viewing, but it's dependent on the skill, references, background lighting, color sensitivity, and the material itself, and is simply a guess. This is not how the best blades made of modern high tech alloys are created; there are instruments called pyrometers that can measure temperatures to a fraction of a degree.

The knifemaker who works with the highest alloy tool and stainless steels should be part scientist, or at the least, a laboratory technician, able to produce specific, controlled, and regulated environments and exposures for accurate and repeatable results in his steels. While I do make some forged blades and use pattern welded damascus steels from time to time, these are chosen for one reason only; the patterned appearance. The very best performers are, of course, high alloy stainless and specialized tool steels.

The diagrams that detail these specific phase and eutectoid transformations specify points at which the eutectoid transformation occurs, which are the points at which one solid transforms into two different solids with different properties and compositions, important to understanding the whole process and how critical temperature is.

Iron/Carbon/Steel phase diagram

While at first glance, the chart seems intimidating, for fine modern high alloy tool steel blades, we are only concerned with the  narrow band of hyper-eutectoid steels. The first thing to remember is that these are equilibrium charts, and that the phases show here occur in slow temperature changes. Start at the very top of the vertical dashed line labeled "B." B is in the middle of the  hyper-eutectoid band of the steel area listed on the bottom of the chart.

  1. At the very top of B, the temperature is 3000° F, and our hyper-eutectoid steel is liquid.
  2. Moving vertically down, when the dashed line of B crosses the first solid line, at about 2500° F, the primary austenite begins to form. You can see our composition is austenite (γ-ferrite) and liquid, this is the slushy stage of welding.
  3. The austenite is liquid until about 2200° F, and then the steel solidifies, becoming a solid solution of carbon and gamma iron (austenite). This is the super-saturated solution of carbon I described earlier. Going down the vertical B line, you'll see a black dot near the bottom of the austenite solid solution phase. This is a general indicator just to signify that at this point, most of the solid solution is austenite, or gamma ferrite.
  4. Further down, the B line crosses the ACM line. ACM means Austenite to Cementite, and the cementite starts to coalesce in the solid solution at about 1600° F. It continues to form down the temperature line. The next black dot is a location of coalescing cementite (FeC3) with austenite.
  5. Down the line further, at 1333° F, formation of cementite is complete and pearlite starts to form. The cementite coalesces in layers alternating with ferrite, forming the pearlite structure with cementite. Cementite is formed in hypereutectoid steels at this point because these steels contain so much carbon, that simple pearlite cannot contain enough of the carbon, so points of FeC3 (cementite) coalesce within the pearlite.
  6. Finally, at 410° F, the cementite in the solution reaches the Curie point of magnetic transformation (designated by AO At this temperature, the cementite in the solution changes from non-magnetic to magnetic.

With this basic understanding of the phase diagram, I encourage you to look over and examine different areas and indicators on this and other diagrams. They are a rather simplistic way of showing how materials transform from liquid to solids, the eutectic points of steel and iron, and the temperatures at which all of this occurs. There are charts showing sublimation, deposition, melting, freezing, condensation, vaporization, and the crystalline structure of all kinds of materials.

Note that martensite is not on this chart, anywhere. This is the really important point here. This chart is describing material transformations at equilibrium, which for steel and iron, means very slow temperature changes. By sudden and deep cooling, we alter this slow migration of carbon and iron, and form astounding structures that drastically affect the steels performance and arrangement.

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Understanding the Eutectic

When water freezes to ice, it happens at exactly 32°F/0°C. At this same temperature, ice turns to water. It's so accurate that thermometers are calibrated in a bath of crushed ice and water because it's exactly 32°F/0°C. This is not true with steel (iron with more carbon), and when we add more carbon, the weirdness factor increases. Steel starts freezing (solidifying) at one temperature and is completely frozen at a lower temperature. So it has a range of solidifying. In this range, steel is mushy like oatmeal or mud. Start adding more carbon, and the range of mushiness gets larger, until about 2% carbon. Add more than 2% carbon and the range of mushiness gets narrower, and then the range goes away at just over 4% carbon.

Solidification is not the only physical action where steel changes. Steel, when heated below the melting temperature to certain specific and critical temperatures, undergoes changes in the internal crystalline lattice structure. These structures are called phases. The magnificence of steel is based on these phase changes or transformations.

The word "eutectic" comes up often in steel discussion, but the word itself simply means "easily melted" in Greek. It may help to understand various applications of the word in the study of the science of materials to clarify the idea of eutectic principles in steel.

Back when I was very young and working in industry, we did a lot of welding. I worked as a maintenance electrician, mechanic, and instrumentation technician. This sounds like a lot, but mainly consisted of a small group of guys handling every single device, machine, driver, power feed, control, and regulation problem in a small-to-medium sized manufacturing plant. This meant wiring up new devices, troubleshooting failed systems, welding broken components, tuning and calibrating every machine and device the plants needed. I worked in half a dozen different plants like this: a plant that made concrete coated steel pipe, a fiberglass manufacturing plant, a secondary aluminum smelter, a pigment manufacturing plant, a circuit board plant, a radio crystal manufacturing plant, and even a large electrical generation station. Though I took vocational welding classes when I was still in high school as an advanced student, the classes were only a brief overview and introduction to real world welding issues we would encounter in the plants.

One of them, a seemingly simple one, was how to weld stainless steel to mild steel, or stainless steel to a high carbon, high alloy steel. A machine would fail, or corrode, and stainless parts were needed because, wisely, the plant maintenance manager wouldn't want to have to shut down the production line and fix the problem (due to corrosion) again. We came up with special welding rod made by Eutectic Castolin®. This is a great company that's been around quite a while, and makes some neat stuff, essential to maintenance and repair as well as welding advancements.

Now, each metal in the bond (lets say high alloy steel and stainless steel) each have a high melting point, and their melting points are different. But when you mix the two metals together in just the right percentages, you get an alloy that has a lower melting point t than both the parent metals! It's an amazing thing, and the welding rod from Eutectic Castolin® already had this perfect mixture in the rod. So when you welded with this stuff, it bonded to both parent metals at a lower temperature than they would ordinarily melt at, allowing a great penetration and bonding of the weld flow.

Simply put, eutectic transformation is a liquid cooling into a solid that has two phases.

Steels aren't the only metals to have eutectic properties. When I got into jewelry work, I quickly learned that eutectic bonding of dissimilar metals was necessary in all parts of the work. For instance, copper has a melting point of 1984°F/1085°C. And silver has melting point of 1763°F/961°C. If you try to fuse copper to silver, the silver will melt and dribble away long before the copper melts. However, if you mix up an alloy of 28% copper to 72% silver, the alloy has a melting point of 1431°F/777°C! This is great! This means that with that alloy between the pieces of silver and copper, you can solidly bond the two dissimilar metals at about 300° F below the lowest (silver) metal's melting point!

Metal alloys aren't the only materials that do this, and it's not only about melting, but also about the phase transformation of a solid. Understanding that in eutectic concentrations in steel means that several components in a specific combination create a whole that has a lower critical temperature than the individual components.

In the combination of steel, the elements iron and carbon, depending on their percentages in relation to each other and the temperature, give some steels strict eutectic points. To get gritty about this, the two atomic species form a joint super-lattice based upon their valence electrons.

What about eutectoid steel?

To throw another term into the mix, we have eutectoid steel. Eutectoid simply means "eutectic-like," but In this case, the word eutectoid describes a process of phase transformation where one solid forms into two different solids. Steel with .8% carbon can transform (with heat) to austenite, and in equilibrium cooling, austenite can then undergo complete phase transformation into pearlite (cementite and ferrite) without a transition zone and without any extra ferrite or extra cementite. This is considered eutectoid steel.

Steels with less than .8% carbon are called hypoeutectoid steels, and hypoeutectoid simply means a mixture of components having less of the minor component (carbon) than the eutectoid composition. Hypoeutectoid steels can transform in equilibrium phase transformation to pearlite and ferrite. Ferrite is a soft component more typically known as mild steel. Railway track is a great example of hypoeutectoid steel. So are railway spikes. You might want to consider that when you see a knife made of a railroad spike. As knives, they are ornamental only, containing carbon in the range of .15% to .30%, creating soft, weak blades at their very hardest. But some folks like the look and they are easy enough to hand-forge.

Steels with more than .8% carbon have so much carbon that they transform into cementite before the eutectoid point and they are called hypereutectoid. Hypereutectoid means a mixture of components having more of the minor component of a eutectoid composition. Hypereutectoid steels can transform in equilibrium phase transformation to pearlite and cementite, with many more abundant hard particles of cementite. These steels are therefore harder, more wear resistant and more durable in long term use as knives and cutting tools. Most of the steels I use in knife blades are hypereutectoid. Carbon is the minor component and they have a lot of carbon, for good reason.

It was brought to my attention that the words hypo-eutectic and hypoeutectoid, and hyper-eutectic and hypereutectoid are different and have different meanings. However, it depends on the country, as the United Kingdom and European English version of the word is not the same as the American English version, so research papers and presentation varies somewhat.

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Hello Mr. Fisher,
I just want to thank you a lot for writing your long detailed page on heat treating. After about 4 days of scrolling internet forums and such, your post laid it out the best. So relieved...!
Thank so much for your time... otherwise all the best!!

Sincerely,
Marc Stanton

Thank you again- like finding the holy grail of treating that cut through all the floating opinionated stuff.


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Understanding Martensite

"Martensite is the most important constituent produced by heat treatments designed to produce ideal mechanical properties."

--George Vander Voort
Martensite and the Control of Retained Austenite
Metallography, Failure Analysis, Archeometallurgy Consultant

Martensite is probably the most important crystalline structure knifemakers and toolmakers are concerned about, so I want to go into it a bit more. Martensite is formed from austenite (gamma-ferrite) detailed above. When heated to transformation temperature (the decalescence point described below), the ferrite (alpha-iron) transforms to austenite (gamma-ferrite). The body-centered cubic lattice structure transforms to a face-centered cubic lattice structure, containing up to 2% carbon in solution. Since carbon atoms are 1/30th the size of iron, there are extra carbon atoms in the lattice structure with even more in solid solution floating around in the interstitial spaces (the spaces in between the lattice atoms). In slow cooling, the carbon atoms will drift back into the alpha-ferrite arrangement (body-centered) alternating with layers of cementite and stabilize. In high carbon steels this slow cooling produces pearlite, layers of ferrite with cementite as the carbon diffuses into coalescing cementite layers (recalescence explained below). This is an equilibrium phase, where physical changes happen at fixed temperatures with the material at thermodynamic equilibrium, without consideration for time. These phase changes happen by diffusion, where atoms move due to thermodynamic and internal motion. The carbon moving to create face-centered, body-centered, cementite, or interstitial locations is a product of diffusion. These phases are represented on an equilibrium phase diagram, but martensite is not.

The reason martensite is not represented is because these particular phase diagrams are for materials at equilibrium. Martensite is not an equilibrium structure because it grows without diffusion, from it's parent austenite, inheriting its chemical composition. It's also highly strained, as a result of thermodynamic temperature changes, again, not at equilibrium.

In fast cooling, the carbon simply has nowhere to go, so it's forced into the crystalline lattices making a new arrangement called martensite. Martensite is actually a distortion of the face centered gamma-ferrite (austenite) structure, making the lattice into a strained and bent tetragon (rectangle), super-saturated with carbon. The crystal structure has lots of dislocations, which can be envisioned by plenty of forced angles from many directions, strengthening the steel.

So the crystals are, in simple terms, messy, out of positions, interfering with others, and knotted up. I know, you metallurgists are wincing, but I'd rather not go into the details of edge dislocations and screw dislocations formed by shear and wave front motion in evolving crystalline bodies. It's enough to know that many dislocations create an irregular lattice, which is much harder to break.

A simple analogy would be plywood. One board with grain one direction would be easy to break. Add another board in another direction, and strength is improved dramatically. Now take it further. Compare OSB (Oriented Strand Board) to the plywood. OSB is made of many small, directionless chips and fibers of wood, compressed and glued into different directions. It's twice as strong as plywood to shear stresses! It's because of all those different directions of orientation.

Martensite doesn't form all at once on cooling. It has a start temperature, Ms and a finish temperature Mf. In this cooling range, austenite transforms to martensite. The cooling has to occur quickly, otherwise, the crystalline structure will convert to bainite, or if cooled even slower, pearlite, the layered alpha-ferrite with cementite. Slower cooling also allows the austenite to stay austenite, or be retained as austenite. While there is always some austenite retained and some converted to martensite, the amount is very important.

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Hello Jay,
I am just delving into knife making as a hobby. Your website is a treasure trove of valuable information that has been a great reference for me. Thank you for investing the time to share your expertise.

Regards,
Charlie Ward Wright IV


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Decalescence and Recalescence
Transformation and Temperature

These are curious properties of steel allotropes, and can help in understanding how carbon migration, phase changes, coalescence, and the whole heat treating process works with steels. Decalescence and recalescence are physical properties based on temperature, and may be used to describe or indicate the critical temperatures where these phasic changes take place. However, they are not something manipulated or changed in any way by the knifemaker, simply something that occurs. Not understanding these reactions may lead to improper, ineffective, or less than optimum heat treating.

Decalescence

As the alloy is exposed to heat, the actual temperature of the steel continues to increase. At critical temperatures, phase change starts to take place. At these points, even with additional heat being applied, the steel temperature does not increase, even with additional thermal loading from the higher ambient temperature in the furnace. In reality, the temperature of the steel decreases instead, as the energy is used to transform the phase of the steel to another form. Applying additional heat will not heat the steel, only encourage further transformation. When transformation is complete, the additional heat will then cause the steel to increase in temperature in a more normal fashion. What's important here is that the energy is being used to change the crystalline lattice structure of the steel. This property of absorbing heat energy while decreasing temperature during heating while phasic change is underway is called decalescence.

For decalescence, the important thing to remember is that plenty of ambient heat and ample time in the furnace is necessary for the phase change to take place, and temperatures must be closely held and applied. This is why the best heat treatment takes place in an electric furnace with accurate pyrometers, regularly checked and calibrated. Too low of a critical temperature, or too short of an exposure to that temperature, and complete phasic conversion will not take place. The blades need to be removed immediately after full phasic transformation, and not heated further.

A critical point is that the decalescence point should not be held too short, nor too long, and the cross sectional geometry of the blade being heat treated plays a large role in this. A thin blade must not be held too long above this temperature, a thicker blade must not be held too short. This varies from blade to blade, and the only way to control this is for the knifemaker to have an understanding and complete control of the process.

Decarburization

If the steel knife blade is heated for too long or for too high of a temperature after the decalescence point,  the carbon will start to migrate to the surface of the steel and bond with any free oxygen, decarburizing the steel. This is a serious fault in heat treating, and it mainly comes from overheating the steel for too long or for too high of a temperature, or exposing the steel to an oxygen-rich environment during heat treating. In my own studio, I've used protective environments for decades, including vacuum, inert gas (nitrogen and argon) purged furnaces and stainless steel foil wraps that create an oxygen-depleted environment around the blades. In all of these types of decarb protection, certain other steps are necessary to ensure the proper and immediate transformations take place, such as adjusted timing, quench methods, post purged environments, and manipulations of the environment of the blade. If a blade is decarburized, two things happen. The first is that a black, hard carbon-rich crust will form on the blade, and will have to be removed. This scale is composed of a significant portion of the carbon that has migrated out of the steel to the surface. This indicates the second and most severe failure of process in decarburization. The carbon content of the steel is lowered, meaning less martensite, less carbides, and a softer, less wear resistant, and somewhat weaker blade overall. It is a very bad thing; truly a failure of heat treating. There is no way for this to be seen in a finished knife blade, no way to correct it if it does happen. The result is a blade that performs less than expected, as lower carbon alloy would because that is what it has become if decarburized. More about decarburization in example at this link.

The Hardening Temperature

This is the temperature slightly above the decalescence temperature that the steel is brought to in order to ensure that complete transformation has occurred. There are all sorts of interpretations of this temperature, some saying 50°F, some saying 100°F above the decalescence point, but it is truly only confirmed by experience guided by the manufacturer's white papers. Even the white papers may give several different hardening temperatures with the different results produced. Here, the experience of the knifemaker is key, along with accurate electric furnaces and calibrated pyrometers.

Recalescence

Recalescence is similar, only an opposite in physical reaction of decalescence. As the steel is cooled at critical points, phase change gets underway and even though the ambient temperature is dropping, the steel increases in temperature at these critical points during phasic change. This is the steel trying to reach entropy, and it has to give up some latent energy while undergoing physical change to do this. So it heats up while cooling during phasic change.

With recalescence, the same concerns exist as in decalescence, but with some more dramatic applications. Effective cooling will absorb latent heat during recalescence and phasic change, and enough coldness (absorptive environment) will allow the extra latent heat to be pulled from the steel, effectively aiding in phasic change. However, in knife blades, during normal heat treatment, we do not want recalescence to occur! Recalescence is an equilibrium reaction, in other words, it happens in slow transformation, with austenite transforming to pearlite and ferrite. Unless we are fully annealing a blade, we don't want this to happen; we want martensite instead. In order for this transformation to occur, we need fast and immediate cooling so recalescence can not happen. This means an even more robust cooling environment with the ability to pull the heat from the steel at the most rapid rate possible without resulting in stress fractures or warping.

Because of these critical temperature reactions, it should become clearer that the most effective means to control this environment is an electric furnace and accurate freezers with calibrated and accurate pyrometers. To try to do this visually is simply a guess, which may be good enough in hand-forged hypoeutectoid blades, but never in hypereutectoid high alloy and stainless tool steels.

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Titled: Knife Heat Treating Article

"Nice article. A college course in itself.
I have learned more about knives from your web site than anywhere else.
Those TV shows like 'Forged in Fire' are somewhat amusing now."

--R. S.


Building the heat exchangers on large aluminum quenching blocks:
Milled aluminum heat exchangers for liquid cooled contact quenching; covers, tube, fittings, plans, thermocouples for control
Milled aluminum heat exchangers for liquid cooled contact quenching; covers, tube, fittings, plans, thermocouples for control

Retained Austenite and Transformed Martensite

You'll undoubtedly read or hear about retained austenite (RA) in knife steel and tool steel discussions. No matter how a piece of hypereutectoid steel is treated, there will always be some austenite retained in the structure when it finally reaches room temperature. Everybody wants to limit or decrease the retained austenite because it's softer than the martensite or carbides in the structure, and decreases the steel's hardness after heat treatment. Retained austenite reduces the mechanical properties of the steel significantly, negatively influencing yield strength, machinability, fatigue strength, and even corrosion resistance. It even affects the size of the steel!

On the other side of the argument, some austenite is necessary for resilience of the structure and during tempering, the austenite is transformed to ferrite and cementite, toughening the structure overall.

There are many significant metallurgical issues about the results of retained austenite, and many differing opinions, since steel technology is a continually evolving science. Generally, though, knifemakers and tool makers want to reduce the amount of retained austenite, and increase the amount of martensite, giving a more durable structure that can then be tempered to the preferred balance of hardness and toughness the blade and knifemaker, and ultimately the knife user requires.

Just what is that balance? It depends on the steel type, the intended use of the knife, the habits and application of the knife user, and the skills and understanding of process by the knifemaker. This is another reason a maker should, at the very least, heat treat his own knives. Sending a knife off to a heat treater who slaps a "58HRC" on the blade is a cheap and fast solution, but not participation of, understanding of, and result-based function of the knifemaker. It is not custom, it is not controlled by the maker, it does not demonstrate to the knife client that the maker has a grasp on the complexity of the process. If he doesn't have a grasp on the basic metallurgy of the blade, what does that say about the geometry of the blade, the fittings and fixtures, the handle? What does that say about his understanding of the knife use and application, durability, and longevity? What does that say about his practice creating the sheath, stand, or display? These are all parts of knifemaking, and they start with the blade.

Consider that chef's knives can vary in temper depending on the style, use, and preference. A boning knife may be favored by the chef to be from 55HRC (springy and flexible) to 62HRC (extremely hard, rigid, and wear-resistant). I've had clients requesting one of each! Knives used in combat and counterterrorism may range from shock resistant and tough to ultra hard, bordering on brittle, and every point in-between. This can depend not only on the user preference, but the blade and knife design and construction overall, including the features of serrations, armor-piercing thickened sections, or razor-thin recurve areas. It is a very delicate balance and it's only learned by experience and feedback, coming from years of custom knifemaking. So it's easy to see how a heat treating contractor farming dozens (or hundreds) of blades through his line at once cannot offer much in the way of variety or specific treatment.

How does one determine the percentage of retained austenite? X-ray diffraction, that's how (by ASTM and SAE standards). Since knifemakers are not typically in possession of this equipment, they must be educated on the standard practices and procedures to produce the desired levels of these structures in their shops and profession. The information is out there, available to everyone, and there is no reason anyone who makes knives and has a access to the internet should not have a grasp of this. Granted, most knife users are not interested in the amount of RA in their knife blade, but they do want to trust their knifemaker or knife manufacturer to supply them with the best knife possible in their budget.

Simply put, we want to limit retained austenite to as small a percentage as reasonable, with as much of the austenite transforming into martensite as possible. It's pretty simple to perceive, but extremely interesting to understand how it all happens.

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Your Web Site-

Dear Sir,
I am impressed; you are the epitome of a professional.

--Alan


CPMS30V high vanadium stainless steel blade:
"Arctica" tactical, combat, rescue, survival knife, obverse side view in CPMS30V high vanadium powder metal technology tool steel blade, 304 stainless steel bolsters, Thunderstorm kevlar with brass handle, locking kydex, aluminum, stainless steel sheath with ultimate belt loop extender and all accessories
More about this "Arctica"

The Quench

The word quench comes from Middle English quenchen, from Anglo-Saxon cwencan, causative to acwincan: to decrease or disappear. The definition currently is to destroy, extinguish, or in our field, to cool suddenly, as in heated steel, by immersion in water or oil. Rather than look at quenching as an act of extinguishment or decrease of the heat, we must take a different view; one of transformation in the suddenness of time. In my view, it's not the heating that is the amazing thing, it's the quenching, and the results of the nature, timing, medium, rate, and depth of the quench.

Quenching, like most aspects of this profession, is a balance. It's a balance of cooling the blade steel as rapidly as possible, but not so rapid that the shock of cooling causes cracking and fracture. Want to know what I mean? Take a piece of 440C high chromium stainless steel and heat it to 1900 F. Pull it out of the furnace and quench it in cold water. It screams, it cries, it shudders, it vibrates as if the hounds of hell are tearing at its soul. And then it shatters into dozens of pieces that settle to the bottom of the bucket. Okay, just a visual for you to consider, but fun, anyway.

The balance a knifemaker walks is one of sudden and deep enough cooling to transform austenite into martensite, but not so sudden as to so severely stress the blade that it cracks. The exact quenching method is specified by the steel manufacturer or foundry, so it's not some mystical question that has to be resolved. Many steels (like O1, oil hardening) are designated by their quenching medium (as are A2 for air hardening and W2 for water hardening). The designation is not consistent and sometimes a particular steel type can be quenched in several different mediums. For example, ATS-34 may be quenched by air or heated oil, with slightly different results. Another reason for knifemakers to heat treat their own blades.

You may note that the medium designation is followed by the word hardening. This is what quenching does, it hardens the steel. It does this mainly by suddenly transforming austenite to martensite, taking a blade from a soft, plastic, glowing hot mass of steel to a cold, hard, stiff, and brittle one.

There are three considerations in quenching, and together, they play a pivotal role in the character and structure of the steel knife blade. They are:

  1. The Austenitizing Temperature. This is the temperature at which austenite is formed, and it's typical to heat the steel just above the critical temperature so that complete austenetizing occurs. It is posted on all commercial steel's white papers and online.
  2. The Quenching Medium (and its temperature). This is simple to determine; it is posted on all commercial steel's white papers and online.
  3. The Quenching Rate. This is how fast to cool the steel, and often depends on the medium. A quench may be interrupted, staged, or controlled in other means. The rate may require first quenching in heated oil, then air, then water. Each steel alloy is different, and the manufacturer's recommendations on the white paper are the guide to this.

While the white paper specifies the recommended treatment and expected results, there are some issues with these documents and their presentation. More on that below. Generally, though, they are a reliable guide to processing individual steels.

Cold, Very Cold, Cryogenics

For centuries the Swiss would take advantage of the extremely low temperatures of the Alps to improve the behavior of their steels. They would allow the steel to remain in the frigid regions of the Alps for long periods of time to improve its quality. Essentially, this was a crude aging process accelerated by the very low temperatures. What we now understand to have happened was the reduction of the retained austenite and the increase in martensite. By performing this once secret process the Swiss obtained the reputation for producing a superior grade of steel.

Lakhwinder Pal Singh, Jagtar Singh
"Effects of Cryogenic Treatment on High-speed Steel Tools"

Cold treatment was well known, both by process and results since the industrial revolution. Old Swiss watchmakers would "age" parts in the snow, and Pierce-Arrow would age engine blocks by putting them outside in the winter. There are other stories of men and companies aging and treating steels to the cold for advantage of higher performance on the market, but they simply did not have access to the modern investigational tooling and devices we have to identify the results of the process. They did realize verifiable known and positive results.

When I started knifemaking, there were no cold treated and no cryogenically quenched knife blades. Guys were just starting on the right track, and we used dry ice and freezers, acetone and alcohol, and whatever we felt improved the rate and depth of cold to harden the steel. Granted, the steels performed extremely well even without this extra step. To this day, if steel knife blades are processed according to the manufacturers recommendations without cold or cryogenic treatments, they will perform extremely well, because simply, the alloys are superior to plain, medium, and high carbon steels.

So why bother with cold or cryogenic treatments at all? After all, it costs money, time, space, materials, electricity, and expendables to do cold treat. We do this because it produces a markedly better steel overall, and we can prove it, particularly in the high alloy and stainless steels.

When I started to expand, refine, and improve my quenching process, I looked at it much like I look at all of the various facets of my work: improvement in steps. As I've explained in bio, at one point my blades were good, but my handles awful, so I tried to improve my handles. Then my sheaths were the worst, so I improved them. When I thought all of that came together, my embellishment was weak, so I worked to bring that up. Then, back to the blades, as the handles were now better than the blades. I look at this profession as one of continual improvement, constant upgrading and refining, coherently creating evolved works, and it was only natural to improve my steel processing as this went on. I started with no cold treatments, moved to long-term sub-zero freezing, then on to cryogenic treatments.

Let's define and identify these quenching treatments.

  1. Conventional Heat Treatment (CHT): This is, simply quenching to room temperature, followed by tempering cycles. Understand that when properly performed, this most commonly performed process will result in extremely long wearing knife blades with good hardness and wear resistance. For high alloy hypereutectoid tool steels, CHT produces blades that out-perform all hypoeutectoid steel blades in every conceivable way! Conventional Heat Treatment is not bad, and knives with high alloy steel blades treated this way can be outstanding performers.
  2. Cold. This term is being used less and less in the field of steel treatment, as it's vague and non-specific. In the old days, simply leaving an engine block or chainsaw bar in the outside cold of Buffalo, New York was truly cold, and yes, they did this for improvement of the machinings and castings. But cold is a relative world, so I'm just identifying it here as a resource. It's colder at room temperature than in a furnace or forge, so it's a relative concept. Yet, some sources talk about "cold treatments" in their advertising and literature today. Some classify cold treatments as -100°F! I'll try to stay away from the word as anything more than a generalized and relative condition as this field has refined enough to identify some more specific ranges of temperature of metal treatments.
  3. Sub-Zero (SZ). Definitely a more specific word or concept, but one wonders: Fahrenheit or Centigrade? Typically, in our part of the world, we are talking about below zero degrees Fahrenheit. Substantially colder than freezing (of water), this is available with some mechanical deep freezes, and some can be specially adapted to get down to -15°F to -20°F.
  4. Shallow Cryogenics (SCT): This is defined as -85°C, or -125°F. Earlier conventions simply called this a cold treatment, and some still do today, but as the science has improved it's now known that this is a critical and distinctive cryogenic range where steel structures are greatly enhanced. This can be accomplished with mechanical refrigeration means. Some institutional researchers simply call this "cryogenic treatment." It's important to know that the most significant improvements in steels are achieved in shallow cryogenic treatments, and somewhat less dramatic increase in deep cryogenic treatments.
  5. Deep Cryogenics (DCT): This is defined as -185°C, or -300°F. This is accomplished with liquid nitrogen in various vessels, distribution means, or chambers. It is the lowest temperature range that tool steels are typically treated to, and can affect the most dramatic change in the performance of the material, but not always. Some institutional researchers call this "ultra-cryogenic treatment."

I'm sure there will be some discussion and emails about this, but the term cryogenic simply means the study of materials at very low temperatures. Some define the temperature delineation of cryogenics at -238°F, but it is an interpretation and not specific. Cryo means cold; a cryogen is a material or solution used for freezing. The word cryo comes from Greek kryo, simply meaning icy cold.

I'm using standard convention detailed by some modern industries, who are well-versed in the technology: The Journal of Materials Processing Technology, The American Iron and Steel Institute, the International Journal of Emerging Technology and Advanced Engineering, and other various research sources (some detailed below).

This, then, is the range of temperatures of quenching for our field:

  • conventional heat treatment (CHT) to room temperature (70°F or 20°C)
  • sub-zero treatment (SZT) with mechanical freezers or dry ice (-15°F to -100°F or -26°C to -73°C)
  • shallow cryogenic treatment (SCRYO or SCT) with mechanical freezers (-125°F or -85°C)
  • deep cryogenic treatment (DCRYO or DCT) with liquid nitrogen (-300°F or -135°C)

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Dear Jay,
Your knife site is frankly dangerous. I have lost myself for countless hours reading and ogling your website and learned more about knives and knife making in the process than I thought possible. I especially love your simple, clean and extremely verbose technical style.
In the world of knife making your site should be listed as a cultural treasure.
Regardless, thank you from the bottom of my heart for one of the best websites on the internet.

Sincerely,
Aaron Young


Shallow/Deep Cryogenic Freezer and safety gear
Shallow/Deep Cryogenic freezer in heat treating area of professional knifemaking studio
Cryogenic processing of knife blades in heat treating area of professional studio

The Cryogenic Quenching Process and Factors

There are three critical factors in cryogenic quenching of steel:

  1. The quenching rate
  2. The quenching temperature
  3. The holding time at that temperature (cryogenic aging)

Understanding these helps to grasp the whole process and what is happening to the steel.

1. Quenching Rate: How does one determine the rate of quenching? It's the fastest cooling possible without experiencing destructive results in the steel. These highly negative results are cracking, distortion, warping, twisting, and uneven cooling that produces cracking, distortion, warping, or twisting. To be specific, first we refer to the manufacturer's white paper and other online resources. Second, we use trial and error. Why is trial and error needed? Because most manufacturers supply information based on 1" thick sections of the steel. Most ASTM, AISI, and other institutional testing and evaluations are based on large, heavy, thick sections of the steels, and knives are none of these. So knives differ from rated and recommended processing because they are relatively thin. Knives in quenching must be handled quickly, with speedy movements and well laid-out and well-designed equipment helps a lot in this.

I remember once having a relative in the studio heat treating his own knife. He was moving slowly, methodically, carefully, gently grasping the wrapped blade from the furnace, leisurely carrying it over to the table, cutting off the foil wrap, digging inside to extract the blade, all while I'm telling him, "Hurry, hurry!" He did not rush, and the steel quenched too slowly, and was barely hardened. We had to heat treat again. Speed is essential, and fluid, continual movement between the mediums is critical. Quenching has to be planned, thought out, even scheduled, particularly if multi-stage quenching to cryogenic temperatures is part of the process.

Specifically, in cryogenic quenching, the sudden and drastic exposure of the steel to shallow or deep cryogenic temperatures can impose such stresses and shock to knife blades that they can crack. In order to quench at an effective and continuous rate, quench staging can be employed. Depending on the steel, an initial quench based on the medium (oil, air, water), followed by freezing to below zero, and then slow cooling to shallow cryogenic temperatures, and finally deep cryogenic temperatures, if required. The rate must be controlled carefully, and each type of quenching has specific means, specially designed devices, and equipment to control this rate so the cooling is continual, even, steady, and uniform.

2. The Quenching Temperature: Again, a critical factor. This one is verified by pyrometer, and it's great to live in a time when we know, or can know, precisely what temperatures our blades are quenched to and when. In multi-stage quenching, each device or environment has a known temperature, and the instruments that measure this should be regularly verified and calibrated. This is another factor that doesn't typically  happen with "thermo-optical emission viewing" (looking at colors in hot steel), typical in hand-forging works. The actual temperatures reached may be recommended on the white papers, and most steels have extremely narrow ranges of required temperatures (as quenched to) necessary for the expected results. Some of these steels quench in stages, some may have to be interrupted or even held at intermediate temperatures during the quench cycle.

3. The Quenching Hold Time (Cryogenic Aging): This is an extremely important factor, as steel transformation does not happen all at once. Curiously, one textbook written in 2006 claims that the hold time at quenching temperatures does not matter, simply that the temperature is reached. This is totally in error. It has been proven time and again that cryogenic aging is critical, and some studies (Lal) have found that the length of cryogenic soaking is more important than the temperature of cryogenic medium! The quenching hold temp/time is critical to success of the process, and special means and devices must be employed to sustain this. Most early experiments had wide and inconsistent results, and it was traced to this critical "hold time soaking," which varied so much amongst the scientists and metallurgists that they weren't even sure cryogenics was worth the trouble! Key studies have shown that it is not enough to cool the steel, but that it must hold a good long while at these low temperatures to realize the benefit of cryogenic treatment. Carbide development, nucleation, and precipitation is a sluggish reaction, and steels continue transformation for a substantial period after reaching the lowest temperature. Experimentation has shown the limits of this and what is also too long to be reasonably beneficial. What are the cryogenic aging times? 6 to 36 hours or more, depending on the steel chemistry, size, geometry, and expected results. This is another reason to keep the process in-house, to assure times are met and not shorted for the sake of economy.

How these factors contribute

To know just how each factor contributes to the steel improvement, I'll cite the studies from a scientific test of a low carbon martensitic stainless steel used in piston rings.

  • The wear resistance was improved 43%
  • The cryogenic soaking temperature was the most significant factor, contributing 72% to the increased wear resistance
  • The cryogenic soaking time (cryogenic aging) was the second most significant factor, contributing 24% to the increased wear resistance
  • The cooling rate was the third most significant factor, contributing 10% to the increased wear resistance
  • The tempering temperature actually showed little significant change to the wear resistance, contributing only 2% to the increased wear resistance.

So what does this study show us? First, considering the steel used by the scientists, this is a fairly low carbon martensitic stainless steel, and I wish they would have done their test with a higher carbon alloy. What we do know is that the effects of this treatment are even more intense and amplified in the higher alloy steels, and this is one of the lowest possible improvement rates. Even so, it shows that the cryogenic temperature was the most critical factor, followed by the aging (soak) time, then the cooling rate, and finally variations in tempering temperature played the lowest role. Remember, the result we are looking at is increased wear resistance, and this is why temperature, time, rate, and overall processing is important to knife blades, as wear resistance is the characteristic we are enhancing. What are the improvements in higher alloy steels? They can be up to eight times the wear resistance of conventionally treated steels!

It's extremely important to know that the processing of the steel during heat treat is one of the largest successful or detrimental factors in blade steel performance. Along with geometry, the knife blade's performance is a result of the knifemaker's understanding and employment of steel heat treating process. While people in this field often generalize the relative performance of steels based on anecdotal tales, poor testing, and popular gossip, most inferior blade steel performance is based on the geometry of the blade and the processing during heat treat. Many, many steels perform well, and properly processed high alloy steels are the very best steels we have access to in the modern world.

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Hi,
I just wanted to take a moment to thank you for sharing your vast knowledge with the world at large, and providing people like me so much insight into knife-making and metallurgy for free.  Even for someone like me who may never be able to afford one of your masterworks, the knowledge you've shared has already made me a wiser consumer where knives (and possibly tools in general) are concerned...Thanks again for all the knowledge and inspiration. If I ever do strike it rich, or end up spending most of my time outdoors, I'll be sure to add even further to your list of back-orders.

Much Respect,
  D. G.


CPM 154CM after deep cryogenic aging treatment:
Deep Cryogenic treatment of Crucible 154CM high alloy martensitic stainless steel blade
Deep cryogenic aging immediately follows austenitizing and hardening

Why are cryogenically treated steels better?
What, exactly, happens during this process?

The sections above have outlined the particular factors of cryogenic quenching in a general way. No specific results were described, and if you wish to dive into the technical side of what happens and what we know (and some of what we don't) this section will help clarify why knife blade steels that are properly cryogenically treated are better performers overall.

Cryogenic treatment results offer more than just a larger volume of martensite in the steel; the cryogenic treatment even increases toughness, which is counter-intuitive to most ides of what happens during this long, cold cycle. Modern study and the capability to examine the micro-structures of steel with improved microscopy and related testing equipment have given us new and continual insight into this amazing process. The process is so fascinating, an in such an evolving state that new research is needed and is currently underway. This means that modernly processed knives are not only the best they have ever been in history because of alloy content and manufacturing methods, but also the best because of treatments available. With equipment crossovers and secondary market of equipment sales, these processes are available even for small volume knifemakers. What an exciting time to be making knives!

First, let's look at high carbon steels (hypereutectoid steels) for the basis and reason for sub-zero (SZT), shallow cryogenic (SCT), and deep cryogenic (DCT) treatments. It's important to know that the most significant improvements in steels are achieved in shallow cryogenic treatments, and somewhat less dramatic improvement in deep cryogenic treatments.

  • Conversion of retained austenite: In high carbon steels, the main reason for cryogenic treatment is a larger proportion of austenite-to-martensite conversion. More austenite is converted to martensite, less retained austenite (RA) remains. This has been proven by x-ray diffraction, and there is no dispute that higher percentages of martensite create much more wear-resistant cutting tools, even after tempering. Martensite conversion occurs over a range of temperature, and on steel charts is designated by Ms (martensite start), and Mf (martensite finish). Steels with more than .3% carbon have a complete conversion of martensite temperature (Mf) that is below zero degrees Fahrenheit. So steels that are not quenched below zero that have more than .3% carbon will have significant retained austenite at room temperature (the temperature at which we use our finished knife blades). Even the low end of the knife blade family, eutectoid steel, with .8% carbon has a Mf of -50°C (-58°F)! The more carbon, the lower the Mf temperature is, thus the lower the temperature at which complete austenite conversion occurs. This is true then, for all hypereutectoid steels. What is the percentage? Depending on the steel and treatment, the retained austenite can vary between 50% and near 0%.

    Why is austenite a problem? We know that it forms at a critical temperature, and we know then, that (from our equilibrium chart) that it does not exist normally at room temperature, so it's metastable. Metastable means it is not stable, and though somewhat stable in our current state, it will eventually decay. Austenite (while tough) is soft, unstable, and its dimensional changes impart stresses in the structure. Heavy mechanical stress (pressure), and  temperature changes can induce additional transformation of austenite, creating dimensional changes and initiating cracks. In knives, this is not nearly the problem as, say, a ball bearing, but it's considerable, depending on the amount of RA. In the tool and die industry, RA is a negative, and a major cause for premature failure. While bearings and gears may work favorably with a 5% to 30% RA volume, a knife is not a ball bearing, and high hardness and wear resistance is critical, particularly at the thin cross section of the cutting edge. The lowest possible amount of RA is desired after quenching, if possible down to less than one percent of austenite retained. This is usually only produced by subzero, shallow, or deep cryogenic quenching.
  • Precipitation of sub-microscopic carbides (η-carbides): When quenched to cryogenic temperatures and correctly aged, steels form eta-carbides (η-carbides). This is a very sluggish diffusion reaction, and another reason that cryogenic aging, or soak times, are critical to success. Eta-carbides are finely dispersed sub-microscopic carbides that tend to fill in areas of the structure giving it greater compressive strength, making it denser, harder, and tougher and more durable overall, improving wear resistance, strength and toughness of the martensite structure. These eta-carbides do not reduce in tempering, and can be physically measured by a particle counter. Technically, iron or substitutional atoms expand and contract, and carbon atoms shift slightly due to lattice deformation as a result of cryogenic treatment. Eta-carbides form in the martensite twinning structure boundaries, and have a considerable diffusing density. Some scientists suggest that these eta-carbides have a more profound effect on wear resistance than the reduction of retained austenite, in other words, they are the primary advantage of cryogenic treatment and the martensitic conversion is a secondary effect! It's is generally agreed that the eta-carbides offer a substantial measureable advantage as a result of cryogenic treatment.
  • Material stabilization: The depth and range of heating and cooling (quenching followed by repeated tempering) increases the overall stability of the steel by thermo-mechanical compression. Like flexing a rubber band, this relieves micro-stresses in the metal, making it less likely to form what could become a crack or fracture. Though it's a simplistic comparison and while there are many highly technical reasons for this, I won't start listing them as it can get pretty deep into crystal morphology, transient states, enlargement of diffraction lines in crystalline lattices, and decomposition effects. Knowing that the average size of the mosaic crystalline lattice blocks increase with a decrease in cryogenic temperatures, and you'll get an idea of the relationship of matter states and temperatures. Add to that the stress fields being reduced in cryogenic treatment, and it's a clear advantage.

With these three main factors, even medium carbon steels benefit immensely from cryogenic treatment, and the effects are more profound in the high alloy steels.

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Beautiful and extremely durable high chromium, high molybdenum powder metal technology martensitic stainless tool steel, processed in deep cryogenics: CPM154CM
"Pallene" khukri, obverse side view in CPM154CM High molybdenum powder metal technology stainless tool steel blade, hand engraved, with hand-engraved 304 stainless steel bolsters, Brecciated Jasper gemstone  handle, hand-carved leather sheath inlaid with rayskin, hand-cast silicon bronze and Imbuya hardwood stand and Paradiso Classico Granite base
More about this "Pallene" khukri

Tempering

For complete transparency, please note that since my first knife made in the 1970s, until the present day, I've heat treated every single one to the best of my knowledge and ability. Know, also, that I've never had one failure, not one return, not one complaint about the hardness and wear resistance of a single knife blade I've made.

Heat treating is not mystical wisdom, not a mystery of scientific knowledge, and not an unobtainable goal: it is simply a process. It's hot, it's cold, it's timing, it's workflow. It's numbers, it's temperatures, it's logical, like any process. And like any process, understanding and repeatability is key for reliable results.

In order to better understand what happens in the entire cryogenic heat treating process, and to illustrate more specifics of the results, it's important to detail tempering. Tempering is part of the heat treating process that is performed after quenching and cryogenic aging. There is a lot of myth about tempering, and abundant misconception, confusion, and even error where the words "cryogenic" and "tempering" are used in the same phrase, for there is no such thing as cryogenic tempering.

It's noteworthy that some companies sell cryogenic processing equipment called names like "cryotemper" or "cryofurnace." These descriptors are their names for cryogenic processors married with tempering ovens that cool the metals to cryogenic temperatures and hold them at those temperatures for cryogenic aging, followed by draining of the coolant and slowly heating to tempering temperatures for automated tempering cycles. Understand that these are separate functions, all done in the same machine, this the blending of the words "cryo" and "tempering." Still, there is no such singular operation as "cryogenic tempering."

Tempering is the process of re-heating the steel to force transformation of a percentage of the crystalline structure into another structure. The reason to temper is fairly straightforward. If steels (knives in particular) are left in an as quenched condition, they are far too brittle and unstable for working tools. All properly heat treated tool steels must be tempered to increase toughness and plasticity, and reduce brittleness and stresses. It isn't because the hardness is too high in untempered steels (most knifemakers think this); it's the lack of toughness and the lack of thermal conditioning resulting in further crystalline microstructural changes in untempered or improperly tempered steels, and these changes can cause micro fractures and cracking.

Tempering is necessary to reduce stresses and balance the hardness and toughness of the steel, and it works in several ways:

  • Tempering transforms retained austenite. This is necessary because retained austenite may not be stable. In cryogenic treatment, most of the retained austenite has been eliminated, but a very small percentage may remain. After a blade is completed and is in use as a knife, austenite will convert to martensite if the temperature of the knife blade goes below the quench temperature (not likely in cryogenically treated steels and another reason to use cryogenics). More significant is that retained austenite can convert to martensite under mechanical stress. Continually mechanically stressing the knife blade (typically at the cutting edge) may force the RA to convert to martensite. Not bad, you might think; martensite is a hard and durable structure. The problem is that martensite has a larger volume than the austenite it replaces, and this creates stress in the structure. What is the volume change? 4-5%! So room temperature stressing of these steels at the cutting edge, with retained austenite available, can create stresses that logically could lead to microscopic chipping or fracture of the micro-structure, or dulling of the cutting edge.
  • Austenite is converted to fresh martensite or bainite in the tempering cycle. Bainite is a plate-like structure of cementite and alpha-ferrite, and is harder than pearlite. It's also tougher than martensite so its presence makes the steel less brittle, without being soft.
  • Untempered martensite is a metastable structure that decomposes when reheated. Martensite has a high dislocation density, and must be stabilized by tempering or the result is a stressed structure, prone to cracking.
  • Unstable retained austenite leads to a shortened service life and fracture. This has been proven by many studies and is well-documented. Effective tempering reduces unstable retained austenite.
  • Tempering reduces the stresses in the steel by thermal conditioning. It works by relieving quenching stresses by precipitating, coalescing, and spheroidization of iron carbide and other alloy carbides, giving the microstructure increased plasticity.
  • Tempering must take place immediately! The longer the time the steel is kept at a temperature between room temperature and 100°C (212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite.
  • Cryo processesing can not wait: if the steel is hardened, cryo must immediately follow in the first quenching cycle. There are some companies claiming to improve already hardened and tempered tool steels with their treatment, but this only applies to thermo-mechanical cycling of the steel. While some RA may be converted, the amount is insignificant when compared to what happens on immediate quench.
Snap Tempering

Performing a "snap temper" is tempering immediately after conventional quenching. The blades are quenched to room temperature, and them put in a tempering oven at an elevated temperature for the purpose of converting some of the newly-formed martensite to cementite and bainite, thus softening the steel!

Why do this? It's done because in some cases, the stress of continued quenching to cryogenic temperatures can crack the metal. But this is only typical on large and irregularly shaped metal pieces such as forming dies, which may have very thick and very thin areas. Knife blades are not forming dies, and it's been my experience that they cool uniformly enough that snap temper is not necessary, and could be detrimental to the steel allotropes formed. The blade that is snap tempered does not have all of the austenite converted to martensite, and less effective and thorough conversion takes place, lessening the reason for cryogenic treatment overall.

I believe that heat treaters who do this professionally do it because it's safer to take this step so that the likelihood of fracture is lessened, since the blades do not belong to them, and they don't really know the alloy makeup or properties (as they are simply told this by the person who sends them the blade to heat treat).

Another reason is listed in the previous section. The longer the time the steel is kept at a temperature between room temperature and 100°C (212°F) after the complete transformation of martensite, the more likely the occurrence of quench cracking from the volumetric expansion caused by isothermal transformation of retained austenite into martensite. In treating large batches of steel, the snap temper allows the heat treater to take his time in the process, by removing stresses that could be caused by volumetric expansion. He can fit the cryo portion of the process into a more convenient time schedule. This may be particularly necessary if different types of steel are heat treated, since the furnace times and temperatures are different, yet the cryogenic treatments are the same. To do large batch processing, it's simply more economical and safe... for the heat treat contractor. This is detailed in the topic about batch processing and cost factors below.

For these reasons, I believe that performing a snap temper is a safer way for the outside heat treating contractor to reduce the possibility of fracturing someone else's knife blade, and thus, incurring financial loss. We can rely upon scientific testing that shows that snap temper, when necessary, permits less conversion to martensite, a lower density of martensite, and a lower density of carbides, curtailing performance, simply for the sake of safety against fracture during the cooling process.

This makes me wonder, then, if this is another source of why so many are skeptical of the process overall. Just like the inadequacy of a proper cryogenic aging time, results can be less than optimum with a snap temper, which is usually only necessary for irregular shaped items with both thick and thin areas, like metal forming or plastic injection molding dies, or when the timing of immediate cryogenic processing is inconvenient for the person processing the knife blade.

"Cryo-treatments are, clearly, the most effective in improving wear resistance if applied right after quenching rather than after tempering."

Zbigniew Zurecki
Cryogenic Quenching of Steel Revisited
Air Products and Chemicals, Inc., 2005

Multiple Tempering

Tempering several times or multiple tempering is a critical process application, and should be carefully researched and performed according to manufacturer's white papers and based on the design criteria of the knife's actual expected use and geometry. Why is it important to have multiple tempering operations?

  • Bainite plays a role in the overall structure of knife blade steels, as it is formed by decomposition of any retained austenite during the second temper. This increases the toughness of knife blade steels, so it's critical that the at the least a second temper is accomplished for increased toughness.
  • Fine precipitates of carbides are formed in tempering, as martensite is converted and the carbon moves to nucleation sites of these carbides. Double tempering assures more complete carbide precipitation and softens the stiff martensite for a workable hardness with less brittleness.
  • Cyclic conversions: There are studies indicating that the second tempering induces further relief of stresses induced not only by the hardening process, but by the conversions that happen in the first tempering cycle. The reason for this is because tempering at low temperatures only affects the martensite, and tempering at high temperatures also affects the austenite. After the first tempering cycle, the microstructure consists of newly formed and newly tempered martensite, carbides, with some retained austenite. During the second temper, newly precipitated carbides are formed and, along with the newly formed martensite, harden the steel overall. This is why in some high alloy hypereutectoid and stainless steels, there is an increasing of hardness during the course of several tempers, fully affecting these changes. Steels that exhibit this interesting characteristic that I commonly use are ATS-34, 154CM, CPMS90V, and D2. This is sometimes called "secondary hardening," and can only happen during multiple tempering cycles.
  • The cooling process between the tempers is also critical! This is something a lot of knifemakers simply ignore, yet it's a critical process application. Since there is newly formed martensite, it makes sense that between heating cycles, the martensite conversion temperature must be reached, which means a low temperature hold, below the Mf temperature.
  • Sometimes, three tempering cycles are required, depending on the steel type and geometry. High speed steels with high carbon require this, large or complex forms require this, and if high stability is desired, it's good practice. D2 can exhibit a 25% increase in toughness when triple-tempered with sub-zero holds between!
  • One more very important point about tempering. Most tempering ovens have notoriously wide hysteresis bands. This means that they heat, and reach a temperature, and shut off power to the heating elements, and then cool until a lower temperature at which the elements are turned on again to start heating. Sometimes, this band (hysteresis band) can be 100°F wide! This leads to very bad process control, and inaccurate and widely variable results. This is the same reason you don't want to use a home food oven, toaster oven, or any other inaccurate oven for tempering. These ovens are not accurate, and the heat inside the chamber is not even, leading to poor and inconsistent results. This is why I my studio, I've switched all of these controllers to PID logic controllers, accurate to one degree Fahrenheit in laboratory grade ovens.

The properties of these high alloy steels are dependent on their individual microstructure and lattice components, which are created and refined during the entire heat treatment cycle. The final hardness alone is not the determinant factor of that microstructure, the entire process is, and from this section on tempering, you can see the role that accurate, meticulous processing plays in that structure and the ultimate durability, longevity, and value of the knife blade.

Page Topics

Proportional/Integral/Derivative Control Accurate within 1° F:
Drying/Tempering oven control panel, chassis. This is a specially modified laboratory oven with high accuracy controls
Tempering/Drying Oven

Austempering and Martempering

These are modifications of conventional heat treating involving interrupted quenching techniques, or more than one quenching medium. This is done to minimize distortion, prevent cracking, and decrease the potential for other stress and conversion problems. I will reveal that these processes have no place in hypereutectoid high alloy and stainless steel knife blades, but may have a place in lower alloy and lower carbon steel types.

Austempering is heating the steel to its critical transformation temperature, and then quenching in a hot medium, usually molten salts, that are high enough in temperature (above the Ms point) to form bainite instead of martensite. This is not typically desirous in knife blades, since martensite is the desirable allotrope. It is typically done industrially to increase shock resistance, not something necessary on most knife blades as wear resistance is diminished from conventional quenching and tempering.

Martempering starts the same, with quenching in a high temperature medium (usually molten salts), and then removing from the medium to allow the steel to cool in air, so that martensite can form.

Both of these processes are then followed by actual conventional tempering after quenching has completed, which brings me to a curious point. Why are these processes called aus-and mar- tempering? The are, in actuality, processes that happen in quenching, not actual tempering of the steel. So maybe they should be called ausquenching and marquenching, as this is more accurate to the step in which these modifications are performed! But this is the terminology, such as it is, and this may be another one of the reasons there is so much confusion in the metals trade about these terms!

In any case, both of these process modifications to quenching have a result. The elephant in the living room is that both of these processes result in high levels of retained austenite, most undesirable in knives! Retained austenite reduces wear resistance, reduces strength, and leads to deformation as the steel is placed in service due to the problem of mechanical transformation, dimensional variations, and distortion at room temperature. All of these results are unwanted, and totally unnecessary for the knife blade heat treating process.

Austempering and martempering do have their place, in ductile and white cast irons, in high silicon shock resistant steels, low alloy carbon steels, and some specialty metals, but I don't see any advantage of either of these processes in the treatment of a durable, wear resistant knife blade. The only reason I can see performing this (based on extensive testing and scientific results published by researchers) is to make an inferior steel (1095, 52100) more shock resistant than conventional heat treating process. When you read the advantage details of austempered and martempered steels from companies who sell this service, you'll see why it's done. Most of these use entirely automated processers, and that is why they are economically preferred. From the austempering and martempering industry, the advantages are:

  • More resistant to shock (the only really valid reason to do these processes, but when is a knife an axe?)
  • Less distortion, distortion control (valid, but not a regular concern as a knife is not a metal forming die or a gauge block)
  • Clean surface for electroplating (not a knife blade concern or issue unless you're a factory producing chrome-plated blades)
  • Resistant to hydrogen embrittlement (not a knife blade concern)
  • Uniform and consistent hardness (this happens with any properly treated blade steel)
  • Tougher and more wear resistant (than conventionally treated low alloy steels, but research does not always support this claim, and if you notice, the comparison is to conventionally treated steels, not cryo treated steels)
  • Hardness target: 38HRC to 52HRC (Crap! That's soft! No thanks!)
  • Greater ductility (I'll stop there, this is NOT something you want in a knife blade!)

The steel types typically austempered and martempered are SAE 1045 to 1095, 4130, 4140, 5060, 5160, 52100, and 6150, distinctly low alloy steel types. Since these steels are inferior in many ways to high chromium, high alloy martensitic stainless steels, I don't use typically use them. Now, when you see the process identified, you'll know more about it and its applications in the world of hand knives.

Page Topics

Post Processing Weight Determination (to determine usage/evaporation rate):
Post processing evaporation and usage rate is determined by weighing residual liquid nitrogen in a container. Here you can see the dramatic coldness of this liquid as it is returned to the Dewar Cryostat
Cryogenic Treatment with Liquid Nitrogen at -320° F, returning Liquid Nitrogen to transport/storage vessel

Normalizing, Annealing, and Spheroidizing

These are, technically, softening processes for steels, but their application depends immensely upon the alloy type!

Normalizing

Normalizing has no place or purpose in processing high alloy and stainless tool steel blades.

Read enough about knives and knife blades on the internet, and you'll come across the term "normalizing" sooner or later. Just what is normalizing and how does it work? More important, does it play any role at all in working and processing modern, high alloy and stainless knife blade steels?

Hey, just what is normal? With steel, everything is variable and changeable, so there really is no normal, so let's just get to the definition of the process of normalizing. Normalizing is heating the steel to a temperature above the transformation range (where alpha-ferrite and pearlite convert into austenite) and then cooling it in still air. That's it.

What? Why? I will firmly declare that in working with high alloy hypereutectoid stainless and tool steels, we call this "hardening!" Yep, if you try this with any of these steels, they won't become normal, they will become quenched, and extremely hard.

So what is the purpose of normalizing? The purpose is to soften the steel and reduce stresses to make it more workable! What? I'll soundly declare that if you try this with these upper-tier high alloy and stainless steels, you won't be doing any work with them at all, as they will be in the lower 60s in Rockwell hardness, and a file and drill bit will just glance off the hardened surface. So in working with high alloy steels, normalizing has no place or purpose at all. Normalizing is then for lower alloy steels and carbon steels.

 Normalizing is a process that's cheaper and a bit faster than annealing, but based on the same idea. You take the steel to its austenitizing temperature, and then cool it slowly. But "cool slowly" is a general term, and needs to be suited to the individual steel alloy. For instance, in one reference, normalizing is done at 100°F temperature drop per hour, in another just sitting in room temperature air. The purpose of normalizing is the same as annealing, to reduce stresses or hardened areas before machining or working the steel. But the normalized blade is not annealed, and the properties of the steel are not uniform (as in annealing). Because normalized steel is not uniform, stress are created, and then the blade may then need stress relieved. So normalizing is not a final condition, but a part of the working process of a typically lower alloy steel.

Normalizing is done with lower alloy carbon steels as a cheaper and faster alternative to annealing, since it doesn't take as long and is not as expensive as having a dedicated oven slowly lowering the temperature of the blade in many, many hours. Because normalizing is not really effective in extremely high alloy tool steels, and annealing is, the word normalizing is an indicator that the knife blade is a lower alloy type. In all my decades of making fine, high alloy tool and stainless steel knife blades, I've never had to normalize a single one of them. Oh, I've annealed a few, but I can count them on one hand. The important thing to note is that high alloy and stainless tool steels are not normalized, they are annealed, and diligent efforts should be made so that annealing is never needed.

 

Annealing

Annealing is full softening of the steel. In annealing, the steel is taken to its austenitizing temperature or a recommended temperature just below it, and then cooled very slowly, extremely slowly, to allow the equilibrium transformations to take place. Every process temperature, time, and step of annealing is different depending on the steel alloy content, and the white papers are a guide to this fully softening process. Annealing is done to create the most ductile, most malleable steel possible, for several reasons.

  • One of the reasons to anneal is to reduce stresses created in machining steels. If you have complex machining or forming operations, stresses can be created and areas can be work-hardened with localized hardening making it difficult to achieve further machining operations. For instance, say you are drilling a hole in steel and overheat the area because of a dull drill bit. The area of contact can instantly harden, since so many of these steels can quench harden in room temperature air. Then, when you try to continue, the steel is too hard to drill, and it must be softened. With the steels I use, the only option is to fully anneal the blade, or use a drill that can drill through the hardened area, usually a tungsten carbide drill.
  • Another reason to anneal is a full-on disaster, like a blade warping out of heat treat. It can't be straightened, it is ruined, unless you can fully soften the steel to straighten it, and start the blade treatment over with.

The important thing for me, as a professional. is to never have to anneal a blade in the first place! Both of these scenarios happen because of other failed steps or mistakes (bad process control or dull cutting tools) and I never, ever purposely want to have to anneal a knife blade. I'll also clarify that in some of these steels, full annealing is almost impossible; they stubbornly refuse to return to the state they arrive from the foundry (fully spheroidized and annealed). This is another reason so many knifemakers do not like working with high alloy tool steels and stainless steels; they are unforgiving of error or casual attention. They need to be made right, the first time, and processed once, for correcting an error may not even be possible.

Spheroidizing

Spheroidized metals are in their fullest, dead soft condition, and this is typically how they arrive from the foundry. The term spheroid refers to the spheroidization of the plates of cementite contained in the pearlite structure, making them big and round and granular and thus, ductile and easy to machine. Spheroidizing is a step beyond annealing, and is expensive achieve, as it takes many hours in the furnace with extremely slow cooling so the equilibrium phase transition can take place. In spheroidizing process, the steel may need to be held for an extended time at the austenitizing temperature, and cycled in the higher ranges before extremely slow cooling results in a fully spheroidized structure.

 Usually, spheroidization is not necessary, in the decades I've been making blades, I've never had to attempt this on a single one. Since most steel billets arrive at the studio in this condition, they are already at their easiest working condition, dead soft, and as soft as they are ever going to be. This might be surprising, though, to those who work with low carbon or low alloy steels, as even in their dead soft condition, these high alloy and stainless tool steels are comparatively tough and difficult to machine.

Now that you know these three important conditions of metal: spheroidized, annealed, and normalized, you'll understand why steels are shipped from the foundry or supplier in "fully annealed and spheroidized" condition, the reasons for annealing, and why no high alloy steels and stainless steels are ever normalized.

Page Topics

CPM154CM knife blades in tempering oven:
Tempering of knife blades in specialized high accuracy tempering oven
Dedicated, laboratory grade, high accuracy stainless steel tempering oven

Cryogenic Processing and High Alloy and Stainless Steels

"When compared with classical quenching, performed to ambient temperature, cryogenic cooling has more effect on the steels with larger amounts of carbon or alloying elements."

Handbook of Residual Stress and Deformation of Steel, ASM International, Totten, Howes, Inoue, 2002

Stainless high alloy steels are the fastest growing steel types made. This illustrates how important these steels are to the world. Martensitic stainless steels constitute the majority of high alloy hypereutectoid steels I and other makers of fine handmade knives use, simply because they are the very best. Even without cryogenic treatment, their performance, strength, wear resistance, corrosion resistance, and durability overall surpass all lower alloy commonly hand-forged steels by many times and in all characteristics. High alloy modern tool steels, martensitic stainless steels, and powder metal technology tool steels benefit greatly from cryogenic processing.

While this information is still being studied, and not all of the effects are well-understood, it's clear that the performance of these steels is terrifically enhanced by cryogenic treatment. It's best to break these properties and results down into individual aspects that have been proven by studies and scientific experimentation:

  • Because these extremely high alloy steels are heavy in carbon, their Mf transformation temperatures are sub-zero. This means that at the very least, they should be quenched below zero Fahrenheit to assure as complete as possible the transformation from austenite to martensite. Cryogenic treatments are the most effective for this transformation.
  • Martensite is a critical component of cryogenically treated steels, and has a hardness of up to Knoop 800 HK. This is four times harder than annealed or non-treated steel, so it's important as the basis for high wear resistance to improve the amount of martensite overall through cryogenic treatment.
  • Martensite plate size is something seldom discussed, but it's understood that a reduction in size of the martensite plates leads to a finer grain, more interlocking boundaries, and a harder steel. What kind of martensite plate reduction are we talking about with cryogenically treated steels vs. conventional heat treating? How about a ten-time reduction of martensite plate size? This is an order of magnitude and astoundingly demonstrative of the cryogenic effect. The smaller size means a harder, tougher, and more wear-resistant blade.
  • Because of the transformation sluggishness of carbide precipitation detailed in the previous section, the hold time (cryogenic aging) at extremely low temperatures should be significant. In my past works, this hold time proved to be a beneficial result, and even though cryogenic temperatures were not reached, holding the blades well below zero for many hours (10, 20, or more) resulted in a superior blade performance. While this undoubtedly aided in the carbide precipitation, cryogenic treatments are much more effective at producing these results.
  • With all carbides, their effectiveness depends on how fine they are, how well-dispersed, how high the volume overall that is precipitated. A critical point is that the three elements chromium, molybdenum, and vanadium have the highest solubility in austenite, therefore they precipitate the highest volume of carbides. This is why these three are big players in high alloy steels.
  • Since there are so many elemental alloys included in these steels, dispersion of these elements within the material becomes a concern. In cryogenic treatment and aging, the element solubility decreases, so molecules move within the structure. The vacancies migrate, and concentrations of single elements disperse, leading to a more even distribution overall.
  • More carbon moves around, bonding with chromium, creating a larger volume of chromium carbides (CR23C6). Since all stainless and high chromium tools steels contain a large amount of chromium (440C, D2, ATS-34, 154CM, N360, CPMS30V, CPMS90V, CPMS35VN) and a large amount of carbon, significant amounts of chromium carbides are formed during cryogenic aging. Iron carbide (cementite) has a Knoop hardness of 1025 HK, but chromium carbides have a Knoop hardness of 1735 HK, 1.7 times harder, leading to higher wear resistance. This is another reason that stainless steels are flat out better performers than carbon steels, which have little or no chromium to form the carbides.
  • Contraction is the physical process that takes place in deep cryogenic processing, in the aging cycle and does not typically occur in shallow cryogenics. The austenite and martensite are so cold that they contract, which physically forces carbon to diffuse, resulting in a greater density of carbides and a more homogenized distribution of carbides.
  • Many of these steels (like ATS-34 and 154CM) are high in molybdenum. Cryogenic treatment helps produce a higher volume of molybdenum carbides, and they are 1.8 times harder than iron carbides, leading to higher wear resistance. Molybdenum has been specifically proven to disperse and move within the crystalline structure at cryogenic temperatures, resulting in a higher volume of molybdenum carbides in high carbon alloy steels, and another reason that long cryogenic aging is critical in these steels.
  • Some of these steels (O1, CPMS35VN) contain significant amounts of tungsten. Cryogenic processing increases the amount of tungsten carbides, which are 1.85 times harder than iron carbides, leading to higher wear resistance.
  • Several of these steels (O1, CPMS30V, CPMS90V, and CPMS35VN) contain significant amounts of vanadium, and cryogenic processing increases the amount of vanadium carbides. Vanadium carbides are 2.6 times harder than iron carbides! This leads to a tremendous increase in wear resistance.
  • Toughness: Since significant cryogenic aging allows more homogenous distribution of the micro-carbides, and since the stainless and high alloy steels have a very large proportion of these carbides, low temperature conditioning produces microstructural and crystallographic changes resulting in an increase in toughness.
  • Hardness: Of course, a dramatic increase in hardness occurs in these high alloy tool steels when cryogenically treated. Consider that the blade will be tempered back, made less hard overall, during the tempering process, and that hardness doesn't contribute as much as in the initial quenched hardness. This is misleading for several reasons: first, because the blades are tougher and more resistant to fracture overall, they can be tempered to a higher hardness without being brittle. This means a much more wear resistant blade. Secondly, the improvement of wear is non-linear; a ten percent increase in wear resistance offers a much greater increase in durability and longevity overall. So when you consider that the cryogenic processes of these tool steels simply produce a higher hardness, that hardness translates to many times the durability and longevity of a tool used to cut.
  • Resistance to cracking or fracture: Conventional considerations about steel suggest that harder steels are more brittle, and there is a persistent idea that cryogenically treated steels are, even after tempering, more brittle and subjected to cracking, but this is not the truth. Scientific metallurgical studies have proven that the abundance of micro carbides created in these high alloy steels assist in enhancing micro-stress distribution, improving (by reducing) fracture growth in the material overall. Simply put, cryo-treated high alloy tool steels are more fracture-resistant than conventionally treated or sub-zero treated steels.
  • Fatigue life: Since the crystalline structure is improved overall in cryogenic processing, it is well-known and established that cryogenically processed steels (and many other metals) benefit from a long-term fatigue life improvement. In considering fatigue life, an important factor is the repetitive forces of stress over a long time, which is much different than a singular, initial force. Studies have shown the springs, particularly valve springs in high performance racing cars under high, continuous, forceful movement have benefitted from cryogenic treatment with many times (up to 7 times) the life of conventionally treated springs! This translates to a longer fatigue life for the knife blade, particularly at the cutting edge, where tremendous forces and deflection are in play.
  • Nickel is limited in these high alloy martensitic stainless steels. I mention this because of the critical effect of nickel on the austenite structure. While nickel improves ductility and machinability (not something you want in a hard, wear-resistant blade) and worse, it's an austenite stabilizer! So less martensite conversion will take place at sub-zero and cryogenic temperatures as nickel is increased. Nickel is not typically alloyed in these steels, but in a few it has a very low volume because of its detrimental nature. I'm assuming it's added so that the steel can be more easily machined.
  • Wear reduction: This is improved in cryogenically treated tool steels by a proven reduction in asperity ( Dr. Sudarshan of Materials Modification Inc. and Dr. Levine of Applied Cryogenics). Asperity is the roughness of the surface, and when steels are cryogenically treated, the wear of the surfaces is typically reduced by half, even though the same polishing methods are applied! This is because the cryo treated steels have less microscopic peaks and valleys, contributing to an improved polish, improved finish, and lower wear.
  • Appearance: Here's a characteristic that you won't find discussed on any scientific paper, because these studies are concentrated on and funded with the intention of examining and improving the physical and material performance, not the appearance of finished steel. However, you will see it discussed in the realm of wear resistance in polished surfaces, focusing on asperity, and it's been proven that after cryogenic treatment, the surface can be highly polished, better polished, with less peaks and valleys leading to a smoother surface overall. Since only artists and fine craftsmen are typically interested in a tool steel's finished appearance, it is up to us to reveal what the steel looks like after cryogenic treatment as opposed to conventional heat treatment. It stands to reason that changes in the crystalline lattices of cryogenically treated steel would change the outward appearance particularly when finished to a high degree of smoothness, as in mirror polished knife blades. Here is what I know:

    High chromium martensitic stainless steels like 440C or ATS-34 are processed with conventional heat treat (CHT) or with sub-zero heat treating (SZT), and the steels are then tempered and finished by grinding and then polishing. These steels are beautiful in their own right, with mirror polishes showing some grain texture. These textures appear like a much diminished and less noticeable version of D2 steel's "orange peel" granularity pattern, seen when held in just the right angle of incident light. While D2 has a much bolder and profound pattern, this same type of effect is seen in ATS-34 and 440C, with curves in the pattern following grind terminations, trailing points, and other geometric features of the blade. If the blades are cryogenically treated, these patterns will not appear! The cryogenic treatment makes the finish of these two steels much more like the finish of powder metal technology tool steel, namely CPM154CM. The surface is extremely clean and uniform, and no grain can be seen at all. This makes sense, considering the greater conversion of austenite, but perhaps more so the precipitation of fine carbides throughout the structure. Simply put, cryogenic treatment produces a more even, uniform, smooth, and beautiful finish than conventional heat treating.
  • Corrosion resistance: This is a complex interaction response to an environment, so I've broken it up into subtopics:
    • Finer finish: The key to this first consideration is in the previous topic, appearance. Because cryogenic treatment produces the possibility of a finer finish (depending on the skills of the metal finishing knifemaker), it stands to reason that the surface is more corrosion resistant simply because the surface is smoother, with less irregularities, and fewer boundaries of different allotropes where corrosion or oxidation could start. This is believed to be due to a larger amount of microscopic carbides in spherical shapes, and a smaller, more refined structure overall.
    • Martensite: There are several differing opinions on the physical corrosion resistance of CHT vs Cryo-treated steels. One consideration is that since more martensite is formed, and martensite is less corrosion resistant than austenite, that the retained austenite would help to increase corrosion resistance. This seems logical, but it is not the case, even in conventionally treated steels! It is well known that conventional heat treat alone increases the corrosion resistance of all steels, particularly high alloy stainless steels. It's also known that the harder the stainless steel is after temper, the more corrosion resistant these steels become. In this comparison, it then seems counterintuitive that corrosion would increase if more martensite is left in the steel, since harder steels with more martensite are clearly more corrosion resistant.
    • Carbides: it's clear that the uniform distribution of micro-fine eta-carbides helped to decrease the corrosion potential of these steels, simply due to the increase in percentage of these carbides. The increased amount of chromium carbides in these cryogenically treated steels further enhance corrosion resistance throughout. Studies have also shown an interesting repassivation effect due to higher levels of chromium carbides occurs, further enhancing corrosion resistance.
    • Water contact angle testing: Here's some interesting stuff! Pure water (deionized) is used to conduct a test of CHT and CryoHT steels. In this test, the contact angle of a water droplet is used as an indicator of the steel's hydrophobic or hydrophilic response. Hydrophobic means having little or now affinity for water, and hydrophilic means having an affinity for water, or to be easily wetted. Of course, since water is the critical factor in most corrosion response, we would want a stainless steel to be more hydrophobic, or resistant to water. This resistance (or acceptance) of water is measured by the angle of contact of a water droplet resting on the surface. It has been proven that cryogenically treated steels exhibit a greater contact angle, and are more hydrophobic than CHT steels. This is believed to be due to large amounts of carbide precipitations in spherical shapes, which allows a smoother surface.
    • Lattice Size and Electrochemical Response: studies have shown that cryogenic treatment can reduce the overall lattice sizes, enabling better corrosion-resistance performance. This is because cryogenic treatment produces a material that is more dense and homogenous, increasing the electro-potential resistance, enhancing corrosion resistance, and preventing corrosive media from directly penetrating deeper downward into the steel.
    • Residual Carbon: since cryogenic treatment produces more carbides and less residual carbon, corrosion resistance is further enhanced. This is due to the a greater amount of small carbides and crystal chromium homogenizing with the crystalline boundaries, with better corrosion resistance overall.
  • Working and sharpening: Is the cryogenically treated knife blade a better performer? Why of course it is, and from the data presented on this page, it's easy to see why improvements of 100 to 800 percent in wear resistance lead to a blade that has much greater longevity, durability, use, and value.
    • But what about sharpening? What about the only service that the knife owner must perform himself during his life, and the life of the knife? Since steels are more wear resistant when cryogenically treated, it stands to reason that they would be resistant to the stone and harder to sharpen. They are. There simply is no way for a knife to be extremely wear resistant and yet be easy to sharpen; that's a myth. But along with advances in steel alloys, we have advanced in our sharpening methods and materials as well, and it's not as complicated as one might think. For instance, you don't need a power driven sharpener, a rack with sticks, a clamping guide, or any other gizmo to effectively sharpen the most modern, super-hard, super-tough high performance alloy tool steel blade. You need diamonds. Now, if you wince at the thought of diamonds and dollar signs go scrolling vertically through your eyes, please know that diamond hone sharpening is actually the most reasonable breakthrough for knives that has happened since these alloys have appeared. Nothing, I mean nothing is as hard as diamond abrasives, and though you might think your cryo-blade is so hard it can't be sharpened, you underestimate the hardness of diamond. Diamond hones will last indefinitely if you take simple care of them: they don't change, don't wear, don't get curved, concave, or clog. The very best hone will cost about $75.00 at the time of this writing, and it will be the last hone you'll ever buy. Realistically, you don't need a whole range of grits, but it's nice if now and then you can get additional grits to upgrade your sharpening tools. $75.00 is a night on the town, a big tank of gas, or the cost of a very cheap suit. But unlike Arkansas stones, silicon carbide, aluminum oxide, India oilstones, ceramics, and other conventional types of stone, diamond cuts all of them, cuts through all steels, and will keep on cutting until your blade wears away, no matter what it's made of. You don't need oil, or water, or a rack, stand, frame, or electricity.
    • Working with cryogenically treated steels is a dream. This seems counterintuitive, since they are so hard and wear resistant. Please bear with me while I get a bit technically descriptive in a language that most metalworkers will relate to. This section is for those who will sharpen, and also for those makers who will grind, shape and finish a cryogenically treated blade.

      First, in sharpening, here is the noticeable difference: When using a diamond hone to sharpen, a conventionally treated blade will smear a bit. What this means is that the blade deposit left by sharpening (the swarf) seems a bit gooey, with larger strings of steel depositing on the diamond stone. They seem a bit clingy and sticky, requiring a vigilant cleaning of the stone so the blade has access to the sharpest of the diamond grits. Effective edge improvement stops at about 400-600 grit, with higher grits just polishing the edge and no marked improvement in edge quality. This is fine for most purposes, particularly when you consider that a medical scalpel is only finished to about 400 grit. But with cryogenically treated steels, there is a noticeable difference. The residue left by sharpening (the swarf) is very powdery and smooth. Instead of a clingy metallic deposit, it seems more like a fine hard powder. This is probably due to the higher martensite content and even more so by the high micro-carbide content of the cryogenically treated blade. Because this powder is very fine and easy to blow off of the stone, sharpening is a bit faster. More importantly, the powder effect extends into much, much higher grits (if desired) and this allows a sharpening up to 1200, 3000, or even 8000 grit! This leads to a super-smooth sharp blade, the smoothest and sharpest I've ever seen, and I've seen more cutting edges than just about anyone alive.

      Second, in working: this refers to working with the cryogenically treated steel blade, and is then more for other knifemakers than the public. After cryogenically treating blades, as a knifemaker, I have to finish them, usually to a bright mirror polish. Some tactical knives have an abraded, media-blasted finish, but in either case, they have to be ground. As with sharpening, conventionally treated blades leave a sticky, clingy residue to the abrasive belt, particularly fine grit belts in the 40 micron to 5 micron range. The cryogenically treated blade grinds easier, is stiffer to deflection (something not studied in cryogenic testing), and leaves a smooth, powdery residue. The only challenge is in mirror polishing, which is more difficult in the cryo-treated blade. But it's worth the extra work; these blades are beautiful.
  • Unusual and remarkable effects: There are some strange effects that happen when a high alloy tool steel is processed cryogenically. Though these characteristics do not affect the typical doubling or tripling (or more) of the service life of the cutting tool, they are of interest to those who make knives particularly, but metalworkers in general, and are interesting and substantial.
    • Dimensional stability: It's proven that the cryogenically treated steel is more dimensionally stable, in that gauge blocks accurate to .0001" can be made and expected to stay that way for many decades, whereas non-cryo-treated steels will change. But there is another very interesting aspect of this treatment. When the knifemaker drills holes in the blade before heat treatment, after conventional and sub-zero treatment, the holes are ever so slightly larger in diameter (perhaps .0005" on a .125" diameter hole), and pin placement is easy with plenty of play. This is due to the volumetric change during heat treat. Properly cryogenically treated blades do not show "growth" and the holes are exactly the same diameter as drilled, which means they are tighter, more accurate, and less forgiving of error. So, in this case, it produces a tighter, more accurate metalwork, and the maker had better be paying close attention, as these dimensions can not easily be changed! In that way, metalwork with cryogenically treated blades can be more difficult than CHT or SZT blades.
    • Harmonic frequencies: The cryo-treated blade is a different allotrope, so it has a different frequency range of harmonics, or ringing. This is noticed when pins are driven and peened in bolsters, when the blade experiences the ringing effect of hammering or impact. Only knifemakers would probably notice this, but these blades will ring at a higher, more harmonious frequency than CHT or SZT blades. This hasn't gone unnoticed in the musical instrument field, and many instruments and bells are now cryogenically treated. The musicians who have this done report a more responsive, brighter tone with higher overtones. This is a specific process called resonance enhancement. It won't make your knife sing; it's just an interesting effect.
  • Using cryogenically treated knife blades: Of course these blades are superior in use, just as sub-zero and aged blades are superior to conventionally treated knife blades. Sub-zero quenched and aged blades have been my mainstay for decades and their performance is well-established and appreciated by the thousands of knife owners who have used and sworn by them for decades.

    When I started cryogenically treating knife blades, I wanted to see something, something profound that I could relate to without all the scientific testing apparatus employed by all of the scholars, metallurgists, scientists, and institutions referenced on this page. I wanted to see some result I could relate to for anyone who has read this far and has an interest in this process. What would that be? In the shop, I heat treated a 440C blade with cryogenic processing. I finished the blade; it was a blade that was being donated (thanks to an anonymous client and my own personal contribution) to a United States Air Force Pararescueman for his use in combat rescue service and duty. I finished the blade and sharpened it to 8000 grit on diamond hones. I started cutting. I cut paper, cardboard, and wire, I cut hemp rope until I simply grew bored, and realized I needed a much more aggressive encounter. I wanted to make the blade fail. I was after a failure, a bending, breaking, or chipping of the edge. Dulling would be nice, too. So I started cutting other metals. I cut aluminum until, after realizing nothing was happening to the edge, I started chopping on the aluminum. Now, this is something I strictly state that hand knives are NOT designed for, in any way. But I wasn't getting anywhere in the cutting stuff, and I was intent on some failure! Please note that this was an extremely thin cutting edge, something you won't see on just about any other maker's handmade knives and something never seen on any factory knife in this steel type and hardness. When nothing happened with the aluminum, I started chopping... brass. This was a 1" diameter hard brass,80B Rockwell, free cutting brass used for screws, bolts and fasteners, nearly as hard as mild steel, and some tough stuff. I wailed on it with the knife, chopping chunks out of it like a little axe! When I examined the thin edge, there were just a few tiny dings, invisible to the eye, but imperfections i could feel with my fingernail none the less. So I stopped, satisfied, and resharpened it. Then, I pushed it further. I clamped a block of mild steel in the vise, and started carving the corner of the steel block away with this very thin cryogenically treated blade. I wasn't hammering wasn't landing blows, but was applying heavy pressure to literally carve away the steel block. It cut the steel with ease, and there was no dulling, not one little bit, of the edge. Cutting steel. With a hand knife. I know this is an anecdotal account, simply a easy visualization, but sobering nonetheless. I don't expect any knife to be used this way, but it's encouraging to experience the result of cryogenic treatment, chopping a hard brass bar and carving steel. There is no doubt if I continued, I'd be able to chop or cut them both it in half. By the way, the blade was 440C, treated to shallow cryogenic temperatures, double tempered, with a final hardness of 60HRC.
  • Cost: From someone who has created more cutting edges in his lifetime than most people have ever seen, I can soundly state that cryogenically treated blades are worth the effort to create a markedly superior knife blade in every conceivable way but one: economy. While inexpensive knife blades have their place in current mass-marketed and primitive hand-forged works, this is not the kind of knife I make, nor do I want to. Since the cost of creating a cryogenically treated blade is born mainly in the equipment and process, once that is established, the cost is fairly low. It takes more knowledge, equipment, more electricity, expendables, and time to create the superior cryo blade, but it's something that my clients deserve, so that is what I offer. And, as with all knifemaking, it's fascinating advanced process that the very finest steels deserve. Below are how some of these steels individually benefit from this process:

O1: This high alloy oil-hardening tool steel is a standard in the industry for a reason. It's a great hyper-eutectoid tool steel with about .9% carbon and the version I use has high tungsten and vanadium with a bit of chromium, though not enough to be stainless steel. I use it when clients want a great performing black colored blade, because the finish and bluing is excellent on this steel. On one website about steels used in woodworker's tools, the writer claims that because O1 has a higher martensitic conversion than other steels, cryogenic treatment is not effective. This is flat-out wrong. While O1 does perform well with conventional heat treating, cryogenic treatment vastly improves this performance. How does it benefit from cryogenic treatment? O-1 can have up to 8.5% retained austenite when quenched to room temperature (20°C). While this does not seem to be a lot, it is significant, and proves that at the very least, O1 should be quenched to sub-zero temps and held there to reduce the amount of retained austenite. So much for the woodworker's assessment of O1. Now here's the really important result and proven by highly specific and controlled technical scientific studies: in treating O1 to shallow cryogenic treatment (SCryo), the wear resistance was improved 221%. In treating O1 to deep cryogenic treatments (DCryo), the wear resistance was improved 418%. Simply put, either of these treatments dramatically and substantially improves the wear resistance while making the blade tougher, and the finish better overall! Why not do this?

440C: This high alloy martensitic stainless steel is a great performer. In SCT, its hardness is increased by 4%, and in DCT, by 7%. and by bringing this steel to a shallow cryogenic treatment, it has 128% improvement of wear resistance. This is a strange respondent in the cryo field, as deep cryogenic treatment does not significantly improve this steel performance, so it doesn't need the liquid nitrogen quenching of DCryo. In fact, in deep cryo, it only has a 121% improvement of wear resistance, so it has better results in shallow cryo. It's interesting to note that the standard by the United States Air Force for all parts made of 440C in any aircraft are that they are cryogenically processed with shallow cryo, and that has been the standard since 1995!

D2: This steel is an incredible steel and benefits undeniably and astoundingly from cryogenic processing. Since D2 has so much carbon 1.7% and so much chromium (12%), it creates abundant chromium carbides even with conventional heat treating. The martensite is also profuse in the structure, due to the extremely high carbon. In shallow cryogenic treatment, D2's wear resistance is increased 316%, which is wonderful. In deep cryogenic treatment, D2 becomes another animal. DCT increases the wear resistance of D2 up to 820%! Over eight times the wear resistance of conventional or even sub-zero heat treatment is an astounding result, and it has been proven over and over again in numerous scientific studies how profoundly D2 responds to this procedure. If you have a conventionally treated D2 blade, you already know that this is a very wear resistant alloy, one that takes and holds an edge for an incredibly long time. Now imagine the same species with eight times more! When properly done, D2 also benefits from the precipitation of finer carbides which lead to increased toughness as well. D2 benefits from multiple tempering cycles (at least three), because they promote the precipitation of secondary carbides, and with triple tempering an increase in toughness of 25% is experienced when compared with double tempering.

ATS-34 (and 154CM) is a great performer all around. When given conventional heat treatment, it results in a high performance blade with high toughness and very good corrosion resistance (though not as good as 440C). When cryogenically treated, some very interesting results take place. The steel as in all high alloy martensitic stainless steels, develops extremely fine microscopic carbide particles. The finish is smooth and excellent, and because of the high molybdenum, two additional results occur. The first is that the creation of molybdenum carbides is abundant, particularly when given a good, long cryogenic aging. This brings up the hardness significantly and the wear resistance is abundant. The second benefit is that because of the high toughness in this alloy due to the high molybdenum, the blade can be tempered to a higher hardness overall, without fear of brittleness. The result is an extreme improvement of high wear resistance and high toughness, along with improved corrosion resistance. This same result can be experienced with 154CM, since technically, they are the same alloy. In other studies of high molybdenum tool steels it can be suggested that a 200 to 300 percent improvement of wear resistance is experienced, even though no certified testing data is available for these steels in the current research literature.

CPMS30V, CPMS90V, CPMS35VN, N360: In all of the other steels I use in making knife blades, the cryogenic treatment certified testing data has not been done. This is probably due to several reasons; mainly the proportional rarity of these steels to common machine tooling steels, and the expense of the studies, along with requests from the steel suppliers for study details. Considering the typical results using high speed, cold work, and high alloy martensitic stainless steels, it is generally expected to achieve a minimum 200 percent increase in overall wear resistance, with up to a 800 percent increase in wear resistance possible with deep cryogenic treatment of these steels.

It should now be clear why cryogenic processing of these particular steels is one of the most important improvements that can be implemented on a high performance, high alloy steel knife blade.

Page Topics

Atmospheric condensation of water falling from knife blades at -320°F
Warming to room temperature after cryogenic treatment and aging
CPM154CM knife blades warming to room temperature after cryogenic treatment and aging

Grain

What is grain, and what role does it play in steel knife blades?

The word "grain" in steel refers to the particles of the crystalline lattice, and how the word is interpreted and in what context it is used changes the definition of grain. For instance, a grain may mean the singular crystalline lattice of a microscopic particle of carbide, or it may mean the group of bonded lattices that are surrounded by another material. Grain may mean the visual appearance of finished steel or freshly broken steel, or it may mean the finest particles visible under an electron microscope.

Literally, the word grain is defined as the discrete particle or crystal determinable in the matrix. So you can see that the type of particle, the size of the particle, and even the viewing apparatus used to see the particle identifies the type of grain being described. Grain study and structure is common in steels, it can determine the material, size, shape, and bonding structure of the crystalline particles, and thus their percentage in determining the effects of various thermal treatments. There are studies about grain size, grain shape, and grain boundaries. There are studies and procedures for lapping the surface, etching the metal, and examining and counting grains under a microscope. From this, you can see that grain complexity is a science into itself, and belongs, in our case, in the realm of the metallurgist and materials scientist.

When knifemakers talk about grain, you should probably take what you read with a grain of salt. Sorry for the bad pun, but in all seriousness, grain manipulation, grain bonding, grain sizing, shape, and structure is beyond the realm of the knifemaker, no matter what forum or venue he is posting on. This strange fascination with grain probably hearkens back to the blacksmithing or hand-forging days, when you could heat treat a piece of metal and then break it in half, and visually examine the grain. A large crystalline grain would mean it wasn't at it's best hardness, a small, fine grain meant you were close to the mark. But this is far and away from scientific grain testing and study, something I will flatly claim is out of the realm of knifemaking.

The reason this is not the knifemaker's realm is that knifemakers only control the shape of the steel blade, its geometry, and the process of heat treating and finishing of the steel, and do not control grain structure. Mistreatment and bad practices on the part of the knifemaker will result in an inferior blade performance, and some of these defects may be visible in the grain structure. Understand that no knifemaker is working under an electron microscope, and no knifemaker imparts some special magic in his process to manipulate grain changes in the steel that are improvements on standard process and cryogenic processes.

The reason I'm detailing this is that I've seen these "grain discussions" for years, on forums, in postings, on websites, and in bulletin boards, and nearly all of them are bunk. Guys are claiming that chromium carbide grains are too large to bond at a cutting edge, that grains are soft, grains are hard, grains are improperly placed. All of these discussions are meant to try to explain why their knives, their idea and interpretation, their choice of alloy is somehow superior than other choices. No where is this more apparent than in the most persistent misrepresentation (lie) spewed in knifemaking, that somehow carbon steel blades are superior to high alloy stainless steel blades. Guys use this grain argument over and over, and in creative yet unsubstantiated ways to claim that these inferior steels are somehow superior, and they use grain discussion to bolster their argument. After all, who is really examining grain, and to what degree? Do these guys have Ph.D.s, are they published scholars, do they have any evidence by such to prove their claims?

The next time you read some claim about grain, consider the source and challenge the source if you must, but it will be a fruitless endeavor. When a knifemaker creates a knife, he only knows what to consider to prevent disasters like grain growth during extended soaking at austenitic temperatures, and non-conversion of allotropes by not reaching or holding the steel until its martensite finish temperature. Correct processing in all steps is critical, but a knifemaker will not improve on the steel apart from the best processing procedure possible. Some things we do know about grain:

  • Grain boundaries hinder the movement of dislocations. The more grain boundaries that exist, the more difficult it is for dislocations to move, and as a result, the steel is harder, stronger, and stiffer.
  • Coarse-grained steel contains larger and fewer grains and grain boundaries than fine-grained steel with many more grains and boundaries
  • Proper heat treating creates fine grain, making the steel harder, stronger, and stiffer.
  • Alloying elements, particularly chromium, block slip planes in grain boundaries, adding to mechanical strength.
  • Proper cryogenic treatment and aging produces even a finer grain, with more boundaries that hinder fracture propagation, making the steel tougher.
  • Some elements are added to these steels (like vanadium) that help to create initialization nuclei for grains to grow, creating a finer grained steel. There is continuous, ongoing research in the microscopic grain precipitation field, with new alloys, new combinations, and new reactions over every horizon. What an exciting time to be alive!
  • Grain sizes in steel are incredibly small. Knifemakers may claim that the grain at the cutting edge has some bearing on edge holding due to grain size, grain bonding, and grain interaction, but this is just total nonsense. To understand the magnitude of sizes and structure in the cutting edge and some humorous notions, please take a few moments and read:
    "Which steel has the greatest "tooth" for the cutting edge?
    --and other carbide particle nonsense
    "
    on by "Blades" page. There, you'll get an idea of actual particle grain size, and how this has no bearing on a cutting edge sharpness. What does matter is the steel type, the steel processing, the geometry of the blade, the sharpening angle and the sharpening media grit size, and little else. Steels do not maintain or lose cutting edge sharpness because of grain bonding, grain sizing, or grain manipulation, and it's time this wives' tale was put to bed.
Bad Boy Chromium Carbides

It's interesting to note that nearly always, when knife enthusiasts and knifemakers are faulting grain in knife blades for bad or inferior performance, they are writing, talking, or posting about chromium carbide. "Chromium carbides are the culprit," they claim, with "large grains that make the steel impossible to sharpen," or "chromium carbides have bad or inferior bonding to other grain particles," or "chromium carbides pull out of the steel," or "chromium carbides lead to an impossible to sharpen knife." Knifemakers on forums claiming to be machinists even state that these carbides are soft! This is clear and obvious ignorance, as any engineering and metallurgy source will quickly and clearly prove that these carbides are extremely hard, and add tremendously to the wear resistance and durability of the alloy in a myriad of ways. This is one of the main reasons chromium is used as an alloy, after all! I'm not talking about simply adding chromium in a lower carbon steel for increased corrosion resistance; I'm talking about chromium in high carbon martensitic steels, where the desired result is chromium carbide because it is so beneficial!

A lot of makers like to cherry pick bits and pieces of data they think bolsters their argument of why chromium is bad in steel. One study suggested that chromium carbide particles were pulled out of the steel in high wear testing. Of course, this does not mean that chromium carbide was in any way less beneficial, yet makers will glom onto any bit of data they think means that the entire machine tool industry is wrong, and they are right for choosing a lower alloy to hammer into a blade. Note that in this one study, the claim was that these were sub-microscopic effects only occurring on high speed, high pressure, high temperature machine cutting tools, at sub-microscopic levels in tribological testing (the science and engineering of interacting surfaces in relative motion).

This is not in hand knives! This is because hand knives in use experience tremendously lower pressures, feed rates, temperatures and abrasive motion stresses than high speed tool steels, hard-surfaced tool steels, or machine-driven specialty steels, where tribological studies are necessary. No one is using a knife blade cutting at 200 surface feet per minute to cut a carbon steel bearing block at high feed rates and elevated temperatures. A hand knife is not a milling cutter, in other words, so these discussions bear little on knives. Again, to understand how minute these particles are and they do not determine the ability of a knife to be sharpened and to cut, please read about carbide particle nonsense on my "Blades" page at this bookmark. It will open your eyes to these ridiculous claims and illustrate just how small grain is, and how comparatively wide the sharpest cutting edge is.

Another common ploy among the ignorant is to claim that high alloy steels in general, and stainless or high chromium steels in particular, are inferior to carbon steels made into knife blades because these high alloys are not used in particular machine tools. They cherry pick (again) to bolster their argument, by citing specific machine tools that may use a lower alloy cutter, or axle, or guide, or runner, or former, or some other component. Then they'll claim, "See? I told you that high alloys and stainless steels are bad, otherwise these machine tools would be equipped with them!"

But what they don't go on to clarify, as I will here, is that the economy of manufacture prohibits the use of higher alloy steels; they are just too expensive to use in machines that are made cheaply! Most of the time, these machines are made in foreign countries, (India, China, Singapore, Taiwan, Pakistan) and other locations by firms that are not known for their use of high quality production, high quality parts, or high quality anything! They are budget-driven and volume-driven firms, not quality-driven firms. So these manufacturers opt for cheaper products overall. Even in machine tools that are not cheaply made, the use of an extremely high alloy is not often justified when a less expensive steel will do. Another limiting factor is that the extremely high performance value may not be necessary or applicable in the range of wear or exposure. For instance why have a 440C stainless steel drive shaft on a planer that has a plain steel chain driving it? The application may not require it, and the use is not appropriate. Just because there are high alloy (and expensive) steels out available, this doesn't mean that they are the best choice for equipment that does not meet that high quality standard, so they use the most economical steels. Is this the reason why a knifemaker would hobble his clients with a lower alloy, lower performing steel?

Now think about this for a moment: would you want these cheaper, lower alloy, less wear resistant, less tough, less durable steels used to make the turbine blades for that aircraft you or your family is flying on? How about the ball bearings for the landing gear, with improved corrosion resistance? That would be (detailed by SAE and AISI, AMS 5880 standards) as the "premium aircraft quality product: 440C." It's got at least 16 percent chromium for a reason. This is not a casual hobby designation, this is the Aerospace Materials Specification (AMS) standard!

Note that these guys typically have something against chromium carbides. Why is that? Could it be that they are fans of carbon steel blades, steels that have very little chromium, and are fans of non-stainless steels in general? Are they looking for excuses to use, promote, and work with non-stainless steel blades because of their own interest and skill level, and can't admit that their blades are inferior? .

Of course this is the reason, because if they knew anything about metallurgy at all, they would know that chromium, the hardest element on the periodic table, is a wonderfully positive addition to steels, used even in low alloy steels like 52100 (the princess of many hand-forging knives) as an addition to improve hardness and wear resistance! Here's an interesting detailed example of this foolishness on my "Blades" page:
Does Chromium hurt or help the blade?

The only thing a knifemaker can do is choose a steel that is the best he can afford for the project, suited to the knife project and expected exposure and use, and heat treat it with the best possible method. The high alloy and stainless high alloy martensitic steels are the best performers made for the applications of fine handmade knives, unless the knife is designed for decorative and primitive appearance with pattern welded damascus or temper lines and rough finishes, or the knife is made with extreme economy (cheap) in mind. There is a reason high alloy hypereutectoid steels outperform all others in these applications in professional, industrial, and military use, and there is a reason that machine shops are not using blacksmith-made products in any type or circumstance. When someone is claiming grain boundary, grain interaction, grain shape, grain inadequacy in any way, he is talking nonsense.

Page Topics

Cryogenically processed for extreme wear resistance:
"Concordia" obverse side view in 440C high chromium stainless steel blade, 304 stainless steel bolsters, Nebula Stone gemstone handle, stand of American Black Walnut, Poplar, Nebula Stone, Baltic Brown granite
More about this Concordia Chef's Knife

Why Cryogenic Processing?

After reading and studying this page, the question in your head about fine knife blades should be "Why not?"

This may be the most complicated topic on the page. The reason is because people don't want to change and grow, they don't want to admit there might be a better option, they may be ignorant of science, scientific process, or metallurgy, they may want to justify their own way of doing things, or their concepts and understanding of the idea of process applications was limited. it simply may be out of financial and investment reach in the studio, as these process and equipment add to the expense of fine knife creation.

These same attitudes nurture the tired, outdated myth that carbon steels make better knife blades than high alloy stainless steels, that hand-forging is somehow better than high alloy machining and laboratory-grade heat treating with cryogenic treatment, and that a primitively-made knife has some durability value rather than decorative only. These are all past myths, and it's surprising how they are defended, propagated, and reinforced by knifemakers who should know better, and should be better educated on their metallurgy. We've got guys selling carbon steel damascus chef's knives for tens of thousands of dollars, simply because of popular television exposure, claiming the experience of every century of blacksmithing as their own, merely to justify their outdated process! Would it help to know that there is no modern machine that endures any measurable force and stress that has one single part that is hand-forged? Would it help to know that there are no blacksmiths at NASA, at any AISI member's business, at no machine shop, at no research facility? Yet these stubborn myths remain, mainly because of money and ignorance.

One scientific study in 2010 relates that while heat treating overall was well understood and employed, cryogenic treatment was still in its infancy. This is a critical point, and I'm very excited about the prospects of what this relatively new understanding of science offers for our field. Even companies who have this performed on knife blades may know little of the process, simply because new and revealing details are being discovered and uncovered every year. Though it was known back in the 1930s and 1940s that cold treatments were beneficial, it has taken decades for the details to be wrung out via studies and advanced microscopy, equipment, communication across fields, and testing, and may take many more years for it to become the standard, though I absolutely believe it will.

  • Financial interests of accounting and the cutting tool industry: It's clear that one of the main areas of drastic improvement of cryogenically treated high alloy tool steels is in the cutting tool industry. These are the makers and suppliers of milling machine cutters, drills, reamers, taps and dies, broaches and every other related cutting tool made of high speed or high alloy tool steels. For example, one might wonder why a manufacturer of milling cutters would want to improve his product. If he improves his milling cutters in a way that doubles, triples, or increases the life eight times on the machine tooling floor, he would, by logic, sell only half, a third, or (yikes!) one-eighth of his cutters. To justify the volume loss, he would have to charge two or three to eight times as much for them. In this highly competitive market based on continual monthly sales flow, this is not reasonable. If he can't sell that idea to his accounting department, and justify the volume loss and equipment cost in the competitive field, it won't happen. Sure, the end user or machine shop would benefit from longer tool wear and less tool changes, less ordering efforts, lower shipping costs, and higher production, but the supplier is not the user. Since the supplier is the one who would have to pay the expense for the cryo treatment while losing volume sales overall, they will fight the change. I'm not the only one to notice this, and one of the leading researchers into cryogenic treatment of steels, professor Randall Barron, was asked why razor blade companies wouldn't cryogenically treat their blades, since studies had proven a minimal doubling of the useful life of the blades after treatment. They said no because "then they wouldn't make as much money." See how this all works? Planned obsolescence is the same in ALL mass-marketed volume sales industries, because sales have to keep going to earn the shareholders profit. Do you then wonder why computer programs and operating systems are constantly and continually upgraded, forcing the computer user to buy a newer computer to run the new programs while support for older programs is discontinued? This is particularly troublesome because usually the existing computer is fine and functional; it's forced obsolescence. All the while, these same large corporations are pushing ads of how gentle they are on the environment, while being horridly wasteful and hoping you won't notice. For the high alloy steel industries, they will only be dragged into this technology kicking and screaming, since it's an investment, a cost, and gamble. The hard economic facts of the manufacturing-use-supply chain are not usually considered by scientists and metallurgists conducting these studies, since they don't actually work in the manufacturing or machining field. So, unfortunately, the industrial application of cryogenics is painfully slow in becoming universal.
  • The steel foundries: It can be considered that the same results of cryogenic treatment in the cutting tool industry could be applied to the steel foundries themselves. If the foundry suggests that all of their steels used in dies, cutting tools, and other high wear applications should be cryogenically treated, what would the result be? It could be that 2, 4, or even 8 times the wear resistance and longevity could occur! This means that there could be a huge demand drop, as all the devices and tools made from these steels would be replaced less often. In a world where volume is king, why would a foundry shoot themselves in the foot by offering a simple way to make the tools made with their steels last longer? They wouldn't; that doesn't make economic sense. So data, testing, verification, and process standards are slow to adapt in these trades, just like the cutting tool trades above, so more mass units of steel can be sold. Consequently, the data on these steels is somewhat limited, even though cryogenic processing has been well-documented and supported in science.
  • Changing and growing: When I was talking to metallurgists about this, they offered that these are new developments (cryogenics), and many industries do not want to employ new processes when what they have been producing was simply "good enough." One even asked me why I would be pursuing this new technology when I had been doing fine for decades. This is a valid argument, and one that I could apply to this very page on my website. I've certainly been doing very well for decades, and am many years in backorders, but still, I took the time and effort to do intensive research and critical time to build this page, word by word, article by researched article, for no obvious benefit apart from building a better understanding, a better knife, a better framework of my tradecraft. This is the reason I love making knives, because the learning never stops. You can take this field as shallow or deep as you prefer, and in this world it's hard to find a field so wide and boundless.
  • Better Options: Everyone who wants a knife wants the best knife possible. Though conventionally treated and sub-zero treated tool steels for hand knife blades perform well, why not offer a better option to the knife user? Better options are offered in fittings, handle materials, geometries, sheaths, stands, and embellishment, so it's only fitting that cryogenic treatment and aging of the blades takes its place in the list of custom options of fine handmade knives.
  • Scientific process, study, and publishing: There are numerous scientific studies that have taken place in the last 7-10 years about cryogenic processing; I encourage you to access them for free on the internet. This is a great time where this knowledge, never before available to the public, can be quickly and freely accessed and assessed by the individual or professional. Because one development begets another, these discoveries and confirmations will continue to accelerate. Another powerful facet of these developments is the ability to apply what has been learned in one profession and science to be applied to another. For instance, studies in cryogenics in microbiology has allowed us the very equipment, supplies, information, and tools for reasonable research in a small metals laboratory environment. This simply was not possible ten years ago, and as the technology grows, so does the result.
  • Varying test results: This is a tough one. Not all scientists are adhering to strict method, and the results then show wide variations. While I'm not a scientist by profession, it's easy to see from the standpoint of a technician that some steps, some variants, some choices to make comparative studies must be backed by strict protocol of process and testing method. For instance, in one study, a comparison of twist drills was made. The control was the same material as the cryogenically processed high speed steel, but the two were tempered entirely differently! The control was austenitized, quenched to room temperature, and then tempered three times without any deep thermal cycling, but then the test piece was cryogenically quenched, and tempered only once. This is a missed test, there are too many variables in the temper cycle to establish a reasonable control. No wonder the test conclusions were negative toward the cryogenic process, when the fault was the tempering! I've read several sources that claim that some industry standards need to be established here, and since the science is relatively new, it's up to people who do cryogenic processing to keep strict and accurate records and protocols, carefully evaluating their results. This is truly leading edge science.
  • Justification of current process: Many metals workers are clinging desperately to their outdated processes. I know this feeling well; I was a photographer who almost resented the digital advances in the craft that allowed inexpensive processing and printing without the necessity of a costly darkroom and lab. I was proud of all I had learned in that field, and had invested tens of thousands of dollars in chemical photography process. It took real skill to make chemical glossy prints of knives, understand all the qualities of light and chemistry, and serious investment to make it work in a business setting. But technology was not stopping for me or any other photographer, and I had to take what I learned in the older process and apply it toward the new one, digital photography.
    Knifemakers can be the same way. It's comfortable to stick with what works and has worked, and what has been tradition for so many years (sometimes decades, or in the case of blacksmithing, centuries). But the development of exciting new alloys has simply made these antiquated processes (like hand-forging) an art of the past. Though the ancient, time-honored technique of hand-forging and working in hypoeutectoid or low alloy steels does and will continue, as the knife-using public becomes educated, these lower-performance steels will not be the preferred performers, but only visual creations of an old art, just like chemically-processed black and white art photography is today. This is because these blade steels can be easily proven to be low-performance, and study and education will march on and build a foundation of truth that cannot be ignored by the knife aficionado. Ask any real knife user (military, counterterrorism, professional chef, butcher, or any other hard-use knife owner) whether he prefers a markedly better knife blade, and I doubt he'll choose a hand-forged carbon steel blade, no matter how pretty the hamon line is.
  • Process applications technology: This is a bit more complicated. Cryogenic processing is a process application. It's something that goes into a knifemaker's shop and studio, just like the grinder nest, the buffing room, or the machine tool setup. A typical example is the grinder, something in every knifemaker's shop. While the tendency is to focus on the machine itself, there is always so much more. Taking up more real estate is the belt rack, the dust collection, the wheel rack, the tool cart, and everything else it takes to maintain, equip, run, service, and operate the grinder. Knifemakers quickly learn that like any process, the environment of the shop itself allows the grinding process to take place, from the power panel that allows electricity from the service drop, to the discharge of the grinder swarf from the dust collector. Good grief, a professional knifemaker must have a spotless and grit-free padded bench to set the blades on between finish grinds so they don't get scratched! All of this requires real estate and equipment.

    The cryogenic process application technology is the same animal. You must have the equipment, specially adapted to the process of knife blade treatment. Knives are not heavy plastic injection molding dies, so the process must be adjusted accordingly. It takes real estate, specialized racks, frames, and cryogenic liquid transport and manipulation equipment. It takes dedicated electrical power feeds. It takes specialized insulation, temperature monitoring equipment, and a smooth workflow direction for expedient momentum. It may take a variety of chambers, pots, buffers, and piping. This represents a sizeable investment in not only equipment, but time, making and arranging the working guts of the process. Taken as a related process and extension of heat treating, the extreme cold environment is positioned right by the extreme hot environment of the furnaces and ovens, and they must not interfere with each other! This is a lot to bite off, and much of this has to be designed and created by the maker, since the exact necessary equipment for the process is not on the market.

    In my studio, while cryogenics is a certain challenge, it's considered as another process. It's logical to consider the cryogenic method applied along with machining, heat treating, leather work, lapidary, anodizing, passivating, photography, textile work, engraving and many other meticulous and specialized fields.
  • All processes must be immediate and concurrent: This is a critical factor in heat treating, one that bears closer examination. It has been proven that timing is critical in all of these processes, in order for successful, reliable, repeatable, and highly effective results to be gained, the entire process must be performed in quick, immediate, expedient order, without delays, in the same facility, oftentimes in the same room. The processing of the steel must not delay in reaching cryogenic temperatures after heat treat; steels change moment by moment in these circumstances. Tempering must not wait, nor the deep cold aging between tempers, every part of the process must integrate with the other. Though you may hear of companies offering "ship and cryo" services, these are unproven and of dubious effectiveness, particularly compared with expedient and continual timing and flow of the profession heat treating regime.
  • Competitive advantage: The cryogenically treated knife blade offers a serious competitive advantage to blades that are not treated this way. The studies and use of these steels proves it, beyond doubt; they are much better performers in wear resistance, toughness, corrosion resistance, durability, and longevity than non-cryogenically treated steels. I want to make the very best knives possible for my clients. My clients don't need to read and understand this page; many of them won't even bother, but it's important to me to supply the very best knife possible within my capabilities. Because the availability of cryogenic processing equipment and supplies has become available in the last few years, it is now possible for me to do this in my own studio. When I began making knives in earnest back in the early 1980s, this would be only a dream, but thanks to technological developments in both steel process and related equipment and supply fields, it's something I can now offer to my clients.

Unlike the company that sells milling cutters by the thousands, I strive to make the very best, and do not function by volume but by quality. This is part of the division between knifemakers as individual artists and craftsmen, and knifemakers as companies that want to make and sell a lot of knives. Realistically, I don't want to sell a knife to every person who comes to the website or hears of my name. I only want the right knife to go to the right person. That is a unique and sobering conversation that I'm lucky to have. The right knife is built in extremely high quality throughout. The performance must also be a robust as I can possibly supply, and that is the reason why cryogenic treatments have a place in my studio.

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"(Cryogenic treatment) has many benefits. It not only gives dimensional stability to the material, but also improves abrasive and fatigue wear resistance and increases strength and hardness of the material. The main reasons for this improvement in properties are the complete transformation of retained austenite in to martensite and the precipitation of ultra fine eta-carbides dispersed into the tempered martensitic matrix. Numerous practical successes of cryogenic treatment and research projects have been reported worldwide. However, the treatment parameters including cooling rate, soaking temperature, soaking time, heating rate, tempering temperature and time need to be optimized with respect to the material and application.

Comparison of Effects of Cryogenic Treatment on Different Types of Steels
--P. I. Patil, R. G. Tated
2012

Oven/Furnace Nest in the studio:
Oven/Furnace nest in Jay Fisher's Enchanted Spirits Studio with burnout ovens, tempering ovens, drying ovens, sword furnace, knife furnaces, combination furnaces
Burnout ovens, tempering/drying ovens, heat treating furnaces, and supplies

What About Dry Ice Baths for Quenching?

How does it work? Dry ice (solid carbon dioxide, CO2) is purchased and crushed, then put in a liquid that will remain liquid at the cold temperature of the dry ice, and a bath is constructed. The knife blades have been brought to their hardening temperature, and then quenched to room temperature, and then are dropped in the very cold bath to complete the quenching.

It's pretty important how the bath is made, chilled, and stored, as are the components of the bath. By the way, these baths are fairly common in use for benchtop laboratory study and process, when a very small amount of cooling is needed, and the lab does not have the funds or resources to purchase a dedicated mechanical chiller with a circulatory bath.

Sounds simple enough, and it has its place, but also has some pretty significant limitations that the knifemaker needs to know and the knife client needs to be made aware of, particularly since there are significant factors that may result in a less than optimum knife blade. While dry ice bath quenching is an improvement over conventional heat treat, it is not the pinnacle of quenching methods and in this section I'll detail the process, advantages, and limitations.

Dry ice is very cold; it sublimates (turns into gas) at −109.3 °F (−78.5 °C). If you can get it in a saturated solution of liquid, that solution will be at a very cold temperature, close to the shallow cryogenic process temperature. Put the knife blades in the solution after quenching to room temp and let them get really cold. Sounds great, but there are some limitations.

In the old and early days, we used acetone for the bath medium, but this is not a good choice due to its instability, flammability, and reaction potential with nearly all plastics. Acetone is a solvent that loves to dissolve things, so other solvents should be used. One of them is ethylene glycol, but this is poisonous, and it's probably not a good idea to keep a volume of it around. Ethanol is another bath solvent, but it's highly drinkable and might disappear when you have visitors in the shop that appreciate fine liquors or Everclear® (just kidding). Denatured alcohol is a common enough agent, though it evaporates quickly at room temp and is flammable so storage has to be carefully considered. Most shops don't have a flammable safety cabinet, and this is the proper way to store this type of agent.

You might read that dry ice sublimates at −109.3 °F (−78.5 °C), so it might be considered that this is the temperature of the bath, but this varies greatly. The dry ice can actually be much colder, depending on how it is made, where it comes from, how it is stored, and how it is transferred. The liquid in the bath may be warmer, unless continuous agitation is employed. So the temperature can vary quite a bit, so to keep accurate records of process, a thermocouple should be employed to determine the temperature of the bath.

There are several substantial issues with using dry ice baths for quenching. While they are a step in the right direction, they are not optimum for quenching steel knife blades and have substantial limitations for these reasons:

  1. They use solvents, which are flammable an/or unstable and require special storage. Since they are used in a quenching process after heat treating, this raises safety concerns, as like-process does not mean like-storage and use. Using a flammable solvent near orange-hot blades can be dangerous and serious precautions must be taken in using flammable solvents near furnaces, ovens, kilns, and other heat-producing equipment. No sparks, no ignition source, no pilots, no open flame of any kind should be in the same room as the flammable solvent bath, and the area should be well-ventilated to prevent any vapor buildup. If non-flammable bath solvents are used, these must be stored according to their reaction potential, toxicity, and with appropriate safety practice. Of course, this is not a major concern for the knife owner, client, or customer, but for the maker and his process.
  2. They require dry ice. This may or may not be readily accessible, and it's a consumable in the heat treating shop that can not be normally stored for any significant length of time without specialized equipment.
  3. They are not accurate. I know of no maker that is regulating the temperature of the bath with control systems, and I don't even know of one who measures the temperature of the bath with a thermal sensor. This is a significant concern for the knife owner, user, or client, as most clients require the absolute best heat treating process possible, and that only comes from accurate control, measurement, and regulation of all parts of the process by the maker. While temperature variation may not matter much for most low alloy steels, in order to understand and define intricacies and specific regimes for reliable results with different steel types, monitoring temperatures is important to fine tune every step of the process. This fine tuning exists in the record, that is, the recorded process of each heat treat, kept in a log, compared to previous results, where tuning and adjustment of process particulars can assure repeatable, reliable results in every heat treating regime.
  4. The bath must be made prior to every heat treat, and made in an insulated container that will contain the bath for a long period of time. The solvent must be stored after the bath is rewarmed to room temperature, in a method where most localities require flammable storage and ventilated cabinets. Remember, we are not talking about a quart of paint remover; baths may end up being several gallons. No sparks, no ignition source, no pilots, no open flame of any kind should be in the same room as the flammable solvent bath, and the area should be well-ventilated to prevent any vapor buildup. If non-flammable bath solvents are used, these must be stored according to their reaction potential, toxicity, and with appropriate safety practice.
  5. The bath must be mechanically agitated, and crushed dry ice added to maintain uniform and stable thermal environment. Where the blades are within the bath is very important; they must be suspended away from the sides of the bath container and not in contact with the container walls. If you stir the bath and let it sit, the area surrounding the blade will be warmer than the rest of the bath. Only continual mechanical stirring will maintain uniform bath temperature during quenching and aging.
  6. Submersion of the blade into the bath can cause thermal shock that is highly detrimental to the blade. This is one that few people discuss, and it's extremely important to the final knife customer, client, or owner. While a knife blade may not openly crack or visibly fracture, this thermal shock of dropping a blade into a dry ice bath may cause sub-microscopic fractures (sub-microscopic means smaller than .2 micrometers), that are unseen but cause premature failure of steel demonstrated by high wear at the cutting edge. The knifemaker or knife user won't even know about this apart from a cutting edge that doesn't seem to last as long as a properly cooled blade. In order to alleviate this problem, thermal staging should be employed so that the blades are cooled relatively slowly but consistently before reaching the final bath temperature. I don't know of any knifemaker who even discusses thermal staging and slow cooling in these baths and processes, but the scientific literature absolutely has limits and specific rates of cooling that are required by testing and steel authorities.
  7. They can't reach deep cryogenic temperatures, and they rarely reach shallow cryogenic temperatures. This means that they can only be used for steels that require mild shallow cryogenic process, which leaves out many of the hypereutectoid high alloy stainless steels completely. Most of these steels require -320°F and that means only liquid nitrogen.
  8. The most important thing is this: Cryogenic Aging. At -78°C, this is just above the temperature of shallow cryogenic processing. By the way, it's not a "cold" treatment, at least according to The Journal of Materials Processing Technology, The American Iron and Steel Institute, and the International Journal of Emerging Technology and Advanced Engineering. We know that in shallow cryogenic processing which is colder than dry ice baths can reach, this is the range where the greatest results and improvements in steel quenching happen. One might then ask, "Why is this a problem, since this seems like it's close to the right temperature for the steel?" It's because that although it is in the right range of temperature, the reaction of carbide precipitation is extremely sluggish, taking many, many hours. I know of no knifemaker who is holding the blade in the dry ice bath for 10, 20, or 30 hours, and this is what is necessary for the most beneficial carbide precipitation! While this dry ice bath temperature will result in less retained austenite, and a greater conversion to martensite, holding the blade in the bath for 10 minutes, 30 minutes, or an hour or so is not long enough! And the baths are not quite cold enough, needing about another -10°F to reach true shallow cryogenics, even at their optimum temperature.

    This is one of the main issues with cryogenics in the metal trades industry: this misconception that simply reaching temperature is enough, and then it's done. It's been proven again and again that cryogenic aging is a hugely beneficial procedure that everyone is trying not to do. They don't want to wait, it costs money, it costs time, it cost in expendables and equipment to keep something very cold for extended periods. Even the manufacturers of cryogenic treatment equipment have told me that this is a big problem for them, as the conception of "reach temp and you're done" has permeated the heat treating field. They've told me that many of the industries that use this equipment are set on the idea that a process must match the worker's shift time, so their production lines aren't delayed! As I've written before, and as is revealed by many studies, even the metalworking trades are confused about cryogenic process and benefit because of the misunderstanding and misperceptions about cryogenic aging and the distinctive benefits of eta carbide formation. By the way,  many scientists, researchers and metallurgists believe that these submicroscopic (smaller than .2 micrometer) carbides are more important to wear resistance than the martensite conversion! Most of these carbides form at shallow cryo temperatures, and are not just limited to deep cryo temperatures. Remember, the greatest conversion and carbide precipitation happens at shallow cryo with additional formation happening at deep cryo.
  9. Deep thermal cycling between temperatures should take place in this temperature range. Yes, the same cold environment of shallow cryogenics should be the same environment that the blades experience between the multiple tempers. This can be realized by secondary hardening, described in detail in the "Tempering" section of this page. Without this SCRYO soak and aging between tempers, optimum conversion of allotropes will not happen.

What to do? While dry ice baths are a step in the right direction, and they are better than conventional heat treating (CHT), there are better methods, and this is why I don't use or recommend dry ice baths for the ultimate treatment of hypereutectoid and high alloy stainless tool steels.

However, if  a maker is determined to use a dry ice bath for quenching, he can attempt to overcome some of these limitations listed above, and then he should be able to explain these to his clients:

  1. He can use a non-flammable solvent, or have a dedicated safe area to work with flammable solvents, including flammable storage safety cabinets. When using these solvents, let's hope he doesn't forget his PPE (personal protective equipment), or his knifemaking days will be shortened!
  2. He can find a cheap and plentiful source for dry ice, as he'll need plenty of it regularly. Perhaps he can buy and install a dry ice generator, but then he'll need a CO2 tank and storage.
  3. He can find a way to regulate the temperature of the dry ice bath. He'll probably need a heater or some method to control the temperature of the dry ice before it's put in the bath, and to control the ramp-down rate. He'll need accurate thermocouple and temperature indication, and a control system to make it all work.
  4. He can make or find a hefty insulated container to store and use the bath in. A lunchbox cooler won't do, and I don't know of any horizontal rectangular Dewars, but one never knows.
  5. He can make and install a circulating or agitation system, plus a cage or suspension method to keep the blades in the core of the bath. This might be linked to the temperature control system detailed in step 3. above.
  6. He can find, make, or create a way to slowly cool the blade down to the temperature of the dry ice bath. I'm not sure how this one would work, but he would need to eliminate the thermal shock of immersion. Most steels require a 4-5°F drop per minute maximum, so from room temp to -78°F should take 28-35 minutes. Can't just drop it in! Maybe he could start with room temperature solvent with the knife blade in it, and then take 28-35 minutes to cool the bath down by slowly adding chunks of dry ice. That might work!
  7. Deep cryogenics required? Sorry, this one's insurmountable. Only liquid nitrogen will work, unless the knifemaker has discovered some unknown dynamic of physics and room temperature superconducting. Let's hope he shares it with the rest of us...
  8. He should maintain the bath at temperature for 30 hours. Yep, he'll need to stay up late to do this one. Or he can create an automated system to keep adding dry ice so he can go beddiebye while the cryogenic aging continues. Or, he could create a really big, big bath with a lot of insulation that lasts for 30 hours...with automated circulation...
  9. For thermal cycling, he should make sure the bath continues to be available while the tempering is taking place, so he can move the blades back to the dry ice bath between tempers. Actually, this one is fairly simple since tempering typically takes only a couple hours per cycle.

If you, as a knife client, customer, or user, now think that dry ice baths are not the most efficient way to quench steel knife blades, congratulations. This is something I learned years ago before I moved to other methods and equipment. Remember, dry ice baths are a step in the right direction, and they are better than conventional heat treating (CHT), but there are better and more reliable methods. Because my clients deserve the very best steel treatment possible for their blades, this is why I don't use or recommend dry ice baths for the ultimate treatment of hypereutectoid and high alloy stainless tool steels.

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Counterterrorism Knife in CPMS30V:
"Taranis" counterterrorism, tactical, combat knife, obverse side view in CPMS30V high vanadium stainless tool steel blade, 304 stainless steel bolsters, Olive/Black G10 fiberglass epoxy composite laminate handle, locking kydex, aluminum, stainless steel sheath with ultimate belt loop extender and accessories
More about this "Taranis"

Cryogenic Treatment After Hardening and Tempering

I've described the role that cryogenics plays in the quenching process of knife blade steels, and how the deep dive and hold into the realms of cold has a positive result and improvement of steels, particularly high alloy steels used in the finest knife blades.

There are companies that sell a service of post-heat treatment cryogenic exposure or immersion. They call it "Cryogenic Processing," "Cryogenic Treatment," and even "Cryogenic Tempering," which are all very confusing terms, as the industry has no standard process definition for what they are doing. If you do some reading into the service they are selling, you'll see that they are offering a post-hardening, post-tempering immersion into either liquid nitrogen, nitrogen gas, or nitrogen spray and a cold cryogenic soak. Are these viable services, and do they help the steel, particularly knife blade steels? It's important to understand where and when this process takes place in the tool creation, and ask some serious questions about why this is considered.

The general sales pitch is this: send us your completed tools and steels, already hardened and tempered, and we'll simply run them through the liquid nitrogen and this will improve the wear resistance, toughness, hardness, etc. That's it. The steel is already hardened and tempered, and at some indeterminate point in the future, you simply get it very cold and hold it there for a while, and the wear resistance is somehow improved.

If this appears a bit questionable to you, I understand and concur. Considering where in the process cryogenics plays a role (in quenching and before tempering), this seems to me also to be an empty promise, particularly when some of these companies cite the statistics of increased wear resistance that occur during cryogenic quenching as being available in their process! I'll flatly claim that this is misleading at best, and a lie at worst. Perhaps they are just ignorant of what happens during actual cryogenic quenching process, but I don't know why, since the results are extremely well documented. Conversely, I can find no studies bearing any important or substantial result of cryogenic processing after hardening and tempering that improves the steel in any way. If there were proof, this would be standard process for all steels, in all tools, and it is not. We would be digging out all our old cutters, chisels, drill bits, metal forming dies, shear blades, and every tool steel item we could find and dragging it through liquid nitrogen to improve it, and we don't. We don't because this is not a proven improvement to the steel, and therefor it is not the standard. The standard and premium process is cryogenic treatment during and as a part of quench, not post tempering! Where is the actual tribological testing to prove the claims of these post-tempering process improvements?

Perhaps the reason for this is that once complete tempering cycles are done, the steel is about as stable as it's going to get. The conversions have taken place, the thermal cycling is done, the steel allotropes have stabilized. While it's possible that some retained austenite may be converted in steels that have inadequate martensite conversion, it's unlikely that the amount is significant to realize any noticeable improvement in wear resistance after tempering.

I've done my own studies and experiments and concluded that once a knife blade is hardened and tempered, at any indeterminate point after that, simply getting it very cold does not markedly change the hardness and performance of the blade. I haven't tried this with all steels, just the most predominant types I use for knife blades, and I've seen no reason whatever to do this.

Of course, there are many types of steels and applications, and a large amount of variables that have to be tested to confirm a viable advantage to post-tempering cryogenics. When considering the heavy sales pitches and promises, I would like to see some viable scientific studies done on the actual steel items treated. While cryogenic processing during quenching is extremely well documented, post-tempering cryogenic exposure is not. Could it be that some companies are simply selling a service that is related to quench process in terminology only, not backed by proof? Where is the actual tribological testing to prove the claims of these post-tempering process improvements? I'll leave that up to you to consider. In my studio, cryogenic processing is part of quenching, not some afterthought that is somehow supposed to change an already stabilized and tempered steel.

I'm only writing about high alloy hypereutectoid and stainless steels, not brass, aluminum, castings, or nylons. I'm not writing about plastics, tissue or food products either, so if you are one of those companies performing this "treatment," please be kind enough to cite studies that prove your claims. Unfortunately, there are few references included in the advertisements for these processes, even though documentation is fairly accessible and common these days.

Now for the few companies that request that you send them hardened and quenched steel items that have not been tempered for them to cryogenically treat and then temper them, it's critical to remember that timing is key in this process. The longer the steel exists in a quenched and untempered condition, the much greater the risk of dimensional instability and stress, leading to micro fractures that may reveal themselves as high wear, even after tempering! Martensite can not sit at room temperature; it is metastable until fully tempered! Add to that the steel isn't at full stability until several thermal cycling operations. Full quenching and tempering must take place immediately for these high alloy steels. There is no time to wait, to ship, to get in line for some distant company to run them through a cryogenic process and then temper them at their convenience.

Page Topics

"Azophi" tactical combat knife, obverse side view in ATS-34 high molybdenum stainless steel blade, 304 stainless steel bolsters, Lignum Vitae hardwood handle, locking kydex, aluminum, stainless steel sheath
More about this "Azophi"

Important Note on White Papers (Data Sheets):

Most manufacturers and foundries have white papers with clear illustrations, charts, and comparisons available, for free, available online. They are also called data sheets and other descriptive terms. Beware, though, that these are advertising documents, and some of the claims can be outright misleading, so some interpretive logic has to be applied by the knifemaker or knife enthusiast seeking a particular steel.

For instance, one supplier claims that their steel has the same or better corrosion resistance than 440C, but that is a gross generalization, as it doesn't even indicate the condition of the 440C. The steel this supplier is selling can not be mirror polished, so that is a huge factor in a steel's corrosion resistance, detailed here. Another factor is that the hardness of the 440C is not specified, and the corrosion resistance of 440C directly corresponds to the hardness of the finished steel. Still another consideration not identified is that steel corrosion resistance can vary depending on how they are quenched and treated!

These documents often feature comparison graphs, but the graphs may not specify ranges or details of the comparison. So you might see a bar graph with bars of the advertised steel towering far above the bars of a comparative steel (typically 440C, since it is the standard others are compared to for a reason). In a related note, be aware of these types of graphs circulating on the internet, without specifics of how steels are compared. Note that in every single case, the steel being peddled is the one excelling in all relative characteristics above others (shocking, no?), and the testing method and treatments are not even specified. More on knife blade testing here.

These advertising documents also may not specify what happens or what is expected in cryogenic treatment. They may mention cold treatment, or freezing, or refrigeration, but typically with little information about the duration, temperature, and rates necessary for an expected result. I've contacted engineers and metallurgists with some of these companies before, and after long discussions, it's clear that extreme testing with scientific method and in substantial lots has not been done on many new steels. This is because, according to them, most steel manufacturing is done overseas, and our colleges and universities (who used to be supplied grants to do extensive experimental testing) are suffering from the same funding limitations as businesses are. The manufacturers relied upon research institutions for the testing and development, and that's not being funded, so many questions are simply unanswered. It then becomes the province of the business experimenter to do all the research and testing, and as a dedicated knifemaker and businessman, I do my own bit of testing in the cryogenic field, with my results being field tested in some of the most demanding occupations and uses in the world. It's an honor to do this, and I'm committed, as all makers should be, to the furthering of their trade, craft, skills, and art.

If properly followed, the results a knifemaker or knife manufacturer experiences will align with those posted on the white paper or data sheet, with refinements and enhancements of the process recorded in the knifemaker's own data set. These guides are proofs and resources on their own, and worth their weight in gold to the researcher and materials developer as well as the knifemaker. They also define the knifemaker's own proof of concepts and skill, with pitfalls and errors in processing avoided and successful results and process expected over the range of steels he uses.

Page Topics

Extremely corrosion-resistant, beautiful and tough high chromium 440C martensitic stainless steel:
"Sirona" chef's knife, obverse side view in mirror polished 440C high chromium stainless steel blade, 304 stainless steel bolsters, Green Orbicular Agate gemstone handle, slip sheath in kydex and 304 stainless steel fasteners
More about this "Sirona"

Hardness Testing

It's critical to understand the importance of hardness testing, not just after the heat treatment process, but all along the way. This is the only practical, frequently referenced, reliable, and logical instrument designed and used to determine correct process of proper and effective heat treating of knife blade steels.

Hardness testing may take place at many times, particularly in establishing a baseline of process if cryogenic treatments, multiple tempers, and various aging and timing steps are used. These baselines, carefully recorded and evaluated, give the knifemaker a greater and specific understanding of each steel he uses. The hardness tester plays a large role in these process adjustments, as does the heat treater's log and record. Though the data sheets and white papers for each steel give an idea of hardness and results, these are only a rough guide, and I've found throughout the years that results can vary tremendously from these supplier-side references. The engineers and metallurgists at the steel company will assure you that results vary greatly from their data sheets, and that the only way to be certain of each item is extensive testing, comparisons, and evaluations of each item processed. The only way to be sure of the individual knife treatment regimes is to accurately and frequently test each knife along the way. This also assures the knives fit the maximum performance criteria that the high-tech alloys are designed for.

Testing can and, depending on the method, should occur before heat treating, after the initial quench, after a shallow or deep or cryogenic processing, and before and after each temper, if necessary. This is a lot of results and data, and can give the maker tremendous insight into how, when, and thus, why changes are occurring in the allotropes and crystalline structure. For example, it's critical to know that 440C, which is not known for secondary hardening after the first temper in any reference, can actually experience this if the maker is pushing his steel process into a refined and extensive custom procedure. Testing can also indicate the important tempering times and temperatures for desired results. From my own experience, I can assure you that the data sheets are not accurate, and typically, no manufacturer or foundry is experimenting with high process control to achieve improved response, due to the reasons listed in the topic "Why Cryogenic Processing?" on this page.

In my studio, I typically use the hardness tester pictured below. How the device works: The blade is placed on the anvil, and tension is adjusted to relieve play. The device uses a specially ground diamond-tipped penetrator for high hardness testing, and this diamond must be regularly checked and examined under a microscope to assure its integrity and geometry. Other penetrators (tungsten carbide) are used for softer metals and lower scales of hardness. A 10kg (22 lbs.) load is first applied, which causes initial penetration. The major load is then smoothly and accurately applied. In the case of a diamond penetrator, at this high hardness range, I use a 150 kg (330 lb. load), and penetration occurs. That's quite a bit of pressure on the tiny diamond, so the integrity of the diamond, the mounts, the anvil, threads, bearings, linkages, and dashpot that regulates the rate of pressure application must be smooth, clean, and frequently checked, serviced and tested for accuracy. The penetrator only penetrates so far, and the penetration stops. The major load (150 kg.) is removed, with the minor load (10 kg.) still applied, and the penetration is measured. This gives an accurate reading of the exact hardness at the point of penetration, and thus the relative hardness and temper of the whole blade. The testing mark is usually placed where it won't be seen, since the mark is permanent in the blade.

Though the hardness tester is a fairly common sight in complete machine shops, it is sometimes neglected. This is a fine instrument, capable of very accurate readings when properly maintained and used. The delineation of this machine is close to  a millionth of an inch, so any error is significant! For example, a slight deviation of the penetrator by the compression of a dust in the bearing surfaces of the anvil that leads to one millionth of an inch displacement can cause significant error of the final reading. This is a delicate instrument and has to be regularly cleaned, calibrated, and maintained.

One more important hardness testing consideration: any malformation, tiny microscopic irregularity, minuscule flaw of, around, or on the diamond penetrator will give inaccurate results. I've seen makers reveal that they have impossibly high hardnesses, and I suspect this is the error. If the penetrator has a chip in it from careless or sudden load application, angled application, or general wear, it will give a reading much higher than the hardness actually is. This is because the carefully ground geometry of the diamond is critical to accuracy. A chipped or worn diamond will tell you a knife blade is 67HRC, when it's actually much lower. The diamond can't penetrate as much, so the instrument gives an inaccurate reading. You'll need a microscope to examine the diamond for flaws, at least 30 power is usually sufficient. More reasons to have the machine regularly calibrated and tested against standards, and the measured hardness of the blades tested on an alternate instrument, by another lab, firm, or entity. There are other issues with hardness testing and these instruments that I may go into in my book.

Condition of the blade for testing

The condition of the steel blade or item to be tested is very important. Steels with a rough finish do not test accurately. Imagine trying to establish a standard measurement of penetration in a surface with a bunch of lines (grind abrasive marks), irregularities, or contaminants on the surface. The steel surface to be tested must be very clean, smooth, and clear of debris, inclusions, or contaminants.

I've read where hardness testing is done on a fire-scale surface, and this is an amateur and glaring mistake. To get an accurate reading, the maker must be reading the steel surface that is the final finished surface that the knife will have, essentially the core of the steel. This can only be reached after heat treating by removing perhaps .0003" to .0005" (half a thousandth of an inch) or more of the surface. This is significant grinding, and I've read where outside heat treating companies who are not keen on grinding on someone else's knife blade surface simply take a Cratex® (rubber impregnated with silicon carbide) wheel and clean off a tiny spot to do the testing. This is a minimum of consideration, and perhaps more cosmetic than accurate. Spot abrading does not produce a highly accurate and valid test, and the reasons are several.

  • Simply abrading away the surface scale does not assure that the finished surface is reached.
  • A rubberized abrasive wheel does not remove enough material to reach the actual core.
  • It also does not assure regularity and flatness of the surface, since these wheels create their own uneven peaks and valleys caused by the abrasive and variable pressure. 
  • Since they work on high speed mandrels, they create localized heat which may substantially effect the point contact of the surface at the abraded spot. Pressure creates localized heat and heat affects the crystalline structure of the steel. You might not think this is important, but remember that the tester is capable of delineating measurements of close to one millionth of an inch, so localized heating can effect the surface considerably.
  • Silicon carbide, the abrasive used in most of these rubberized abrasives, can be deposited into the actual surface of the steel. While you might not think this happens, please understand the microscopic nature of what is being measured. Adding some incredibly small contaminants might not effect the visible appearance of the steel, but remember the magnitude of the measurement.

Another factor in establishing an accurate reading is the degree to which the surface is finished.  Some ASTM standards require "lapped and polished" specimens! None of this "grind the spot to 400 grit and you're done." Remember, this is in the final test that is done, the number that will accompany the knife. While in my experience the degree of finish can vary the final reading at up to one point depending on the finish, in this world one point is significant variation. Standards also recommend that the underside of the tested piece is also finished, as any microscopic movement or deviation against the anvil (think sliding or slight displacement or compression) will affect the measurement.

The surfaces to be measured must be parallel, testing surface to anvil contact surface. This seems obvious, but an angled surface will cause sideways pressure as the geometry becomes a wedge which will effect the reading as pressure is applied.

Thickness must be considered; a blade that is 1mm (.040") is too thin to test in the standard Rockwell tester. This is not often seen but on thin chef's or fillet knives, but is also a reason that the blade is not tested in a thin region (near the cutting edge or on thin tangs).

Testing must occur away from any edge (or hole or any irregularity). It stands to reason that since the metal is deformed, if the test is near the edge of the material, it will be deformed at an angle, and the reading will not be accurate.

Time is seldom considered, but did you know there are standards as to how long the pressure is maintained at the contact point of the penetrator? This is because steel deforms plastically, even very hard steel, and it may take minutes for this to occur.

All of these reasons add to the logic of a maker testing his own knife blades. The tests should be standard, an average of multiple tests, and the maker can control all of those tiny yet significant factors that contribute to the viability of the tests. Farming out knife blades for heat treating will not establish these practices, an outside heat treater does not get paid to experiment or vary process, or test along the way, or establish standards in his own testing regime. This may be well and good for the hobbyist knifemaker, but for the professional, a higher standard of measure is desired by both the maker and knife owner.

I'll go into this in greater detail in my book, but I've included it here to help the reader understand just what is required in establishing an accurate guideline and framework of testing for knife blade hardness to aid in processing the steel to its highest performance, and establishing accurate benchmarks for the client and knife owner and user.

Page Topics

Hardness testing is a critical process of heat treating knife blade steels:
Hardness testing using a Rockwell Hardness Testing apparatus for a CPM154CM knife blade
Testing CPM154CM knife blade after hardening, cryogenic aging, and multiple tempering

Outside Heat Treating Contractors

For complete transparency, please note that since my first knife made in the 1970s, until the present day, I've heat treated every single one to the best of my knowledge and ability. Know, also, that I've never had one failure, not one return, not one complaint about the hardness and wear resistance of a single knife blade I've made.

Heat treating is not mystical wisdom, not a mystery of scientific knowledge, and not an unobtainable goal: it is simply a process. It's hot, it's cold, it's timing, it's workflow. It's numbers, it's temperatures, it's logical, like any process. And like any process, understanding and repeatability is key for reliable results.

In this section, I use the word outside because this refers to people who heat treat outside of the premises of the knifemaker's or knife manufacturer's own studio or shop.

There is quite a bit of money to be made by contracting heat treating, with very little relative expense. Once the equipment is paid for, it's only electricity and expendables (mainly liquid nitrogen and gases) to pay for, and the business can be quite lucrative in large batches. This is because there are many people and companies who make knives (and other cutting tools) and few who understand or who can perform the process of proper heat treating and processing. More about this in the Equipment topic below.

There is nothing wrong with outside contractors, particularly if the maker can not reliably heat treat his own work. But us older guys who see this in the trade know that if the maker doesn't heat treat the blades himself, he's handicapped and limited in the knowledge and scope of his work, and is simply not in charge of the entire process of making a knife. I'm sure I'll get some hate mail over this, as guys frequently write to me to justify what they are doing and how it's the best, most proper, most reasonable way to do things, and it may well be, at least for them.

If he's in the business of knifemaking and calls himself a professional, his clients simply hold him to a higher standard, one that assures he knows all of the particulars of the field, and steel work is the entire scope of the blade. If he doesn't perform all of the necessary operations of the blade making, how will his client know that his blades are worth their salt? Will they then question his ability to interface a permanent handle with the blade, create and install the fittings, the handle material, and the sheath? Every part of this operation that is contracted or farmed out means that other hands are involved, other ideas, other skill levels, other process and procedures, and other levels of quality are in play.

Cheaper and Easier

This concept of outside help can then be logically extended. Since it's easier to send a blade out for someone else to heat treat, it's clear to the client that the maker is interested in the easier way. Most guys won't get this, but understand: IF A KNIFEMAKER DOES THIS, his potential clients will know that he is taking an easier step, and will wonder what other easier steps he may be taking. This will affect his reputation and standing as a knifemaker, and it's been demonstrated over and again in this field.

A manufacturer or maker contracts out his heat treating, it's cheaper and easier. He may then contract out his blade construction, logically to foreign shops (in India, Pakistan, Taiwan, or China) because it's cheaper and easier. He contracts out the handle supplier, and the sheaths, all the while making things easier and cheaper. Reasonably, he's then competing with all the other makers and manufacturers who do this, and the clients know he's making easier and cheaper, so they will only pay easier and cheaper prices for his work. This continues a downward spiral called "lowballing," where the only option is to make a knife easier and cheaper. That way, you can compete with easier and cheaper knife sources, but that eventually means moving all of your operations to a foreign company because, here in the United States of America, we have extremely high median annual earnings, and people don't want to work for cheap, yet most people buy cheaper and easier. This is particularly true of the majority of knife interests (and represented in our national trade deficit!) and that is why most of the knife information, companies, exchange, forums, websites, postings, and conversation is focused on the low-end market of cheaper and easier mass manufactured knives, who inevitably, contract their work to offshore labor.

Cheaper and easier is fine, if that's the direction one wishes to go, but know this; it becomes a business based on cheaper and easier volume, and this is not how fine works are created. Worse, this is not how fine works are purchased by those who seek them out.

For the sake of argument, let's say that a maker is not interested in fine works, owning and understanding the whole process, or meaningfully contributing to the art, craft, and science of knifemaking. That's fine, but don't expect him to create a knife better than any other mass-marketed product, and don't expect to be successful at it, unless he's building a company about cheaper and easier. Also, don't compare cheaper and easier knives to those that are completely made, in house, by a professional, because the two are in different leagues altogether. And don't expect a cheaper and easier knife to ever be of value to a client, as it will end up a poor performer both in the physical sense, and in the investment sense.

Simply put, if a knifemaker starts contracting out for the sake of expense, there is no logical stopping point, and I've seen it over and again. A knifemaker makes knives, and that means he makes the blades.

For Knifemakers:

Every maker who uses an outside heat treating contractor swears they are the best ever! If you don't believe this, I challenge you to find one maker who complains about the result of their chosen heat treating contractor. After all, this would be detrimental to the sales of their knives, wouldn't it? It's always been curious to me that every single knifemaker crows about the results of their chosen heat treating contractor's results, no matter who they are. Are there any realistic comparisons of each contractor's process or results? Does the knifemaker even know or understand the process and timing of the procedure, since different procedures (like snap tempering) produces lesser results? Or, as knifemakers, are they simply taking what they get and then using the heat treating contractor's name to bolster their own knives value and performance expectations? Perhaps, just perhaps, they can justify why they don't do the heat treating themselves, because, after all, they could never be as good as old (insert name here)'s heat treating process...

It was shocking to read in a forum posting about a knife blade that was treated by an outside contractor, and the maker then had to deal with rounded moons of different appearance of the steel on the cutting edge and spine of the blade. These "blemishes" were there after grinding and sanding, and the maker wondered if there was something wrong with the steel. Then, another maker who used the same heat treater chimed in, claiming that the marks were there from the "torch process" that the heat treater used to straighten the blade! What? This is an atrocious screw up on the heat treater's part; no torch should ever be used to straighten a blade; hell, no torch should even be in the same room with a knife that's been heat treated! This is a ruined blade, a blade where localized heating has completely changed the integrity, crystalline structure, and the entire allotrope of the steel in spotty locations, and that is why it looks different. It's botched, it's ruined, it's garbage and unlikely to be saved, and the guys discussing it shrug it off as it it's something that's normal and routinely done! How terribly sad, not for the maker (who is plainly ignorant of the process) or the guys who think it's normal (who are plainly ignorant of the process), but mostly for the guy who buys the knife and finds that it has soft spots, or high wear areas, or reduced toughness in spotty locations, or finds that the blade eventually cracks from dimensional instability. Yet the heat treating contractor goes on to treat blades this way as if he is doing the right thing, and the guys who use his services claim he's the best... truly sad.

What if the heat treater screws up your blade? What will they do? You'll have to get a clear contract from each individual business to find out. Most of them will give you a value (coupon or money or purchase potential) for the size of the blade in raw stock. Yep, you get another bar of steel, raw, so you can start over. This may be fine for some makers, but it's not how I would want to do business. After all, the value in the blade is in the labor that has (hopefully) gone into the blade before heat treat. That may mean profiling, drilling, grinding, filework and in some cases, even engraving! Yikes; that could be many, many hours and that's a hellofa hit! As a maker improves his work, this can be a substantial amount of the value in the knife, and I'll flatly claim that it's usually many times more than the value of the raw steel. But if a heat treater screws up, a bar of raw steel will be all you'll get for your trouble. That is, if he admits he made a mistake at all! I disclose this because typically the heat treater will blame the maker: you've ground it too thin, you've ground it unevenly, you've got the wrong type of steel, or other reasons that a blade may be "less than optimum" after heat treat. By the way, less than optimum means warped, cracked, bent, curved, wavy, or damaged.

Heat treating overall is a fairly simple and straightforward process, clearly outlined in the manufacturer's white pages and through endless online sources. This is another reason to do these operations in the knifemaker's own shop or studio, so the maker himself is responsible for the very best treatment of knife blade throughout the entire process, without delay, with complete control of the process and thus the results. This is why many of us old-timers claim that if you don't heat treat your own blades, you're not really a knifemaker.

Heat Treating Purist Knifemakers

In the 35 years I've been doing this (full time professionally since 1988), I've heard a continuous, unsolvable argument about heat treating by the knifemaker, and farming out this process step to others. It's sad to see that it has caused such grief and embarrassment, complaint and conflict among makers, and here's how it goes:

A guy wants to get into knives. He makes some with simple hand tools and then quickly realizes he needs more equipment. He gets what equipment he can as time goes on, and the improvement and enjoyment of his craft builds. This is how it should be. I don't know of any maker who was gifted an entire functioning shop or studio, all at once. So the modern knifemaker is then building his skill set, his tool set, his process understanding, and the result of this should be obvious and apparent in his works, his completed knives, sheaths, stands, cases, and accessories.

I'm different than most guys, I suppose. I didn't get into making knives because I wanted to make and have knives; I got into this because I was fascinated by the process of heat treating. That you could create such wide and variable ranges of steel hardness, toughness, flexibility, corrosion resistance, and appearance of steel is what captured my interest. As I stated in my bio, an old welder told me if I wanted to understand heat treating, then make a knife. The reason he knew this is because a knife blade is truly a special case. It's not structural steel that must support a piece of equipment or building, and it's not tooling steel that must be made to its absolute highest hardness for extended wear resistance at high feed rates. Knives are special, they have to be hard and tough, wear resistant and tenacious, a bit flexible, and (in most cases) corrosion resistant.

I started with a torch; a #12 rosebud (heating tip) of an oxyacetylene rig. I quickly learned that the process with this rather crude tool was tenuous at best, and highly uneven, but it was all I could afford. It didn't take long to come up with the resources to buy my first burnout oven, because at the time I started, there were no dedicated knifemaker heat treat furnaces. I modified the oven by doubling the element size and modifying the controller, to make it into a rapid ramp furnace. I then built a furnace in a discarded refrigerator body, and it had an internal furnace and internal quenching chamber, was evacuated by vacuum pump and infused with dry nitrogen! This was a real beast, it could gain 500 degrees F a minute when empty! I progressed beyond that, and am still progressing.

My point is that I've always believed that the heat treatment of steel was the basis of knife making. The very basis. It's what differentiates the knife blade from the raw stock and separates the cold chisel from a scalpel; it is the very foundation of why a knife is a knife and not just a piece of metal with a cutting edge on it. Most makers (and clients) agree.

There are those who, for whatever reason, do not wish to do their own heat treating. I know why: it's too expensive, it takes to much room, too much equipment, too much effort. They may not be confident in their capabilities, they don't understand the process, they simply may not be able to afford it or justify the expense of the equipment. I understand; I've stated I can barely afford my own shop, and this is how most creative artists and fine craftsmen work! It's okay to have knives treated by someone else, as long as it's disclosed, and as long as the knifemaker can explain what heat treating is and how it works for his clients. Most clients don't want to know the intricacies of carbide nucleation and propagation in evolving crystalline bodies; that's the knifemaker's realm.

The Purist vs. The Absolute Purist
-and the comparative argument

Most artists, craftsmen, and creative people would prefer to have as much control of the process as is possible, particularly involving the core of what it is they are making or creating. For a knifemaker, the very core of his work is the performance of the blade which is established by steel choice, steel geometry, and steel heat treating. All of these are the responsibility of the maker of the knife.

Frequently, I've seen makers become highly defensive about this topic. Old guys like me might claim that you're not really a knifemaker if you don't do your own heat treat, and the sparks fly! You'll hear or read the argument of the absolute purist, which seems to be a go-to sarcastic and defensive posture based on this principle: "If you don't dig your own ore out of the ground, smelt it yourself, then you aren't a knifemaker either. Take that, you heat treating knifemakers!"

What an embarrassing comment, trying to compare mining with knifemaking. That's like saying to the fine artist painter that unless he shovels the titanium bearing ore out of his own mine (remember, to the absolute purist everything must originate by the hands of the maker), then he cannot call himself a fine artist. Perhaps Michelangelo wasn't a real sculptor, because he didn't hoist his own marble from the quarry by himself. A jeweler could not call himself a jeweler because he didn't mine his own gold bearing quartz. See how this goes? So the absolute purist shovels this cynical argument back on the maker who does his own heat treat, as if heat treating your own knives is somehow wrong!

This is an argument of building or creating raw stock vs. building or creating a knife. In insulting comparison, the absolute purist claims that unless you build your own raw stock, you cannot call yourself a knifemaker, thus equating building raw stock to building a knife. Of course, no one believes this, so the guy making this argument has just excused himself from heat treating because, in his mind, the two are equivalent!

But the comparison and technique is based in total falsehood. Let's get this clear. This is not a discussion about heat treatment; this has somehow become a comparative evaluation of raw stock production vs. treatment of a machined, forged, and/or hand-worked piece of raw stock and the two are of totally different operations, concepts, and even different fields! Yes, mining is a different field than knifemaking, foundry creation of steel is not knifemaking, but heat treating is knifemaking! If you don't believe so, then just quit heat treating your blades altogether, after all, it's not part of knifemaking, right?

Guys will go on to claim that cutting out a blade is the same thing. Have someone else cut out your blades, after all, it doesn't affect the "quality" of the knife. Why stop there? Why not have someone else grind them, shape them, sand them, polish them? Why stop there? Why not have someone else attach a handle, sand and finish the handle, polish and embellish the knife? Why stop there? You are still calling yourself a knifemaker, right? Why not have someone else make the sheath, make a stand, and sharpen the knife? Why not just have every single facet farmed out to different companies, people, organizations, and groups, in many different countries, and have the knives delivered to your door, and then call yourself a knifemaker? Why not?

There is no clear definition here of what constitutes a "knifemaker," specifically, exactly, and repeatedly. There is no designation of "knifemaker" on the United States Government's IRS Principle Business or Professional Activity Code which is how this business is classified by our government. So, really, who defines what is a knifemaker? You could bake apples at a travelling renaissance fair and call yourself a knifemaker! You could glean old newspaper from the county dump and call yourself a knifemaker! Why not? Where is the line? There is none, really, unless some authority decides to make one. So, go ahead, call yourself a knifemaker, no matter what you do... right?

This, then, becomes a discussion of integrity. That's a powerful word, right? It opens up all sorts of concepts like morality, virtue, reason, and truth. Man, those are those tough words that are borne out through years of service, concepts that involve others. I'm not saying that if you don't heat treat your own knives, you don't have integrity. I will say that if you make the base comparison that raw stock manufacture is the same as heat treating a knife blade for the purposes of justifying why you don't heat treat, you don't have integrity.

Then there is the work scope argument: I've seen these guys take it a step further, claiming that unless you're a professional heat treater, you can't treat your own knives adequately! That's the ultimate in farming-out justification.

Okay, let's accept that premise just for fun. This means that you can't drill an adequate hole in a knife blade, because you're not a machinist. A typical machinist has completed a certified, often regulated level of training or apprenticeship recognized by an official governmental organization. The machinist fixes the work to a trammed table on a dedicated machine, centers his hole using a centering indicator, or digital readout, from blueprints, and starts with a spotting drill progressing to a combination drill/countersink and then follows with screw machine drills, larger and larger drills, followed by a drill just slightly smaller than the final required hole size, and then he reams the hole with a chucking reamer, and then measures it with a calibrated pin gauge. A knifemaker therefor can't drill his own hole; he doesn't drill professionally; he's not a machinist! By the way, ask a machinist try to grind a knife offhand, and he'll tell you to go to hell, because he's not a knifemaker!

More fun: a knifemaker can't possibly fit a handle to his blade, because he's not a carpenter. The carpenter would carefully store the wood, making sure that the moisture content is correct, using dedicated tools to plane the raw stock and determine the specific grain arrangement desired. He then profiles out the stock, kiln-dries or kiln-ages the stock to the specific required condition, and then may hand-plane it and hand-scrape it in just the right direction and orientation for the application. He may use special chemical stains that react with the tannin in the wood to create the appearance he desires, then after this chemical treatment, uses stabilizers to stop any further reaction. Then, he starts sanding... a knifemaker could not possibly do this; it's not in his scope.

On the other hand, is a knifemaker who claims you're not really a knifemaker unless you heat treat your own knives a purist? Perhaps we can compare this to other trades, say, a jeweler. A jeweler may not mine his own ore, but he does his own cutting, fitting, soldering, and casting. A woodworker or carpenter may not grow his own trees, but does all of the cutting, shaping, fitting, bending, and treatment for surface and appearance. The potter or ceramics artist may not dig his own clay, but he does all his own shaping, forming, and firing.

Reverting back to the absolute purist, he may argue that some jewelers do not do their own casting, that some ceramics artists do not do their own firing, and that some parts can be farmed out! Do you see how this argument festers in unsolvable circles?

Here is the really important thing: most artists, craftsmen, and creative people would prefer to have as much control of the process as is possible, particularly involving the core of what it is they are making or creating. For a knifemaker, the very core of his work is the performance of the blade (after all, he's not a letter opener maker or a spoon maker). The performance of the blade is established by steel choice, steel geometry, and steel heat treating, and all of these are the responsibility of the maker of the knife. If you don't think so, ask a client this: "Who is responsible if one of these factors fail in a knife they purchase?" For instance, if the heat treating is missed and the blade is soft or has soft spots, or cracks from dimensional instability, you won't get away with telling the client that it's the heat treater's fault. More importantly, If the quality of a sharpened piece of steel even matters, why wouldn't the maker want to control this essential step? Why is it then that people take the stand of the absolute purist (if you don't dig your own ore, you're not a knifemaker) as a moral equivalent to justify why they don't heat treat their own blades?

It's because they want to be called knifemakers, that's why. If the name "knifemaker" did not matter, why not just assemble kits and call yourself a knifemaker? Why not buy the entire knife assembled and created overseas in a boxed form (these are available) and glue on a piece of wood and sand it, and then call yourself a knifemaker? Why not just buy the knife already completely assembled and then just etch your name on it and call yourself a knifemaker? Don't laugh, I've had people ask for this! You can easily see the progression of logic, and it's all based on the fact that the word, "knifemaker" means something, and means a lot!

To argue these concepts are a baseless, useless endeavor. A guy who doesn't want to heat treat his own blades will not change; the guy who believes that knifemakers should heat treat their own blades will not change. It's all about integrity of the individual. If you do it, explain why; if you don't do it, explain why: that's all the client wants to know. The client will make and understand the distinctions between the two. And if you feel some discomfort in telling a client that you don't heat treat your own blades, you probably need to examine that a bit more and make some adjustments in your process, abilities, and knives. That way, your client will be confident in your abilities and understanding of what it takes to make a knife.

Look, it's okay to have blades heat treated by someone else, particularly if if the maker can't afford the equipment, space, time, and effort of doing this critical operation himself. But in doing so, let's hope the knifemaker doesn't simply rely solely upon the heat treater's name in the trade. Let's hope the maker wants to educate himself on the process, and start heat treating because that is the core of working with knife blades and the foundation of building a good knife.

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Counterterrorism Push/Punch Dagger in ATS-34 high molybdenum martensitic stainless steel blade:
"Vindicator" Counterterrorism Tacical Combat Push/Punch dagger, obverse side view in ATS-34 high molybdenum-chromium stainless steel blade, 304 stainless steel bolsters, Camo coyote, blacck, olive drab G10 fiberglass/epoxy composite handle, hybrid tension lock sheath in coyote brown kydex, brown stainless steel, titanium, anodized brown aluminum
More about this "Vindicator"

Recipes for Steel Processing

Look around on the internet and you'll see plenty of recipes for heat treating and processing of knife blades steels. You can find them on forums, bulletin boards, hobby sites, and knifemakers' own websites, but you won't find any here. The reason that I don't include recipes is that they can only be generalized, and heat treating and processing, including hardening, tempering, annealing, or spheroidizing is vastly different with each steel type, and with each circumstance.

What are these differences?

  • The steel type and alloy is the first determinant factor in process steps. From this very page, and on the descriptions of steels on my Blades page at this bookmark, you can easily see that steels vary tremendously in their alloy content, their use, and their applications even if specifically and only used for knife blades! There is no standard recipe for steels; even the process itself can vary. For instance, some steels require staged quenching, some do not. Some require secondary hardening experienced only in temper alternated with deep cryogenic thermal cycling, some do not. Some simple steels require only one tempering cycle, some steels require at least three tempers for the best performance results the steel was designed for. Some will anneal, some will not, some can anneal just by heating them up and letting them air cool, some require cycling at high temperatures to achieve full softness or spheroidization. There is no standardized process that can be given by a recipe.
  • The steel manufacturer plays a very large role in steel processing. Even with the same steel classification, designation, and alloy set, the steel varies greatly between manufacturers or foundries. This can be seen in the wide variations of treatment and processing on each manufacturer's data sheets and white papers, and clearly, there is no standard for each steel type. While different sources of steel may have similar generalized ideas of treatment, no universal recipe exists.
  • The date the steel was manufactured is seldom, if ever, discussed or considered, and I will flatly claim that this is a significant factor in the processing of the steel. It's not that the steel ages or changes in any way by simply sitting around waiting to be treated and processed, it's that each manufacturer varies their own foundry process and steel alloy content periodically, and you can read about this on the disclaimers on their own websites and their own data sheets. They simply tweak their steels, and the method used to handle them varies accordingly. You'll read (from them) that they reserve the right to change these process variables and the steel alloy content, so that their own data sheets are simply a current or dated guide. It's expected that as they roll out new steels, lots, and batches, that they will tune and adjust not only the alloy, but also the alloy's processing steps to refine and improve their product.
  • The steel geometry is a considerable factor in processing steps. I've mentioned before that guidelines for steel processing are typically established by processing 1" thick sections; this is standard in the industry, and knives are not 1" in all dimensions. Knives are a very special case, with relatively thin, long pieces of steel that require a unique blend of hardness and toughness, and even that varies depending on the type of grind, the thickness of the cutting edge, spine, and even the geometry of the ricasso area where forces are transferred through the handle to the blade. Even the inclusion of serrations and the geometry of the serrations must be considered in the process results. The tip type, cross section, and geometry, the expected use of the knife, the associated fittings, materials, and mounting method must all be individually considered, and all these factors vary tremendously between knife blades.
  • The foundries also offer guidance on processing based on a safe margin of error or safe ranges. What this means is that they don't want a machine shop, fabrication shop, manufacturer, or maker using their steels to suffer from catastrophic failure of their steels, so they give very safe and carefully marginal guides. Of course, they don't want their steels to crack, warp, or deform; they want them to be durable and tend to offer process instructions on the side of toughness, not wear resistance. Most of them don't even suggest deep cryogenic processing, opting for the term "cold treatment," and I'll suggest that this is a generalized term for a reason. Many of them won't detail the range of "cold treatment," and this may be done for several reasons. They may not be sure of established testing of their steels in these various ranges, since the testing of new steels is slow and expensive, and scientific and institutional settings for this type of testing is budget-strapped. It may be questioned if they are up on the current processing technology, or it's possible they don't want to offer a process that would tremendously increase (as much as 8 times) the life of the items created with their steels!. If they did, logically, they would sell considerably less! By the way, the term "cold treatment" is not the current standard, it's only a generalized term; the standard terminology is listed above and is used by The Journal of Materials Processing Technology, The American Iron and Steel Institute, the International Journal of Emerging Technology and Advanced Engineering and other scientific and professional resources.
  • The processing environment and equipment is one of the largest factors in this discussion of why a "one size fits all" recipe for steel processing is a useless endeavor. In knifemaking (and in every machine shop or fabrication facility) the environment, the equipment, and the people performing the processing of steels in every step are vastly different. In knifemaking, you've got guys adapting toaster ovens for tempering, not understanding localized heating and thermal transmission, and not even understanding the difference between radiation, conduction, or convection. You've got a wide range of equipment in play, in all different environments in different parts of the world. Variations of timing, movement, and even humidity and ambient room temperature will all play a role in how various steels respond to treatments. Good grief, there are makers using window fans to air quench blades, completely against the foundry's process recommendations for still air quenching, when even a slight draft in a room can have a deleterious effect on the steel!
  • Evaluation of process and tuning of the process can also vary wildly. For instance, an inexperienced maker may claim that a cutting edge is fracturing because he thinks it's too hard, when in actuality, it's been overheated in post treatment grinding, and is too soft, making breakage easier because it's ground thin! These mistakes in process control and the understanding and evaluation of issues and events can vary widely among craftsmen, and to offer a standard recipe for processing would assume that the user is capable of evaluating and understanding all of the intricacies of the process in his particular environment. Even when this is necessary, good scientific method must be employed, varying only one aspect of the process at a time to tune it, and recording, testing, and evaluating the results.

What to do? This is not so simple, but it is obvious. While I can't speak for other makers, in my studio and shop, I have an established log of heat treating, keep accurate records of each step of the process, and develop specific methodology based on my particular environment, equipment, materials, and knife geometries. It's logical to have and employ some basic testing equipment that any machine shop would have, and to keep records of evaluations along with the recommended process sheets for each type of steel used. It's critical to have knives field tested (to failure if necessary) to understand the results of process adjustments, and have experience in these evaluations that can only take place by making many knives with a wide variety of materials. These knives should be field and use tested in the hands of professionals, and that should be reflected in the type of knife, the wide assortment of knives, and the client basis that uses them. There is no better testing than an satisfied client, and decades of satisfied clients is a sobering goal.

I understand that many (perhaps most) makers only use one or two steel types, but even if so, it makes sense to have intimate knowledge of how those steels respond in every way including even long term exposure to the elements, usage, sharpening, and finish potential, something few makers consider but all clients do. And, above all, it's important to be able to explain to knife clients what these processes are, what they do, and how they effect the blades that the client desires, because the knife client and knife owner are really the important factor in this discussion.

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Unique and specialized geometry:
"Nunavut" custom skinning knife, obverse side view in 440C high chromium stainless steel blade, hand-engraved 304 stainless steel bolsters, Musk Ox boss horn handle, hand-tooled leather sheath
More about this Nunavut Skinning Knife

Equipment

In my own studio, the equipment is built around the process, and the process is built around the desired result.

This section is directed more at knifemakers who may wonder about the equipment used in proper processing of these modern, high alloy, and stainless steels. Knife clients and users may learn a little about why this process is important and the maker's understanding and control of it is even more important.

The equipment used in proper processing of high alloy and stainless steel alloys varies depending on the size, amount, and budget of the individual shop, studio, or maker. I won't go into manufacturing or mass-processing, because you can find all you want about these large pieces of automated equipment on the internet. Manufacturing interests are well-covered in the large tool processing field, and examples and suppliers of this equipment are abundant. It's enough to know that a decent small cryogenic processor can be acquired, brand new, for about $10,000, with some smaller models available and plenty of larger ones to be had for the price of a new vehicle. That's just the cryo processor, not the furnace, quenching stage mediums, or tempering ovens and chillers.

This doesn't work for most small shops, as knifemakers and artists are typically processing dozens of blades a year, and not hundreds. Small volume is the problem, and this is why it's usually uneconomical for the individual maker to outfit his own shop. Small volume equipment is simply not made, not offered for sale, and not available, which is sad, because if someone was making these smaller units, they would probably do well! For instance, furnaces are now available for the small shop and studio, but when I started making knives, they weren't available. Most of us who wanted to heat with electric furnaces were adapting and modifying casting burnout ovens and pottery kilns to do the job. Then, several companies started making knifemaking heat treating furnaces and they are doing quite well today fulfilling this need in our trade.

This means that currently, for the modern small shop, the maker must make first the equipment to do the job. This means custom adaptation of existing or available equipment, equipment borrowed from other industries and professions, equipment not designed for, but adapted to use in the small metals lab, studio, and shop.

There are those who claim that without a "proper" cryogenic processor, the results achieved cannot be as good as shipping the knife blade off to a heat treater who is outfitted with the dedicated equipment. This is not true; if the process is followed accurately and specifically, it doesn't matter what brand of equipment is used to do it; it's about the accuracy and control of the process. Just as it doesn't matter which brand of furnace is used to heat the steel to it austenitizing temperature, just as it doesn't matter what brand of oven is used for tempering: what matters is the rate, temperature, and time.

I will suggest that a reasonable attempt should be made to avoid having an outside contractor heat treat and cryogenically treat knife blades because of the reasons detailed above and throughout this page:

  • The maker is uncertain of the process done to the knife
  • The blades are often snap tempered, leading to substantial retained austenite and an overabundance of pearlite.
  • The austenitizing temperature and timing is unknown as is the decalescence point.
  • The cryogenic process aging time is unknown.
  • To have the most dramatic advantage, cryogenic processing must be immediate and concurrent, and not delayed for the sake of economy or batch processing.
  • The timing of the knife blade processing is dependent on the heat treating contractors batch. Steps like snap temper are performed so blades will become "stabilized" while batch volume is built up for cryogenic processing.
  • The tempering process is unknown, and some steels must be triple tempered with deep cooling between tempers. Timing is critical and varies with the size of the blade!
  • The most important part: the knifemaker does not have the confidence, understanding, or dedication to learn about the very process that makes handmade knife blades better than manufactured knife blades in every conceivable way.

That last one is a real kicker. Say you are confident that your outside contractor has the equipment, know-how, and reputation of reliable heat treating. You are dependent on his name, his work, his method. Fine, but what about if you have a client who asks you to make a knife that is non-standard? What about a client who wants a knife made of a steel that has a wide range of heat treating options and methods (like D2, ATS-34, or CPM154CM, CTS®XHP, or CPMS30V)? These, and many other high alloy and stainless steels have a variety of treatment methods, all to create different results. You should be able to explain these to your knife client, as someone who orders or purchases a fine custom handmade knife expects more knowledge, more information, more savvy from their maker, as these knives are not cheap! As a knifemaker, at the very least, the tuning and tweaking of the process to achieve a fine, specific result is an ongoing learning affair, and it allows a vast improvement over typical "safe" methods of processing. I use the word safe because it's safe for the heat treating contractor to perform a less aggressive heat treat regime to avoid damaging a knifemaker's blade. This, I will claim flatly reduces the performance of the knife blade and it's clearly described above.

Batch processing and cost factors: for the professional heat treating contractor, consider this: a modest and small cryogenic processor can eat up 5 liters of liquid nitrogen per hour. This may not seem like much, but think about this a moment. Consider that it will require at least 30 hours or more of soak time, and a ten hour cool down at recommended rates to get to this temperature. This means that 40 hours at 5 liters per hour and the processor is chewing up 200 liters of liquid nitrogen. There are about 1.8 pounds to a liter of liquid nitrogen; this translates to 360 pounds of liquid nitrogen. In our location liquid nitrogen is currently costing about $2.00 per pound. So this means that using this particular processor rate, it will cost $720.00 to do one run. A conservative heat treating contractor may charge between $10.00 and $20.00 per blade, so in order to just break even with the nitrogen cost, the heat treating contractor must batch process at least 70 blades. But this is not the whole cost; he must pay for the equipment, the electricity, the facility, the transportation and overhead of the business and his own labor. I'm using the keystone rule to suggest that he needs to do 200 blades at once, just to break even and make a payment on his equipment.

If your blade is in the 200 in the batch, do you think that he will do it when it's optimum in timing and without process delays? This doesn't make sense. This is why outside contractors handle huge lots of knives, many from knife manufacturers, and they include some handmade works in the batch. Not a lot of personal attention to your knife; this is expected, after all, it's not the maker carefully controlling the process of each knife blade heat treatment.

Look, I'm not here to slam heat treating contractors, they perform a critical service to the industry. I'm just showing how this works, so that you know what to and what not to expect in this process. If a ten thousand dollar processor and hundreds of dollars of liquid nitrogen per heat treat is not in your budget, there are other means and reasons to build your own equipment for this procedure.

Most makers are familiar with heat treating, and my own decades of experience has proven that simple tools can do the job quite well. The issue is understanding the process. For instance, in cryogenic processing, it's not important what container is used to hold the knives submerged in or suspended above the liquid nitrogen for dozens of hours during aging, what is important is the rate at which the steel reaches that low temperature. Simply dropping a blade into -320°F liquid from room temperature is sure to be a disaster. You don't need a sophisticated cryo processor to slowly lower the temperature; there are other methods, including building quench staging, buffer chambers, and even building your own small batch knife-sized processor if you are inclined! The knifemaker often has to build his own equipment and methods, and as long as the focus is the process itself, he can control his results to a fine degree. In my own studio, the equipment is built around the process, and the process is built around the desired result.

I won't go into great detail about the technical side of the equipment here; that's a discussion for my advanced book, and every artist and craftsman has his own idea about how he would like to proceed, based on the knives he's building. I will acknowledge that a good background and understanding of physics, electronics, and machinery is essential if you are to take this on. You won't find this information on a knife forum; it's too far-reaching and complicated. As with most things, a little knowledge is a dangerous thing; immersion in deep background study must take place first. I will state that if you have read the entire page up to this point, you already have an understanding of heat treating and cryogenic processing greater than most knifemakers or knife manufacturers!

In my studio, the equipment is accurate, verified, dedicated, and tested, and much of it is either handmade, or adapted from other industries for the specific purpose of making knives. It's also like the rest of my knifemaking: continually evolving, improving, adapting, and changing to suit continual advances in steels, in knives, in materials, and in the requests of my clients, who want the very best knives possible from me. This is a tradecraft and art where the learning never stops, and I'm proud and happy to be a part.

Page Topics

Milling the heat exchanger on 9 x 18" contact quenching block:
Milling the heat exchanger pathways in a contact quenching block of aluminum
Custom equipment dedicated to knifemaking in the studio

Misconceptions, Myths, and Lies
  • "Cryogenic quenching embrittles the steel." This is wrong. They think of a banana dropped in liquid nitrogen that shatters when it hits the floor. This is not steel. The banana undergoes a sudden, drastic freezing of its water molecules, expanding them into large ice crystals which is a brittle structure. Steel is slowly and evenly cooled, there is no radical change (from a liquid to a solid), and the harder structure of martensite that results is then tempered for the correct and required toughness.
  • "NASA created cryogenics." No, not true. people have been freezing things for many decades before NASA. In fact, the Germans were industrially using cryogenics on parts for aircraft engines as early as 1930. The United States started using cryogenics in the 1940s. NASA didn't even exist until 1958, so when someone says this, they are trying to make their product, service, or themselves look cool. Don't believe it.
  • Wikipedia states that "Very little research into this technique has been published in the scientific literature, and the papers published to date are contradictory." Wow. Whiskey Tango Foxtrot. This is just a lie, and whoever wrote this was plainly ignorant of the massive amount of research and results performed in this field. Do tell the all the major steel industries about WIKI's open source findings, so we can return to pre-World War 1 status. Sigh.
  • "If it were good, the cryogenic treatment of steels would be more widespread." Really? Just because the person who thinks or writes about this is out of the loop, it's no excuse to deny that currently, some of the many items that are considerably improved by cryogenic treatment by manufacturers and the machining industry are: saw blades, drills, cutting pliers, knives, and punch and die sets, milling cutters, taps and dies, gear cutters, broaches, files, scissors, trimmers, slitters, woodworking tools, chain saw blades, drawing dies, and stamping dies. It is widespread. Do some research, for goodness sake.
  • "You can make a perfectly good knife without cryogenic processing." This is true, but personally, I am not here to make merely good enough; I'm here to make the best. I can make a perfectly good knife with hypoeutectoid (lower carbon) steels, a perfectly good handle with leather, make a perfectly good knife handle without bolsters, make a perfectly good sheath with single thickness kydex and hollow eyelets holding it together. Perfectly good. And good is, I suppose, good enough. If you think so, you're on the wrong website. Sigh.
  • "Cryogenic Processing is only for machine cutters: mills, drills, indexes, etc." This goes along with the claim that "a knife is not a valve seat, not a planer blade, not a forming die or cutting tool." Okay, this is true. But both are steel, and wear resistance in all uses can be improved drastically by this process. Even though you may not use a hand-knife at 200 surface feet per minute, you will cut with it, so why not improve the wear resistance, and toughness, and corrosion resistance as much as possible? Why not have your knife user, owner, and client sharpen knives only half as often? This is a conservative improvement potential in cryogenically treated hypereutectoid steel alloys: doubling the wear resistance. If you were the knife buyer and owner, what would you want? Would you say, "Hey, don't bother with the cryogenic stuff; I'll take mine only half as wear resistant; I like to sharpen more often. And that way, I can cut my blade life in half." ...Ahem.
  • "You can't do it unless you're a professional/metallurgist/scientist" The reason most makers don't do it, and some justify their lack of process participation while verbally spanking other makers for attempting it. This is some shallow game that a lot of knifemakers do, and it's just juvenile. They throw insulting terms like "garage cryogenics" and one even claimed the old adage, "sleeping in a garage does not make you a mechanic." This is jealous, shallow, spiteful ignorance. Cryogenic treatment is simply a process, just like heat treating, only different, with different equipment and technique employed. Would the jealous complainer fault the maker (and himself) for operating a professional grinding machine? I know many machine shops that would love to have a knifemaker's grinder, but they are just not skilled enough to use a belt grinder, opting for a bench grinder instead. Isn't the knifemaker trying to be a professional in his field? Doesn't that mean every part of the process? There really is no technological mystery to the process of cryogenic treatment of metals, it's not voodoo science; it does not require a Ph.D., and it's becoming more commonplace every day.
  • "It's too dangerous." Yep, fooling with metals and machines that cut them is like playing with baby rabbits. Grabbing a handful of bright yellow orange metal with tongs and thrusting it in a hot bucket of oil is something a child could be trusted with. Turning on a bench lathe that runs at 5 horsepower and has enough torque to twist off a one inch bar of steel in three seconds is a cake walk. Grinding with white hot sparking metals flying off into clouds of dust is namby-pamby kid stuff. Touching a blade to a rag wheel spinning at over 150 miles an hour is something a novice can try.... and it goes on and on. What? I thought we were talking about cryogenics! Yet these things we knifemakers do daily are much more dangerous and deadly, and we learn, understand, and apply safety practice in all of them. So that's just an excuse. By all means, it can be dangerous, so as adults, we know that and take safety precautions and care.
  • "Cryogenic tempering:" I saw a website of a company that cryogenically treats razor blades, nylons, and other finished products (what about my underwear?) and the owner of the company called his treatment "cryogenic tempering." What? There is no such thing. And that's the owner of the company! Boy, talk about not knowing your own business. Cryogenics may be applied (in metals) in quenching and in aging, but NOT in tempering. Tempering is reheating of hardened metals to convert a portion of the crystalline lattice into other allotropes, and getting something very cold is not heating... I don't know how this phrase even got started, but by the ignorant. Of course, their company is claiming nylons won't run if you "temper" them with cryogenics, so maybe they are even colder to begin with and liquid nitrogen heats them up.... I'm suddenly disinterested in clothing apparel, no matter who is wearing it.
  • "Cryogenic treatment is not a different type of quenching method but is an additional treatment normally used after quenching." Technically incorrect, and this is from an article about cryogenic knife blade treatment in one of the leading knife publications! Cryogenic treatment is part of quenching; as the blade is quenched from its austenitizing temperature, the quench continues into the sub-zero, shallow cryogenic, and in some cases, deep cryogenic temperature range. Then, it is held (aged) for a period of time for the results painstakingly and clearly described above. While some people separate the quenching from the cryogenic soaking, in knife blades this is not particularly necessary, as they are not thick blocks of metal forming dies, and do not suffer quench fracture that would require "snap temper" operations.
  • "Cryo treatment is a fad whose time had passed." Really? How much out of the loop are you? This is a claim made by the ignorant, as cryogenic processing is growing every year, and has become a mainstay of many high-tech industries. The truth is that conventional heat treatment is becoming a fad whose time has passed! For instance, as early as 1995, the STANDARD by the United States Air Force for all parts made of 440C in any aircraft are that they are cryogenically processed! A fad? Really?
  • "O1 doesn't benefit from cryogenic treatment, so it's not necessary." Wrong, wrong, wrong. While O1 may be adequately quenched in oil and only to room temperature, and it still makes a fine knife blade, read this detailed text above to understand why this comment is so, so wrong.
  • "Eta-carbides don't form at shallow cryo, only in deep cryo." Again, a misconception, and it's surprising to note that even a lot of metallurgists aren't up to current knowledge on this one, though the studies (some listed below in the references) clearly explain this. The error that eta-carbides were not formed in shallow cryo was probably based on bad or inconsistent testing, where blades were simply left to reach the shallow cryo temperature, and not aged in a continual, long time period. These carbides definitely do form at -86°C/-125°F (shallow cryogenic treatment and aging). Scientific studies have also shown that most of the benefit of cryogenic quenching takes place in shallow cryo (at -125°F/-86°C) with some steels having further enhancement in deep cryo.
  • "Fine eta carbides don't do anything in a knife blade." A knifemaker made this statement, and it's just foolish. Carbide production and benefit is extremely well-known and documented in the cutting trades, and knives are cutting tools. The claim is just wrong, and you can get the details on this page and in countless studies available, for free, all over the internet. This is another example of how knowledge is changing the world through this amazing medium.
  • "It can't be proven that cryo helps." Another amazingly ignorant attitude. Read, for goodness sake, educate yourself some!
  • "The studies and research are no good because they're performed in (India/Japan/Taiwan/Great Britain/Spain/etc.)." Just because there is no great industrial iron and steel behemoth in the United States any more, this doesn't mean that all other research from other countries has stopped! That's a sad commentary on research in general, and technological progress specifically. Researchers are many races, from many nations. If the studies are clearly described and documented, they are easy enough to assess. Read them, understand them, acknowledge that most of the researchers are not selling a product, bloviating their curriculum vitae, or falsifying facts. It's all pretty clear; don't let racism cloud your mind. It might help to know that some of these researchers have Ph.D.s in Cryogenic Treatment of Materials. Wow!
  • "The idea is nonsense" Sigh; I read this on a post about cryogenic treatment. If this oblivious person simply read this very page, he would more become educated than most people on the physical properties and reactions, and understand why cryogenic treatment is such and important part of metalwork. But the internet is open to all comers, uninformed and knowledgeable, and as the information resources continue to grow, the ignorant will be (hopefully) weeded out or too embarrassed to post.
  • "I'm an old metallurgist, and back in the old days, we didn't do this stuff, and I know what is and what was, and what will be and I know all the details...etc." Sadly, I've seen this attitude far too often. First, studies on cryogenics are current and evolving, and a lot of new information has happened in just the last few years (this page was written in 2015-2016), and continual studies demonstrate the quickly evolving nature of this science. New materials are being tested every day, and new delineations of study details are constantly revealed and underway. So unless the person claiming to know these things in immersed in current study, it might be best to defer to someone who is, someone who actually applies cryogenic processing to the specific materials (like some of the finest custom knife blades available, being used by some of the top military, combat, and counterterrorism operatives in the world). That way, the knives are made by a skilled craftsman, who, although he may not be a metallurgist, understands the current state of materials technology required by the most demanding users of knives. While you may think this a bit arrogant, know that I defer to all of the published scientific professionals whose works are accredited, peer reviewed, and detailed below. I'm not making some spurious claim, this is science. If someone disagrees with what I present here, their issue is not with me; it's with the sources below. Write to them and straighten them out, please!
    By the way, be very careful about someone claiming to be a metallurgist and offering free advice. Just like knifemakers, they should have their own online curriculum vitae, listing of their accomplishments and study, and they should refer to scientist's and published data to back up their claims, just like any profession. Please note that all of the references listed on this page and below do have just such credentials, and you can easily access them.

"Cryogenic treatment is not proven or accepted method."

It's sad when I read comments with this bent. It shows that the person who made them did not educate himself on the widespread use and benefit of the process, borne out in countless studies, articles, and intensive research. The person who claims this sticks stubbornly to his claim, and no one is going to convince him otherwise, no matter what the facts are, and what reality is proven to be.

"The consequences of a claim that something is true are entirely irrelevant to the issue of whether the claim is true.

--Steven Goldberg

This is how claims and misinformation is spread around the internet and in conversations. Claims have consequences, and those consequences are shared and reinforced by others who repeat the claim, but, after all, it's just a claim.

The reality is much different, and in making fine knives, the reality of results can be physically distinct and inevitably proven, particularly by professional knife users. It's going to be harder and harder to substantiate false and outdated claims in the modern information age, and I'm glad to be living in a time when knowledge is so abundantly available at our fingertips! You just have to be able to distinguish who is presenting facts supported by hard data and research, and who is making excuses and uneducated claims.

Thanks for reading this, and thanks for stopping wives' tales, misinformation, and lies in our tradecraft, interest, and art.

Page Topics

Balancing a disc grinding plate to high accuracy:
Static wheel balaning of disc grinding plate
Frictionless static wheel balancer

Glossary of Terms
alloy (steel)
A substance composed of two or more metals intimately mixed and united. Typically, in knife blades, these alloys are included to enhance mechanical properties, aid in fabrication characteristics, and add specific attributes to the steel. I use a dozen different alloys in my current work; all of them are hypereutectoid and high alloy tool steels and stainless steels.
anneal (annealing)
A treatment of steel to convert austenite and martensite to pearlite, softening the steel, relieving stresses, and making the steel ductile and malleable for easy machining and working. With proper work method, this is rarely, if ever needed in the modern knife shop, and I can count on one hand the times I've done this in 35 years of knifemaking. Annealing is done by heating steel to a predetermined temperature, and cooling slowly over many hours to allow equilibrium phase transformation to take place. The exact time, temperatures, and rate depend on the steel alloy type.
asperity
This is the rough surface or edge of metal, particularly defined when surfaces are polished (or not!). Asperity is improved (reduced) in cryogenically treated steels, and these same steels can be made sharper due to the fineness of the carbide structure created when these steels are cryogenically treated.
austenite (gamma-ferrite)
A crystalline phase of non-magnetic steel created at high temperature conversion, necessary to form martensite, cementite, pearlite, or bainite, depending on the treatment process. More about austenite at this bookmark.
bainite
Bainite is a combination of cementite and ferrite, stronger than pearlite. It's formed from austenite below the temperature that will form pearlite, and above the temperature than which will form martensite. More on bainite at this bookmark.
carbides
Extremely hard particles in knife blade steels. These carbides are sought-after in knife blade steels, they are beneficial to extremely high wear resistance. Some metallurgists believe that they play a more critical role in the durability and wear resistance of steel than martensite. There are many types of carbides, and all of them are formed with carbon and a less electronegative element. In these steels, some iron carbides are Fe3C, Fe7C3 and Fe2C. Some chromium carbides are Cr23C6, Cr3C, Cr7C3, Cr3C2. Other carbides are molybdenum carbides Mo3C2, vanadium carbides, niobium carbides, tungsten carbides and complex carbides that are combinations of other carbides! Some carbides have complicated crystalline structures, some form in interstitial locations of other crystalline lattice structures. With all carbides, their effectiveness depends on how fine they are, how well-dispersed, how high the volume overall that is precipitated. A critical point is that the three elements of chromium, molybdenum, and vanadium have the highest solubility in austenite, therefore they precipitate the highest volume of carbides. This is why these three are big players in high alloy steels.
cementite
Iron and carbon with the chemical compound Fe3C. It is a brittle, extremely hard ceramic substance. More on cementite at this bookmark.
critical (temperature)
In knife blades and heat treating, this is the temperature at which phase transformation takes place, the temperature when austenite is formed from the base allotrope. Also known as the austenitizing temperature. These temperatures vary depending on the steel alloy. In the old days, all the temperatures of transformation were called "critical."
cryogenic
Simply means: of or relating to extremely low temperatures. Cryogenic references do not have a specific temperature, no matter what you may read on open source definition guides and encyclopedias. Each science and realm of cryogenics is different, but in knife blade discussion, it means colder than sub-zero treatment of blades to impart higher wear resistance, toughness, and corrosion resistance. Further specification must be made, such as shallow cryogenics or deep cryogenics, or specifying the temperature to clarify the context of the idea, range, or discussion.
crystal, crystalline
In this context, a body that is formed by the solidification of the combination of steel alloy elements that has a regularly repeating internal arrangement of its atoms and molecules with strictly defined and identifiable external plane faces.
decalescence
The property of absorbing heat energy without increasing temperature while phasic change is underway in steel. Technically, a decrease in temperature when compared to ambient thermal loading.
decarburization
A very bad thing; knife blade steels are overheated, or heated too long, or heated in an oxygen-rich environment, and the carbon migrates to the surface of the steel, bonding with the free oxygen to form scale. The scale is ground off, and the knife owner does not even know that the steel has been rendered to a less than optimum alloy by carbon loss. Carbon is the most important alloy in all steels, so this is no small error. Read about the horrors of decarburization by an established and experienced knifemaker at this bookmark.
equilibrium
In this context, equilibrium means with all physical structure at rest, in balance, and with changes slow and static, with no dynamic forces. In steel, the phasic changes occur slowly with the physical form at rest, and this is not what knifemakers do, unless we are after full annealing of steels!
ferrite (Alpha-ferrite)
Iron with a body-centered cubic crystalline lattice form, magnetic, soft, a major constituent of mild steel. More about ferrite at this bookmark.
hardening temperature
This is the temperature above the decalescence point to which steels are heated for complete transformation before quenching during hardening process. The hardening temperature (and time that the blade is exposed to this temperature) depends on the steel alloy, the manufacturer's guidelines, the cross-sectional thickness of the blade, and the knifemaker's own experience for the desired result of complete austenitizing.
hysteresis (hysteresis band)
Also called "dead-band". In this context, it's the range of cycling in ovens between the temperature that the heating element turns off after reaching the set temperature, and the oven cools to a lower temperature and then the element turns back on until the set temperature is reached again. This creates a cyclic effect in a range of temperature, and this is called hysteresis. In most ovens and furnaces, this range can be extremely wide, between 50° and 150° F, creating wide swings in temperature and inaccurate control of the process. Attempts should be made with equipment to narrow this band for greater accuracy in the process. In my own studio, switching tempering and drying ovens to PID controllers will result in a hysteresis band of about 1°F! Mechanical freezers can suffer from wide hysteresis bands as well, applied to their cooling control rather than heating control.
interstitial
In this context, the word refers to the holes between larger metal atoms in crystalline lattices where smaller atoms or ions occupy. It also refers to the spaces in the larger molecular arrangement that carbon and small carbides occupy.
lattice
A regular geometrical arrangement of objects constituting volume; specifically: the arrangement of atoms in a crystal in a clear and definite physical and mechanical form.
martensite
Martensite is a very hard, corrosion resistant wear resistant crystalline structure created by sudden quenching transformation from austenite. More about martensite at this bookmark, and further explanation of understanding martensite at this bookmark.
metastable
In the context of steel phases, this means stable for the moment, if no outside forces or conditions act upon the metallic structure. So in knife blades, this is not really reasonable, as force on the structure of the steel (applied by cutting and pressure), aging (inevitable), and temperature changes can all force the metastable material into another phase or condition. The idea is to get the blade into a "stable metastable" condition, so that normal knife use and exposure does not induce changes in the steel knife blade.
normalizing
A treatment in lower alloy steels to relieve stresses caused by machining and forging, involving heating the steel to its austenitizing temperature or somewhat below, and then letting cool in room air or by a fairly fast rate. This cannot be used in high alloy martensitic stainless hypereutectoid steels, because they will quench and harden, but is typically performed in lower alloy or hand-forged blades.
pearlite
Pearlite is a layered structure of ferrite and cementite, formed in steels by slow cooling. It is very tough, but not particularly hard or wear resistant. More about pearlite at this bookmark.
PID controller
PID stands for proportional, integral, derivative, and this is an industrial process controller that is programmed to high accuracy with internal feedback capabilities. What this means is that the controller is not simply a thermal switch, turning heating (or cooling) on and off; it calculates or can be set up to work with the individual application, controlling the rate, timing, error, and expected heat loss (in the case of a heating application) to anticipate the load, process, and needs of an individual device. Without going into specifics, these controllers allow very accurate temperature control, once set up and programmed for the specific use. In the case of my tempering/drying ovens, variations of set temperature create a narrow hysteresis band of 1°F.
precipitation
In steel phase transformation, precipitation occurs as a substance (usually carbide) is produced from a solution (in our case a solid solution: austenite).
quench
To cool suddenly. In knife blades, this forces transformation of austenite to martensite, and precipitation of carbides, the basis for hardening steel. Quench types, mediums, rates, and temperatures differ depending on the steel type and alloy undergoing quenching.
recalescence
The property of losing energy without a drop in temperature during cooling of the steel at equilibrium during phasic change. The steel actually increases in temperature as the physical structure changes and the steel attempts to reach entropy.
spheroidized (spheroidizing)
A treatment of steel to convert plate-like cementite into spheroid cementite, resulting in extremely soft, malleable, ductile condition of steel for ease in machining and working. This is done by heating steel to a predetermined temperature, and cooling slowly over many hours to allow equilibrium phase transformation to take place. The exact time, temperatures, and rate depend on the steel alloy type.
stainless steel
A steel containing a substantial amount of chromium, which adds strength and inhibits corrosion. While in the United States of America, we classify stainless steels as generally having more than 10% and up to 13% or more chromium, in Europe and other parts of the world, they classify stainless steels specifically as having more than 10.5% chromium. Since it has been convention in the past to classify steels having as little as 4% chromium as stainless steels, it can be very confusing to classify with only the simple designation of "stainless." Therefore, it's best to describe stainless steels by their grade (austenitic, ferritic, martensitic) and by their trade name or SAE/AISI designation when describing them. Simply identifying a steel as "stainless" does not accurately identify the steel.
steel
A strong, hard metal made of iron and carbon with alloys of other elements all included to produce specific effects and results in the final use of the steel item.
sub-microscopic
Definition: too small to be seen by an ordinary light microscope. In this context (knife steel) it means that a structure is too small to be seen by an ordinary light microscope. Sub-microscopic is then a definition of the size of an object. This doesn't mean the structure can't be seen (actually imaged) by a microscope that uses other methods, such as a scanning electron microscope. The sub-microscopic size limit is about 1500X and the resolution of .2 micrometers.
super steel
There is, simply, no such thing. This type of term is non-specific and a sales-directed descriptor, inserted to make one think one steel is superior to others. Like steels with mystical, generalized, or popular name created as business advertisers, this has no place in the context of steel discussion, unless you are discussing comic book characters Superman, Supergirl, Superboy, Superdog... hey, were is Superwoman? How come it's Wonder Woman and not Superwoman? Ahh, I get it: the wonders of women...
tribology, tribological
Tribology is a branch of mechanical engineering and materials science. Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. In knifemaking, tribological studies play a role in determining steel wear characteristics and this is the only scientific, accepted method to determine the relative wear resistance of steel. Cutting tests do not; they are too variable, and knife blades can not be consistently created to any high degree of accuracy. In tribological testing, wear surfaces, indentation, loss of mass, and friction are all considered and calculated. This is ASTM and AISI approved testing of the wear resistance of steels and it is the only recognized standard. More about cutting tests of knife blades on this page.

Page Topics


Milling machine VA power indicator, DC drive controller, RPM, SFPM speed indicator, and digital readout positioner:
Milling machine VA power indicator, DC drive controller, RPM, SFPM indicator, and Digital Readout

References:
  • "Elements of Metallurgy and Engineering Alloys," Edited by F.C.Campbell, ASM International, 2008
  • "Out of the Fiery Furnace, The Impact of Metals on the History of Mankind" Robert Raymond, 1984
  • Machinery's Handbook, Editions 1-29, A Reference Book for the Mechanical Engineer, Designer, Manufacturing Engineer, Draftsman, Toolmaker, and Machinist," Oberg, Jones, Horton, Ryfeel, and Green, 1914-2012
  • Dr. Randall Barron, Professor at Louisiana Technical University, Department of Material Engineering, 1970-1990
  • Case Study: Design of Bainitic Steels, , Bhadeshia, Materials Science and Metallurgy, Cambridge
  • Cementite precipitation during tempering of martensite under the influence of an externally applied stress, Bhadeshia, Cambridge, 1994
  • TWI (The Welding Institute), Cambridge, UK 2015
  • Uddeholm®, et al., 2015
  • Tool Steels, 5th Edition,, 1998, George Adam Roberts, Richard Kennedy, G. Krauss
  • Materials and Processes, Crystal Dislocations, NDT Resource Center, Fundamentals of heat treating steel, ASM International, 2006
  • Light Microscopy of Carbon Steels, Leonard Samuels, 1999
  • The Effects of Alloying Elements on Steels, M. Maalekian, Christian Doppler Laboratory for Early Stages of Precipitation, 2007
  • George Vander Voort, Metallography, Failure Analysis, Archeometallurgy Consultant, numerous articles
  • Steel Heat Treatment: Metallurgy and Technologies, George E. Totten, 2006
  • Cryogenics, Basics and Applications, Linde, et al.
  • The Journal of Materials Processing Technology, Influence of shallow and deep cryogenic treatment on the residual state of stress of 4140 steel, D. Senthilkumar, 2011
  • Defects and Distortions in Heat Treated Parts, ASMI, Sinha
  • Cryogenic Quenching of Steel Revisited, Zbigniew Zureki, 2005
  • Cryogenic Treatment and its Effect on Tool Steel, Yugandhar, Krishnan, Rao, Kalidas,
  • Heat Treating Process and Principles, Krauss, ASM International, 1990
  • Below Zero Chilling Toughens Metals and Increases Tool Life, Machine and Tool Blue Book, Morris, 1995
  • Effect of Subzero Treatment on Microstructure and Material Properties, Karthikeyan, Raj, Dinesh, Kumar, International Journal Of Modern Engineering Research, 2014
  • Optimization of cryogenic treatment to maximize the wear resistance of 18% Cr martensitic stainless steel by Taguchi method, Darwin, Lal, Nagarajan, Journal of Materials Processing Technology, 2008
  • Handbook of Residual Stress and Deformation of Steel, , ASM International, Totten, Howes, Inoue, 2002
  • Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel, Das, Dutta, Topo, and Ray, 2007
  • Effect of cryogenic treatment on microstructure and wear characteristics of AISI M35 HSS, International Journal of Materials Science and Applications, Candane, Alagumurthi, Palaniradja, 2013
  • Deep Cryogenic Treatment of Cold Work Tool Steel, Molinari, 2014
  • Deep Cryogenic Treatment of Tool Steels, A Review, Collins, Heat Treatment of Metals, 1996
  • Influence of Deep Cryogenic Treatment on the Mechanical Properties of AISI 440C, Idayan, Gnanavelbabu, Rajkumar, 12th Global Congress on Manufacturing and Management, 2014
  • Corrosion Resistance of ASSAB Stavax ESR Stainless Steel by Heat and Cold Treatment, Materials Transactions, The Japan Institute of Metals and Material, Han, Lin, Shih, 2013
  • Comparison of Wear Properties of Tool Steels AISI D2 and O1 With the Same Hardness, Bourithis, Papadimitriou, Sideris, Tribology International 39 (2006)
  • Improving Component Wear Performance Through Cryogenic Treatment, Wurzbach, OMA-1, CLS, DeFelice, Maintenance Reliability Group Laboratories

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30 liter cryogenic Dewar cryostat transportation cooler
30 literCryogenic Dewar Cryostat transportation cooler
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