Jay Fisher - World Class Knifemaker
Quality Without Compromise
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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.
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.
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.
"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:
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.
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.
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."
"Out of the Fiery Furnace;
The Impact of Metals on the History of Mankind"
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.
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.
"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
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!
There are many other alloy elements in modern tool steels, but just for the carbon steel discussion, these are the important and prevalent players.
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:
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.
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.
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:
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.
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:
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.
The predominate allotropes, constituents, and crystalline structures for our specific discussion of fine knife blade steels are :
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,
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.
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.
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.
Thanks for the great site, sharing your beautiful knives, and your knife knowledge and philosophy. I was raised as a mechanic and welder in my family’s heavy equipment business and 30 years ago, the knowledge you are freely sharing was handed down father to son and not shared to the world.
My wife is a professional pastry chef, food blogger, and teacher. I thought she needed a custom knife for valentines or her birthday and looking at all the $500 ‘customs’ I thought, hell I could do better than that I have a metal shop in the garage. I started reading the knife forums and the usual drivel about real knives being forged. Being disabled, my hammer swinging days are over. Then I ran into your site. I spent the last week and a half studying as much as of your site as I could digest and as importantly as how, the why's.
Thanks again for the copious knife knowledge, I help my wife on her blog so I know how involved building and maintaining a site is. If I was blessed with riches, instead of free time and enough knowledge and tooling to be dangerous I would put her name on your list for gorgeous Concordia, instead she will get a well-crafted RogboBilt O1 chef and paring knife, plain but made with love.
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.
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.
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!!
Thank you again- like finding the holy grail of treating that cut through all the floating opinionated stuff.
"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 does this by displacive transformation. It's also highly strained, a kinetic product brought about as a result of rapid 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.
Did you know that:
"Martensitic transformation occurs in other metals and materials, not only in steel. It occurs in, for example, nonferrous alloys, pure metals, ceramics, minerals, inorganic compounds, solidified gases and polymer."
-H. K. D. H. Bhadeshia
Martensite in Steels
Materials Science & Metallurgy, 2002
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.
Charlie Ward Wright IV
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.
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.
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.
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 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.
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."
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.
Your Web Site-
I am impressed; you are the epitome of a professional.
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:
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.
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.
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:
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.
There are three critical factors in cryogenic quenching of steel:
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. Even if the crack is not visible to the human eye, too fast of a drop in temperature can have a detrimental effect on the actual crystalline structure of the steel with microscopic fractures, and that failure will present itself as high wear of the steel, and a markedly less-than-optimum condition. 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.
What is the specific rate of cooling for most of these steels? 4-5 degrees Fahrenheit per minute. That means in order to reach -100°F, it should take about 40 minutes (from room temperature), and to reach -325°F should take about an hour and a half (from room temperature). This is why simply dipping blades into cryogenic baths of dry ice and alcohol or liquid nitrogen is a huge and destructive error, yet knifemakers who are uneducated in this process frequently do this, and tell others that it's the proper way to quench! Sad, truly sad for the knife client. The cryogenic process cooling rate is absolute and critical.
Like too fast of a quenching rate, too slow of a rate is not as effective. Remember that quenching quickly is the goal, and a continuous curve in the downward temperature scale is essential. This is because allotropes converting are metastable, and it's important to continue the cooling process without delay. While not as damaging or limiting as quickly dropping the blade into the deeply cold environment, the blade does need to keep cooling at a good rate. Being that knife blades are thin, relatively small pieces of metal, the cooling rate should be as fast as possible while adhering to that 4-5 degrees per minute Fahrenheit rate. Another consideration is uniformity in the cooling jacket, environment, buffer, and cycle. Once the rate is established and reliable, other factors can be tweaked, adjusted, and changed for a variety of distinct performance levels in the custom shop.
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.
People have asked me about what appears to be a conflict of process about the hold time at cryogenic processing. Here's a part of an email from a man trying to navigate his way through the processing:
In your link I just read a recommended soak time of 6 to 36 hrs. Would you suggest the full 36hrs of soak time? I find it ironic that I often read how important it is to perform the first temper immediately after quenching to reduce chances of experiencing stress cracks, yet some of the latest heat treating studies for knives suggest deep freezing (which imparts more stress on the steel) for as long as an extra 36hrs of high stress.
I can see how this can be confusing, and I want to be clear. When I claim that the soak time is 6 to 36 hours, it's a generalized statement. I'll break this down for clarity:
First, this is a minimum recommended soak time, and actual soak times may be much longer. In my own work, if it's possible, I've maintained some blades at 60 hours or more. The time is the minimum, and what happens is that the improvements taper off, and eventually, it becomes more costly to hold the knives in this environment than realize additional benefits which would offset the continued cryogenic soak. Refrigeration, liquid nitrogen, the cost of continuing operation of the processor, and the practice of creating the super-cold environment outweighs what beneficial results happen. After a few days at this temperature, not enough gains are realized to justify the expense. So, as long a soak as possible, but don't put the steel away for weeks, there isn't enough improvement after a few days to justify the cost of the process. Percentage points of improvement are noticeable, but when fractions of a percent take days to achieve, it's not worth the cost.
The second part of this statement is the wide range (6-36 hrs.) I've included this range because of the variety of steels used for knife blades that may require cryogenic processing. Not all alloys are the same, and each one requires a different regime. I keep a detailed log of all my process operations and results, and that's the best way to hone in on specific, repeatable processing.
The third bit of confusion is understandable. In one section, you read that tempering cannot wait; it needs to happen immediately, and then you read that the blades are held in the cryogenic processor for days before tempering! The statement that tempering cannot wait is first based on studies of conventional heat treatment. Quench a steel like D2 to room temperature, and you'll see a hardness of 62HRC, but leave it at room temperature overnight and it will be 52HRC! So, in conventional heat treatment, you can see how tempering must not wait. The longer the time the steel is kept at a temperature between room temperature and 100°C (68°F to 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.
What about in cryogenic processing? It's important to understand that room temperature is not cryogenic temperature. Leaving a blade at a static 68°F is not the same as holding it in an environment of -325°F. At room temperature, allotropes are metastable; they well change in time with (or without) mechanical pressure; this metastable nature is why it's critical to get tempering underway immediately. At cryogenic temperatures, steel is not metastable to any degree that would affect a change in the structure, apart from compression which forces precipitation of carbides at nucleation sites. In other words, conversion is still taking place. Once the steel blades are warmed to room temperature; all bets are off, and tempering must take place immediately, or the allotrope will convert. Again, the longer the time the steel is kept at a temperature between room temperature and 100°C (68°F to 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. I hope that clearly explains the differences between holding at room temperature and compressing and converting at cryogenic temperatures.
The warm-up rate for the blades from deep cryogenic temperatures (-325°F) to room temperature before tempering is important to know. This is done by just letting the blades sit in still, room temperature air. On this page and several others of this website, you'll see photos of my blades on racks with condensate (steamy air) falling off of them or blades on a rack covered with dense ice. This is how they warm up. It's not a good idea to slowly warm them (leaving them in the cryogenic environment to slowly reach room temperature as the processor or environment warms); this may allow metastable austenite to convert. Conversely, hurrying the process along by sticking them in a pre-warmed oven is also not a good idea, since this is highly stressful. The balance between these is simple; they are pulled from the cryogenic environment, sit in still room temperature air until condensate on them turns liquid (above 32°F), and then moved into a cold oven to evenly ramp up to first tempering temperature.
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.
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.
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.
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 conventional ideas 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, but the exact results vary depending on the steel type, which is another reason for the knifemaker to heat treat his own blades.
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.
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, even on a microscopic scale. This presents as lower wear resistance, or a less than optimum cutting edge retention.
Tempering is necessary to reduce stresses and balance the hardness and toughness of the steel, and it works in several ways:
Performing a "snap temper" is tempering immediately after conventional quenching. The blades are quenched to room temperature, and then 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.
Several people have asked me if I ever had any cracking of blades in stress riser areas (around holes, filework, in radical geometrical changes of cross-sectional areas) in any of my blades. This is a great question, because heat treating contractors will tell you that this is the reason to perform a snap temper. I'll be very clear on this: I've never, ever, had one single crack of any kind in cryogenically processing any knife blade steel! Not a single incident. Take a look at some of the radically shaped blades I make, even with full engraving, and know that not a single incident of stress fracture has occurred. This is because references state that snap temper is done for two reasons:
From this, it's clear that since knives are not wildly complex dies with thick and thin areas, the reason to snap temper is number two: convenience. Know that this is detrimental to the final steel allotropes and condition.
I believe that heat treaters who do this 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). Also, consider that heat treatment contractors do not typically know who the manufacturer or foundry of the steel is, and there are some variations of treatment between steel blades coming from different sources.
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 (68°F to 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."
Cryogenic Quenching of Steel Revisited
Air Products and Chemicals, Inc., 2005
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?
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.
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:
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.
These are, technically, softening processes for steels, but their application depends immensely upon the alloy type!
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 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.
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.
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.
"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:
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 normally 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 wear resistance improvement in shallow cryo. However, there's a very important thing that is seldom mentioned by researchers who evaluate these things, and something I've learned from working with 440C in both shallow and deep cryogenic processing. When 440C is processed in deep cryogenic processing, with multiple tempering and DCRYO immersion between tempers, it's markedly more dimensionally stable in DCRYO than in SCRYO. If I'm using 440C on critical applications where dimension, hole sizes, hole spacing, and alignment is critical, I'll use DCRYO. Think folding knives and their parts, spine, spacers, pivots, bolsters, and blades and locks. When it has to be the right size, and spot on, DCRYO excels in dimensional stability when using 440C. 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, 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.
Here's a query about a particular steel (D2) and the cryogenic holding time:
I’m sorry to bug you so much regarding heat treating. But you seem to be extremely knowledgeable on the subject of heat treating, and I appreciate having intelligent dialogue with you on the matter.
I looked up TTT curves for D2 last night. I read that for D2 the Mf temperature is in the range of -112F to -166F. I remember that you mentioned that at -321F chemical transitions are very sluggish or slow…hence requiring extended time in this very cold state.
“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”.
So, if the reaction/precipitation time is directly proportional to temperature, (in theory), would it be better to only bring the blades down to ..say 170F and hold it at that temperature and consume less Nitrogen? At this temperature, all the Martensite should be converted to Austenite, and the chemical reaction might be faster…requiring less soak time. I guess the problem is that I can’t quantify how much faster the reactions would be, and I might be missing out on the Eta Carbides that form in colder temperatures.
G. N., P. Eng
Senior Maintenance Engineer
This is a great query, because it's an opportunity to clarify some effects of the process, to the best of my understanding, from what I have learned in study and practice. The first thing to note is that the Mf, the temperature at which martensite is finished forming upon quenching, is about -150°F. Each steel manufacturer is different, so it's best to make sure that one knows the makeup and recommendations of the individual foundry or steel supplier first.
I believe that Mr. N. suspects that conversion takes place slowly, sluggishly because of the temperature, and this is not quite the concept. The beneficial reaction of carbide precipitation is caused by the temperature, so I'll go into it a little bit deeper here.
Martensite formation is what happens when the steel quickly reaches its martensite finish temperature. This may not be a true cryogenic temperature; in the case of the D2 described above, it's colder than the shallow cryo (-125°F) but not as cold as deep cryo (-325°F). At the martensite finish point, the steel has as much martensite as it's going to have, and that part is pretty much done. But this is only part of the story; the formation of eta carbides is critically important, and some studies suggest that they contribute to the wear resistance of steels more than the martensite.
Much of what happens at this temperature is due to compression. If you are studying this page, you probably know I've mentioned compression before. While the studies are complex and detailed, I'll try to put them in plain English. After the martensite is formed, expansion and compression play a role in precipitating carbide development. The martensite formed is, in simple terms, cramped, expanded, forced, bent, and strained, but the cooling continues. Due to laws of the states of physical matter, the steel is compressed by the cold. The metal shrinks in the extreme cold, pressures increase within the structure, and this physical state forces lattice deformation, shifting carbon atoms, stacking, arranging, and shuffling the crystal form. Carbides then form by the spinodal decomposition of some of the martensite.
While this can get pretty complicated very quickly, and there are great minds and works on the process at the crystalline level, the concept is this:
The point here is that the extreme cold does not slow the process in a detrimental way; the extreme cold forces the process to continue past martensite conversion on to eta carbide precipitation. Now consider this; at least one study confirms that deep cryogenic treatment for extended times (20 hours was used in the experimental process) actually increases the inherent driving force of carbide conversion that takes place in tempering. This means that there are structural changes that happen in Dcryo that enable more carbides to be formed when the blades are tempered! And this study was done on D2.
Very neat and exciting, isn't it? Okay, maybe it's a cheap thrill; it's not like the day you bought your new car or your boat, but for guys like me, it's a thrill to be alive at a time when we are learning, improving, and sharing all this data.
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:
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? .
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.
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.
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.
"(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
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:
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
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.
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:
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.
"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.
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.
As you might guess, I get plenty of emails questioning me about heat treating and cryogenic processing of knife blade steels, since is something I professionally do. Please remember, I don't perform this service for others; I'm not pushing a business in heat treating; I have nothing to gain in the heat treatment of other people's knife blades. The reason I present this information here is like the rest of this website: I do this as a service to my community, to give realistic, valid information to the reader. Whether the reader is a past, present, or future client of mine, whether the reader is another knifemaker, beginner, hobbyist, part time, or full time, whether the reader is a person who has a passing interest in heat treating and blades or he is a dedicated knife enthusiast who wants to know the facts about the subject, the information I present here is a gift to my profession. Anyone interested in knives in service in real world use, knives made in the best possible way our modern technology can dictate deserves to know what is on this page. While the reader can find this information in professional sources, text, documentation, studies, and guides, I've compiled it here with real world knife blade creation, and include misperceptions, erroneous processes, and mistaken ideas, so that you can know the facts and the hyperbole together. Then, the reader can make his own conclusions and decisions based on this information.
As the traffic has grown on this page and many others related to specific blade steels and knife related process, more questions, curiosities, and interest has grown, and that's a good thing. I've decided that, rather than answer some of these questions and inquires on a singular level, I will post pertinent queries here, if they have any bearing on the topic, for others to learn by. I detail this on my page "Learning About Knives." Please know that I don't ordinarily answer questions not related to direct knife orders; I simply don't have time to answer them all. So if you write me and ask a technical question, please don't be surprised if I don't answer you. Sometimes, a query will strike me as important, in that I can offer what I have learned and know and many others who are reading the same pages will find the answers they are looking for as well. Again, I do this as a service to my profession, and you can learn about the service aspects of the professional knifemaker at this link.
Here are some important emails with the answers I've given. Note that names are omitted to protect the privacy of the individuals, and other knifemakers, companies, and entities are also not given, mostly to protect them from embarrassment. I'll add to this section as I feel is necessary; thanks for helping to stop misinformation, wives' tales, and misconceptions in our tradecraft through learning and education.
I have been reading your website regarding heat treating and cryo treatment.
AWSOME write-ups. I’m not finished reading it yet.
I see you are now using some 154CM steel.
Make knives part time, and have been using D2 tool steel. I am still persistent on
having my blades tempered on the
primary hardness point of 450’ish F rather than 925is F. Recently I have read that N***** C********* has been playing
around with various heat treating technics for D2 and CPM3V. He has been deepfreezing his blades prior to any tempering at all.
Have you tried experimenting with heat treatment like this and having the same positive outcomes?
I ask you because you seem to have the proper studio and equipment to alter your heat treating methods at will.
Hi, G. Thanks for your nice words about my site and articles about Heat Treating and Cryogenic Processing of Knife Blade Steels and my page about D2. I actually use over a dozen different steels currently.
A lot of guys try a lot of things, but the very best way to treat high alloy hypereutectoid tool steels is to first follow the manufacturer’s recommendations, followed by a lot of experience (maybe decades) of heat treating. Here are some things that are important about D2, particularly related to the forum you linked: Acetone and alcohol is shallow cryogenics, and barely that, particularly since long soaks are not possible or done with this method. While cryogenic treatment at -100 F will improve the steel’s as-quenched state, it is not the premium treatment for this steel. Deep cryo is, at -325F followed by a long, very long soak at this temperature. Try 35-50 hours minimum! This long time soak is critical in carbide precipitation, and carbide grains are not large in D2 with this method, but extremely small and more profuse. What proper, regulated deep cryogenic produces in D2 is astounding, with an increase of over 800 percent in wear resistance and 25% increase in toughness over conventional treatment. Notably, this is a 400% increase in wear resistance over shallow cryo (or dry ice) and yet forum posters claim this does not happen, which is flatly incorrect, since it's been proven over and again in actual tribological studies by degreed, peer-reviewed professional metallurgists and scientific research. I don't know how a knifemaker can honestly discount what metallurgical scientists have proven, inserting his own conclusions when he has none of the training, equipment, knowledge, funding, or backing of real scientists. This is why it's so important to read, study, and learn from verified sources, not simply post one's ideas and thoughts on a forum comment section. I defer to those sources who actually know metallurgy and have thoughtfully published their results for all of us to read. But you've got to read, research, and study, and a lot of these makers simply don't or won't make that effort.
Tempering is time-process critical, and I won't go into high or low levels of tempering curves, since it depends on the desired allotrope balance. What is missing in these discussions is that for higher performance and the best allotrope conversion, multiple tempers are necessary in the correct stages, with deep cryogenic soaks in between. By the way dipping D2 into any cold solution (water, dry ice and alcohol, or liquid nitrogen) is absolutely the wrong way to handle this or any air-quenched steel! What will happen is micro fractures from shock, invisible to the eye, but contributing to lower wear resistance. All metallurgical references will clearly state that the maximum cooling rate is 4-5 degrees Fahrenheit a minute, not 100 degrees below zero in thirty seconds! I really wish guys who claim to know how to heat treat would at the very least, do some research, plainly presented in industrial sources and by AISI, ASM, ASTM, and SAE….
It’s good you are researching the best methods. D2 is a very finicky steel to process, and most D2 in knife blades is not processed correctly. By the way, D2 should never be hand-forged; if it is, it is ruined steel. Die shops and machine shops professionally dealing in D2 for high performance industrial dies are very specific about their heat treat processes, and most of them will tell you it’s a hard steel to get right. By the way, snap temper is wrong; it’s a crutch for some other failure (too high a cooling rate or timing conveniences leading to fracture-prone blades on a microscopic scale, showing as high wear, being the main reason).
The best of luck in your endeavors; I look forward to seeing your knives one day!
It amazes me how many knifemakers claim to know what can only be seen with an electron microscope or x-ray diffraction (retained austenite, carbide structure, etc.), while not having the access to, training about, or the very equipment used to make these determinations. But, as I've stated before, it's truly sad that our tradecraft is so filled with misperception, ignorance, and myth. Here's some logical, simple and clear help:
If you are a maker and are using D2, start with the manufacturers’ recommendations. They know their steel and will politely tell you how it is best treated. If you wish to improve on their process, research needs to be done in earnest, through AISI, ASM, ASTM, SAE, and other sources detailing real metallurgical studies. There are some great treatises out there about the process, and I encourage you to purchase them, read them, and study them, and they are not available for free, and this is perhaps one of the reasons makers don't study them; they don't see the need to purchase them, but that is where the information resides.
Forums are not typically the place for professional information (sorry, but true), because the depth of data, information, experience, and process applications can not be presented in a couple paragraphs of a post, or multiple posts for that matter. Take this very page for instance; I suppose it will print out to the equivalent of over 100 pages of text, yet barely scratches the surface of steel knife blade treatment information, and each steel is different! This is why textbook-type resources are the best source of information; most of them are over 600 pages long. For some reason, most knifemakers won't bother to purchase these $100-$400 dollar information-rich engineering sources, much less read them several times, frequently access them for the applicable information, and then apply this data to their own work. I frequently use these necessary resources, but could not convey the scope of study on this website, or this page would be many thousands of pages long!
Again, keep it simple. Start with the manufacturer's recommendations, which are usually the best for basic, reliable performance, and if you wish to improve on them, talk to the metallurgists and engineers at those companies, be prepared for the discussion by studying as much as you can; as most are professionals with degrees and are accredited in their field, and won't appreciate wasting time educating you.
Here's a great formula for success:
Study, study, study, practice,
Study, study, study, practice,
Study, study, study, practice,
Study, study, practice, practice,
Study, practice, practice, practice,
Practice, practice, practice, and then write.
Knifemakers who do little study, minimal practice, and then write on forums are shortcutting a few steps...
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