Jay Fisher - World Class Knifemaker
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An air hardening die steel. It's composed of these major alloy elements (I don't list the minor elements since they don't play a major role) :
These basic elements (along with iron) are a simple combination that works well together, and has for many decades.
Some anonymous posters say that I'm wrong to title this page "D2: Wear Resistance King" and claim that there are other steels that are also highly wear resistant. They say that the title is wrong, therefore I don't know what I'm talking about. This is typical of internet geniuses posting on internet forums created by non-professionals, who scan around and read titles and headlines in their frantic hurry to post their own precious comments based on their extensive and exhaustive research of page titles, without the bother of taking the time, nor the intellect to read and understand page content and context.
For those who do read, understand that I'm not making some claim that D2 is the absolute best wear resistant material (it's hard to beat tungsten carbide and diamond), but that because D2 has been around so long, and because it is so highly proven through many decades of use in the die steel realm, it is a highly regarded, reliable, longstanding performer. Like 440C, it is not the new kid on the block; it's been around long enough to have a serious reputation, one of extremely high wear resistance and tough durability, with good stain resistance. New steels on the block, or steels not normally used in knife blades like cobalt steels, high speed steels, and powder metal technology steels may be processed to be more wear resistant, but of course, may also lack toughness necessary on knife blades without tempering them back to a lower hardness, thus defeating the purpose of using them in the first place. Other steels can be highly corrosion resistant, something haters of high chromium steels tend to selectively ignore when it suits their particular argument of the day. The reality is that D2 can be made extremely hard and wear resistant, and very tough, and that is why there is a huge industrial following of this long-standing, grizzled old royal in the tool and die steel field.
If you don't like this claim and don't support D2's long and recognized value in the tool steel field and industry, please don't just post your uninformed opinion on some third rate forum of hobbyists. Go to industry, into the field of machining, and tell the hundreds of thousands of machinists, users, metallurgists, and professionals that D2 has no place in any machine, tool, or part, and demand that they immediately switch over to a newer steel, one with more curb appeal to the internet title cruiser. Explain to them that their ways are dated, old, and passé. After all, you've learned your metallurgy from the very best internet chat forums, where it's known that the real professionals (the actual leading edge of scientific and industrial minds) spend their time, generating 25,000 posts every couple years just proving what they know, while calling themselves "dizzybob123" (my apologies to the real dizzybob). And those guys actually doing the work, building the tools, equipment, and devices: why they're just losers, with industrial jobs with metals and things...
D2 has a lot of carbon. A lot. This varies somewhat between manufacturers, but know that the extremely high amount of carbon is critical to this steel's very long and proven reputation. Simply put, it has some of the highest carbon content in tool and die steels. Other types of "D" steels have up to 2.25 percent carbon (D3, D4) which is more carbon than cast iron! The leader of carbon in high chromium-high carbon tool steels is D7, which has 2.35 percent! D3, D4, D5, and D7 are not used to make knife blades because they are too brittle, and not tough enough to resist fracture in thin cross sectional areas.
Though you might think these small percentages (most less than two percent) of carbon seem like they're not much, in steel this makes a huge difference in the properties of the alloy. With a large percentage of chromium, the pair (chromium and carbon) make up quite a contender in the field of wear resistance in die steels. Add a bit of vanadium and this steel (D2) has quite the reputation of super-long cutting edge retention, or wear resistance. The averages are used in the chart because of the large range of manufacturer variations.
|Critical Alloy comparison between 440C and D2 (in percent, averaged)|
|Percentage Difference||+165||-69||No Comparison!|
I left out the less critical alloys to illustrate the profound differences in these steels. While the two steels both have elevated levels of chromium and carbon, they are quite different in performance. While 440C excels in corrosion resistance with good wear characteristics, D2 is much less corrosion resistant, but highly more wear resistant, so much so, it's a die steel.
For more information, link to my Heat Treating and Cryogenic Treatment of Knife Blades page.
D2 is a die steel, thus the designation "D." It's part of the family of Tool Steels, steels used to work and form metals, woods, plastics, and other industrial materials. These steels must withstand shock loads, rapid temperature changes, high wear uses, and are generally the toughest, most wear resistant steels made. They are far and above the high carbon alloys or the standard steels. I mention standard steels because standard steels are typically used to make most tools, like saws, hammers, files, and common drill bits. Standard steels are also the choice of individuals who hand-forge (open forge) knife blades, as they can be easily worked with low temperatures and in open atmospheres.
The family of tool steels has several subgroups, and I've read that D2 was originally created to perform as a high speed steel, but was not temperature stable enough. It's difficult to find any references on D2 before the 1960s, however high speed tool steels were known by the early 20th century. The details I can find suggest this is the case as in machinist's references, D2 is used in applications requiring sharp cutting edges where no high speed is involved. D2 does excel in heat treatment stability, not important to the knife owner, but critical to the knife maker. In industry, it's used in hot trimming dies, drawing dies, coining tools, forming and bending dies, thread rolling dies, as well as lathe centers, arbors, bushings and gauges.
Dies are an interesting tool. Many dies are used to shape metals by stamping, pressing, and forming under tremendous pressures available in hydraulic presses. There are many configurations for dies, but basically, they reshape the material they are working by shearing, bending, stretching, extruding, piercing, broaching, bulging, and blanking. These materials being formed by dies are mostly metals, and D2 is often used to make the dies that do the actual contact work. You can imagine the tremendous forces, wear potential, and stresses that take place in these dies as they work, and many times the materials and dies are hot while this is happening! So, while D2 is not used in high speed applications (motor-driven rotating tools that continually generate heat at a cutting edge), D2 has no problem working in the hot range. That is, if it's heat treated, quenched, and tempered (processed) correctly. This makes it easy to understand why D2 makes a superior knife blade.
There is a colloquial saying about D2 that I've heard all my life, "D2 holds an edge forever, and is impossible to sharpen." This is a clear indication of D2's reputation, well deserved. When I was young, my father asked me to sharpen his D2 Kabar knife, a knife used in WW2. It took literally hours by hand on the stone, and I was sore from the effort. D2, when properly processed, is incredibly wear resistant. By the way, no modern Kabar knife (now made in the People's Republic of China) is made with D2; the last one I looked at was made of (horror) 420 stainless steel.
D2's wear resistance comes from several alloys working together. An old metallurgist explained to me that in heat treating high chromium and high carbon tool steels, extensive chromium carbides are formed that are highly wear resistant, but more so, the amount of the carbides overall and their distribution density is extremely high, as are dislocations when properly processed in cryogenic temperatures. Add a bit of vanadium and D2 has vanadium carbides, and this steel can form an edge that is difficult to dull, even when cutting metals!
For more information, link to my Heat Treating and Cryogenic Treatment of Knife Blades page.
Hand-forged D2 is ruined steel.
An added feature and benefit of D2 steel is that it has very high heat resistance, when processed in the higher temperature range. The processing of D2 is interesting, and timing is critical as well as tight control of temperatures, within a few degrees Fahrenheit. This is crucial for the knifemaker to understand well; a slight variation yields a completely different quality of blade. This is why when I read that someone is hand-forging D2, the blade quality and performance is definitely compromised, if not ruined.. You simply cannot hand-forge D2 and maintain the temperature, timing, and atmosphere critically necessary to yield maximum performance of this steel. Like the exotic and high alloy hypereutectoid martensitic stainless steels, this steel must be processed in narrow ranges and controlled environments. Hand-forged D2 is ruined steel.
D2 has a very precise knee in its tempering graph chart, and hitting the knee just right is paramount to performance. For instance, you can temper the blade at a lower range and achieve a 60HRC hardness, but this is not where D2 shines. It will be brittle there, and it will also be subject to temperature extremes in an adverse way with lower overall corrosion resistance. Then, high temperature exposure will soften the blade!
However, if D2 is tempered correctly (and at the correct time), the heat treating processer (the knifemaker, I hope!) can achieve the supreme balance of hardness and toughness that will yield incredible blade performance in both cutting, wear resistance, toughness, and even, to a degree, corrosion resistance, at the same 60HRC! What this means to the knife owner is that just because a knifemaker (or manufacturer) offers a D2 blade at 60HRC, it doesn't mean he has processed it right. This is why it's best to query the knifemaker about his heat treating practice, and true professional knifemakers should be able to answer every question you may have about your blade and how it was processed.
Interestingly, this steel is process time critical. While many steels may be hardened and tempered at a later time convenient to the knifemaker, a delay in processing has disastrous results in D2. Pull a D2 blade out of an oven to air quench from its critical austenitizing temperature, and as soon as it cools to 120°F, it will (and should be) about 64HRC. Leave it to sit at room temperature overnight, and the blade will be 52-54HRC! This is a fascinating property, and just like the curious curve of the knee that proves that heating this steel actually increases the hardness during tempering is one of the reasons that heat treating has fascinated me for so many decades.
One more thing: D2 benefits greatly from triple tempering, with cryogenic holds between! Most knifemakers are unaware of this and how it affects toughness (see the next topic below). This critical action in processing can result in a blade that has a 25% higher toughness (measured by the Charpy process) and better plasticity. This is another reason to do your own heat treating and educate yourself about the process.
If correctly processed in the higher range, D2 has a temperature resistance in use and exposures on the knife blade to 950°F (510°C). That's pretty hot, so the knife blade won't lose its temper even when exposed to periodic flame! Also, when sharpening, motor-driven sharpening tools (wheels, buffs, abrasive) would have to bring the edge up to above this temperature to affect the temper. D2 processed in the high range is very temperature-stable.
For more information, link to my Heat Treating and Cryogenic Treatment of Knife Blades page.
I've noticed that this page gets a lot of attention from knifemakers, and discussion goes on and on about this steel, how to heat treat it and what to expect. This can get out of hand, with guys claiming to know special bonding structures of the grain, grain size and shape, and describing various methods to achieve certain invisible, unproven, and ridiculous results. Here's my take on this:
Heat treat each piece of D2 according to the manufacturer's directions. THAT'S IT! You don't have to try to better their process; the foundry knows how they made the steel and they politely and effectively tell you how to heat treat it for the best performance.
All D2 is not the same! Since minor variations in the alloy content are certain to occur as a result of foundry process, as a knifemaker this is not your concern. Just heat treat the steel according to the manufacturer's directions.
Good grief, it's easier than baking a cake! When you bake a cake, you have to measure and mix the ingredients, and the foundry has already done this for you!
If, as a maker, you think that you have some better process than the steel supplier, please do tell them your discovery, maybe you can become a metallurgist or engineer. Also be sure to tell every machine shop and industrial manufacturer of machine tools, dies, valves, shears, forms, presses, and every other industry that uses D2. Wait. Do you suppose that these professional industries already know how to achieve the best performance in their steels? Then, who do they get their heat treating information from? Could it be the supplier of the steel? Ahem.
Just follow the steel supplier's directions.
I don't know how this could be simpler. And then spend some time on Fit, Finish, Balance, Design, Accessories, and Service, as these are the real limitations for most knifemakers today.
Some people think that because D2 can be hardened and tempered to have very high hardness and high wear resistance that it won't be tough. Toughness is the resistance to breakage or fracture, or more microscopically, the ability for the steel to withstand its molecular bonds from being torn apart, leading to fracture. Another way to perceive this is to think of toughness being the antithesis of brittleness. The more tough, the less brittle, the less tough, the more brittle. If you think that properly hardened and tempered D2 is not tough, then why in the world would it be used for some of the most demanding industrial applications there are, such as dies to cut, form, shape, bend and pierce other materials, even metal? Of course D2 is tough, that is, if it is properly hardened and tempered.
The reason I emphasized that last phrase is because, like so many other steels, mis-processing, missed steps, incorrect timing, out-of-sequence actions, uncalibrated equipment, slow ovens, and bad atmospheres can cause outright failures in the processing of this steel, and this is a steel that is unforgiving of error or inattention. Please re-read the section just above. A lot can happen in the heat treatment of this steel, but only a narrow range of exposures, temperatures, and timing will give reliable, predictable success. If this steel is processed at the lower tempering temperatures, for instance, it will yield a blade that is not so tough, and a bit more brittle. Again, if you think that it isn't tough enough, why are planer blades made from D2, blades that may spin at over 15,000 rpm and 7800 Surface Feet per minute, hogging through rough, abrasive hardwoods? Do you honestly think that your use on a hand knife can compare to this?
As stated in the previous section above, toughness can be increased by 25% by triple tempering with a cryogenic hold between steps. This has been proven by testing methods and electron microscopy (Arain, 1999, Heat Treatment and Toughness Behavior of Tool Steels (D2 and H13) for Cutting Blades). From my own experience, this is not how most D2 is handled, and perhaps why some knifemakers consider it not very tough. But if a die is being made in a professional die shop where it is expected to perform under tremendous pressures and abrasive conditions, with thin, tough sections, you can be assured that D2 is their choice for a reason, and you can be certain that they know how to heat treat it and process it correctly for optimum performance.
It's all about the process...
As always and with any tool steel, please consider that if there were a viable replacement for D2, D2 would be erased from production, and never used in any application. D2 fills a distinct position in the steel world, and it's been there a very long time and will continue to be!
Because D2 has such a high level of carbon with chromium, it forms a very distinctive grain pattern when the steel is polished. D2 can be polished, unlike the high vanadium S30V and S90V tool steels, which have too much vanadium to create an attractive finish. The finish on polished D2 is not smooth or glassy, but has a distinctive granularity commonly described as orange peel. Orange peel is a great description, because that is almost exactly the texture appearing in the finish.
Of all the steels that are used for polished knife blades, D2's characteristic orange peel has achieved a level of acceptance and even preference, as knife owners can instantly identify the steel, and that has its own appeal. Knife owners may even look for this granularity as an indicator of value.
It's important to note that not all D2 has this grain structure; high sulfur D2 does not exhibit this. But because of the higher sulfur content, the performance of high-sulfur D2 is somewhat less than low sulfur D2. The purpose of adding sulfur to steel is to improve machinability, and the sulfur content does not improve the performance of the knife blade. In fact, most steel makers are strictly required to limit the sulfur as it is considered detrimental. My advice is to stay away from any sulfurized steel for knife blades.
While D2 may not always be finished to a high polish (most tactical combat knives made in the studio have media blasted or satin finishes), it will benefit from polishing in increased corrosion resistance. Typically, the smoother the surface, the harder it is for surface corrosion (rust and pitting) to gain a foothold.
D2 is sometimes considered a stainless steel, however it's on the borderline of this classification. The designation "stainless" in steels is given to steels that are passive in aqueous solutions. According to the ASM International, for a steel to be considered stainless, it must contain at least 11.5% of chromium by weight, with at least 12% chromium to be passive in aqueous solutions. D2's UNS classification lists it has having an 11% to 13% content of chromium. So, depending on the manufacturer, D2 is right at the borderline of stainless steels. Most scientists, engineers, and metallurgists consider stainless any chromium percentage of over 12% a stainless steel. However, some organizations, manufacturers, and even foundries consider 10 percent chromium a stainless steel. You can then understand that the word stainless can be a generalization, so it helps to have the actual alloy content from the verified supplier.
For D2, it's important to understand the limitations. D2 is corrosion resistant, but not as corrosion resistant as many other stainless steels. If the blade is left with a coating of citrus juice, blood, or salt water, it will darken and start to show initial corrosion effects fairly quickly, sometimes within minutes. Simply exposing it to water periodically will not show rust, if dried immediately after. Like most blades, storing in a dry area, out of the sheath, and a light coating of wax will preserve the blade finish for many decades. This steel is for someone who will take care of their blade and understand its limitations.
In the photo below, look in the medium dark areas of the blade as I tried to photographically capture the characteristic orange peel granularity of this polished D2 knife blade.
Sharpening D2 is tough. If you're wanting a blade to hold an edge an incredibly long time, then you can expect that it will take a very serious method and time to sharpen it. A knife blade's cutting edge that is wear resistant during cutting will also resist the abrasive of the sharpener; it's simple physics. The old claim of having a very long lasting edge and yet being easy to sharpen is a horrible myth, still used today by unscrupulous knife makers, manufacturers, boutique shops, semi and pre-production shops, and knife dealers. It's time this myth is laid to rest.
If you're going to sit down with a stone (Arkansas, silicon carbide, aluminum oxide, India, oilstone, or water stone) then you better have a pot of coffee, perhaps a sandwich for a the three hour break, and plenty of beefy forearms and grip strength. It will take a long while to scrub up a bur for the primary edge, and plenty more to refine the secondary cutting edge of properly hardened and tempered D2. Wear resistant means wear resistant to all things, including the stone. Even ceramic stones will meet with plenty of resistance in sharpening properly hardened and tempered D2. If you have a D2 blade and it is easy to sharpen, it's either not D2, or it's been miss-processed!
My answer? Diamond. Diamond. Diamond, followed by Diamond. No steel is harder, no steel can withstand the cutting action of the hardest material on earth. On my tactical combat knives with an Ultimate Belt Loop Extender, a small diamond pad will handle the touch ups, but for a full sharpening regime, you'll need some serious diamond abrasive stones. I never recommend any motorized sharpening operations; they are just not controllable to a fine degree. As long as you understand that this steel will not easily or quickly be honed up, you'll be far ahead of the game. Incidentally, this steel is also known as "that damned D2," because of the sharpening effort. Diamond erases all that. It's one of those developments that wasn't available when I started making knives, and it is profoundly appreciated!
D2 has been around longer than 440C, ATS-34, and most of the other high alloy steels. It's not a new, exciting, or proprietary steel, it's not a gimmick, not a flash in the pan. It's been around a very long while strictly because it is a very good steel; it has staying power. D2 is not a new steel, and thus, is not as pop-exciting as some of the newer alloys, though it has a very stout following and recommendations.
Are there other good alloys? Of course there are, and some of them have marked advantages over D2. You may be surprised to hear that at the time of this writing, I use over a dozen different steels, and every one of them has certain advantages and certain (and sometimes defeating) limitations. Take the CPM steels, made by the crucible particle technology process. They are great steels, they are very expensive, and each one has its own attributes. CPM154CM is a beautiful steel, but when compared to D2, is not as wear resistant. CPMS35VN is tougher, but it can't be mirror polished, has a lower tempering temperature, is expensive, and limited in available sizes. You won't see this on their data sheets.
By the way, don't always trust manufacturer's data sheets, Crucible Steel reps have told me face to face that there are "misprints" on these sheets. One such misprint is claiming that CPMS35VN can be mirror polished, when it can not be mirror polished due to the high vanadium carbides that are present after heat treating. Whoops! Okay, it's just a misprint. So if you can't entirely trust a manufacturer's data sheet, what can you trust? Who you should be able to trust is someone who has had decades of using many steel types for many knives; they are the guys with the know-how. More on that below.
Another example of painting newer steels as superior can be found in what is not said. Many of these newer alloys have totally ignored finishing as a benefit. Finishing is the ability for a knife blade to be polished to a high, even mirror luster. This is critical in art pieces and investment grade knives, and also critical to keep the highest corrosion resistance possible. A rough blade corrodes easier, simply because of increased surface area. More on that topic here.
There simply is no ultimate steel for knife blades, no superior, magnificent, ultimate blade material. Each choice of steel for a knife blade is a decision of balance of many properties.
Knife buyers are just like everybody else; they can be swayed towards a particular knife buying decision based on advertising. Who wouldn't want the newest truck, the newest tool, the newest snack, or the newest knife? How do you make a knife, a tool that has existed for longer than any other tool in the history of man sound and seem new? Sure, people need knives, they use knives, and they collect knives. What could be more appealing than the newest, most exotic, most astounding performing steel made today? Why, even I would want to buy that! One problem though: it doesn't exist. See the next topic.
There simply is no ultimate steel for knife blades: no superior, magnificent, ultimate blade material. Please understand this: If there were a steel superior to all other exotic metals, all others would be cast aside, no longer made at any foundry, and they would be sentenced to the ashbin of obsolescence. Every steel has its pros and cons, its advantages and disadvantages. So advertisers need to make something sound new and better (even if it is cheaper and worse), so they claim a new steel, a better steel, a rarer steel, a superior steel is used in their product. Isn't it funny that they never claim new or superior handles, bolsters, or sheaths, only the blade steel! That's because they know that you, as a consumer, have limitations that prevent you from knowing the truth. How does this happen?
Okay, so is most of what you read about knife blade performance hype? It really depends on who is presenting it, and this is very important. If you are a knife client, buyer, or user, there is a simple way to know whether your knifemaker, knife factory, and knife supplier is selling the hype or knows what he is talking about. Look at his knives!
Most knifemakers and manufacturers who overly hype their steel type produce an inferior product.
D2 is an older, proven tool steel, relied upon in industrial settings for making some of the most durable cutting and metal forming tools possible. It's hard, wear resistant, and fairly tough (when properly processed) and makes an extremely long-lived and edge-holding knife blade. So why don't you see D2 used more often, and what are some of its limitations?
From this list, you can see that the number one issue with D2 is the expense of the steel, in acquiring and machining and finishing, and the particular nature and high care required. It's an expensive steel, and will always have its place at the top of the wear resistance kings of knife blades.
D2 has been around a long while, and it is here to stay. With its extreme wear resistance and good toughness, with proper processing and attention to detail, this steel makes a knife blade that has an incredible cutting edge that just won't stop.
When you want a knife edge to last and last, and you're not exposing the blade to corrosive or salty environments, and you trust the maker to understand the particularities of this steel type, D2 can offer a supreme cutting edge, and a fantastic knife that will last for many generations.
For more information, link to my Heat Treating and Cryogenic Treatment of Knife Blades page.
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