Originally used by alchemists to describe the process of extracting metals from rocks and minerals, the term Metallurgy encompasses a host of science and engineering disciplines which study, test, and create new metal alloys. In the knife world, “blade steel” refers to a constantly-expanding list of alloys that each have unique chemical compositions and physical properties ideal for use in cutting tools.
When we design a knife, we select the materials based on several factors. A knife designed for self-defense doesn’t necessarily need phenomenal edge retention, but it certainly needs to be strong and tough. On the other hand, an EDC blade may be used all day for a myriad of different cutting tasks, so edge retention may be most important to the user. In any case, knowing about your blade can help you to make informed choices when buying, using, and maintaining your knives.
The internet has an endless supply of spec sheets, charts, analyses, and forum discussions on the topic of blade steel. For those looking to take the plunge, Knife Steel Nerds is a wealth of information, and highly recommended for anyone with an interest in blade-related metallurgy. For now, we’re going to break down some of the basics, and give you a better understanding of the steel mark on your blade.
Before we go any further, there are some important key terms we should define first:
- Edge Retention: The ability of a blade to hold a useful cutting edge.
- Corrosion Resistance: A steel’s ability to resist rusting.
- Strength: A steel’s ability to resist breaking or shattering on impact.
- Toughness: A steel’s ability to resist fracturing, chipping, or deforming under strain.
- Wear Resistance: The ability of a steel to resist abrasion, erosion, or other damage caused by friction. Has a direct effect on edge retention.
- Carbide: Hard but brittle metallic particles composed of carbon and other metals present in the steel. The hardness of carbides affects the wear resistance of a steel, while the size, quantity, and brittleness of different carbides impact the toughness of the steel.
What is Steel, and How is it Made?
In the simplest terms, steel is an alloy – a blend of different metals – made of iron and carbon, as well as various other metals – called alloying elements – which are added to give the steel certain properties. Most steels used in production manufacturing today are produced either through a traditional molten casting process, or through a powder, or particle, metallurgy process.
In conventional steel casting, metals are combined in a furnace and the molten steel is poured into a mold and cast into ingots. From those ingots, the steel is then forged or rolled – or both – into the final shape. In its molten state, the steel is mostly homogenous, with an even distribution of all metals throughout the mixture. However, once it starts to cool and harden in the mold, a process called segregation begins, where the alloying elements begin clumping up with carbon to form large carbides, resulting in a less uniform mixture and uneven distribution of the desired chemical properties. The process of forging and rolling the steel works to remix and redistribute the carbides for better consistency.
Powder metallurgy, such as Crucible Industries’ CPM process, allows for much greater control over the properties of a given steel, eliminating segregation. Rather than casting it in a mold, the molten steel shoots through a high-pressure nozzle, atomizing into very tiny droplets which quickly solidify. These tiny spherical micro-ingots cool so quickly that they form smaller carbides without segregation, and the mixture remains homogeneous. The final step is to bake the powder at high pressure, forming it into a solid piece of steel that retains its uniform structure.
A Few Words on Carbides…
Controlling the size, distribution, and composition of carbides is of utmost importance in knifemaking, as all these factors can affect edge retention, machinability, strength, toughness, and corrosion resistance. In basic terms, a very hard carbide, like Vanadium Carbide, will increase the blade’s wear resistance, allowing it to stay sharp longer. But because harder carbides are also more brittle, too many result in a blade that stays sharp but may chip more easily.
The metals that don’t end up in carbides exist in-solution, filling the space between carbides and contributing to several key characteristics of the final blade.
Let’s use Chromium as an example. Chromium is one of the most common alloying elements in steel, and most steels are a careful balance of carbon and chromium content. In solution, chromium aids in corrosion resistance, and so steels with as much as 12% chromium or more are considered “stainless” steel, capable of resisting the onset of rust and corrosion better than steels low in chromium. However, Chromium may also bind with carbon to form chromium carbide. These carbides can be relatively large and strong, so they can add to a blade’s strength. But as we said earlier, larger, harder carbides can have other effects that may or may not be desirable. And if the chromium is bound up in carbides, then it isn’t of much use as a rust-repellent, so you need to add another alloying element, like Nitrogen or Molybdenum, to get the job done another way.
Heat treatment processes can have a profound effect on carbide formation as well. Heat treatment is the process that knifemakers must use to give the steel its desired hardness, expressed as an HRC rating. How the heat treating is done, what temperatures are used, and the final HRC rating hugely impact the performance of a blade. Between metallurgists and knifemakers, the choice of steel and the processes used to harden and machine it into a blade must be carefully controlled to ensure that the steel performs as intended.
Now that your head is spinning, what does this all mean to you? In short, your blade is made of a steel that was created to have certain characteristics that are desired for a variety of purposes in given situations. We are careful to select from a stable of blade steels that we prefer for various reasons, which are usually related to the intended function – or range of functions – of the knife in question. Without further ado, let’s get to the breakdown.
D2: First patented in 1927, D2 steel is what’s known as a “high carbon, high chromium” tool steel, which is a type of steel that is well-suited for use in making tools of all kinds. D2 is known for its high strength and wear resistance, owing to the large amount of chromium carbides, making it useful for high-impact cutting tools like axes and hammers. The low amount of chromium in solution means that D2 is prone to patina and may rust relatively easily without proper attention.
D2 has become a commonly used blade material over the years, and has proven to be a reliable go-to for many knifemakers. Crucible Industries produces a CPM version of D2.
154CM: Created by U.S. steel producer Crucible Industries in 1959, 154CM was one of the first “high-speed” stainless steels ever made. High speed steels are capable of retaining their hardness at the high temperatures caused by the friction of moving parts, such as the bearings in a spinning turbine engine. Some chromium in 154CM forms chromium carbides, reducing the amount of chromium that can repel rust, but the addition of molybdenum has the effect of improving “hot hardness” while also aiding in corrosion resistance.
Legendary knife maker Bob Loveless introduced the first knife made of 154CM in 1972, making it a staple of the industry that is still lauded for its toughness, wear resistance, and corrosion resistance. It’s a well rounded steel that’s perfect for tougher applications or for general purpose blades.
440C: First created in the 1930s, 440C steel was one of the early high-hardness stainless steels, popularized in knives due to good strength, wear resistance, and corrosion resistance. Easy to hone to a fine edge, we offer it as an option for our Table Knife blades, but we use it most commonly for rescue hooks and defensive tools, like the SOCP Dagger Family, due to the ease of sharpening and lower cost compared to a similar steel like 154CM.
N680: Produced by Böhler, N680 is a high chromium and molybdenum steel that has had nitrogen added to the mix. Nitrogen is difficult to add to steel, so there’s only a small amount of it, but nitrogen has the effect of significantly improving corrosion resistance by helping to keep more chromium in solution. As result, N680 is a popular choice for those working in wet conditions, particularly in or around saltwater. Our 112SBK-BLK H2O Fixed Dive Knife is a good example.
Damascus: As much a style of steel as it is an ancient steel-making process, Damascus steel is a combination of two similar steels which have different levels of corrosion resistance. The steels are forged together into a final shape by hand through a process called Forge Welding, where the two steels are heated and folded together, creating multiple layers in various patterns. The final forged material is then ground into a blade and acid etched by dipping into an acid bath. The acid corrodes the two steels at slightly different rates, which reveals the signature damascus pattern.
We don’t use Damascus steel very often, choosing to reserve this unique, handcrafted and beautiful material for some of our top-end Gold Class releases. Damascus steel is a difficult product to produce at high volume with a high level of quality, requiring careful attention and time-intensive work, but the results are truly outstanding.
CPM–154: This powder version of 154CM is produced using Crucible’s CPM (Crucible Particle Metallurgy) process. While the chemical composition is identical to 154CM, CPM-154 sees an improvement in most characteristics. Smaller, more numerous, and evenly distributed carbides maintain the strength and wear resistance of its conventional counterpart, while toughness and corrosion resistance see some gains.
As our choice for the Table Knife, this steel is well balanced for use in the kitchen. Edge retention and corrosion resistance reduce maintenance, and better toughness allows for greater flexibility and durability.
CPM-S30V: The very first steel ever designed for use as a pocket knife blade, CPM-S30V was a refinement of similar steels like S90V and S60V, renowned for their high vanadium content, which creates very strong vanadium carbide, freeing up chromium to aid in corrosion resistance. Developed by Crucible in collaboration with acclaimed knifemaker Chris Reeve, CPM-S30V is regarded as a well-rounded, all-purpose blade steel.
Balancing strength, toughness, corrosion resistance, and edge retention, this steel is the most common in our lineup. CPM-S30V is great for EDC or general purpose blades, where no one characteristic may be more or less important than another. Its excellent corrosion resistance and edge retention makes it ideal for a pocket knife that may be used for a wide variety of tasks periodically, or for a limited range of tasks every day. Being fairly easy to sharpen and hone in the field and tough enough for most chores, CPM-S30V is a prime choice for our hunting knives as well.
M390: Conceptualized as a high-hardness, high-chromium stainless super steel, M390 was created in Austria by the famed steel production company Böhler Edelstahl. Born out of a desire to create a stainless tool steel that was relatively easy to machine, M390 is an extremely popular blade material most often found in high-end knives produced outside the U.S., renowned for its ability to take an ultra-thin edge and hold it against prolonged use. There is a large amount of chromium in solution, which provides phenomenal corrosion resistance. The high chromium content also results in a large amount of chromium carbide, resulting in greater wear resistance and strength, but reduced toughness compared to something like CPM-S90V. M390 is a fantastic low-maintenance blade option.
CPM-20CV: In the early 2000s, M390 was beginning to rise in popularity, but it was not commonly used in knives due to scarcity. Latrobe steel in Pennsylvania, later purchased by Crucible Industries, created what they called Duratech 20CV, essentially an American version of M390. Today, Crucible’s CPM-20CV is as popular as M390, and widely available to U.S. manufacturers.
While there are ever-so-slight differences between them, CPM-20CV is considered analogous to M390, excelling as a high-hardness super steel with great edge retention and corrosion resistance. We’re especially happy to offer it as another blade option in our Custom Knife Builder.
CPM-S90V: A precursor to CPM-S30V, CPM-S90V was developed by Crucible in 1996 while attempting to create a new high-wear-resistance stainless steel tougher than similar steels like M390 and CPM-20CV. The result is a steel that helps to dispel the myth that harder steel is better steel. Our CPM-S90V is hardened to 59-61 HRC, very close to the 58-60 HRC rating of CPM-S30V, yet the higher vanadium content of CPM-S90V gives it a significant advantage in edge retention without sacrificing toughness.
Because of that greater wear resistance, CPM-S90V is also more difficult to grind, bevel, and sharpen, making it more time-consuming and labor intensive to work with. But the benefits of CPM-S90V far outstrip those concerns, and we employ it in top-tier, premium blades in our Blue Class, Black Class, and HUNT series knives, as well as various limited releases, and as a premium option in our Custom Knife Builder.
CPM-M4: When the original M4 steel was first created, high-speed tool steels were still new to the world of metallurgy, and there was a lack of consensus on the use of tungsten versus molybdenum as a key alloying element used to improve “red hardness,” or the ability of the steel to maintain its hardness at temperatures which would make it red hot.
Tungsten is incredibly tough, but molybdenum can do everything tungsten does with half the material. Conventional M4 uses a mixture of both. Several years and lots of tests later dispelled remaining doubts about the less-expensive and more readily available molybdenum, making it preferable to tungsten and leading Crucible to produce CPM-M4 with only molybdenum.
With the benefit of modern powder metallurgy, CPM-M4 stands as one of the most popular, if not the newest, blade steels in the knife world. It is characterized by exceptional wear-resistance at high hardness, with edge retention rivaling that of CPM-20CV, but having the robust durability of D2. While it is an exceptionally hard steel to machine, it’s a perfect fit for heavy users, but low chromium content means keeping it dry and oiled on the regular.
CPM-3V: One of the newest additions to our lineup, CPM-3V was created by Crucible as a high-toughness steel. The high vanadium content of CPM-3V, combined with the wonders of powder metallurgy, result in a desirable low volume of super hard vanadium carbides. These carbides give a blade good wear resistance, while the low density of carbides improves toughness. Essentially, if you bend the blade, fewer hard carbides can contact each other and causes stress fractures or deformities.
When we first tested CPM-3V in our test lab, we were unable to break it, a first for Benchmade. The blade flexed to nearly 45 degrees before the testing machine failed, and the blade sprang back almost to its original shape. This exceptional level of toughness makes CPM-3V the optimal choice for hard-use outdoor knives such as the 202 Leuku, or tactical knives like the 537GY Bailout.
Damasteel: If Damascus steel was invented today, this is how it would be done. Produced by Damasteel in Sweden, Damasteel® is a stainless powder metallurgy version of Damascus steel. The powder metallurgy process removes many of the challenges presented by forge-welded damascus, while also granting much more control over the consistency, physical properties, and even the pattern of the final product.
Available in an expanding selection of patterns, we love Damasteel for its toughness, corrosion resistance, and ability to take a very keen edge. Damasteel empowers us to have even more control of quality and aesthetics, while also being available for full production scale.
The End… Or is it?
You made it! There is a lot of information here, and this is just the simplified tip of the iceberg. Any conversation on blade steel can evolve into the deepest of dives, and resources like Knife Steel Nerds are hugely informative and enlightening.
With our 2021 product announcements approaching, we’re excited to expand our assortment with new materials. We are constantly driving forward and exploring the newest and most dependable steels at our disposal, and we carefully vet new steels, always with an eye toward providing the best performance, quality, and reliability.
Special thanks to Dr. Larrin Thomas of KnifeSteelNerds.com for his wonderful repository of all things knife steel. Check out his new book, Knife Engineering: Steel, Heat Treating, and Geometry.