Understanding Tool Steels

To truly understand knives, you must have some understanding of blade steel. This is particularly important if you want to develop a deeper appreciation of knife performance beyond the most basic cutting tasks.

With that said, the topic of blade steels is a deep rabbit hole that is often overwhelming for the average knife user. Truth be told, Spyderco does not make this challenge any easier, as we have worked tirelessly to become the industry leader in the use of exotic, high-performance steels in our knives. Our current product line includes more than 30 different blade steels, including several ultra-high-performance steels that can only be found on Spyderco knives. If you broaden that scope to include discontinued products, limited-edition Sprint Run® knives, dealer and distributor Exclusives, and our remarkable Mule Team™ project, that number easily doubles.

To help our customers navigate this broad selection of steels, we also strive to be the industry leader in informational resources through our longstanding Edge-U-Cation® initiative. For most of our history, we have shared detailed insights into the nuances of blade steel metallurgy through our catalogs, steel charts, website, byte newsletters, and now, this blog.

As part of that ongoing educational effort, this article will give you a deeper understanding of tool steels, the extreme performance characteristics they can offer, and the compromises that sometimes come with them.

The first step in understanding the complexities of any category of steel is to define a clear set of performance criteria. Traditionally, these include three primary properties: wear resistance, toughness, and corrosion resistance. Since knives are cutting tools, it also helps to add a fourth criterion: ease of sharpening.

Metallurgically, wear resistance relates to a steel’s ability to withstand abrasion and impact without deforming. However, when it comes to knife blades, its meaning becomes more specific. Since the goal of a knife is to take and retain a sharp cutting edge, wear resistance translates more accurately as “edge retention.”

Toughness is the ability of a steel to resist fracturing, chipping, or breaking under impact and structural stress. Essentially, it is a measure of how much energy a material can absorb before failing.

Corrosion resistance is the ability of a steel to resist degradation, rusting, pitting, and discoloration caused by oxidation, moisture, salt, and acidic environments.

Ease of sharpening is self-explanatory. Typically, this quality is diametrically opposed to wear resistance and edge retention. Steels that hold an edge exceptionally well normally resist abrasion and are the most challenging to sharpen; however, that’s not always the case.

These criteria provide a basic framework to evaluate different types of steels, especially as those steels apply to different applications.

As its name implies, stainless steel is a type of steel purposely formulated to resist corrosion. Stainless steels accomplish this primarily by adding chromium, although other alloys can contribute to corrosion resistance as well. In industrial applications, the threshold for stainless steel is generally considered to be 10.5 percent chromium by weight. However, by American cutlery steel standards, 13 percent chromium is generally required for a steel to be classified as stainless.

When chromium exists “in solution” in a steel, it is not combined with other elements. In this form, it attracts and combines with oxygen molecules on the steel’s surface to form a protective outer layer. This chromium oxide layer forms a barrier that prevents oxygen in the environment from interacting with the iron in the steel to form rust.

However, when chromium combines with carbon in the steel’s molecular matrix, the result is the formation of chromium carbides. Although hard and wear resistant, chromium carbides are relatively large. They therefore weaken the structure of the steel and reduce its toughness. Chromium also tends to “steal” carbon away from other, more desirable, carbide-forming elements, like vanadium and niobium.

As noted earlier, the American knife industry generally cites 13 percent chromium as the threshold for a steel to be considered stainless. In reality, the exact amount of chromium necessary for reliable corrosion resistance also depends upon the steel’s carbon content and the presence of other contributing alloys. Nevertheless, if we consider the three basic qualities of a knife steel—edge retention (aka, “wear resistance”), toughness, and corrosion resistance—corrosion resistance is arguably the most problematic. While chromium, in small amounts, does contribute to the hardenability of a steel, adding a large volume of chromium typically compromises its toughness and edge retention. Instead, if we purposely remove corrosion resistance from our list of desirable qualities—and make the commitment to maintain our knives diligently—we open the door to ultra-high performance in the other two categories. That’s exactly what advanced tool steels offer the knife enthusiast.

In simple terms, tool steel is a category of carbon and alloy steels used to make tools, dies, molds, and wear-resistant machine parts. They generally contain 0.7–1.5 percent carbon, but may also be enhanced by alloying elements, including chromium, tungsten, molybdenum, and vanadium. The addition of these elements can increase the steel’s hardness, toughness, and wear resistance—properties that are achieved through precise heat-treatment processes.

Tool steels are classified by their chemical composition, the means through which they are hardened, and their intended industrial applications. The American Iron and Steel Institute (AISI) classifies them by letter grades, including W, O, A, D, S, H, M, and T. Their basic qualities and intended uses are as follows:

W-Type (Water-Hardening)

These plain high-carbon steels are hardened by water quenching. They are cost-effective options for cutting applications that do not generate high heat, but are vulnerable to warping. Water-hardening steels are most often used for cutlery, hand tools, and woodworking tools.

O-Type (Oil-Hardening)

These medium-alloy steels are formulated to be quenched in oil. They offer a good balance of wear resistance and toughness and are most often used for punches, dies, and cutting tools.

A-Type (Air-Hardening)

Air-hardened steels are medium-alloy steels that harden effectively in air, without the need for quenching in water or oil. They are very dimensionally stable and lend themselves well to use in the manufacture of blanking and forming dies and punches.

D-Type (High Carbon, High Chromium)

These air-hardened steels feature high carbon and chromium content, which gives them excellent hardness and wear resistance. They are widely used in blanking and forming dies as well as shear blades.

S-Type (Shock-Resisting)

These steels are specially formulated for impact resistance in both hot and cold environments. Extremely tough, they excel in applications like chisels, punches, and jackhammer bits.

H-Type (Hot-Working)

Hot-working tool steels are designed to retain their hardness and strength in applications that expose them to prolonged heat. They excel at applications like hot-forging tools, extrusion dies, and casting dies.

T- and M-Type (High-Speed Steels)

Tool steels alloyed with tungsten (T) and molybdenum (M) retain their hardness at high temperatures and are well suited to machining operations that involve fast cutting. They are ideal for use in drill bits, milling cutters, and saw blades.

These categories of tool steels were all developed for specific industrial applications; however, many of their unique properties also make them excellent blade steels. While some qualities, like hot hardness, are not relevant to knife blades, their most important attributes—wear resistance and toughness—definitely apply.

Traditional tool steels like 1095, 1084, and 5160 have been a mainstay of knifemaking for as long as they have existed. Others like 52100, W-1, and W-2 were popular among bladesmiths, but rarely seen in production knives because they were not available in sheet form. As the alloy compositions of tool steels became more sophisticated and they became more readily available in sheet and stock form, they were quickly adapted to use in both custom and factory-made knives. The performance of these steels was limited, however, by their manufacturing process.

Conventional tool steels are ingot steels. They are created by melting steel and other alloys in a crucible to create the desired mix of elements, then pouring the molten steel into a mold to create a solidified shape. The resulting ingot is then processed through forging or hot rolling to yield the desired shape.

The primary disadvantage of ingot steel is that its alloy composition has natural limits. If the concentration of alloying elements exceeds a particular threshold, it will cause micro-segregation during solidification. In other words, the elements will segregate or “regroup” rather than remaining uniformly distributed throughout the steel. This not only prevents the alloys from effectively contributing their unique benefits to the steel, but can lead to inferior mechanical properties.

Powder metallurgy (PM) steels, also known as particle metallurgy steels, solve this problem in a unique way. They begin the same way as ingot steels, melted in a crucible to achieve the ideal, uniform mix of alloying elements. Rather than being poured into a mold, however, the molten steel is passed through jets of nitrogen gas that instantly cool and solidify the steel into a fine powder. This “gas atomization” process freezes the steel in its perfectly mixed form, with each grain of powder forming a tiny ingot. The resulting powder is then sealed in a mild steel canister and heated under extreme pressure and temperature. This “hot isostatic pressing” (HIP) process fuses the powder into a solid ingot that can then be processed into the desired form.

By rapidly cooling the steel during its perfectly mixed state, the powder metallurgy process effectively prevents alloy segregation. It also allows the creation of alloy-rich formulations that are impossible to achieve by the conventional ingot process. Equally important, the rapid solidification of the powder particles restricts carbide growth. This produces a fine, extremely homogeneous microstructure that greatly enhances the steel’s toughness and wear resistance.

Spyderco has a long, proud tradition of showcasing high-performance tool steels in our production knives. These have included everything from ingot steels like O-1, 52100, Aogami Super Blue, V-Toku2, and D2 to particle metallurgy tool steels like HAP-40, CPM® CRU-WEAR®, Micro-Melt® PD#1, CPM M4, and our extensive family of knives showcasing K390 blades. Our limited-edition Sprint Run® and Exclusive knives have included other advanced tool steels like CPM REX® 45 and CPM REX 76. Currently, we hold the distinction of being the only production company regularly producing knives with full blades featuring ultra-hard, ultra-high-performance steels like Carpenter® Technology Corporation’s Maxamet®, CPM REX® 121®, and CPM 15V®.

In addition to providing our customers with more options, our passion for tool steels is an integral part of our approach to the Edge-U-Cation process. By offering the same model with several different steel options, we provide end users with a scalable platform for discovering and understanding steel performance. Starting with a “base” version of a knife, they become comfortable with its carry, operation, ergonomics, and the performance characteristics of its standard steel. If they then upgrade to a more sophisticated, higher-performance blade steel in that same model, all other aspects of the knife remain unchanged and they are able to accurately quantify the differences in its cutting performance, edge retention, and ease of sharpening. The more they experience and learn, the more able they are to understand and appreciate the nuances of ultra-high-performance steels like CPM REX 121. Through that process and the knives that support it, they have a path to becoming expert knife users.

If you are passionate about knives and disciplined enough to maintain them well, you may be ready to explore levels of edge retention and toughness that only tool steels deliver. And when you’re ready, Spyderco has you covered.