Spyderco byte September 2024 - Edge-U-Cation®- CPM Rex 121
Edge-U-Cation® - CPM Rex 121
Crucible Industries describes their CPM Rex 121 as “a high-vanadium, cobalt-bearing tool steel designed to offer the highest combination of wear resistance, attainable hardness, and red hardness available in a high speed steel.” Those simple words, however, don’t do it justice—especially when it is used as a material for knife blades.
In industrial applications, Rex 121’s extreme wear resistance and superior “red hardness”—its ability to maintain its hardness at high temperatures—allows tools made from it to be run at higher cutting speeds than those made from other high-speed steels. In this way, it bridges the gap between high-alloy tool steels and carbide materials without suffering from the brittleness and fragility often associated with cemented carbides. Rex 121 does this with an extremely sophisticated alloy composition that includes over three percent carbon and large volumes of vanadium, tungsten, molybdenum, and cobalt. In fact, Rex 121’s high cobalt content (9.0%) elevates it to a special alloy group, designated with the abbreviation HSSE or HSS-E. Transcending conventional HSS (High Speed Steel) alloys, HSSE steels offer higher hardness, superior hot hardness, and tempering resistance.
According to Zapp metals' CPM Rex 121 datasheet, “CPM® Rex 121 is the Super-HSSE of the CPM tool steel family. Maximal hardness and a compressive strength equal to solid carbide, combined with a high wear resistance and hot hardness make CPM Rex 121 the 'tough' alternative to solid carbide for a wide array of applications. CPM Rex 121 is frequently used as the ultimate phase in tool performance optimization of Super HSSE grades for fine blanking and cold forming…”
The key to achieving CPM Rex 121’s extremely rich mix of alloys is the Crucible Particle Metallurgy (CPM) process—state-of-the-art technology that quickly transforms molten steel into powdered form. Instead of being cast into an ingot, the molten steel is forced through a narrow nozzle where high-pressure gas atomizes it, rapidly cooling it into small droplets or particles. This rapid cooling “freezes” the steel’s alloy mix in perfect solution, preventing segregation and creating “micro-ingots.” The particles are then collected and placed in sealed canisters. Vacuum pumps evacuate all the air from the canisters before the steel powder is reheated at extreme pressure and temperature to fuse it into a solid billet. This “hot isostatic pressing” (HIP) process, together with the other advantages of CPM technology, allows alloy combinations that are impossible to achieve with conventional steelmaking methods.
Wear Resistance and Edge Retention
In the steel industry, wear resistance is generally defined as “the ability of a material to resist the progressive loss of volume from its surface through mechanical actions exerted by another solid, such as repeated rubbing, sliding, or scraping.” When steel is used to craft knife blades, wear resistance generally translates to “edge retention.” The more resistant to wear a steel is, the longer an edge made from that steel will typically last before dulling.In describing Rex 121’s wear resistance, Crucible’s official data sheet states: “Depending on the application, the wear resistance of CPM Rex 121 is approximately 50-100% better than CPM Rex T15, CPM Rex 76 or CPM 10V.” If that sounds impressive, it is. To put it into even better perspective, Zapp’s data sheet rates Rex 121 at a maximum 10 out of 10 on the wear-resistance scale, and some metallurgical experts consider it literally “off the scale.”
The best way to scientifically quantify the effects of CPM Rex 121’s extreme wear resistance is to test knife blades made from it with a CATRA (Cutlery Allied Trades Research Association) machine. This sophisticated machine precisely measures the cutting performance and edge retention of test blades by cutting through a stack of special silicon-impregnated paper card stock. Multiple passes are made through the card stock, each with calibrated pressure and stroke length. The cutting depth of each stroke is progressively measured to quantify the degradation of the edge throughout the process. In both Spyderco’s in-house testing and independent testing by leading metallurgical experts, Rex 121 outperformed all other steels—including Maxamet—by a significant margin.
While Rex 121’s edge retention is extremely impressive, like most things in life, it comes at a price. Specifically, that trade-off comes in terms of toughness. Crucible’s data sheet states, “Depending on the heat treatment, the impact toughness of CPM Rex 121 is approximately comparable to that of CPM 15V or conventionally melted T15.” Zapp’s data sheet includes a chart of toughness test results for CPM Rex 121 and four other similar ultra-high-hardness tool steels. Based on standard Charpy impact test protocols, Rex 121 toughness is about one third that of Rex 76.
CPM Rex 121’s exact alloy composition includes: Carbon – 3.4%, Chromium – 4.0%, Vanadium – 9.5%, Tungsten – 10.0%, Molybdenum – 5.0%, Cobalt – 9.0%, and Sulfur – 0.03%. At an austenitizing temperature of 1925 degrees Fahrenheit, Rex 121 takes full advantage of this rich alloy composition to achieve an incredible 32% carbide volume. All those hard carbides are directly responsible for its unparalleled edge retention.
Crucible’s CPM Rex 121 is truly in a class by itself when it comes to its edge-holding properties. It is an elite, high-performance steel that also presents unique challenges in the manufacturing and heat-treating processes. Spyderco is extremely proud to be one of the only companies to feature this remarkable material in our knives, and we truly appreciate the passion of the “steel-obsessed” knife enthusiasts who will invest in them.