CPM REX 45 is best known today as a high-hardness blade steel found in limited-production folders, but knives were never the reason it was created. REX 45 began life as an industrial high-speed steel, part of a family of alloys developed to keep cutting tools working under the heat, abrasion, and stress generated by machining metal. Its eventual appearance in pocketknives came decades later, after knife manufacturers began looking beyond conventional blade steels and experimenting with materials originally intended for machine shops, dies, milling cutters, and other industrial tooling.

During the late nineteenth century, cutting tools were limited by heat. As a tool cut metal at increasingly high speeds, friction heated the cutting edge until ordinary carbon tool steels began losing hardness. Robert Forester Mushet's air-hardening steel of the late 1860s was an important step forward, but the major breakthrough came around the turn of the twentieth century through the work of Frederick Winslow Taylor and Maunsel White. Their experiments showed that highly alloyed tool steels could be hardened at temperatures previously considered excessive and still retain useful hardness when the cutting edge became extremely hot. The new class became known as high-speed steel because machine tools equipped with it could operate at previously impractical cutting speeds.

High-speed steels developed rapidly during the decades that followed. Tungsten played a major role in early grades by helping steel retain hardness at elevated temperatures, while chromium, vanadium, molybdenum, and eventually cobalt were used to control hardenability, wear resistance, carbide formation, and hot hardness. The familiar AISI classification system eventually divided these materials largely into tungsten-rich T-series steels and molybdenum-rich M-series steels.

Among the latter was M3, a family closely related to the better-known M2 but containing more carbon and vanadium. M3 Class 2, often abbreviated M3:2, typically contained around 1.2 percent carbon, 4 percent chromium, 5 percent molybdenum, 6 percent tungsten, and 3 percent vanadium. Compared with M2, the additional carbon and vanadium increased the amount of hard carbide in the steel and gave it greater resistance to abrasive wear.

The chemistry alone, however, is only part of the REX 45 story. Highly alloyed high-speed steels have historically been difficult to produce through conventional ingot metallurgy. As a large mass of molten steel slowly solidifies, alloying elements can segregate into different regions rather than remaining evenly distributed. Large carbide structures can also develop, and later forging and rolling cannot completely erase every effect of the original solidification process.

In the late 1960s, Crucible Industries developed a different way to manufacture these demanding alloys. Commercial production using the Crucible Particle Metallurgy process began in Syracuse, New York, in 1970. Instead of allowing an entire mass of molten steel to solidify as a conventional ingot, the CPM process atomized molten alloy into tiny droplets that cooled rapidly into powder. The powder was then collected, sealed, consolidated under heat and pressure, and worked into usable steel.

Because each particle solidified so quickly, alloying elements had far less opportunity to segregate. The resulting steel contained a finer and more uniform distribution of carbides, improving toughness and grindability while allowing metallurgists to work with highly alloyed compositions that were increasingly difficult to manage through conventional methods.

Establishing exactly when REX 45 entered Crucible's lineup is more difficult than tracing its metallurgy. Crucible's trademark history does provide some information, though: the company filed a federal trademark application on November 9, 1998, and registration followed in July 2000, but those dates relate to trademark protection rather than the steel's introduction.

By then, REX 45 already had an established industrial role. It was used in gear hobs, milling cutters, end mills, broaches, form tools, shaper cutters, extrusion punches, and taps. These were jobs that could take advantage of the steel's combination of high hardness, strong wear resistance, and the ability to resist softening as cutting temperatures increased.

Its heat treatment reflected those industrial demands. Crucible recommended austenitizing temperatures beyond 2100 degrees Fahrenheit, with 2175 degrees Fahrenheit being the standard heat treatment to achieve 66 to 68 HRC. The steel required rapid cooling through the critical portion of the quench, followed by repeated high-temperature tempering. Triple tempering was specified, while the overall heat treatment could be adjusted depending on whether the application required maximum hardness or a somewhat greater margin of toughness.

By the 1990s and 2000s, the knife industry had become increasingly willing to experiment with steels developed outside the cutlery world. D2 and A2 had long histories as tool steels, while powder-metallurgy alloys such as CPM 10V and CPM M4 showed that steels developed for dies, punches, and industrial cutting tools could perform exceptionally well in a blade. REX 45 fit naturally into that trend, although it would take longer to make the jump into production knives.

As a blade steel, REX 45's greatest strengths are its high attainable hardness and strong wear resistance. Production knives can reach the mid-to-upper 60s HRC, allowing the steel to hold a stable edge and maintain cutting performance through extended use. That performance comes with relatively low corrosion resistance, however. With only about 4 percent chromium, REX 45 is not stainless and can develop staining, patina, or rust if moisture and corrosive residue are left on the blade.

Its hardness and carbide content can also make sharpening more demanding than with simpler blade steels, particularly during extensive sharpening or reprofiling. Diamond and cubic-boron-nitride abrasives are well suited to the job. Interestingly, the red hardness that was so important to REX 45's original industrial purpose offers little practical advantage in a pocketknife, where the edge never approaches the temperatures generated during high-speed machining. Its value in a knife instead comes from the hardness and wear resistance that accompanied its development as an industrial cutting steel.

In December 2024, Crucible Industries filed for bankruptcy, and large portions of its assets, equipment, trademarks, and intellectual property were acquired by French high-speed-steel producer Erasteel in 2025. Erasteel was already familiar with this family of steels through ASP 2030, its own M3 Class 2 plus cobalt high-speed steel.

REX 45 spent decades cutting metal long before anyone started carrying it in a pocket. Its move into knives did not require the steel to become something different; it simply gave an old industrial alloy a new job. The conditions were far less extreme than those it had been designed for, but its hardness, wear resistance, and fine particle-metallurgy structure turned out to be just as useful at the edge of a folding knife as they had been in the machine shop.