Spyderco byte October 2023 - EDGE-U-CATION
Crucible CPM M4
According to Crucible® Industries’ data sheet, “CPM® REX® M4 is a special purpose high speed steel designed to give high wear resistance in tools.” While that’s certainly true, it’s a bit of an understatement.
Among tool steels, CPM M4 offers a remarkable combination of malleable strength, high impact toughness, and extreme wear resistance. These qualities enable it to perform exceptionally well in cold-work tooling applications like punches, die and broach inserts, and taps, as well as premium cutting tools for use on abrasive alloys, heat-treated materials, and castings. In these demanding industrial uses, CPM M4 easily outperforms most other high-carbon, high-chromium die steels.
As its name indicates, CPM M4 is produced by the state-of-the-art Crucible Particle Metallurgy (CPM) process. The basic formulation of the steel, however, goes back to 1937 and pioneering metallurgist James P. Gill. The author of one of the first and most definitive books on tool steels, Gill was a lifelong employee of the Vanadium Alloys Steel Company (VASCO) in Latrobe, Pennsylvania. Not surprisingly, he was also a leader in the development of vanadium-enriched steels.
Early efforts to add vanadium to tool steel revealed that once the vanadium content exceeded 2%, the performance of the steel actually declined, as it would draw carbon from the main mass of the steel to form vanadium carbides. Metallurgists later realized that this problem could be solved by increasing the percentage of carbon in the steel. Starting with a baseline carbon content of 0.80%, they found that for every 1% of vanadium added to the steel, the carbon content would have to be increased by 0.16%.
Using this concept, Gill introduced a steel called Neatro in 1937, which contained 1.25% carbon and 4% vanadium, as well as 8% molybdenum. That same year, he filed a patent application on his formulation that was issued as U.S. Patent No. 2,105,114 on January 11, 1938. A year later, that patent was modified to reduce the molybdenum content to 4.5% and add 5.5% tungsten. Neatro was also officially renamed M4.
Despite its impressive performance qualities, as an ingot steel, M4 is difficult to machine in its annealed state and challenging to grind once hardened. However, when M4 is produced by the powder metallurgy process, it allows the addition of 0.06% to 0.08% sulfur. The small sulfides that result are uniformly dispersed throughout the structure of the steel, enhancing its machinability. The particle metallurgy process also promotes a finer, more homogenous carbide structure, making it significantly easier to grind in its hardened state.
Introduced in 1973, CPM M4 was actually recognized by Crucible’s metallurgists as one of the steels that most benefitted from the particle metallurgy process. According to Dr. Larrin Thomas’ excellent book, The Story of Knife Steel, in 1978, Crucible’s August Kasak and Edward Dulis wrote:
As a CPM product, M4 has proven to be a pleasant surprise: its toughness, in terms of both impact and bend fracture strengths, is higher than that of any high speed steel grade known to us… The unexpectedly high toughness of CPM M4 is attributable to an optimum combination of relatively high carbide density (number of carbides per unit volume), small size of carbides, and their uniform distribution. This combination of desirable carbide characteristics creates favorable conditions for obtaining a stable, fine-grained microstructure that leads to a high-toughness product without sacrifice of other desirable properties.
For the record, Crucible and Niagara Specialty Metals’ data sheets list the specific alloy composition of CPM Rex M4 as follows: Carbon: 1.30%, manganese: 0.30%, silicon: 0.30%, chromium: 4.30%, tungsten: 5.60%, molybdenum: 4.50%, and vanadium: 4.00%. This composition summary does not, however, include trace elements—like the critically important sulfur cited earlier. Adding those, its complete composition also includes 0.03% phosphorous and 0.06% sulfur.
CPM M4 as a Blade SteelCPM M4’s remarkable balance of wear resistance, toughness, and impact strength make it ideally suited to hard-use cutting tools. For that reason, it has long been the blade steel of choice for the elite knives used in cutting competitions—like those organized by BladeSports International, Inc.™ Their demanding competition courses include everything from cutting delicate paper straws to hacking through 2x4s and thick, free-hanging ropes. These challenges require a knife that is razor sharp and will stay that way despite repeated, full-power chops through seasoned wood and other resilient materials. The fact that CPM M4 is a go-to choice of experienced BladeSports competitors is therefore a true testament to its unique properties as a blade steel.
In the world of production knives, Spyderco was one of the first companies to feature CPM M4 in our blades, largely because of the influence of renowned custom knifemaker and design collaborator Gayle Bradley. In addition to crafting exquisite custom folders and fixed blades, Bradley is also a veteran BladeSports champion, so it’s no wonder he had a strong affinity for CPM M4 in his knives. We faithfully followed his lead, using this elite steel in the manufacture of his Bradley Folder™, Air™, Advocate™, and Bradley Folder 2 models.
Spyderco has also showcased CPM M4 in many of our Exclusive knives—distinctive expressions of established designs specially manufactured for longstanding dealers and distributors. Perhaps most notable among these are the multiple M4-bladed Exclusives we’ve created for Blade HQ, which feature either natural-colored G-10 scales or mint-green injection-molded handles.
CPM M4 is an elite, high-performance tool steel with a long and fascinating history. For serious knife users who want to “up their game,” the unparalleled combination of edge retention, toughness, and impact resistance it offers in knives is hard to beat. Spyderco is proud to include it in our stable of high-performance blade steels.