HOW TO HEAT TREAT CPM M4

 

CPM M4 is popular in competition cutting blades as it is high-speed steel exhibiting better grit and wear resistance than M2 or M3 in cold work punches, die inserts and cutting applications. 

While it may chew up belts grinding it after hardening, its exceptional edge retention and strength make it impossible to ignore and worth the inconvenience. If you are used to hard-to-grind and polish steels, you will be prepared to work with M4.

For hardening, knives should first be cleaned with soapy water. If you are not using oxygen-free heat treat tools, use a high-temperature tool wrap foil (2,200°F+ rated) pouch before heat-treating. Missing stages like preheating and equalizing or cryo will result in reduced hardness, higher amounts of Retained Austenite (RA), weakened stain resistance or other problems. As fast as possible, ramp AFAP between preheating and austenitizing temperatures. To avoid thermal shock-induced warp, be sure to clamp flat after the quench during cryo or tempering.

 Figures represent quenching under positive pressure with aluminum plates and compressed air to at or below 125°F / 50°C. Alternative quenching methods may result in reduced hardness, high RA, or other problems.

 

Before or after the first temper cycle, cryogenic treatment is suggested to convert retained austenite. Although liquid nitrogen is favoured, a sub-zero bath with dry ice and kerosene will work for -100°F / - 74°C. Depending on the number of blades and thickness, immerse in sub-zero treatment for 4-6 hours. A cryogenic treatment can be done immediately after the quench. However, to avoid a thermal shock-induced warp, clamp blades flat. Cryo treatment should always be followed by a tempering cycle. 

Once the blade is quenched and near ambient temperature, blades should be tempered appropriately. The recommended times are to provide an even, consistent heat. The final hardness values range based on the initial as-quenched hardness and conversion percentage to Martensite. Only predictable testing methods, such as the calibrated Rockwell Hardness Tester, can grant actual hardness values. Comparative testing methods like hardness calibrated files and chisels are inaccurate for hardness value reading. 

 

*Temper three times for two hours. 

**The included Heat Treat Schedule on this page is formulated based upon Industry standards and data from ASM International, Crucible and other foundry spec sheets, and Kevin Cashen (independent researcher, ferroalloy metallurgist, and bladesmith of Matherton Forge). 

To view other steel types and recommended heat treatments, click here.