As shown in the graph for an M2 steel austenitized at 2225°F, the hardness of high-speed steel is directly affected by tempering temperature and time. From the slopes of the curves, it can be seen that M2 undergoes secondary hardening at temperatures above approximately 700°F, and that secondary hardening proceeds at higher temperatures up to about 1100°F, depending on time at temperature.
These temperatures approximate the practical limits for most tempering operations; lower temperatures do not evoke the secondary hardening response, and higher temperatures produce hardness considerably lower than usually desired.
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of High-Speed Tool Steels -> Recommended Heat-Treating Procedures
