High speed steel that should be cold treated may be hardened in accordance with the usual practice for that steel. During the actual heat treatment, the P8 steel tool is heated in the range of 1260 ~ 1290 ℃, while the P9 steel tool is heated in the range of 1220 ~ 1240 ℃. It is recommended to use the lower limit of heating temperature to harden the small and complicated tools. Cold oil (below 35 ℃), hot oil (100 ~ 160 ℃), molten nitrate or strong alkali (480 ~ 530 ℃) are used for cooler.
The composition of hardened high speed steel is composed of Martin body, carbide and 25 ~ 35% austenite. Austenite is a soft and tough structure, and its existence will deteriorate the cutting and grinding properties of the steel. Therefore, it is pointed out in the section “relationship between steel and tempering” that the allowable allowance of retained austenite is limited to 5%, which means that its quantity is much less than that after quenching.
The transformation of retained austenite occurs in the process of cold treatment and fire matching. The cold treatment is carried out immediately after quenching. When the heat treatment operations here are interconnected, attention must be paid to the cooling method in the temperature range from 100 ℃ to 20 ℃.