High temperature thermo-mechanical treatment of 12−0−2+C type high-speed steel

Abstract

This investigation of the influence of high temperature thermo-mechanical treatment (HTTMT) involved 20 ∼ 60% deformation in the temperature range of 1000 ∼ 1150°C or conventional heat treatment (CHT), and examination of the phase transformations, the structure and the properties of W-Mo-V high-speed steel. The HTTMT, comprising austenitizing at 1190°C, drop forging at 1000 ∼ 1150°C and tempering at 540°C, may be employed for making particular tools from 12−0−2+C high-speed steel. The method can be recommended for the manufacture of parts of gear-cutting hobs. It is possible also to use HTTMT for parting-off tools and other high-speed steel tools of simple geometrical shapes. The method, with deformation by hot rolling, is suitable for the manufacture of twist drills, which are usually hot rolled. After HTTMT, as in the case of CHT, the structure of 12−0−2+C steel consists of fine lath martensite twinned in some regions, precipitations of cementite, retained austenite and undissolved M 4C 3 and M 6C carbides. In the martensite of the steel after the HTTMT, apart from the cementite there are fine M 4C 3 carbides, increasing the hardness with respect to CHT. The HTTMT of this steel at 1150°C caused refinement of the austenite grain by dynamic or metadynamic recrystallization of austenite, finally resulting in refinement of the martensite laths. After HTTMT carried out at 1000°C, the austenite grains were elongated and their size was similar to that after CHT. The martensite laths had an irregular bent shape caused by dynamic recovery and polygonization of the austenite. Tempering of the steel both after HTTMT and after CHT caused autonomous precipitation of fine M 4C 3 carbides during holding at the tempering temperature with a consequent secondary hardening effect, which was also influenced by martensitic transformation of retained austenite during cooling from the tempering temperature. After CHT a secondary hardness of 66.5 HRC was produced on tempering at 570°C. HTTMT caused the temperature of the maximum secondary hardness of 63 ∼ 65 HRC to shift to 540°C. Other properties after HTTMT were similar to those produced by CHT.