SPECIAL ASPECTS OF HEAT TREATMENT OF STEELS FOR THE PRODUCTION OF HOT DEFORMATION STAMPS

The authors identified the influence of heat treatment parameters on the structure and properties of the advanced 70H3G2FTR die steel for hot deformation. The optimal heat hardening mode for the hot working hammer die produced from the suggested steel is recommended, which contains spheroidizing annealing at the temperature of 780 °C with the slow furnace cooling up to 550 °С and the following still air cooling up to the normal temperature; hardening at 1000 °С with oil cooling; high-temperature tempering at 550–600 °С with the still air cooling. During the hardening process, the structure of lath martensite with the high density of dislocations and the hardness HRC 57–62 is formed, however, 0.5–0.6 % of carbide phase remains insoluble.    

It is shown that at the stage of high-temperature tempering at 550–600 °С, the process of hardness stabilizing by special complex carbide and intermetallic compounds inclusions can be observed in 70H3G2FTR steel. The analysis of carbide phase at the tempering shows that the compounds of Ме₇С₃ and Ме₃С types are the carbides, which composition changes in the process of thermal influence. So, for example, part of Fe and Cr atoms are replaced with Ti and V atoms in various ratios.    

The authors simulated the temperature-stress condition of hammer die during thermal treatment. It is shown, that the suggested mode of thermal treatment of 70H3G2FTR steel leads to the production of the dispersed ferrite-cementite structure with the distributed special carbides, high level of mechanical properties, and provides the uniform distribution of temperature fields along the volume of hot deformation stamp and, as a consequence, the insignificant internal residual stresses. At the stage of high-temperature soak, the temperature gradient is minimized and does not exceed 25–30 °С.