Change in structure, properties, and accumulation of damage in Kh12M and R6M5 steel in elastoplastic loading

Abstract

1. An increase in hardening temperature of Kh12M steel to 1050°C leads to strengthening of its sensitivity to hardening in elastoplastic loading and intense formation of microcracks at the grain boundaries as the result of transformation of a large quantity of residual austenite into martensite. Therefore for tools operating under high specific loads treatment by preliminary high-temperature thermomechanical working is preferable. 2. Low-cycle loading of hardened Kh12M and R6M5 steels leads to gradual relaxation of first order residual stresses as the result of formation of microcracks for R6M5 steel and for Kh12M as the result of deformation of the residual austenite, which causes the formation of martensite and also leads to the appearance of microcracks, the quantity of which steadily increases. 3. Fracture of tools of hardened Kh12M and R6M5 steels is a multistage process and is determined by the length of each of the five stages (accumulation of damage, formation of a system of microcracks in local areas, formation of the initial macrocrack, cyclic growth of the initial crack, and transition to the unstable condition), the lengths of which depend upon the heat treat cycles and the effective loads.