Thermal stabilization and destabilization of austenite in tungsten-molybdenum high-speed steels

Conclusions

• 1. Processes that take place in the martensite exert a major influence on stress relaxation in the austenite of steels R6M5 and R6M5K5 and its stabilization:

  Preliminary tempering at 300–400°C almost completely stabilizes the austenite of the hardened steel owing to vigorous carbon impoverishment of the martensite and its compression, and subsequent heating and high tempering lead to destabilization of the austenite;

  Holding of the hardened steel at 20°C for two months stabilizes the austenite virtually completely;

  Holding at 20°C also stabilizes the austenite, lowering the tendency to secondary martensitic transformation after high tempering; the most vigorous stabilization is observed after holding for 1–5 days; the stabilization effect is reduced for extended holding times (to 30 days); and,

  Segregation of carbon (carbides) from the martensite during isothermal holding in the 20–400°C interval, or at temperatures of the secondary transformation result in stabilization of the austenite; segregation of cementite-type carbides from the martensite during cooling after high tempering in the 420–380°C interval activates a secondary transformation of the austenite.

The accompanying mechanical stabilization and the transformation kinetics after high tempering exert an influence on the thermal stabilization of the residual austenite. The bainitic transformation contributes to stabilization of the austenite, while the martensitic transformation intensifies decomposition of the austenite during subzero treatment.