Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M 23 C 6 and Mo‐rich M 6 C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models.