Microstructure and tempering softening mechanism of modified H13 steel with the addition of Tungsten, Molybdenum, and lowering of Chromium

The rapid development in the advanced manufacturing industry asks for better tempering softening resistance (TSR) of Hot work die steels. In this work, a modified H13 steel (CXN03 steel) with additional tungsten, molybdenum, and lowering chromium was prepared. The TSR of CXN03 is significantly better than H13. After quenching at 1040 °C, the hardness and strength of H13 were larger than those of CXN03. However, the hardness and strength of CXN03 exceeded those of H13 after 2 h tempering at 600 °C. A mathematical model was utilized to correlate microstructural characteristics with yield strength during tempering. The calculated results indicated that the superior tempering softening resistance of CXN03 steel mainly results from the excellent stability of dispersive nano-sized M2C, which could prevent dislocation recovery. Recrystallization softening was observed in H13 but not in CXN03. The recrystallization of H13 is driven by dislocation movement, and the rearrangement of dislocations contributed to the formation of sub-boundaries. These sub-boundaries could divide martensite lath as well as form sub-grains. As the tempering time increased, sub-boundaries transformed into high-angle grain boundaries by absorbing the vicinal dislocations. Therefore, martensite lath collapsed, and massive recrystallized grains occurred. The massive stable M2C in CXN03 hindered the dislocation rearrangement, thus preventing the recrystallization.