Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M6C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M6C and martensite caused by the inconsistency deformation, M6C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness. •The type of carbides and interface relationship was obtained.•Evolution of microstructure during heat treatment was studied.•The role of carbides in impact toughness was investigated.•The variation of impact toughness value has been explained by the characteristics of martensite.