The microstructure

impact performance and fracture morphology of 42CrMo steel after pre-heat treatment (annealing

quenching

tempering) and treatment of quenching from dual phase range + tempering at high temperature ( also referred to as quenching from critical range + tempering) were observed and analyzed by means of metallographic micro¬scope

hardness tester and scanning electron microscope. The results show that the residual ferrite is uneven in distribution between tempered sorbite when the preparatory heat treatment is tempering

whereas the residual ferrite is small and even when the preparatory heat treatment is quenching and hardening and tempering. The hardness difference of sub-heat treatment is very small when different preparation heat treatment is used. After sub-temperature treatment

the impact performance of 42CrMo steel is higher than that of conventional hardening and tempering. When pre-heat treatment is hardening and tempering

the impact energy after sub-temperature treatment is the largest

and it can be seen from the fracture morphology. The cracking area and crack fiber expansion area of hardening and tempering account for a larger proportion than annealing and quenching. Therefore

the hardening and tempering treatment is more suitable as a preparatory heat treatment for the 42CrMo steel.