Abstract: Effect of heat treatment process on microstructure and properties of the carburized 15MnNi4MoA steel was studied. Three different post-carburizing heat treatment processes were designed, i.e.: quenching+low temperature tempering, once high temperature tempering+quenching+low temperature tempering and twice high temperature tempering+quenching+low temperature tempering, and the mechanical properties and microstructure of the tested steel after heat treatment were compared and analyzed. The microstructure and impact fracture morphologies of the tested steel after heat treatment were observed by means of scanning electron microscope. At the same time, the Vickers-hardness and impact absorbed energy (U-notch) at room temperature of the specimens under different heat treatment processes were tested. The results show that after heat treatment, the microstructure of the all specimens has little difference, which is martensite+retained austenite. The specimen treated by twice high temperature tempering treatment has a finer martensite structure and better mechanical properties. The core hardness is reduced to 358 HV, the surface hardness is increased to 664 HV, and the impact absorbed energy at room temperature reaches 143 J.

Key words: post-carburizing heat treatment, high-temperature tempering, martensite, impact property, surface hardness