Numerical Simulation of Microstructure Distribution in Tooth of 42CrMo Steel Gear during Step Quenching

42CrMo steel gear model was established, and then the microstructure distribution in the gear tooth during step quenching was simulated with Sysweld software and verified by experiments. The results show that after being cooled in the nitrate solution until the surface temperature was about 175℃, the bainite content in structure of the gear tooth increased with the distance from the surface increasing; the martensite content in the subsurface layer was the highest (90%, area fraction, the same as below), followed by that in the core (66%), and that in the surface layer was the lowest (58%). After step quenching, the surface microstructure of the gear tooth consisted of 90% martensite and 10% retained austenite, and the core microstructure was composed of 66% martensite and 34% bainite. The retained austensite content at surface of gear tooth after quenching, measured by experiments, was 7.5%, the martensite and bainite content in core was about 69% and 31%, respectively. The experimental results were consistent with the simulation.