Effect of deep tempering on microstructure and hardness of carburized M50NiL steel

Through the deep medium-temperature tempering treatment, microstructure observation and hardness test of the carburized M50NiL bearing steel, the influence of the temperature and times of deep tempering on the microstructure and hardness of the steel were studied. The results show that the deep medium-temperature tempering is beneficial to the dissolution of large-size carbides in the carburized layer of the steel at the initial state, uniform distribution of carbides, and the precipitation of granular carbides from lath martensite. During deep tempering treatment, some granular carbides are connected to form rod-like carbides. With the increase of the number of deep tempering, the lath martensite in the steel is decomposed and the size is obviously refined, and more carbides in the core area are precipitated, whose distribution is more dispersed uniform. The deep tempering at 460 °C and 480 °C can slightly increase the surface hardness of bearing steel, but five times of medium temperature tempering at 500 °C makes the hardness of the steel slightly decreased. There exists the lowest point of hardness in the subsurface layer of the initial-stated M50NiL steel, which is however eliminated after 5-time deep tempering at 460 °C and 500 °C, and this is related to the elimination of stripe-like carbides after the two deep tempering processes.