Effect of Austempering Treatment on Microstructure and Mechanical Properties of M50NiL Bearing Steel

By means of microstructure observation, microhardness, tensile and rotating bending fatigue tests, the effects of austempering treatment on the formation of lower bainite, carbide precipitation and mechanical properties of M50NiL bearing steel were studied. Results showed that multi-layer composite microstructure was formed in the steel after austempering treatment. The microstructure of the surface layer was mainly consisted of lower bainite, martensite and carbides, that of the sub-surface layer contained granular carbides, a little lower bainite, lath martensite, twin martensite and thin film retained austenite, and that of the core region mostly contained lath martensite. In contrast with the traditional martensite quenching treatment, there was more lower bainite formed in the surface layer after austempering treatment. The appearance of acicular bainite dramatically decreased the effective grain size by refining the microstructure, and there were more finer carbides precipitated in the lath martensite. After austempering treatment and tempering, the average hardness in surface layer of the steel increased from 746.5 HV_0.5 to 786.4 HV_0.5, and the rotating bending fatigue limit of the steel increased from 870.9 to 1170.0 MPa, but the tensile strength of the steel decreased slightly. The effects of austempering treatment on the mechanical properties had a close relationship with the carbide precipitation behavior and the formation of lower bainite.