Influence of Prior Cold Rolling on Bainite Transformation of High Carbon Bearing Steel

The use of cold ring rolling as a typical forming method for bearing steel has shown considerable effects on the development of microstructures in subsequent heat treatment. However, the impact of prior cold rolling on the bainite formation remains unclear. In this study, the influencing mechanism of prior cold rolling on the bainite transformation kinetics for aviation M50 bearing steel has been investigated. It was observed that cold rolling shortened the incubation period of bainite and limited the width of the bainite sheaves. The bainite transformation process was first promoted and then inhibited by cold ring rolling. To investigate the decisive factors affecting the bainite transformation kinetics under cold rolling, in-situ X-ray diffraction analysis and microstructure characterization were conducted. The results showed that although there was a recovery effect during austenitization, the dislocations introduced by cold rolling were partially retained and the carbide dissolution is enhanced. The dislocation density, carbon content, and compressive strength of the high-temperature austenite were increased, and the austenite size was refined, thereby directly affecting the bainite transformation process under cold rolling. Accordingly, a kinetic model for bainite transformation considering cold rolling was established, which revealed the promotion effect of increasing grain boundaries and low-angle grain boundaries on bainite nucleation in the early stage and the inhibition effect of high-density dislocations and high carbon concentration on bainite sheaves growth in the late stage of austempering under cold rolling.