Residual Stress Control on Bearing Steel by Surface Cooled Induction Heating Fast Tempering

In order to reduce the amount of CO₂ emission during carburizing while keeping the advantages of case hardening, a surface cooled induction heating fast tempering process has been developed for the production of bearing elements. This process is applied to SUJ2 bearing steel, followed by conventional hardening. By surface cooled induction heating fast tempering, high temperature at the center of the test piece and relatively low temperature at the surface cause a difference in tempering temperature, leading to compressive residual stresses at the surface because of the difference in volume change during tempering. As a result, the bearing made by this process has the same rolling contact fatigue life as a carburized bearing due to high compressive residual stress. Process simulation of surface cooled induction heating fast tempering shows that residual stress is mainly caused by the difference in volume change of martensite during this process between the surface and the center. However, the decomposition of retained austenite appears to shift residual stress toward tensile stress.