Evolution of subsurface plastic zone due to rolling contact fatigue of M-50 NiL case hardened bearing steel

Accelerated rolling contact fatigue (RCF) experiments on M-50 NiL case carburized bearing steel were conducted using a three-ball-on-rod tester at high contact stress levels (5.5 GPa) and over several hundred million contact cycles to investigate the evolution of plastic deformation zone characteristics. A microindentation technique was used to measure the changes in the material hardening and softening characteristics within the RCF affected zone and represent the changes in the form of microhardness maps. These maps revealed regions of considerable material hardening inside the plastic zone and softening of material outside the hardened region. The results indicate large amount of plastic strain accumulation beneath the contact surface via ratcheting due to RCF cycling. The hardened and softened regions continue to evolve in severity and size with increasing number of RCF cycles. Using the Hertzian contact theory, a rationale for the evolution of hardened and softened regions during RCF loading is provided. The entire plastic zone is shown to be dominated by four different principal stress states which contribute to the evolution of these regions during RCF.