Work hardening response of M50-NiL case hardened bearing steel during shakedown in rolling contact fatigue

The work hardening response during shakedown phase for case hardened M50-NiL bearing steel in rolling contact fatigue (RCF) is presented. Rolling contact fatigue testing is performed using a ball rod tester for varying cycles and Hertzian stress of 5·5 GPa on multiple test tracks of a case hardened rod. Longitudinal sectioning and polishing of the test rod reveal the RCF affected zones. Micro-Vickers indentation mapping of the subsurface zones is used to measure workhardening. A maximum increase in hardness from 7·4 to 8·3 GPa was observed at ∼13·5×10⁶ cycles, after which hardness saturated. A finite element simulation of the RCF test was performed to predict the increase in hardness. It is hypothesised that plastic strain accumulation around the hard carbide inclusions results in increased hardness. Cyclic hardening likely saturates when the ratchet strain per cycle approaches zero. Plastic shakedown follows with no further accumulation of plastic strain and marks the beginning of steady state response.