Rolling contact fatigue of case carburized steels

Abstract Case carburized steels are widely used in high performance ball and rolling element bearings. They are characterized by the hardened exterior and gradient in the material properties as a function of depth. In this investigation, an elastic-plastic finite element model based on micro-indentation tests was developed to investigate the rolling contact fatigue of case carburized steels. A series of micro-indentation tests were conducted to obtain the hardness gradient in the case carburized 8620 steel. The results demonstrated that the hardness varies linearly from the surface to the core of the material. The finite element modeling approach employs Mises based plasticity model with kinematic hardening to incorporate the effect of material plasticity. The hardness gradient in the material was modeled by changing the yield strength as a function of depth. Linear relationship between hardness and yield strength was assumed. The FE model was coupled with continuum damage mechanics approach to capture material degradation due to fatigue damage. It considers both; stress and accumulated plastic strain based damage evolution laws for fatigue failure initiation and propagation. The residual stress distribution due to carburization process was modeled by modifying the damage evolution law. Material dependent parameters used in the damage evolution laws were determined using the SN results for torsional fatigue of the bearing steel. The effects of topological randomness in the material microstructure are accounted in the model through the use of Voronoi tessellations. The model was used to compare the rolling contact fatigue (RCF) lives of through hardened and case carburized bearing steel with different case depths at contact pressures ranging from 2 to 3.5 GPa. The effect of residual stress distribution on RCF lives was also investigated. The results show that there is an optimum case depth for which maximum RCF lives can be attained. The spall shapes and the depth below the surface where damage initiates were found to be dependent on the case depth. The model was also used to study the effect of initial material imperfections. The fatigue lives and their dispersion quantified by Weibull slopes obtained from the model correlate well with the experiments.