Rolling contact fatigue in refurbished case carburized bearings

Abstract

This paper presents a continuum damage mechanics-based elastic-plastic FE model developed to quantify the rolling contact fatigue (RCF) life of refurbished bearings made from case carburized steel. Using the model developed for this investigation, case carburized steel fatigue performance was compared to that of through hardened steels. To simulate the characteristics of case carburized steel, a series of micro-indentation tests was performed to determine the hardness gradient and the case depth for the case carburized 8620 steel.The hardness gradient in the material was modeled by changing the yield strength as a function of depth. Therefore, the finite element modeling approach employed the von Mises plasticity-based model with kinematic hardening to incorporate the effect of material plasticity. Furthermore, the residual stress distribution resulting from the carburization process was modeled by modifying the damage evolution law. In order to simulate the refurbishing process, damage accumulation was calculated for a set number of fatigue cycles with the original bearing geometry. A layer of the original surface was then removed, but the fatigue damage accumulated prior to refurbishing was preserved. The refurbished geometry was then subjected to additional fatigue cycles until the damage was detected. The model as developed also accounts for the effects of topological randomness in the material microstructure through the use of Voronoi tessellations.The model was used to compare the RCF lives of refurbished bearings made from through hardened and case carburized bearing steel at contact pressures ranging from 2 to 3.5 GPa. The number of fatigue cycles prior to the refurbishing and the depth of material removal were varied to analyze their influence on refurbished life. It was found that greater regrinding depth (more than 0.5 times the half-width) and more fatigue cycles prior to refurbishing enhanced the total fatigue life of refurbished bearings.The model predicted that the ratio of the total RCF life of refurbished bearing to that of unrefurbished bearing is more for through hardened bearings than case carburized bearings with case depth of 500 μm. This is as expected, because in through hardened steel there is no case, so material properties are not affected by refurbishing. In the case hardened bearing steel, however, the refurbishing process removes part of the hardened case region and exposes the softer core to stress which reduces the fatigue performance of the refurbished bearing. Since this effect diminishes as the case depth increases; the increase in the fatigue life after refurbishing was larger for case carburized bearings with case depth of 1000 μm than through hardened bearings.