Effect of retained austenite on micropitting behavior of carburized AISI 8620 steel under boundary lubrication

The objective of this study was to investigate the effect of retained austenite (RA) on the micropitting behavior of carburized AISI 8620 steel under boundary lubrication condition. Samples with RA ranging from approximately 0–70% were prepared using specific carburizing schemes. Rolling contact fatigue tests were carried out at maximum contact pressure 1.5 GPa using a benchtop test rig. Samples were subsequently analyzed using noncontact white light profilometry and micro X-Ray Diffraction to observe the evolution of micropitting and RA, respectively. An increase in RA% resulted in increased micropitting life. The failure mechanism for the lowest RA samples was dominated by early crack initiation and rapid crack propagation, whereas samples with medium and high RA showed initiation and propagation of micropitting with clear evidence of RA transforming to martensite. Higher levels of RA% to ensure a stable amount remains after this transformation is desirable for extended micropitting life. An extended finite element method (XFEM) which included a Voronoi tessellation to randomly generate the steel microstructure was used to simulate the experiments, and showed that consistent with experimental findings, crack propagation was increasingly hindered in the case of microstructure with increasing RA%.