Influence of Retained Austenite on Fatigue Performance of Carburized Gears

SAE 8620 and 20CrMo materials were subjected to carburizing process to obtain the identical hardened layer of HRC 61-64. The carburized surface and core properties of the materials were examined and characterized through optical microstructure to measure the presence of cementite carbides and Retained Austenite (RA). From the results, it was found that the SAE 8620 and 20CrMo materials have 10 % and 14% of RA respectively. Whereas, the core and case structure were free from network carbides. The fatigue test was conducted to correlate the RA and fatigue strength of the materials. It was revealed that material with lower RA has higher fatigue strength than material with higher RA. Higher amount of retained austenite leads to reduction in amount of martensitic and compressive residual stress attributed to lower the contact fatigue strength. Untransformed austenite is metastable and is transformed to untempered martensite, which causes brittleness to the component and leads to premature failure.