Influence of Retained austenite on fatigue performance of carburized gears 2019-28-0102
SAE 8620 and 20MnCr5 are widely used materials for gears in automotive application. The 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 20MnCr5 materials have 10 % and 14% of RA respectively. Whereas, the core and case structure were free from network carbides. The fatigue test were 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 the 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 it is transformed to untempered martensite, which causes brittleness to the component and leads to premature failure.