Effect of Retained Austenite on Sub-surface Initiated Spalling during Rolling Contact Fatigue in Carburized SAE4320 Steel

The effect of retained austenite (γR) on rolling contact fatigue (RCF) properties was carefully investigated using carburized SAE4320 steel. We intentionally prepared specimens with four different volume fractions of γR from 6% to 39% controlling subzero heat treatment. The effect of γR on RCF was examined by means of hardness measurement, X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). RCF test revealed that sub-surface initiated spalling life is prolonged as volume fraction of γR increases. At the region of the depth z0 where the orthogonal shear stress becomes maximum, γR mostly transformed to martensite, resulting in the significant increase in Vickers hardness. SEM observation showed that the region of initially γR exhibited high resistance to RCF. Moreover, TEM analysis revealed that the initially γR region changed into mixed structure of very fine hard martensite with still remaining γR. This suggests that the transformation of γR to fine hard martensite during RCF contributed to the improvement in RCF life.