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

The effect of retained austenite (γR) on the rolling contact fatigue (RCF) properties of carburized SAE4320 steel was carefully investigated. We prepared specimens comprising four volume fractions of γR from 6% to 39% by controlling the subzero heat treatment. The effect of γR on the RCF was investigated using hardness measurements, X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy. The RCF test revealed that the sub-surface initiated spalling life was prolonged as the volume fraction of γR increased. In the area at the depth z0 where the orthogonal shear stress was maximum, the majority of the γR was transformed to martensite, thus resulting in a significant increase in the Vickers hardness. The result of SEM observation showed that the region initially comprising γR exhibited a high resistance to RCF. Moreover, the TEM analysis revealed that the initial γR region changed into a mixture of very fine hard martensite and some unchanged γR during RCF. This suggests that the transformation of γR into fine hard martensite during RCF contributed to the improvement of the RCF life.