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

The effect of retained austenite (gamma(R)) on the rolling contact fatigue (RCF) properties of carburized SAE4320 steel was carefully investigated. We prepared specimens comprising four volume fractions of gamma(R) from 6% to 39% by controlling the subzero heat treatment. The effect of gamma(R) 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 gamma(R) increased. In the area at the depth z(0) where the orthogonal shear stress was maximum, the majority of the gamma(R) 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 gamma(R) exhibited a high resistance to RCF. Moreover, the TEM analysis revealed that the initial gamma(R) region changed into a mixture of very fine hard martensite and some unchanged gamma(R) during RCF. This suggests that the transformation of gamma(R) into fine hard martensite during RCF contributed to the improvement of the RCF life.