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

Abstract

The effect of retained austenite (γ_R) on the rolling contact fatigue (RCF) properties of carburized SAE4320 steel was carefully investigated. We prepared specimens comprising four volume fractions of γ_R from 6% to 39% by controlling the subzero heat treatment. The effect of γ_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 γ_R increased. In the area at the depth z₀ where the orthogonal shear stress was maximum, the majority of the γ_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 γ_R exhibited a high resistance to RCF. Moreover, the TEM analysis revealed that the initial γ_R region changed into a mixture of very fine hard martensite and some unchanged γ_R during RCF. This suggests that the transformation of γ_R into fine hard martensite during RCF contributed to the improvement of the RCF life.

(a) TEM bright field image of flat structure (high magnification of Fig. 12) in DEA of S₃₉ after RCF (N = 3.7 × 10⁶), (b) SADP, (c), (d) indexed diffraction patterns of (b) indicating α phase and γ phase, respectively. (Online version in color.)