Microstructural-based analysis of the unexpected differences in rolling contact fatigue behavior between SAE52100 and SAE4320

The rolling contact fatigue (RCF) of both a high carbon through-hardening bearing steel (SAE52100) and a low carbon case-hardening bearing steel (SAE4320) have been investigated using RCF. The RCF-life for the SAE4320 steel was found to be & AP;7.0 x 107 cycles, about 7 times higher than the RCF-life found for the SAE52100 steel of & AP; 1.0 x 107 cycles. Measurements from fracture surfaces of specimens tested to failure in rotatory bending fa-tigue tests revealed an average inclusion size of 27.3 & mu;m for SAE52100, and 38.2 & mu;m for SAE4320, indicating a lower purity for case-hardening bearing steel, and suggesting SAE4320 should have a shorter RCF-life, contrary to the measured RCF-lives. However, the carbide size, the prior austenite grain size and the retained austenite (RA) volume fraction for SAE4320 were measured as 0.20 & mu;m, 10.8 & mu;m and 25.0%, respectively, all of which are better than those for SAE52100 (0.59 & mu;m, 16.1 & mu;m and 3.1%). The enhanced RCF-life can be understood as a combined result of the detrimental effects of inclusion size and the beneficial contribution of the refined carbide and prior austenite grain size and of the increased austenite volume fraction. Based on this study, it is proposed that a double refinement of both carbide and prior austenite grain size, together with control of the austenite fraction, may be a promising way to extend RCF-life without the requirement for further inclusion refinement during the fabrication high-performance bearing steels.