The significance of ultrafine film-like retained austenite in governing very high cycle fatigue behavior in an ultrahigh-strength MN-SI-Cr-C steel

We elucidate here the very high cycle fatigue (VHCF) behavior of an ultrahigh-strength medium carbon Mn-Si-Cr-C steel processed using the approach of bainite-based quenching and partitioning (BQ&P). The microstructure of BQ&P process comprised of bainite, carbon-depleted martensite, retained austenite (RA) and small amount of martensite/austenite island (M/A). The tensile strength (Rm) and fatigue limit strength after 109 cycles (σw9) and in the non-failed condition were 1688 MPa and 875 MPa, respectively such that σw9/Rm exceeded conventional steels and was 0.52. Two types of failure modes were observed depending on the surface and microstructure, notably surface-induced failure and non-inclusion-induced failure, where the non-inclusion-induced failure was influenced by the microstructure. Inclusion-induced failure was absent. The study underscores that film-like retained austenite was the underlying reason for superior fatigue properties, hitherto not previously obtained.