Chemical heterogeneity enhances very high cycle fatigue resistance of carbide-free bainitic steel

In this study, we investigated the very-high-cycle fatigue (VHCF) behavior of high-carbon bainitic steel with and without chemical heterogeneity. The only processing difference was the initial microstructure: pearlite for chemically heterogeneous steel (PUB-320) versus martensite for homogeneous steel (MUB-320), with all subsequent steps being identical, including fast heating and bainitic austempering. The heterogeneous steel exhibited an ultrafine bainitic structure characterized by Mn-enriched film-like retained austenite (RA) and Mn-depleted bainitic ferrite. A significant enhancement in the VHCF strength (σ_w9, at 10⁹ cycles) was achieved, increasing from 728 MPa (MUB-320) to 812 MPa (PUB-320), corresponding to a fatigue strength ratio (σ_w9/UTS) of 0.52. This enhancement is directly attributed to the chemical heterogeneity, which improves the mechanical stability of RA and modifies its plastic deformation mechanisms. Under cyclic loading, the RA ahead of the microcracks either remains stable or transforms into stacking faults, mechanical twins, or nano-sized martensite, thereby alleviating the local stress concentrations and impeding microcrack propagation. These findings underscore the potential of chemical heterogeneity as an effective microstructural strategy for enhancing the fatigue resistance of bainitic steels without requiring costly ultra-clean steelmaking processes.