Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (N-i/N-f) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.