Assessment of microstructure- and inclusion-induced fatigue crack initiation in bainitic/martensitic rail steels: Competing and synergistic effects

•Competing and synergistic effects of inclusion and microstructure on fatigue crack initiation are discussed.•Both inclusion-induced and microstructural small cracks are related to slip in the microstructure.•Coarse and soft microstructure has a small critical inclusion size and high sensitivity to inclusion.•Bainite/martensite microstructure with small-sized lath thickness and block length has superior VHCF performance. We emphasized the competing and synergistic effects of microstructure and non-metallic inclusion on subsurface fatigue crack initiation behaviors of bainitic/martensitic rail steels through multi-scale characterizations on the microstructure and inclusion features in the crack initiation area. Results show that the emergence of two failure modes, namely non-inclusion- and inclusion-induced crack initiation, is an outcome of the interaction between microstructure and inclusion. The refined bainite/martensite microstructure with small-sized lath thickness and block length can enhance the very high cycle fatigue performance and reliability via increasing the critical inclusion size and reducing the sensitivity to microstructure-induced crack initiation.