Crack initiation mechanism of carbide-free bainitic steel during impact loading

Carbide-free bainitic (CFB) steel is widely utilized in industrial applications, notably as semi-autogenous grinding (SAG) mill liners, where they are subjected severe impact loading conditions during ore processing. A comprehensive understanding of the crack initiation mechanism under these extreme conditions is critical for enhancing the durability and safety of critical industrial components. This study investigates the crack initiation mechanism of CFB steels under impact loading, with a particular focus on the role of key microstructural features, namely bainite blocks and martensite-austenite (M-A) constituents. The findings reveal that stress concentration, induced by dislocation pile-up at boundary between M-A constituent and bainite block, triggers microcrack nucleation within the comparatively brittle M-A constituent. The propagation of these microcracks across bainite block boundaries is identified as the critical event governing the crack initiation process. It is demonstrated that the finer bainite blocks effectively enhance the resistance of the crack initiation, thereby improving the overall impact toughness of CFB steels. These findings provide fundamental theoretical insights and practical microstructural design principles for developing CFB steels with superior impact resistance in demanding industrial applications.