Fatigue crack propagation behavior of multiphase microstructure in a quenched and partitioned medium‐carbon bainitic steel

A medium‐carbon steel was treated by the bainitic isothermal transformation plus quenching and partitioning (B‐QP) process to obtain bainite/martensite/retained austenite multiphase microstructure, and its fatigue crack propagation (FCP) behavior was evaluated in contrast with BAT (bainite austempering) sample with fully bainite microstructure. Results show that B‐QP sample exhibits a lower FCP rate and higher fatigue threshold ΔKth (12.6 MPa·m1/2). Moreover, the FCP path of B‐QP sample displays a strongly tortuosity and more crack branching due to more filmy retained austenite (7.2%) and higher percentage of high angle misoriented boundaries (68%). The larger crack tortuosity and the secondary cracks as result of crack branching are primarily responsible for the lower FCP rate of B‐QP sample. In addition, the FCP rate curve of B‐QP sample shows a pronounced small plateauing at the near‐threshold zone, which can be ascribed to the mechanical twinning that occurred in the filmy retained austenite. Highlights The B‐QP sample exhibits a lower FCP rate and higher fatigue threshold. The crack tortuosity and secondary cracks are the mainly reasons for the lower FCP rate. The B‐QP sample shows a pronounced small plateauing caused by the mechanical twinning.