Effect of Microstructure on Fatigue Crack Propagation and S–N Fatigue Behaviors of TMCP Steels with Yield Strengths of Approximately 450 MPa

In the present study, stress (S) – number of cycles to failure (N) (S–N) fatigue and fatigue crack propagation behaviors of three thermomechanical control process steels with different microstructures but similar yield strengths of approximately 450 MPa were investigated. The P + F steel was predominately pearlite plus ferrite, whereas B1 and B2 steels were both bainitic steels with martensite-austenite and pearlitic islands. Despite the significant difference in microstructural features, the resulting fatigue crack propagation rates and near-threshold ΔK values were comparable with each other. The hard phases, such as pearlite colonies in the P + F specimen, tended to affect fatigue crack propagation behavior in a similar manner, and severe crack branching was observed in intermediate and high ΔK regimes. Despite similar fatigue crack propagation rates and near-threshold ΔK values, the resistance to S–N fatigue was substantially different for each steel specimen. Depending on fatigue crack initiators, such as the ferrite/pearlite phase boundaries for the P + F specimens and the cracked martensite-austenite and/or small pearlitic islands for the bainitic specimens, the cycles for crack initiation varied greatly.