Alloying impacts on austenite stability and fatigue crack propagation in medium-carbon direct-quenched and partitioned steels

Medium carbon steels with varying levels of Si were subjected to direct quenching and partitioning (DQP) treatment, employing a quench stop temperature (T Q ) of ~150°C to attain around 80% primary martensite prior to the partitioning step. An optimized partitioning temperature (T P ) of 200°C was chosen based on previous research (Ghosh et al. (2022), Ghosh et al. (2023)). The resulting microstructure featured predominantly dislocated martensite laths interspersed by fine lath-like retained austenite (RA) films, yielding remarkable mechanical properties comprising high yield strength (~1000 MPa) and tensile strength (~2100 MPa) combined with good ductility (~12–14%). Based on our previous findings (Kumar et al. (2023)), fatigue crack propagation in DQP steels under stress intensity control (ΔK) at an R-ratio of 0.1 exhibited lower crack growth rates in the Paris regime than in the case of directly quenched (DQ) steels, primarily due to the amount and stability of RA. In the current study, a medium-silicon (M-Si; 0.7 wt.% Si) steel displays an even slower FCG rate in the Paris regime compared to a high-silicon (H-Si; 1.5 wt.% Si) steel. This phenomenon could be linked to the partial decomposition of untransformed austenite into bainite during the partitioning treatment of M-Si steel. The enhancement in FCG resistance was attributed to the synergistic effects of plasticity-induced crack closure, crack deflection mechanisms, and transformation-induced plasticity (TRIP) of RA, facilitated by the presence of small fraction of bainite along with martensite and RA.