Effect of tempered sorbite/bainite microstructures on fatigue crack propagation and closure in Fe-Cr-Mo-Mn steel

The large-scale wind power spindles are prone to fatigue failure under cyclic loading, particularly, due to microstructure inhomogeneity of tempered large-scale forgings, it is of great significance to explore the influence of non-uniform tempered sorbite/bainite (TS/B) on fatigue crack propagation and closure effects. In this paper, the fatigue crack propagation behavior of coarse TS/B and fine TS/B were investigated. The results show that the fine TS/B with higher microstructure refinement have higher fatigue life compared to the coarse TS/B due to the tortuous fatigue crack propagation path. In the low ΔK region, due to roughness induced crack closure (RICC), the coarse TS/B exhibits a more tortuous fatigue crack propagation path, thereby leading to higher resistance to fatigue crack propagation. In the medium ΔK region, the coarse TS/B exhibiting a large plastic region size (Δr_p) tends to absorb dislocations, enhance energy release at the crack tip and reduce fatigue crack propagation rate (FCPR). In contrast, the fine TS/B are prone to crack deflection at the phase boundaries of refined TS/B due to higher misorientation (MO), resulting in decreasing FCPR. In addition, compared with the coarse TS/B, the fine TS/B is more likely to generate microcracks and reduce the FCPR of the primary crack in the regions with high Schmid factor (SF) and low Taylor factor (TF). The {110} < 111 > slip system has a low slip resistance in the fine TS/B, and the primary crack propagation of the fine TS/B is preferentially in the {110} < 111 > slip system at high SF, which accelerates the FCPR.