High cycle fatigue properties, failure mechanisms and microstructure evolution of 9Cr3W3Co steel under different stress ratios at 650 °C

The study investigates the high cycle fatigue properties, failure mechanisms, and microstructure evolution of 9Cr3W3Co steel at 650 °C under stress ratios R of 0.1, 0.3 and 0.5. S-N curves exhibit a continuous decline, with corresponding fatigue strengths at 10⁷ cycles of 116.6, 99.6 and 64.8 MPa for R of 0.1, 0.3 and 0.5, respectively. Surface or sub-surface crack initiation failure (SSCIF) is the only observed failure mode at R = 0.1, while necking induced failure (NIF) dominates at R = 0.5. At R = 0.3, both failure modes coexist. SSCIF mode is characterized by localized plastic deformation and fatigue damage accumulation, with crack propagation and facet formation influenced by the activation of slip systems with relatively high Schmid factors and the grain orientation and local deformation distribution of martensite structures. NIF mode is marked by widespread plastic deformation, void nucleation and coalescence, resulting in the rapid diminution of the effective load-bearing of specimens and ultimately leading to necking failure.