Crack growth behavior, fracture mechanism, and microstructural evolution of G115 steel under creep-fatigue loading conditions

Creep-fatigue crack growth (CFCG) behavior, fracture mechanism, and microstructural evolution of G115 steel under different hold times and initial stress intensity factor ranges (Delta K-in) were investigated. The crack propagation durations increased significantly with decreasing Delta K-in and increasing dwell time. In da/dN vs. Delta K plots, a hook was formed, and it entered the fast crack growth zone at approximately Delta K = 36 MPa root m, which corresponds to sigma(ref) = sigma(0.2) under plane stress. In (da/dt)(avg) vs. (C-t)(avg) plots, data for a specific hold time in a wide load range exhibited a unique relationship. It appears that the parameter D-0 in this relationship is affected by hold time and constraint, while phi is independent of these factors. Scanning electron microscopy analyses revealed that with increasing hold time (60-600 s), the fracture mode changed from transgranular-dominated to intergranular-dominated. Meanwhile, the fracture mode changed from intergranular-dominated to predominantly transgranular with high ductility when Delta K-in increased (17-29 MPa root m). According to electron backscatter diffraction observations, many substructured and recrystallized grains formed after CFCG and the deformed grains further decreased as Delta K-in increased. The increased crack resistance with decreased deformation grains may combine with the ductility fracture effect to decrease the CFCG rates under high load levels. Additionally, the local misorientation angle reduced significantly after CFCG, and the differences among various load levels were limited.