Research on Microstructure Dependence of Near-Threshold Fatigue Crack Propagation by Combined Analyses of Fracture Surface and Fatigue Crack Growth Path

Near-threshold fatigue crack growth behavior was investigated in a newly developed steel 25Cr2NiMo1V with different heat treatments to meet different property requirements of high-pressure (HP) and low-pressure (LP) parts in the combined steam turbine rotor. The load-shedding method was adopted in the near-threshold fatigue crack growth experiment at room temperature with a constant load ratio of 0.1. Combined analyses of crack surface and fatigue crack growth path were carried out to identify the dominant crack growth mechanisms in both HP and LP. Results show that in the threshold regime, fatigue crack growth resistance of the HP is clearly superior to that of LP and hence shows strongly dependence on the microstructure of 25Cr2NiMo1V. The distributed bainitic microstructures and larger prior austenite grain size in HP result in more tortuous crack propagation path than that in LP. Compared with ferritic blocks in HP, the tempered martensitic laths in LP do not play a dominate role in stopping the fatigue crack advance.