Effect of microstructure on the fatigue crack growth behavior of 9%Cr martensitic steel at different stress ratios under 630 ℃

To understand the effect of mean stress and microstructure on the fatigue crack growth (FCG) behavior of 9%Cr martensitic steel under high temperatures, the FCG experiments are conducted on three stress ratios (R = 0.1, 0.3, and 0.5) at 630 ℃. The results indicate that the high-stress ratios have a higher fatigue crack growth rate, and the Paris region can be divided into two regions by the transition point (da/dN=1×10-4 mm/Cycle) with different fracture morphology. For da/dN>1×10-4 mm/Cycle, due to the influence of high crack driving force and the microstructure, the FCG path presents in a zigzag way. For da/dN⩽1×10-4 mm/Cycle, the FCG path presents the characteristic of purely horizontal and no obvious crack deflection. Meanwhile, the martensitic lath fracture angles near the FCG path have an obvious statistical relationship with the stress ratio, which is mainly concentrated at 60° to 80° and 100° to 130°. Additionally, according to the EBSD map of secondary cracks, when the secondary cracks encounter the HAGBs, the larger angle of secondary crack deflection occurs.