Investigation on the resistance to fatigue crack growth for weld metals with different Ti addition in near-threshold regime

The resistance to fatigue crack growth and its correlation with microstructure for weld metals with different trace Ti element addition was systematically investigated in this paper. It is found that there were some obvious differences in grain size, block ferrite and precipitations for Ti-free and Ti-containing weld metal. Fatigue crack growth tests were conducted in weld center under different stress ratios, the results indicate that weld metal without Ti addition performed better on fatigue resistance in near-threshold regime at each stress ratio. Transgranular growth with crack deflections and intergranular growth with bifurcations are the typical crack growth behavior in Ti-free and Ti-containing weld centers respectively. It is revealed that large size of prior austenite grains in Ti-free weld center lead to more crack deflection via analysis on the fatigue crack growth path. Moreover, the carbides scattered on ferrite boundaries also lead to crack deflection and reduce the possibility of intergranular fracture at near-threshold for Ti-free weld center. Compared to crack bifurcations, the crack deflections contributed more to crack closure inducted by contact surface roughness, and the fatigue growth resistance is enhanced for Ti-free weld center. The correlation between fatigue resistance performance and misorientation of ferrite boundaries in weld centers was further clarified by electron back-scattered diffraction (EBSD) characterization.