The effect of microstructure on the fatigue crack growth behavior of Ni-Cr-Mo-V steel weld metal

Fatigue crack growth (FCG) rate, which could significantly affect fatigue lifetime of thick wall components, has been studied extensively in forgings with homogeneous microstructure. However, there were few studies on the effect of inhomogeneous microstructure on FCG rate in weld metal. In this work, the effect of microstructure on FCG behavior of Ni-Cr-Mo-V weld metal was investigated. The FCG rate in the Paris regime was similar to that reported in the literature. However, in the near-threshold regime, the FCG rate decreased rapidly at first with the decrease of stress intensity factor range (AK) and then increased slightly. Finally, the FCG rate slowly decreased with the decrease of AK. It was found that when the monotonic plastic zone size around the crack tip was equivalent to the ferrite grain size in reheated grain zone at a certain AK, the crack propagated along the ferrite grain boundaries. The occurrence of intergranular fracture increased the fracture surface roughness, and enhanced the roughness-induced crack closure effect, which increased the FCG resistance and reduced the FCG rate drastically at the early stage. At a lower AK level, frequent deflection and crack branching were observed and the mechanisms were investigated by crystallographic analysis.