In-situ SEM study of crack initiation and propagation behavior in a dissimilar metal welded joint

In this paper, the in-situ fracture tests were carried out to study the fracture mechanism of different regions (base region, weld region and interface region) in a dissimilar metal welded joint of nuclear power plant. The local microstructural effect on short crack initiation and propagation behavior was investigated. The results indicated that the mechanism of crack initiation and propagation was strongly depended on the microstructures of the difference zone in the dissimilar metal welded joint. There were large numbers of twins in the base region, and the cracks tended to initiate along the twin boundary. The base region exhibited the characteristics of ductile fracture. There was no twin existed in the weld metal. The crack tended to initiate at the slip band and propagate along the columnar grain boundary due to existence of the second phase particles along the grain boundary. The fracture mode of the weld metal was intergranular. In the interface region, the crack tended to propagate from the weld zone to the epitaxial zone, and then along the grain boundary of the epitaxial zone. The fracture mode was the combination of the brittle and ductile fracture. Meanwhile, the weld region has a lower resistance of crack initiation and propagation compared with the base region and interface region.