Fracture mechanism of a dissimilar metal welded joint in nuclear power plant

In this paper, fracture tests and microscopic observations were conducted on an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plant. The fracture mechanism of different regions within the joint was investigated, and the relationship between fracture mechanism in local microstructures and local fracture resistance of the DMWJ was analyzed. The results show the fracture mechanism of A508 and 316L base metals and heat-affected-zone (HAZ) of 316L is typical ductile fracture of nucleation, growth and coalescence of voids, and different crack growth resistance is mainly related to microstructures of them. The fracture mode in A508 HAZ and A508/52Mb interface region with predominant martensite microstructure is mixed brittle and ductile fracture, which leads to lower crack growth resistance. The columnar austenite crystal orientation relative to main crack growth direction apparently affects fracture mechanism and crack growth resistance of buttering Alloy52Mb and weld Alloy52Mw. The cracks in Alloy52Mb propagate across columnar crystal boundaries in a ductile mode, and higher fracture resistance is produced. While the cracks in weld Alloy52Mw propagate along the weak columnar crystal boundaries in a brittle mode, which leads to lower crack growth resistance. The cracks in interface regions and HAZs generally grow towards the material sides with lower strength, which is caused by local strength mismatch and affects local crack growth resistance.