Fracture analysis on dissimilar metal welding zone of vessel CPP power pipe

Abstract The power oil pipe of a vessel adjustable pitch propeller fractured after running for about 25000 hours. The fracture causes were analyzed by macro fracture morphology, micro fracture morphology, chemical composition, metallographic analysis, numerical simulation analysis, mechanical property and hardness test. Fractographic analysis shows that the fracture mode of oil pipe is high cycle fatigue fracture. The crack starts from the intergranular brittle fracture zone in the heat affected zone of dissimilar metal welding. The intergranular crack is mainly formed in the welding process of dissimilar metals, which is caused by intergranular embrittlement due to the deep diffusion of low melting point welding fluid mainly composed of Cu along the grain boundary in the heat affected zone of steel. The temperature gradient of the welding zone is analyzed by finite element method, and the reason why the low melting point welding fluid can diffuse to the farthest depth of 1 mm on the steel side is explained. The fracture mechanics analysis shows that when the crack depth reaches 1 mm, the crack has a greater probability to continue to expand forward, which leads to the risk of unstable fracture. Therefore, both experiments and numerical simulation show that the intergranular brittle crack formed by the diffusion of liquid metal to the heat affected zone of the steel side is the cause of fatigue crack initiation and propagation.