The power oil pipe of a vessel adjustable pitch propeller fractured after running for about 25,000 h. 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.

船舶可调螺距螺旋桨动力油管在运行约25000小时后断裂。从宏观断口形貌、微观断口形貌、化学成分、金相分析、数值模拟分析、力学性能和硬度试验等方面对断裂原因进行分析。断口分析表明，油管的断裂方式为高周疲劳断裂。裂纹始于异种金属焊接热影响区的晶间脆性断裂区。晶间裂纹主要是在异种金属的焊接过程中形成的，它是由以Cu为主的低熔点焊接液沿钢热影响区晶界深度扩散引起的晶间脆化。采用有限元法分析焊接区的温度梯度，解释了低熔点焊接液能扩散到钢侧最远1mm深度的原因。断裂力学分析表明，当裂纹深度达到1 mm时，裂纹继续向前扩展的概率较大，存在不稳定断裂的风险。因此，无论是实验还是数值模拟都表明，液态金属向钢侧热影响区扩散形成的晶间脆性裂纹是疲劳裂纹萌生和扩展的原因。断裂力学分析表明，当裂纹深度达到1 mm时，裂纹继续向前扩展的概率较大，存在不稳定断裂的风险。因此，无论是实验还是数值模拟都表明，液态金属向钢侧热影响区扩散形成的晶间脆性裂纹是疲劳裂纹萌生和扩展的原因。断裂力学分析表明，当裂纹深度达到1 mm时，裂纹继续向前扩展的概率较大，存在不稳定断裂的风险。因此，无论是实验还是数值模拟都表明，液态金属向钢侧热影响区扩散形成的晶间脆性裂纹是疲劳裂纹萌生和扩展的原因。