某电站锅炉末级再热器12Cr1MoVG/TP304H异种钢管焊接接头在服役过程中发生断裂，通过宏观形貌观察、化学成分分析、力学性能测试、显微组织观察、微区成分分析等对断裂原因进行分析。结果表明：该异种钢管焊接接头沿12Cr1MoVG钢管侧熔合线发生脆性断裂；在高温工况下长期服役后，焊接接头发生碳的扩散迁移，导致12Cr1MoVG钢管侧熔合线附近集聚的碳化物沿晶界分布并形成微观蠕变裂纹，从而严重削弱了该区域的强度和韧性，同时12Cr1MoVG和TP304H钢的导热系数和线膨胀系数差别很大，导致高温环境下形成较大的热应力，在高温蒸汽压力和热应力的综合作用下该异种钢焊接接头沿12Cr1MoVG钢管侧熔合线发生脆性断裂。

12Cr1MoVG/TP304H dissimilar steel tube welded joint of final boiler reheater in a power plant fractured during service, and the fracture cause was analyzed by macroscopic morphology observation, chemical composition analysis, mechanical property test, microstructure observation, micro-area composition analysis. The results show that the brittle fracture occurred in dissimilar steel tube welded joints along the fusion line on 12Cr1MoVG steel tube side. After long-term service at high temperature, the carbon in the welded joint diffused and migrated, and then the carbide concentrated near the fusion line on 12Cr1MoVG steel tube side distributed along the grain boundaries; meanwhile, micro creep cracks formed, which seriously weakened the strength and toughness of the region. The thermal conductivity and the coefficient of linear expansion between 12Cr1MoVG steel and TP304H steel was very different, resulting in the formation of large thermal stress in the high temperature environment. Finally, the dissimilar steel welded joint underwent brittle fracture along the fusion line on 12Cr1MoVG steel tube side under the combination of high temperature steam pressure and thermal stress.