12Cr1MoV钢蒸汽管环焊缝开裂原因分析

Cracking Cause Analysis of 12Cr1MoV Steel Steam Pipe Ring Weld

某12Cr1MoV蒸汽管道对接接头在520℃运行90000 h后发生泄漏。通过宏观观察、金相观察、扫描电镜观察、能谱分析等方法对断口进行分析,发现在焊缝一侧3~9点钟位置存在沿环向分布的裂纹。宏观上断口面无塑性变形,且完整复制了焊缝熔合形貌,裂纹面覆盖了致密的高温氧化物;微观上裂纹开裂位于焊接热影响区的细晶区和临界晶区,同时主裂纹附近可见沿晶界分布的微裂纹和蠕变孔洞,由此判断该环缝发生了Ⅳ型蠕变开裂。失效现场同时还发现管系存在一定的位移,结合环向仅有180°开裂的现象,判断管系服役期间在环缝位置可能存在异常弯矩,这是导致环缝发生早期Ⅳ型蠕变开裂的主要驱动力。

After operation at 520 ℃ for 90000 h, the leakage of a 12Cr1MoV steam pipe butt joint occurred. After analysis through macroscopic observation, metallographic observation, SEM and EDS analysis, it is found that there is circumferential crack on one side of the weld from 3 o’clock to 9 o’clock. The fracture surface has no plastic deformation, and it completely copies the morphology of weld fusion line, and the crack surface is covered with dense high-temperature oxide.The crack propagation region is located in the fine grain heat affected zone and intercritical heat affected zone, at the same time, the microcracks and creep holes distributed along the grain boundary can be seen near the main crack, which indicates that the circumferential crack is type Ⅳ creep crack. Some displacement of the pipeline was also found at the failure site.Combined with the phenomenon that the pipe has only 180° cracked in the circumferential direction, it can be concluded that there may be abnormal bending moment at the circumferential joint during the service of the pipeline system, which is also the main driving force leading to the early type Ⅳ creep cracking of the pipeline.