某装置S30408不锈钢法兰服役时发生开裂泄漏,采用宏观观察、化学成分分析、硬度测试、金相检验和扫描电镜分析等方法分析其开裂原因。结果表明:法兰材料的碳元素含量超标,大量碳化物沿晶界析出,显微组织呈敏化态;S30408不锈钢法兰与管道对焊时,其颈部受热,导致敏化加剧,晶界脆化;法兰颈部形状突变,颈部成为应力集中区,在焊接残余应力和工作压力的作用下,颈部逐渐萌生沿晶微裂纹,随着时间的推移,微裂纹扩展并汇聚形成宏观裂纹,导致法兰贯穿开裂。

The cracking and leakage of S30408 stainless steel flange in a device occurred during service. The cracking causes were analyzed by macroscopic observation, chemical composition analysis, hardness test, metallographic examination and scanning electron microscopy analysis. The results show that the carbon content of flange material exceeded the standard, a large number of carbides precipitated along the grain boundary, and the microstructure was sensitized. When S30408 stainless steel flange was butt welded with the pipe, its neck was heated, resulting in increased sensitization and grain boundary embrittlement. The shape of flange neck was abruptly changed, and the neck became the stress concentration area. Under the action of welding residual stress and working pressure, the neck gradually initiated intergranular microcracks. With the passage of time, the microcracks expanded and converged to form macroscopic cracks, resulting in flange penetration cracking.