Abstract: 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.