关键词: 304不锈钢, 氯化物应力腐蚀, 焊接残余应力, 晶界敏化

Abstract: A crane pipe elbow broke after being idle for 2 years at the seaside loading and unloading yard. In order to find out the root cause of the elbow cracking, its macroscopic morphology is carefully studied and the fracture, microstructure and chemical composition are analyzed. The analysis results indicate that there's a large amount of chlorine at the fracture of the elbow and the fracture is intergranular cracking; there's welding residual stress at the source of the crack. Further force analysis and calculation on it reveals that the outermost side of the crane pipe elbow generates tensile stress under its own weight. However, this stress value is much less than the yield strength of 304 stainless steel and the empirical value of weld residual stress (which is generally greater than 50% of the yield strength). Therefore, it is concluded that the reason for the fracture of the elbow is that the grain boundaries of 304 stainless steel sensitization occurs due to the presence of chloride ions, and eventually chloride stress corrosion cracking occurs under the action of welding residual stress. According to the cracking causes, this paper puts forward the corresponding countermeasures.

Key words: 304 stainless steel, chloride stress corrosion, welding residual stress, grain boundary sensitization