Failure Analysis of a Cracked Stainless-Steel Steam-Water Separator

Currently, austenitic stainless steel has been widely used for the pressure boundary, including reactors, separators and storage tanks serviced in energy, petrochemical, chemical and food industries in view of its inherent corrosion resistance. However, the corrosion resistance may deteriorate under some circumstances such as field welding and inappropriate post-weld heat treatment. A steam-water separator serviced in a power plant was found cracking and a large amount of steam leaked outside. The cracking was located in the heat-affected zone (HAZ) of the joint on the head side of the pressure vessel. The material of the head was SUS 304 austenite stainless steel. Failure analysis was conducted to investigate the cause of cracking. The testing and measurement included chemical composition analysis, metallographic examination, fracture surface observation and deposit elements analysis. Results showed that the cracking was intergranular and stress corrosion cracking (SCC) was the primary cause of failure. During the fabrication of the separator, the HAZ of the joint was overheated by the thermal input of welding. Brittle carbides such as M23C6 precipitating at the grain boundary, resulted in a narrow belt lack of chromium nearby known as sensitization. The corrosion resistance of the austenite stainless-steel decreased obviously there, and cracking failure occurred rapidly under tensile stress. The influencing factors discussed in this paper mainly focused on material performance, post-weld heat treatment, and corrosivity of medium. Austenitic stainless steel containing stabilizing elements or with low C content was recommended for the new vessel design in order to avoid similar cracking failure.