Evaluation of crevice stress corrosion cracking of austenitic stainless steels in oxygenated high temperature water

Abstract

Socket welds are commonly used in small bore pipes to connect valves, elbows, and flanges in various piping systems in pressurized water reactors (PWRs). During the operation, these welds are subjected to high cyclic fatigue, which may result in fatigue cracking and leakage of primary system coolant. For a temporary measure, an installation of a clamping device surrounding the damaged area is being considered to retain the leakage until the permanent repair by weld overlay or replacement of damaged components. Once clamps are installed, crevices and local tensile stresses would be developed in the bolted region. Therefore, susceptibility to stress corrosion cracking (SCC) of the clamp materials such as types 304, and 316 austenitic stainless steels should be properly evaluated. In this study, crevice bent beam (CBB) tests were performed in simulated pressurized water environment containing up to 8ppm of dissolved oxygen to simulate the anticipated conditions in clamping devices where the trapped air in the clamp could be dissolved into the leaked water. According to the analysis, the results shows high susceptibility to SCC in case of SS304 compared to SS316 that shows efficient resistance even in the oxygenated PWR water. Martensite transformation was found to be a main factor to SCC initiation and propagation in SS304 which showed high susceptibility to such a transformation. The crack propagated both IGSCC and TGSCC, where most of TG cracks branched from IG cracks were associated with the presence of martensites which could have been formed under tensile strain.