溶体化316系ステンレス鋼の高温水中応力腐食割れ感受性に対する加工方向の影響

Abstract

Recently, stress corrosion cracking (SCC) was observed in L-grade (low carbon) stainless steel (SS) components such as core shrouds and primary loop recirculation pipes in boiling water reactors (BWR). Since a shroud was fabricated from several parts machined from thick plates and connected by welding process, both the hot rolled surface and the cross section were exposed to the high temperature water environment. Hence, SCC susceptibility should be examined after considering the hot rolling direction during the production process of SS thick plates. Creviced bent beam test was conducted on type 316L and type 316 SS plate specimens machined from the longitudinal/transverse (LT), the transverse/short-transverse (TS) and the longitudinal/short-transverse (LS) planes. Each specimen was cold rolled by 10% to 30% just before machining to the final shape. After exposure to the test environment for 1000 h, each specimen was observed by a scanning electron microscope to measure SCC length on the surface. SCC susceptibility was higher in the LS and TS planes than the LT plane for both type 316L and type 316 SS, although the susceptibility of type 316L SS was much lower than type 316 SS. The SCC length conformed to the log-normal distribution irrespective of the cold rolling ratio and, hence, a statistical analysis could be applied by assuming the distribution when a life prediction of SS components would be necessary.