L-grade stainless steels as 316NG, SUS316L and SUS304L are used for the BWR reactor internals and re-circulation pipes. The L-grade stainless steels are known for typical SCC resistant materials because they are hardly thermally sensitized in usual welding process due to its lower carbon contents. However SCC of the L-grade material components were reported recently. The detail investigation of the components showed that the hardness of the surface was very high where the SCC was observed. There was no grain boundary segregation of Chromium. Therefore the fabrication process such as welding, machining and surface finishing are strongly assumed to accelerate the SCC initiation.
The test samples were made from the mock up shroud to simulate the fabrication process especially welding and surface finish process and the SCC tests were conducted. The result shows that hard machining and grinder work to the surface could induce SCC to the L-grade stainless steel. The polishing, water jet peening and laser peening can improve the surface residual stress from tension to compression, thus the SCC initiation is suppressed. Hydrogen water chemistry (HWC) also reduces the susceptibility of SCC initiation.