Chracteristics of work hardened surface layer on austenitic stainless steels and its relation to SCC susceptibility in high temperature water

It is widely accepted that cold worked austenitic stainless steels have shown high stress corrosion cracking (SCC) susceptibility in both actual nuclear power plants and experimental studies. However, the dominant factor in the work hardened surface layer for SCC initiation is not fully understood. In this study, detailed characterizations of the work hardened surface layers formed on type 316L stainless steels with different surface finish methods (grinder, emery paper, and colloidal silica) were carried out using transmission electron microscopy (TEM) and an electron backscatter diffraction (EBSD) technique. The mechanistic relationship between these characteristics and the SCC susceptibility in high temperature water was also investigated. The ultrafine-grained layer and plastic flow region were clearly formed on the top surface, even by the emery paper finish, which is considered to be a relatively mild surface finish method. There is no significant difference in the top surface microstructure between grinder and emery paper finished specimen. However, for the grinder finished specimen, a highly deformed zone corresponding to the hardened layer was observed. The EBSD analysis revealed a significant local strain accumulation in the grinder finish specimen, while the local strain concentration was very limited in the other two specimens. Only the grinder finished specimen showed significant IGSCC susceptibility. The results suggest that an ultrafine-grained layer and a plastic flow region should not be detrimental to SCC resistance. The significant local strain accumulation near the grain boundary just beneath the recrystallized layer can strongly affect SCC susceptibility from the microstructural point of view.