CHRACTERISTICS OF WORK HARDENED SURFACE LAYER ON AUSTENITIC STAINLESS STEELS AND ITS RELATION TO SCC SUSCEPTIBILITY IN HIGH TEMPERATURE WATER

摘要

It is widely accepted that cold worked austeniticstainless steels have shown high stress corrosioncracking (SCC) susceptibility in both actual nuclearpower plants and experimental studies. However, thedominant factor in the work hardened surface layerfor SCC initiation is not fully understood. In this study,detailed characterizations of the work hardenedsurface layers formed on type 316L stainless steelswith different surface finish methods (grinder, emerypaper, and colloidal silica) were carried out usingtransmission electron microscopy (TEM) and anelectron backscatter diffraction (EBSD) technique.The mechanistic relationship between thesecharacteristics and the SCC susceptibility in hightemperature water was also investigated. Theultrafine-grained layer and plastic flow region wereclearly formed on the top surface, even by the emerypaper finish, which is considered to be a relativelymild surface finish method. There is no significantdifference in the top surface microstructure betweengrinder and emery paper finished specimen. However,for the grinder finished specimen, a highly deformedzone corresponding to the hardened layer wasobserved. The EBSD analysis revealed a significantlocal strain accumulation in the grinder finishspecimen, while the local strain concentration wasvery limited in the other two specimens. Only thegrinder finished specimen showed significant IGSCCsusceptibility. The results suggest that an ultrafinegrainedlayer and a plastic flow region should not bedetrimental to SCC resistance. The significant localstrain accumulation near the grain boundary justbeneath the recrystallized layer can strongly affectSCC susceptibility from the microstructural point ofview.