Effect of martensite on SCC initiation in austentitc stainless steels in simulated PWR primary water environment

Metastable austenitic stainless steels are susceptible to martensitic transformations during cold-working; however, their implication on SCC initiation in PWR primary water is unclear. In this study, cold -rolling and warm-forging were applied to austenitic stainless steels type 304L and 316L in order to introduce different amounts of martensite. Multiple specimens were then extracted from these rolled/forged materials and tested under slow strain rate tensile (SSRT) test conditions in PWR primary water to assess their SCC initiation behaviors. These specimens were characterized prior to and after the tests, using SEM and EBSD. The results showed that a plastic strain of ~3% during SSRT testing induced extensive intergranular SCC initiation in the steels which did not contain martensite. However, cold-rolled 304L, which contained martensite, showed few SCC cracks even when strained to ~4.5% and only limited cracking occurred on specimens strained to ~5.8%. Post–test characterization revealed that extensive preferential oxidation of martensite occurred in the cold-rolled 304L whereas only intergranular oxidation was observed for the steels in which no martensite was detected. The correlation between martensite and oxidation behavior/SCC initiation is discussed.