IASCC susceptibility of type 304, 304L and 316L stainless steels

Slow strain rate tests (SSRT) in simulated Boiling Water Reactor (BWR) environments were conducted to estimate Irradiation Assisted Stress Corrosion Cracking (IASCC) susceptibility of irradiated type 316L series (standard 316L and-its variations of which minor element content are different), type 304 and type 304L stainless steels. Type 316L series include some type 316L of law phosphorous (P) concentration, which were suggested to be better than standard type 316L in IASCC susceptibility and mechanical property in the previous reports. In this study; for the ratio of the intergranular area to the entire area of the fracture surface (IG ratio), type 304 showed 0%, whereas type 316L series showed 0 - 47%. The figures of type 316L series seemed to indicate whether each of them had IASCC susceptibility or not, although no significant relationship between IG ratio and P concentration was observed. It was a concern that 0% IG of type 304 did not correspond to its susceptibility. Type 304 was found to be hardened due to irradiation compared to type 316L series. It was possible that type 304 showed 0% IG fracture surface because it failed too early in SSRT to initiate SCC due to low deformability, whereas some of type 316L series lasted-longer and therefore showed higher figures. Therefore, it could not be said that type 304 had lower IASCC susceptibility than some of type 316L series merely because it showed 0% IG. Strain at crack initiation (SCI) for the specimens with IG fracture surface was proposed as an another measure for IASCC susceptibility. It was defined as the strain at which the stress-strain curve of SSRT began to depart from that of:tensile test (TT), when those were plotted using the same scales. SCI stood for the strain up to which the material could elongate without SCC. The higher SCI was, the less IASCC susceptibility the materials seemed to have. For the specimens of 0% IG, lower limit of SCI was estimated instead. In this paper, IASCC susceptibilities of the stainless steels were estimated using SCI. Type 316L series showed:lower IASCC susceptibility than 304L. In addition, it was found that the low P type 316L series had lower IASCC susceptibility than standard type 316L.