Microstructural Effects on Stress Corrosion Initiation in Austenitic Stainless Steel in PWR Environments

Abstract

Although service experience of austenitic stainless steels exposed to PWR primary coolant has been good, stress corrosion crack propagation has been observed in laboratory tests in the presence of 15% cold work. Data on crack initiation are much more limited and this study therefore aims to improve the understanding of the conditions under which crack initiation and subsequent development of stress corrosion cracking might be possible. Testing was performed on two heats of Type 304/304L stainless steel under slow strain rate tensile loading. A range of analytical techniques were used to characterize the resultant precursor features and cracking, and digital image correlation before and after testing was also used to evaluation the influence of localized deformation on SCC. The results indicate that crack initiation can occur in austenitic stainless steels exposed to good quality primary coolant under dynamic straining conditions; additional testing underway under more plant-representative conditions will be reported later. Significant influences of steel microstructure on crack initiation susceptibility were observed.

Oxidation and SCC Initiation Studies of Type 304L SS in PWR Primary Water

“…The material used in this investigation was not homogeneous, and exhibited meso-scale segregation bands. Such bands, which have been reported to be δ-ferrite free due to the locally elevated Mn and Ni content [21,22] also had an increased density of complex multiphase inclusions that appeared to react during the test. These complex inclusions appeared to be capable of initiating intragranular defects.…”

“…Subsequently, the material was forged to a 20% reduction in the temperature window 360-170°C to simulate the heat shrinkage that can occur in the heat affected zone of a weld. The same material was also used for the work reported in two other papers presented in this conference [21,22].…”

“…Furthermore, whilst the grain size in the δ-ferrite containing matrix was approximately 35 µm, a coarser grain structure was observed in the band with no δ-ferrite (Figure 2a). SEM-EDX microanalysis of the bands that contained no δ-ferrite were enriched in Ni and Mn [22], which would promote the austenite phase stability. Furthermore, in these δferrite free bands an increased proportion of inclusions was observed.…”

Oxidation and SCC Initiation Studies of Type 304L SS in PWR Primary Water

“…The material used in this investigation was not homogeneous, and exhibited meso-scale segregation bands. Such bands, which have been reported to be δ-ferrite free due to the locally elevated Mn and Ni content [21,22] also had an increased density of complex multiphase inclusions that appeared to react during the test. These complex inclusions appeared to be capable of initiating intragranular defects.…”

“…Subsequently, the material was forged to a 20% reduction in the temperature window 360-170°C to simulate the heat shrinkage that can occur in the heat affected zone of a weld. The same material was also used for the work reported in two other papers presented in this conference [21,22].…”