In order to evaluate the SCC susceptibility of cast austenitic stainless steels which are often used for the main coolant piping of pressurized water reactors (PWRs), the slow strain rate test (SSRT) and the constant load test (CLT) of materials were performed in simulated primary water at 360°C. The cast austenitic stainless steels contain ferrite phase ranging from 8 to 23 % and its mechanical properties are affected by long time thermal aging. Therefore, the influence of the ferrite content and thermal aging on the SCC susceptibility of this stainless steel was evaluated with three materials ranging ferrite contents of 23%, 15% and 8%.
The reduction in area observed by the SSRT in simulated primary water at 360°C was smaller than that obtained by the tensile test in air at the same temperature. The SCC susceptibility was evaluated by reduction ratio defined by the ratio of the reduction in area by the SSRT to that by the tensile test. The reduction ratio was not clear for low ferrite specimens, but apparently decreased with increasing aging time for the specimen with 23% ferrite. This change by aging time can be explained as follows: (1) the brittle fracture in the unaged specimens is mainly caused by quasi-cleavage fracture in austenite phase. (2) After aging, it becomes a mixture of quasi-cleavage fracture in both austenite and ferrite phases and phase boundary fracture of both phases. INTRODUCTION
Only a few researchers have studied SCC of austenitic stainless steel in high-temperature and high-pressure water with dissolved hydrogen. Totsuka et al. (1~ and Smialowska et al. (2~(3~ mention that austenitic stainless steel has lower susceptibility to cracks than does Alloy 600 but shows similar cracking behavior in their tests. As for SCC of stainless steel in high temperature water with high concentrating of dissolved oxygen, Tsubota et al. (4) have reported that the SCC susceptibility increases with increasing material hardness. According to Speidel (5~, crack propagation rates in stainless steel increase with increasing hardness of the steel induced by cold working. General Electric's research group has also reported that stainless steel has SCC susceptibility in deaerated and hydrogenated water environment, indicating the necessity for re-evaluating the conventional knowledge on the SCC susceptibility of stainless steel, especially severally cold worked material, in de-oxygenerated high-temperature and high-pressure water.
The cast austenitic stainless steels, which are often used for primary coolant piping of pressurized water reactors (PWRs) are also the austenitic stainless steels. But, little is known about the SCC susceptibility of the cast austenitic stainless steels in PWRs primary water, especially the influence of ferrite content and thermal aging. Therefore, in order to evaluate the SCC susceptibility of cast austenitic stainless steels, the slow strain rate test (SSRT) and the constant load test (CLT) of the materials were performed in simulated primary water at 360°C. The materials of main coolant piping contain ferritic phase ranging from 8 to 23 % and its mechanical properties are affected by long time thermal aging. Therefore, the influence of the ferrite content and thermal aging on the SCC susceptibility of the cast austenitic stainless steels and prepared three kinds of castings with different ferrite contents (23%, 15% and 8%). In this present study, the SCC susceptibility in a simulated PWR primary water of cast austenitic stainless steels were evaluated using unaged and aged ferrite-containing cast austenitic stainless steels.
