Effect of sodium environment on creep and fatigue properties of FBR Grade Type 316 stainless steel
Furukawa Tomohiro; Yoshida Eiichi; Komine Ryuji; Kato Shoichi
Abstract:To clarify the effect of sodium environment on mechanical properties of FBR Grade Type 316SS (316FR), creep and fatigue tests were carried out both in sodium and in air at elevated temperatures. The creep strength in sodium was slightly lower than that in air at the stresses corresponding to the rupture time of 1 000 to 10 000 hours. At the lower stresses the strength was equivalent in both the environments. The fatigue strength in sodium was higher than that in air at total strain ranges above 0.5%, and was equivalent to that in air at the strain ranges below 0.5%. Metallurgical examinations showed that the strength reduction in sodium was caused by the precipitation of s-phase on grain boundaries close to the surface of a specimen, and that in the case of creep tests at the lower stresses, rupture in sodium was caused by grain boundary cracks in the bulk of a specimen. In the main loading mode of FBR's, which is creep-fatigue loading at low stress levels with long hold periods, the mechanism of failure is considered to be similar to that of creep at low stress levels rather than creep at high stress levels or fatigue. Therefore, it was considered that the sodium environment has limited effects on the creep-fatigue strength of 316FR in FBR's. This was verified by the fact that internal grain boundary cracks were also found in the specimens that were subjected to creep-fatigue tests in sodium. Key Words:316FR, Stainless steel, Sodium, Creep strength, Fatigue strength, FBR