A Comparison of Creep Deformation and Rupture Behaviour of 316L(N) Austenitic Stainless Steel in Flowing Sodium and in Air

Abstract

Abstract Type 316L(N) austenitic stainless steel is used for the fabrication of Proto-type Fast Breeder Reactor (PFBR) under advanced stage of construction at Kalpakkam, India. The influence of flowing sodium on creep rupture behaviour of the steel has been investigated and results are compared with those obtained on carrying out creep test in air environment. Creep test on the material both in flowing sodium and in air were carried out at 873 K over a stress range of 225- 305 MPa. Sodium velocity across the creep specimen was maintained around 2.5 m/s. The testing environment was found not to change the rate of steady state creep deformation significantly. The tertiary stage of creep deformation of the steel was found to change significantly by the testing environment. The tertiary stage of creep deformation in the steel started much later for testing in sodium environment than that in air environment. The steel possessed higher creep rupture life for testing in liquid sodium environment that in air environment. Higher creep rupture strength of the material in flowing sodium was companied with higher creep rupture elongation. Optical micrographic investigation of the creep ruptured specimen in air showed extensive intergranular creep cavitation both in interior as well as on the specimen surface, whereas specimen tested in sodium showed relatively less creep cavitation. Almost no oxidation was observed on the specimen surface creep tested in flowing sodium and also no evidence of surface damage due to possible carburization and decarburization was noticed. SEM fractrographs of the creep ruptured specimens tested in flowing sodium showed predominantly ductile dimple failure whereas predominantly intergranular creep failure was observed in the creep ruptured specimen tested in air.