Evolution of creep resistant 316 stainless steel for sodium cooled fast reactor applications

Sodium cooled fast reactors (SFRs) are designed to operate at high temperatures with an initial design life of about 40 years. Austenitic stainless steel (SS) types 304 and 316 and their variants have been generally used for out-of-core structural components of the reactor assembly system. The choice of these two grades of stainless steels is decided by several important factors such as high temperature mechanical properties like creep, low cycle fatigue and creep-fatigue interaction, compatibility with liquid sodium coolant, weldability, fabricability and cost. The components which operate in the creep temperature range are made of 316 SS. This material has been used extensively in the early SFRs. Studies on long term creep properties of 316 SS have clearly established the good creep resistance of this material and the microstructural stability at temperatures below 873 K. In view of the susceptibility of welded components to stress corrosion cracking, low carbon grades of 304 and 316 SS with alloying addition of nitrogen (designated as 304L(N) SS and 316L(N) SS) are used for structural components of later generation of SFRs. Nitrogen addition in the range of 0.06-0.08 wt% produces significant improvement in the creep properties of this material through solid solution strengthening and lowering of stacking fault energy. In view of the recent trends to increase the design life of SFRs to 60 years and more, it is necessary that non-replaceable structural components of reactor assembly have sufficient high temperature mechanical properties over such very long periods of operation. Increasing the nitrogen content from 0.06-0.08 wt % to levels of 0.12-0.14 wt% has been found to increase creep rupture life of 316LN SS by an order of magnitude. The beneficial effects of nitrogen are also extended to type 316 SS weld metal. This paper discusses the progressive improvements in the creep properties of 316 SS grade by varying the amounts of interstitial elements carbon and nitrogen.