High temperature design curves for high nitrogen grades of 316LN stainless steel

Nitrogen alloyed low carbon grade 316L(N) stainless steel (SS) is a major structural material for high temperature structural components of sodium cooled fast reactors. With a view to significantly enhance the high temperature mechanical properties of 316L(N) SS and thereby increase the design life of structural components from 40 years to 60 years, the influence of nitrogen content on the tensile and creep properties of this steel has been investigated. Four heats of 316LN SS with 0.07, 0.11, 0.14, and 0.22 wt.% nitrogen were used in this investigation. Tensile tests were carried out at various temperatures between room temperature and 850 deg. C. Creep tests were carried out at 650 deg. C at various stress levels in the range of 140-225 MPa. The maximum rupture life in these tests was 16,000 h. The tensile and creep data were analysed according to RCC-MR nuclear code procedures and the design curves have been generated. The tensile and creep strength of 316L(N) SS have been found to improve significantly by increasing the nitrogen content.