Notch creep rupture strength of 316LN SS and its variation with nitrogen content

Low carbon, nitrogen-alloyed, 316L(N) stainless steel (SS) is a major structural material for high temperature components of sodium cooled fast reactors. With a view to increase the design life from 40 to 60 years and beyond, studies are being carried out to develop 316LN grade austenitic stainless steels with superior high temperature mechanical properties. As a part of this development programme, four laboratory heats of 316LN SS containing 0.07, 0.11, 0.14 and 0.22 wt.% nitrogen are being evaluated extensively. Creep tests have been carried out on smooth and notched sample geometries at a nominal stress level of 200 MPa and at a temperature of 923 K. The notched specimens contained a V-notch at an angle of 60° with root radius of 0.19 mm, which provided theoretical stress concentration factor of 4.2. The gauge diameter of the notched specimen at the notch root was 6.4 mm and was 10 mm for the smooth specimen. It was found that the presence of notch increased the creep life for all the four heats. In the case of smooth specimens, rupture life increased with increase in nitrogen content. In the case of notched specimens, rupture life showed a peak value at 0.14 wt.% nitrogen content. The ratio of rupture life of smooth to notched specimens decreased with increase in the nitrogen content from 18 for the material containing 0.07 wt.% nitrogen to 3 for the material containing 0.22 wt.% nitrogen thereby implying that the notch strengthening effect decreased significantly at high nitrogen levels. The notched specimens showed a lower value of ductility, measured in terms of reduction in area, as compared to the smooth specimens. Finite element analysis of the stress and strain distribution in the notch region was carried out as a function of nitrogen content. The stress concentration factor (Kt) evaluated through stress analysis was found to be 4.2 for the geometry used in this study. The creep analysis was run for a sufficiently long time to achieve the steady state creep condition. The influence of nitrogen content on the values of triaxial stresses under steady state conditions for the given notch geometry was evaluated. The presence of notch with the investigated notch geometry lowered the maximum principal stress and this lead to higher rupture life in all the four heats.