Evaluation of the effect of nitrogen on creep properties of 316LN stainless steel from impression creep tests

Impression creep testing technique is an innovative tool to study creep deformation behavior of materials mainly because it requires relatively short test time and small volume of material for evaluating the creep properties when compared with conventional uniaxial creep testing which is both material and time consuming. The effect of nitrogen on creep deformation behavior of type 316LN stainless steel has been studied using impression creep testing technique. Impression creep tests have been carried out at 923 K up to 1000 h on 316LN stainless steel containing 0.07, 0.11, 0.14, and 0.22 wt.% nitrogen, in the punching stress range of 400–800 MPa. The impression creep curves were characterized by a loading strain, a primary creep stage, and a secondary creep stage similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep tests was absent. The steady state impression velocity was found to increase with increasing punching stress. The calculated stress exponent values varied between 3.3 and 8.2 with increasing in nitrogen content. It was observed that impression velocity was sensitive to the variation in nitrogen content in the steel. The steady state impression velocity was found to decrease with increasing nitrogen content. Correlation between the impression creep parameters and conventional uniaxial creep parameters has been established based on the laws of mechanics for time dependent plasticity.