Effects of Nitrogen and Copper on Creep Properties of 316L-based Stainless Steel

Abstract

Type 316L stainless steel is widely used as the bipolar plate in MCFC fuel cells due to its excellent high temperature performance. Although 316L steel is able to survive up to 900oC against oxidation，the relatively weak creep resistance limits the service time inadequate to 40,000 operative hours. In order to promote high strength austenitic stainless steel, Cu-added and nitrogen contained type 316L steels are selected in this study. Tensile properties at elevated temperatures and creep properties at 650oC under various stresses of the selected steels were studied. OM, SEM, TEM and STEM accompanying with thermodynamic calculations have been used to investigate the precipitation behavior of Cu-rich phase and secondary phases such as the M23C6 carbide, intermetallic sigma (σ), Laves (η) and chi (χ) phases. Cu has been used as a strengthening element in austenitic stainless steel by solid solution strengthening and Cu-rich phase precipitation hardening. Cu has a modulus strengthening effect due to the different modulus between Cu atoms and the Fe matrix. During the creep tests at 650oC, Cu-rich precipitates nucleate and distribute homogeneously within the matrix. The precipitation of Cu-rich phase is a gradual compositional change process without crystallographic structure transformation due to the same fcc structure and close lattice parameter of the Cu-rich phase and the austenitic matrix. High density Cu-rich precipitates remain fine in grains after long exposure at high temperature, leading to an increase of precipitation hardening. Nitrogen alloyed steel showed a remarkable higher creep rupture time under all the stress levels with respect to the nitrogen concentration. The improvement in creep properties is attributed to the combined influence of grain boundary M23C6 precipitation retardation and strengthening arising from the nitrogen in solid solution. Nitrogen delays the formation and decreases the coarsening rate of the grain boundary precipitates which causes the grain boundary embrittlement. The strengthening effects also come from the dislocation drag resulting from nitrogen being carried along and the nitrogen-chromium complexes/clusters which hinder dislocation motion. No nitride precipitation was found in this study