Effect of Nb and Cu on the high temperature creep properties of a high Mn-N austenitic stainless steel

The effect of Nb and Cu addition on the creep properties of a high Mn-N austenitic stainless steel was investigated at 600 and 650 °C. In the original high Mn-N steel, which was initially precipitate-free, the precipitation of M23C6 (M = Cr, Fe) and Cr2N took place mostly on grain boudaries during creep deformation. On the other hand, the minor addition of Nb resulted in high number density of Z-phases (CrNbN) and MX (M = Nb; X = C, N) carbonitrides inside grains by combining with a high content of N, while suppressing the formation of Cr2N. The addition of Cu gave rise to the independent precipitation of nanometer-sized metallic Cu particles. The combination of the different precipitate-forming mechanisms associated with Z-phase, MX and Cu-rich precipitates turned out to improve the creep-resistance significantly. The thermodynamics and kinetics of the precipitation were discussed using thermo-kinetic simulations.