Precipitation reactions caused by nitrogen uptake during service at high temperatures of a niobium stabilised austenitic stainless steel

Abstract

Precipitation phenomena in type 347 austenitic stainless steels have been investigated after long-term heat treatment and creep in air at 700 and 800 °C. Nitrogen uptake was observed during long-term creep testing at 800 °C. No such effect was observed at 700 °C although times up to about 70,000 h were used. The major phases precipitated after long time exposure at 800 °C were primary Nb(C,N), Z-phase, Cr 2N and M 23C 6, while primary Nb(C,N), secondary Nb(C,N) and σ-phase were the major phases at 700 °C. Z-phase precipitated in both intragranular and intergranular form at 800 °C. Large precipitates exhibiting a core/rim structure showed a rim of Z-phase surrounding undissolved primary Nb(C,N). The microstructural evolution during creep deformation in air at 800 °C was modelled thermodynamically. The model satisfactorily predicts nitrogen uptake and the essential features of the evolution of the microstructure with time. The precipitation sequence could be qualitatively described, although it was not possible to model the formation of all precipitates.