Sigma-phase formation in Nitronic 50 and Nitronic 50W stainless steels

Abstract

The formation of the sigma-phase has been studied in two heats of a nitrogen-strengthened austenitic stainless steel, Nitronic 50 and Nitronic 50W, a weld filler wire composition. It was found that the presence of cold-work greatly enhanced the rate of sigma precipitation in the temperature range of 600–1000° C. The nucleation and growth of the sigma-phase was accompanied by recrystallization of the deformed austenite. The final austenite grain size was generally very small (0.5–4μm), since grain growth was inhibited by the intergranular sigma precipitates. Precipitation of M₂₃C₆ carbides was suppressed by the rapid formation of the sigma-phase. In samples solution annealed prior to ageing, the kinetics of sigma precipitation were much slower, and for a heat containing 0.04wt% carbon (Nitronic 50W), the sigma precipitation was retarded by the formation of intergranular M₂₃C₆. Grain boundaries in solution annealed and aged samples of both alloys also contained Z-phase (NbCrN) precipitates, which formed in the range of 700–1000° C. Retained delta-ferrite in the Nitronic 50W heat transformed directly to sigma-phase during ageing of the cold-worked specimens, but under-went more complex reactions in the solution annealed plus aged samples. Precipitation sequences at the ferrite-austenite interfaces in these latter specimens varied with ageing temperature. Sigma-phase chemistries in all samples were determined in the analytical electron microscope. In the solution-annealed and aged specimens, the sigma-phase chemistries were generally not of equilibrium values, and a wide variation in sigma composition was seen in each sample. In the cold-worked and aged samples, equilibrium sigma chemistries were attained. On comparing data from different samples, compositional changes in the sigma phase were observed, with molybdenum increasing and chromium decreasing as ageing temperatures increased.