σ-PHASE PRECIPITATION IN STABILIZED AUSTENITIC STAINLESS STEELS Original Research Article

Experimental observations of σ-phase precipitation in two stabilized austenitic stainless steels, AISI 321 and AISI 347, aged up to 80 000 h at temperatures between 500 and 800°C, are compared with the results obtained from a simple model of the process. The model is based on the assumption that diffusion of alloying elements from inside austenite grains to σ-phase particles at grain boundaries limits the rate of formation, and this is supported by the presence of zones depleted in chromium at grain boundaries revealed by TEM/EDS. The full multicomponent thermodynamic behaviour of the system is taken into account using the DICTRA software and the influence of grain shape is discussed. The predictions of the model are in fair agreement with experimentally estimated volume fractions of σ-phase.