Simultaneous oxidation and sigma-phase formation in a stainless steel

Oxidation of 347 austenitic stainless steel and the simultaneous formation of σ phase have been investigated, in the temperature range 650 °C to 816 °C. Oxidation rates for both 0.09-mm foil and 2.29-mm sheet were found to obey parabolic kinetics in the early stages of growth. However, significant departures from parabolic growth leading to accelerated oxidation occurred at 816 °C, after about 2500 hours in the thin foil and 10,000 hours in the thicker sheet material. Sigma-phase precipitation occurred throughout the temperature range, with a pronounced precipitate-free zone (PFZ) near the metal surface. The PFZ (which occurred as a result of Cr depletion) increased in width as a function of time and temperature. This observation was taken as evidence that the surface oxidation reaction was not limited by the Cr depletion associated with the formation of σ phase. Mathematical modeling of the Cr content at the metal/oxide interface together with an experimental observation of a “breakaway” condition of accelerated oxidation (at high temperature) was used to predict the oxidation-limited lifetime for the 0.09-mm foil at lower temperatures. Calculated Cr-depletion profiles, which corresponded closely with experimental data, resulted in depletion profile widths that corresponded closely with those for the σ-phase PFZs.