Interdiffusion in the type 316 austenitic stainless steel/iron system

Interdiffusion behaviour in the type 316 stainless steel/iron system in the temperature range 1023 K to 1373 K was studied using electron probe microanalysis, optical and scanning electron microscopy and X-ray diffraction techniques. Annealing of the stainless steel/iron diffusion couples at temperatures higher than 1163 K led to a smooth variation of concentrations of different elements across the interface. The interdiffusion coefficients obtained at different temperatures in the range 1223-1373 K decreased with increase in the chromium content at least up to 10 wt.%. For the range of chromium concentration 4-12 wt.%, the temperature dependence of the diffusion coefficients followed Arrhenius behaviour: both the frequency factor and the activation energy for diffusion increased monotonically with an increase in chromium concentration. Annealing of the diffusion couples at temperatures below 1163 K produced copious precipitates of the sigma phase in the stainless steel portion. While the number density of these precipitates decreased with the distance away from the interface, the mean size of the precipitates increased with the distance from the interface. The observed enhancement in the kinetics of the sigma-phase precipitation in diffusion couples is attributed to the vacancy wind effect. (orig.)