In-situ XRD investigation of σ phase precipitation kinetics during isothermal holding in a hyper duplex stainless steel

Abstract

The precipitation kinetics and formation mechanisms of σ phase during isothermal holding of a hyper duplex stainless steel in the temperature range from 750 °C to 1025 °C were studied by means of in-situ high energy X-ray diffraction experiments. Rietveld analysis of the data determined the nose temperature as 900 °C where a σ phase fraction of 1 wt% is attained after 15 s. The measurements revealed a distinct change from initial interface controlled nucleation of σ phase to a later stage of diffusion controlled growth during holding at 1000 °C after the σ phase fraction reaches half its final value. At lower holding temperatures the change from interface control to diffusion control happens more gradual and at later stages of the transformation. At these lower temperatures σ phase precipitation is accompanied by the formation of secondary austenite. Changes in σ phase formation mechanisms and kinetics were correlated with variations in the morphology of the precipitates through SEM analysis of quenched samples. Isothermal holding at 1000∘C and 1025 °C as well as above the solvus temperature of σ phase at 1120 °C leads to local fluctuations of austenite and ferrite phase fractions and the concomitant movement of austenite/ferrite phase boundaries even when no overall change in the phase fractions occurs.