Ferrite formation in a duplex stainless steel during controlled heating: X-ray diffraction and modeling using para-equilibrium diffusion kinetics

The kinetics of the austenite to ferrite (γ→δ) transformation in a 2205 duplex stainless steel during controlled heating is examined using a combination of experimental and modeling techniques. The model of the γ→δ transformation is based on the assumption that this transformation is driven by the diffusion of nitrogen under para-equilibrium conditions. This one-dimensional diffusion model uses a moving grid method to track multiple moving interfaces and is capable of calculating the nitrogen diffusion across several grains during heating. The effect of different starting microstructures on the transformation kinetics is simulated by considering two y and two δ grains of varying sizes consistent with those in the base metal. The model predictions are compared with those measured experimentally using an in-situ x-ray diffraction technique. In this experimental technique, high energy synchrotron radiation is used to monitor the phases present in real-time under controlled heating conditions at 25 K/s. Comparison between calculations and experiments indicates that the model predicts δ fractions up to 30% higher than those observed experimentally at the beginning of the transformation. The difference between the calculated and measured δ fractions then starts to decrease, and the completion time predicted using the model agrees reasonably well with that measured. The possible cause of such discrepancy is discussed. Transformation kinetics from three different starting microstructures is calculated using the diffusion model and the effect of hard and soft impingements on the transformation kinetics is investigated.