The influence of nitrogen on the austenite/ferrite interface mobility in Fe–1at.%Si

The relation between the nitrogen content and the mobility of the austenite/ferrite interface during the austenite to ferrite transformation have been investigated. Samples of an Fe–1at.%Si alloy were given various internal nitriding treatments, leading to nitrogen concentrations between 20 and 3000 appm. Dilatometric measurements during cooling indicate a clear shift in the transformation curves. The transformation is shifted to lower temperatures and is slowed down at higher nitrogen contents. The transformation kinetics were modelled using an interface mobility model in combination with a tetrakaidecahedron as a representation of the austenite grain. For nitrogen concentrations smaller than 500 appm there is a good agreement between experimental and modelled fraction curves, using an exponential relation for the nitrogen dependence of the pre-exponential mobility factor, M 0. For higher nitrogen levels, M 0 decreases less rapidly. Moreover, only the first stage of the ferrite formation is accurately modelled. Towards the end of the transformation the discrepancy between experimental and modelled fraction curves increases. This suggests that for the determination of the driving force in the case of nitrogen concentrations larger than 500 appm long range diffusion of nitrogen should be taken into account.