Diffusion and phase transformations in steels: A tool for experimentation and a method for modelling

The contribution of diffusion couples to the study of kinetic transitions in phase transformations is outlined. The use of diffusion equations for modelling the kinetics of the austenite to ferrite transformation in steels is demonstrated, with a special emphasis on ternary system and the specific question of the interfacial conditions for the substitutional element. A kinetic transition from paraequilibrium to LENP interfacial conditions is demonstrated, and a model is presented allowing to describe quantitatively in Fe-Ni-C alloys both the classical isothermal experiments , and the decarburization kinetics. A classification of alloying elements in steels is provided, according to their interaction with carbon, and to their ability to slow down moving interfaces via a solute drag effect.