The mechanical properties of materials are controlled by the microstructure which is obtained by an adequate heat treatment. This treatment determines a first time scale which, in the case of steels, is often too short to yield equilibrium. If the material is then subjected to long term service conditions at higher temperatures the microstructure changes, leading to a deterioration of the mechanical properties. It is thus important to know about the kinetics of these reactions.
Furthermore, it is by no means granted that only those phases have to be considered which are present at equilibrium. Metastable phases may kinetically be favoured in comparison to stable phases such that, at intermediate stages, metastable phases also appear in the microstructure. These phases will eventually disappear, but their temporary existence influences strongly the kinetics of formation of the microstructure. It is thus important to know about their occurrence and their life time in the microstructure.
The growth kinetics of stable and metastable phases in ferritic steels is treated with the program DICTRA using a multi-cell approach. The thermodynamic aspects of the growth conditions of the phases M23C6, M7C3, M3C, M6C and Laves phase are discussed with model alloys of Fe-Cr-C-X. With these simulations it is possible to provide quantitative information on the transformation kinetics, in particular for long term applications. The results show remarkable effects on the growth kinetics due to competition of stable and metastable phases during simultaneous growth.