AbstractAn attempt has been made to model the kinetics of changes of composition in cementite during the aging of creep resistant low alloy steels of the type used in power plant. In the model a finite difference method is used for calculating the rate at which alloying elements redistribute between ferrite and cementite, subject to the thermodynamic constraints which determine the equilibrium compositions of these phases. It has also been possible to derive approximate analytical solutions which give good physical insight into the factors controlling the approach to equilibrium. Particle size has been found to have a strong influence on the rate at which the cementite composition is changed. A theoretical basis for the time dependence of cementite composition is also provided. The method is easily adapted to multiple or anisothermal heat treatments. Further work is suggested to account for simultaneous coarsening and changes in composition and also to allow for the formation of alloy carbides at long aging times.MST/874