If the different components in a compound material are chemically incompatible the heat treatment of such a material will present some difficulties. There may be strong driving forces for the transfer of atoms by means of diffusion between the two component materials. This transfer may result in a drastic change in the properties. Here we will present a detailed study of diffusion in a compound steel by means of numerical solution of the diffusion equations. We will also present a more approximate calculation where the mobile carbon is equilibrated between the steels and the less mobile substitutional elements are only equilibrated within each steel but not between steels. Thus, complete equilibrium is established within each steel. In the latter calculation both steels are multicomponent and multiphase materials.