Thermodynamic modeling of carbon diffusion in manganese steels

Decarburization of steels is a diffusion-controlled thermochemical process that is largely dependent on the process conditions, such as the process atmosphere and the reaction temperature, as well as the composition of the steel alloy. When modeling such diffusion processes, Fick´s law is commonly used that is based on a concentration gradient as the driving force for diffusion. However, this approach holds only for “ideal mixtures”. Steel alloys are far away from ideal mixtures, for which complete mutual solubility of all components without formation of intermetallic phases are necessary requirements. In order to overcome this problem, a thermodynamic approach for describing the diffusion is proposed, which takes into account the real mixing behavior of steels. Especially, the thermodynamic modeling of carbon diffusion in the martensitic hardenable tool steel 22MnB5 compared to a binary Fe-C steel is presented. Results are discussed with regard to the necessary process conditions for successful edge area decarburization of this steel grades.