Thermodynamics of stable and metastable structures in Fe–C system

The thermodynamic properties and the phase diagram of the Fe–C system are reviewed by means of the CALPHAD method and Gibbs energy functions valid from 0 K upwards are presented. The Fe–C system has been evaluated previously by Gustafson. The information on thermodynamic properties and phase equilibria have now been updated and used as a basis to re-optimize the model parameters. In addition, thermodynamic properties of metastable cementite, Hägg and eta carbides are evaluated on the basis of available experimental data and taking into account the magnetic nature of these carbides. Moreover, a model is proposed for carbon ordering phenomena in martensite. Structural changes during early stages of aging of martensite are described using the proposed model and tempering equilibria with cementite, Hägg, and eta carbides are well reproduced. It should also been mentioned that the present description represents experimental data on the equilibrium with the liquid better than Gustafson׳s thermodynamic description.