The knowledge of activity of the carbon in liquid iron is one of the most fundamental problems of iron-and steelmaking. But accurate values in the high-carbon range are not easily obtainable. because of experimental difficulties. So a statistical thermodynamical method was developed to estimate the activities in the whole system on the basis of available data.
Previous works assumed the “interstitial” model for the molten Fe-C system by analogy to austenite. Comparing the previous theory with experimental values, an inconsistency of the theory was found.
Paying special attention to the characteristics of liquid, that is, the absence of a long-range orderedness in the arrangement of atoms, the “substitutional” model was assumed which was in good agreement with experimental values in the whole system.