Phase Diagram of Fe–100% C Alloys. Part One. Basic Inconsistencies of Fe–Fe₃C Alloy Phase Diagram

The knowledge and control of phase transformations according to the Fe–Fe₃C alloy phase diagram form the theoretical, process, and physical chemical basis for metallurgy, metal science, and thermal treatment of steels and cast iron and allow producing essentially new alloys. At the same time, the diagram in question is far from complete—it is fragmentary and contains quite a number of inconsistencies which often contravene the basic rules and laws of the theory of phase diagrams for alloys. It has been established that the conventional Fe–Fe₃C alloy phase diagram has remained largely unchanged for more than 90 years and turned into a sort of research and teaching procedural cliché circulated without exception in both, training and research educational environments. The basic inconsistency of the Fe–Fe₃C diagram is that the diagram field contains two superimposed phase equilibrium systems, including the stable graphite equilibrium system and the metastable cementite equilibrium system, which is inconsistent with the basic provisions of the theory of phase diagrams for alloys and, most importantly, raises doubt about the correctness of this superimposition. The presence of stable (graphite) and metastable (cementite) equilibrium in a single diagram field expresses the contradictory and superficial comprehension of the structuring in the Fe–Fe₃C system and, as a consequence, breaks the rules of constructing phase diagrams. The article considers certain areas of theoretical calculations of the Fe–Fe₃C diagram on the basis of the thermodynamics of ferricarbonic alloys and constructing the iron–diamond diagram as well as the position of the cementite liquidus line in the region of cementite and in the hypercementite part. It is also shown that several authors actually make a revisionist reconsideration of the conventional provisions of the diagram in the form of impermissible theoretizing and alternative variants of the Fe–Fe₃C diagram, while ignoring the main laws of the theory of alloys, solid state physics, and physical material science.