The Iron-Carbon Phase Diagram for the Solid State

The part of the binary Fe-C phase diagram that deals with the solid state is an impressive manifestation of the complexity of pure iron and the consequences of carbon additions. Its origins reveal a story that inspires today, how ferromagnetism caused confusion, and yet stabilizes the most important phase in the microstructure of colossal quantities of structural steels. There are unresolved issues—if graphite is ignored, is the stable phase field at ambient temperature defined by a mixture of cementite and ferrite? Some studies favor mixtures of iron carbides (χ, η, ǫ) and ferrite to represent equilibrium at low temperatures, but it is shown that this may not be correct. Should we limit ourselves to the body-centered cubic form of ferrite when interpreting equilibrium, or does the stability of mixtures of body-centered tetragonal ferrite and austenite take precedence when the lattice change is generated by the Bain deformation? A case is made for the routine inclusion of the T 0 curve on the phase diagram, on the basis that it is no different in its thermodynamic definition from the equilibrium phase boundaries of pure iron in a pressure-temperature plot.