The Iron-Carbon Phase Diagram (One Minute Mentor)

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The Iron-Carbon Phase Diagram

The basis for understanding the heat treatment of steels is the iron-carbon (Fe-C) phase diagram. The Fe-C diagram is really two diagrams in one, showing the equilibrium between cementite (iron carbide, or Fe3C) and the several phases of iron, as well as the equilibrium between graphite and the other phases. Steels are alloys of iron, carbon and other elements that contain less than 2% carbon (usually less than 1%), therefore the portion of the diagram below 2% C; that is, the iron-cementite (Fe-Fe3C) diagram, is more pertinent to steel heat treatment. In cast irons, high carbon content (1.75-4.0% C) and high silicon content promote graphite formation. Therefore, cast iron technology is based more on the Fe-graphite diagram.

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