The iron–carbon phase diagram

Supplemental material for the Quick Study "The physics in your fork" PHYSICS TODAY, May 2007, page 88.

Iron-Carbon Equilibrium Phase Diagram

Carbon is almost insoluble in ferrite; its maximum concentration peaks at only 0.0218% by weight at a temperature of 727 °C. But austenite containing 0.77% dissolved carbon by weight is stable at that temperature. That strong dependence of solubility on crystal structure leads to interesting phase separations in steel.

Below 727 °C, all but the smallest quantity of carbon is accommodated as cementite (iron carbide) so that at equilibrium, cool carbon steel consists of phase-separated ferrite and cementite. Above 727 °C, some or all of the carbon can be accommodated in austenite. Steel with the eutectoid composition, 0.77% carbon by weight, becomes pure austenite above 727 °C.

Carbon steel with less than the eutectoid composition—hypoeutectoid steel—has too little carbon to become pure austenite at 727 °C. So it forms a phase-separated mixture of ferrite (the carbon-poor phase) and austenite (the carbon-rich phase). Above the eutectoid composition, hypereutectoid steel has too much carbon to become pure austenite at 727 °C, so it phase separates into austenite (carbon poor) and cementite (carbon rich).

To harden steel, it must be heated until it is fully austenized. That transformation is completed just above 727 °C for eutectoid steel, while both hypo- and hypereutectoid steels require higher temperatures. Quenching then destabilizes the austenite and leads to steel consisting of nonequilibrium composition; in particular, quenching allows martensite to form.