The iron–carbon system is a central example of how composition and temperature control metallic microstructure.

For ordinary steels, the most useful part of the equilibrium diagram involves iron-rich compositions containing relatively small amounts of carbon.

Important phases

Ferrite is a low-carbon body-centred-cubic phase. Austenite is a face-centred-cubic phase that dissolves more carbon. Cementite, $\mathrm{Fe_3C}$, is a hard iron carbide.

Eutectoid transformation

At the eutectoid composition and temperature, austenite transforms into a mixture of ferrite and cementite. Under suitable cooling this mixture forms the lamellar microstructure called pearlite.

Steels below and above the eutectoid composition contain different proportions of proeutectoid ferrite or cementite in addition to pearlite.

Equilibrium versus real heat treatment

The equilibrium phase diagram explains which phases are favoured, but cooling rate matters. Rapid cooling can suppress equilibrium transformations and form metastable structures such as martensite.

The iron–carbon diagram therefore provides a framework for connecting steel composition and thermal history with mechanical properties.