English: Iron–carbon phase diagram, at atmospheric pressure

This diagram is limited by pure iron on the left and by iron carbide on the right. The mains phases are:

• iron: ferrite, ferritic steel
• iron: austenite, austenitic steel
• iron carbide: cementite, Fe3C.

It shows a eutectic and a eutectoid; these phases crystallise as a stacking of fine strips of pure phases (iron and cementite) in case of the eutectoid, or a pure iron containing small balls of cementite for the eutectic. Although it is heterogeneous, these phases behave like homogeneous pure bodies.

Steel is between 0 and 2.06 mass percent of carbon. Cast iron is between 2.06 and 6.67%.

Black lines indicate the metastable Fe/Fe3Cphase, red is the stable Fe/Carbon phase

This diagram is for illustrative purposes only, and is not thermodynamically accurate.