Iron–carbon phase diagram

The iron–carbon phase diagram maps stable phases and phase transformations in iron–carbon alloys against carbon content and temperature. Its metastable iron–iron carbide version underpins most descriptions of steel and cast iron.

Ferrite: Ferrite is a soft, body-centered cubic solid solution of carbon in alpha iron. This low-carbon phase occupies a major region of the diagram below the eutectoid temperature.
Phase diagram: A phase diagram displays stable phases or phase assemblages as functions of variables such as temperature, pressure, and composition. The iron–carbon map applies this general tool to alloy composition and temperature.
Steel: Steel is an iron-based alloy containing carbon, usually below about 2.1 weight percent, with possible additional alloying elements. Its compositions lie on the steel side of the diagram’s conventional dividing line.
Stable iron–graphite phase diagram: The stable iron–graphite phase diagram describes equilibrium with graphite as the carbon-rich phase instead of cementite. It differs from the commonly used metastable diagram by treating graphite as the stable carbon phase.
Austenite: Austenite is a face-centered cubic solid solution of carbon in gamma iron. Its larger carbon solubility explains why heating can dissolve carbon before cooling transformations.
Alloy: An alloy is a material containing a metal combined with one or more other elements. Iron–carbon compositions are alloys whose carbon content sets their position along the horizontal axis.
Cast iron: Cast iron is an iron–carbon alloy with carbon content generally above the steel range, often containing substantial silicon. Its higher carbon content places it near the eutectic region and promotes casting behavior.
Graphite: Graphite is a crystalline form of carbon with layers of hexagonally arranged atoms. It is the stable carbon phase in the equilibrium iron–carbon system, although cementite often forms in practice.
Cementite: Cementite is the hard, brittle iron carbide compound Fe₃C. It is the carbide phase in the metastable diagram used for steels and cast irons.
Weight percent: Weight percent is a concentration measure equal to a component’s mass divided by total mass, multiplied by one hundred. The diagram’s carbon axis is conventionally expressed as weight percent carbon.

Linked from 13 pages