Ferrite (iron)

Ferrite or alpha iron (α-Fe) is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material.

It can be considered as pure iron practically (strength = 280N/mm2). Ferrite can be strictly defined as a solid solution of iron in body-centered cubic (BCC) containing a maximum of 0.03% carbon at 723oC and 0.006% carbon at room temperature.

Most "mild" steels (plain carbon steels with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amounts of pearlite (a fine lamellar structure of ferrite and cementite) as the carbon content is increased. Since bainite (shown as ledeburite on the diagram) and pearlite each have ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if it is allowed to reach equilibrium at room temperature.

In pure iron, ferrite is stable below 910°C. Above this temperature the face-centered cubic form of iron, austenite (gamma-iron) is stable. Above 1390°C, up to the melting point at 1539°C, the body-centred cubic crystal structure is again the more stable form of delta-ferrite (δ-Fe).

Only a very small amount of carbon can be dissolved in ferrite; the maximum solubility is about 0.02wt% at 723°C. This is because carbon dissolves in iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes", so that each carbon atom is surrounded by a strong local strain field. Hence the enthalpy of mixing is positive (unfavourable), but the contribution of entropy to the free energy of solution stabilises the structure for low carbon content. 723°C also is the minimum temperature at which iron-carbon austenite (0.8 wt% C) is stable; at this temperature there is a eutectoid reaction between ferrite, austenite and cementite.