Ferrite (iron)

Ferrite, is a body-centered cubic (BCC, alpha iron) form of iron. It is this crystalline structure which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material.

It has a strength of 280 N/mm² and a hardness of approximately 80 Brinell.

Below 910 °C (1,670 °F) the body-center-cubic allotrope of pure iron is stable. Above this temperature the face-centred cubic allotrope of iron, austenite (gamma-iron) is stable. Above 1,390 °C (2,530 °F), up to the melting point at 1,539 °C (2,802 °F), the body-centred cubic crystal structure is again the more stable form, as delta-ferrite (δ-Fe). Ferrite above the critical temperature A₂ (Curie temperature) of 771 °C (1,044 K; 1,420 °F), where it is paramagnetic rather than ferromagnetic. The term is beta ferrite or beta iron (β-Fe). The term beta iron is not any longer used because it is crystallographically identical to, and its phase field contiguous with, α-Iron.

Only a very small amount of carbon can be dissolved in ferrite; the maximum solubility is about 0.02 wt% at 723 °C (1,333 °F) and 0.001% carbon at 0 °C (32 °F). 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 at low carbon content. 723 °C (1,333 °F) 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.

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