Ferrite

Units

Ferrite, also known as α-ferrite (α-Fe) or alpha iron, is a materials science term for pure iron, with a body-centered cubic B.C.C crystal structure. 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.

Mild steel (carbon steel 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 at the bottom of this page) 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. The exact amount of ferrite will depend on the cooling processes the iron-carbon alloy undergoes as it cools from liquid state.

In pure iron, ferrite is stable below 910 ºC (1,670 ºF). Above this temperature the face-centred cubic form of iron, austenite (gamma-iron) is stable. Above 1,390 ºC (2,530 ºF), up to the melting point at 1,539 ºC (2,802 ºF), the body-centred cubic crystal structure is again the more stable form of delta-ferrite (δ-Fe). Ferrite above the critical temperature A2 (Curie temperature) of 771 ºC (1,044 K; 1,420 ºF), where it is paramagnetic rather than ferromagnetic, is beta ferrite or beta iron (β-Fe). The termbeta iron is seldom used because it is crystallographically identical to, and its phase field contiguous with, α-Fe.

Only a very small amount of carbon can be dissolved in ferrite; the maximum solubility is about 0.02 wt% at 723 ºC (1,333 ºF) and 0.005% carbon at 0 ºC (32 ºF). 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 ºC (1,333 ºF) 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.

Because of its significance for planetary cores, the physical properties of iron at high pressures and temperatures have also been studied extensively. α-ferrite, which is the form of iron that is stable under standard conditions, can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form termed ε-iron, which crystallizes in a hexagonal close-packed (hcp) structure.