|Steels and other iron–carbon alloy phases|
|Other iron-based materials|
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.1
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 °C (1,670 °F). Above this temperature the face-centred cubic form 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 of 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, is beta ferrite or beta iron (β-Fe). The term beta 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 °C (1,333 °F) and 0.005% carbon at 0 °C (32 °F).3 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 (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.
- Maranian, Peter (2009), Reducing Brittle and Fatigue Failures in Steel Structures, New York: American Society of Civil Engineers, ISBN 978-0-7844-1067-7.
- Structure of plain steel, retrieved 2008-10-21.
- Smith & Hashemi 2006, p. 363.