Ferrite (iron)

Iron-carbon phase diagram, showing the conditions under which ferrite (α) is stable.

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.[1]

It has a strength of 280 N/mm2 and a hardness of approximately 80 Brinell.[2]

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;[3] the maximum solubility is about 0.02 wt% at 723 °C (1,333 °F) and 0.001% carbon at 0 °C (32 °F).[4] 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.

Mild steel (carbon steel with up to about 0.2 wt% C) consist mostly of ferrite and increasing amounts of cementite (iron carbide) in a laminar structure called pearlite. Since bainite 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 process the iron-carbon alloy undergoes as it cools.

Molar volume vs. pressure for α-ferrite at room temperature.

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.

See also

References

  1. ↑ Maranian, Peter (2009), Reducing Brittle and Fatigue Failures in Steel Structures, New York: American Society of Civil Engineers, ISBN 978-0-7844-1067-7.
  2. ↑ Structure of plain steel, retrieved 2008-10-21.
  3. ↑ Alvarenga HD, Van de Putte T, Van Steenberge N, Sietsma J, Terryn H (Apr 2009). "Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels". Metal Mater Trans A. doi:10.1007/s11661-014-2600-y.
  4. ↑ Smith & Hashemi 2006, p. 363.

Bibliography

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