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Austenite and ferrite

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Austenite was originally used to describe an iron-carbon alloy, in which the iron was in the face-centred-cubic (gamma-iron) form. It is now a term used for all iron alloys with a basis of gamma-iron. Austenite in iron-carbon alloys is generally only evident above 723°C, and below 1500°C, depending on carbon content. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Similarly, ferrite was a term originally used for iron-carbon alloys, in which the iron was in the body-centred cubic (alpha- or delta-iron) morphology, but is now used for the constituent in iron alloys, which contains iron in the alpha- or delta-iron form. Alpha ferrite forms by the slow cooling of austenite, with the associated rejection of carbon by diffusion. This can begin within a temperature range of 900°C to 723°C, and alpha-ferrite is evident to room temperature. Delta ferrite is the high temperature form of iron, formed on cooling low carbon concentrations in iron-carbon alloys from the liquid state before transforming to austenite. In highly alloyed steels, delta ferrite can be retained to room temperature.

When iron carbon alloys transform from austenite on cooling, the solubility limit of carbon in ferrite is commonly exceeded. Under slow cooling conditions, carbides are formed, and at faster cooling rates carbon may be trapped in solid solution.




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