Allotropes of Iron

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Allotropes of Iron

Iron as a solid exists in different allotropic crystalline forms. The most important are alpha iron (ferrite) and gamma iron (austenite). The crystal lattice arrangement of alpha iron is one which is body centred whereas that of gamma iron is a face-centred cubic arrangement. The iron-carbon equilibrium diagram (technically the iron-iron carbide meta-stable phase diagram) shows the temperature ranges in which each form of iron is most stable as well as the carbon solubility range for these allotropes or phases. 

The great importance of the two crystalline forms of iron lies in the difference of the solubility in solid solution of carbon between the two. Carbon is virtually insoluble in the body-centred ferrite form, the lattice structure of which will only accept a maximum of 0.04% of carbon. The face-centred cubic lattice of austenite will accept carbon atoms more readily and up to about 2.1% carbon is soluble in alpha iron.