Allotropy

The property possessed by certain elements to exist in two or more distinct forms that are chemically identical but have different physical properties. In the case of iron, the crystal structure has one form at room temperature and another at high temperature. When heated above 910C, the atomic structure changes from 'body centred cubic' to 'face centred cubic' but reverts again when cooled. The allotropy of iron modifies the solubility of carbon and it is because of this that steel can be hardened.