Iron has got the following three allotropic forms of crystal at different temperatures. When iron is heated from normal temperature to high temperature (molten state), it undergoes all the allotropic forms.
1. Alpha Iron:
It occurs from normal temperature of 910°C and has got body-centred cubic lattice crystals.
(a) Ferromagnetic alpha iron which occurs from normal temperature to 770°C.
ADVERTISEMENTS:
(b) Paramagnetic alpha iron which occurs from 770°C to 910°C.
2. Gamma Iron:
This occurs from 910°C to 1400°C. It has got crystal structure of face-centred cubic lattice.
3. Delta Iron:
ADVERTISEMENTS:
This form occurs from 1400°C to 1539°C (molten state). The crystal structure is body-centred lattice.
All these forms have been shown in graphical form in Fig. 1.9.
ADVERTISEMENTS:
In the graph there appear points of discontinuity which are called Points of Arrests or simply A points. In order to distinguish between the various A points, these are numbered as A1, A2, A3, and A4.
The change from ferromagnetic alpha iron to paramagnetic alpha iron is called A2 and it occurs at 770°C. The change from paramagnetic alpha iron to gamma iron which occurs at 910°C is called A3. When gamma iron changes to delta iron at 1440°C the change is called A4.
The point A1 does not represent any allotropic change and does not occur in pure iron. It occurs in iron containing carbon, that is, cast iron and steel.