In actual practice it is very difficult to trace the cooling of iron from 1600°C to ambient temperature because particular cooling rate is not known. Particular curve can be traced from temperature, time and transformation (TTT) curve. However allotropic changes observed during cooling of pure iron are depicted in Fig. 8.4. When iron is cooled from molten condition up to the solid state, the major allotropic changing occurs which are:
(i) First changing occurs at l539°C at which formation of delta iron starts.
(ii) Second changing takes place at 1404°C and where delta iron starts changes into gamma iron or austenite (FCC structure).
(iii) Third changing occurs at 910°C and where gamma iron (FCC structure) starts changes into beta iron (BCC structure) in form of ferrite, leadaburite and austenite.
(iv) Fourth changing takes place at 768°C and where beta iron (BCC structure) starts changes into alpha iron in form of ferrite, pearlite and cementite.
Therefore, the temperature points at which such changing takes place into allotropic forms are called critical points. The critical points obtained during cooling are slightly lower than those obtained in heating. The most marked of these range commonly called the point of recalescence and point of decalescence