The uniqueness of iron is not only that it is one of the most common elements, and the production of its derivative (steel) «exceeds the production of other metals by 14 times», not only in its polymorphism, but also in the following: «conversion of BCC α-Fe (К = 8) into a denser FCC γ-modification (K = 12) with heating is quite unusual and along with the thermodynamic interpretation requires a special physical explanation, especially in connection with the fact that it lies at the basis of the metal science and the thermal treatment of iron alloys» ( V.K.Grigorovich). «Unusual» iron is also confirmed by the fact that the hardness of Fe at 440 °С is 1.15 times higher than at 20 °С. Other metals of this unique quality do not – as the temperature rises, the hardness decreases. Only manganese with a tetragonal lattice exhibits a hardness maximum at 650-750 °C; a maximum hardness is observed near the α D β transformation of manganese. The absolute maximum of hardness at 440 °C for iron allows (by analogy) to assume a transformation in iron at this temperature. Especially note: at the temperature of unconditional a→g → </s> transformation (910 °C) there is an absolute minimum of hardness. A curious situation arose around the temperature interval 400-500 °С. M.V.Belous and his co-authors simply do not notice it in the classification of four transformations during drawing back process, although back in 1925 P.Oberghoffer, basing on a minimum at 400-500 °C of the thermoelectric power of the iron-platinum pair, wrote: «Whether we are here dealing with further transformations in pure iron, should be investigated in new detailed studies». Based on the maximum etchability value, acceleration of the graphitization of steels, the maximum corrosion rate of gray cast iron, anomalies in the temperature dependences of the physical and mechanical properties, changes in the solubility of cementite, the maximum lattice parameter, the extremum on the resistance curve of the pure iron deposit, the conversion in iron at ~ 450 °C is justified.
