⟩ What does allotropic mean what does iron and steel have to do with it?
Iron is a metal with polymorphism structure. Each structure stable in the range of temperature, for example delta-iron with bcc structure in the range of 1538-1394C changes to gamma -Iron with fcc in the range of 912-1394C and gamma iron to Alfa-Iron etc. For hardening of the metal, this quality of iron is exploited.