After most metals crystallize, their lattice type will not change, but there are also a few metals, such as iron, cobalt and titanium, whose lattice type will change with the change of temperature in the solid state. This phenomenon is called allotropic transformation. Pure iron has allotropic transformation, which can form allotropic crystals of body centered cubic and face centered cubic. Allotropic transformation belongs to solid-state transformation. For metals with solid-state transformation, recrystallization can occur through heating and cooling in the solid state to change their microstructure and properties, which is of great significance for heat treatment. For example, the microstructure and properties of steel and cast iron can be changed by heat treatment, which is one of the main reasons why steel and iron materials have various properties and wide applications.