At which temperatures does iron change between ferrite and austenite crystal structures?

Iron exhibits allotropy, meaning it changes its crystal structure based on temperature. The transition between ferrite and austenite occurs at specific thermal thresholds:

• Alpha-ferrite to Austenite: At room temperature, iron exists as body-centred cubic (bcc) alpha-ferrite. As it is heated, it transforms into the face-centred cubic (fcc) structure known as austenite at 912° C (1,674° F).12
• Austenite to Delta-ferrite: Upon further heating, the fcc austenite structure remains stable until it reaches 1,394° C (2,541° F). At this temperature, the iron reverts to a bcc structure, which is referred to as delta-ferrite.12

This delta-ferrite phase persists until the iron reaches its melting point of 1,538° C (2,800° F). These structural transformations are fundamental to the heat treating of steel, as the fcc austenite structure provides more space to accommodate alloying elements in solid solution than the bcc ferrite structure.123