Pure iron (along the y-axis of the diagram) can exist with two crystal structures. At temperatures below 1674°F (912°C), iron is body-centered cubic (BCC) and is called ferrite (a) or a-iron. At temperatures between 1674°F (912°C) and 2541°F (1394°C), iron is face-centered cubic (FCC) and is called austenite (g) or g-iron. Above 2541°F (1394°C) and below the melting point of pure iron at 2800°F (1538°C), iron returns to BCC and is designated as delta (d) ferrite or d-iron. Steel can be heat treated as a result of the austenite to ferrite phase transformations.
Iron can form a solid solution with carbon. The carbon atoms dissolve into the solid by filling the interstitial sites of the FCC and BCC crystals. Due to the size and shape of these interstitial sites, the austenite (FCC iron) can dissolve up to 2.14 wt% C while ferrite (BCC iron) can only contain 0.022 wt% C. This 100-fold difference in carbon solubility in the two iron crystal structures is also necessary for the heat treatment of steel.
Part 3: Pearlite formation
Part 4: Martensite formation and tempering
Part 5: Time-temperature-transformation (TTT) diagrams