The basis of steel hardening lies in the fact, that iron exists in two crystal structures:
Below 912°C and from 1394°C to its melting point iron is body centered cubic – bcc – called ferrite. In the lower temperature range ferrite is also referred to as alpha iron, in the higher temperature range as delta iron.
At a temperatures from 912°C to 1394°C iron is in the face centered cubic crystal structure – fcc – called gamma iron or austenite. Heating pure iron above 912°C transforms the structure from Ferrite into Austenite. Cooling the iron from the austenitizing area below 912°C results in the original bcc iron structure, no matter what cooling rate is applied.
Pure iron can not be hardened.
The addition of carbon converts iron into hardenable steel. (Alloying elements such as manganese, molybdenum and chromium enhance the hardenability).
Carbon is present in iron both in solid solution and in the form of carbides. It is significant that the sides of the face centered cubes of the austenite are about 25% larger than the sides of the body centered cube of the ferrite. The solubility for carbon is therefore much greater in austenite than in ferrite.
When a steel with say 0.4% carbon is heated above the ferrite.austenite (alpha-gamma) transformation point, carbon and the other alloying elements can go into solid solution in the spacious austenitic fcc structure. Subsequent cooling through the gamma – alpha transformation point leads into the narrow ferrite structure. There is not enough space in this structure to keep carbon in solid solution.
So, if the cooling rate is low, carbide is formed in connection with the transformation process. As a result the microstructure at room temperature consists of ferrite and carbide. (The fine lamellar structure of ferrite and iron carbide is called pearlite - see Fig 6).