Steel can be defined as an iron-carbon alloy with a carbon content less than 2.06%. In practice, however, other elements are always present in non-negligible quantities, so to constitute a complex alloy.

Without going into detail, because it is a complex subject that requires knowledge of chemistry and physical-chemistry, it is necessary to know that crystalline grains structure of the steel is closely related to the temperature at which the material is treated and with its cooling speed.