Steel can be heat-processed to enhance hardness and ductility.
Quench hardening - When steel is heated and cooled rapidly it becomes very hard. This is a process that is termed "quenching". Steel has become widely used because of this simple factor that is has a "quenching property". This is one of the main reasons that steel is so popularly used, as there is no real need to melt it down and alloy it with other elements to obtain high hardness. Quenched steels are hard but they are also weak. Additionally, when the workpiece is heated and cooled rapidly there can be problems with work dimensions and cracking.
Tempering - As mentioned when quench hardening steels the steel becomes very hard but weak, and as such the steel can not be used in this state. In order to be able to use the steel effectively it is necessary to carry out a "tempering" process. The tempering process is one that reheats the quenched steel and gives it toughness.
The higher the tempering temperatures, the tougher the steel becomes. However the hardness of the steel does deteriorate. Thus making it necessary to give the appropriate treatment for each particular purpose.
Thermal refining - Thermal refining is a term given to steels that have undergone a quench hardening and tempering process. This is carried out at relatively high temperatures of about 400°C or higher. Once the steel has been through this kind of process it is generally termed a "heat treated steel". Thus both hardness and toughness are maintained.