Hardening and tempering steel is done through a process of heating, quenching and reheating steel. The purpose is to increase the level of hardness and the strength of the steel.
Steel contains carbon, and the amount of carbon determines whether or not it can be hardened. Steel with low concentrations of carbon cannot be hardened because there is insufficient carbon to change the crystalline structure. However, high carbon steel can be hardened and tempered.
To harden the steel, the metal is heated to very high temperatures. After heating the steel higher than its critical temperature, the metal is cooled fast through a process called quenching. This involves dipping the heated metal into water, oil, or any other liquid to rapidly lower its temperature.
Since steel contains carbon along with other metals, the heating of the alloy above its critical temperature causes the carbon and the metals to go into solid solution together. When this solid solution is cooled rapidly through quenching, the solution “freezes” and preserves the micro structure obtained during the heating process, thus making the steel harder. This also makes the steel more brittle, which is why it has to be tempered to maintain the strength of the alloy.
Tempering is the process of reheating the steel to a relatively lower temperature compared to the temperature used for hardening. The reheating allows for the precipitation of the carbon in the steel. Depending on the strength desired in the final product, the temperature and duration for which the steel is tempered can be controlled.