Heat treatment is a group of industrial, thermal, and metalworking processes used to alter the physical and sometimes chemical properties of a material. The most common application of heat treatment is metallurgical, but it is also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing, and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Heat treatment is often used to alter the mechanical properties of a metallic alloy, manipulating properties such as the hardness, strength, toughness, ductility, and elasticity. There are two mechanisms that may change an alloys properties during heat treatment: the formation of martensite causes the crystals to deform intrinsically, and the diffusion mechanism causes changes in the homogeneity of the alloy. Many heat treating methods have been developed to alter the properties of only a portion of an object.

The three stages of heat treatment are heating, soaking, and cooling. The metal or alloy is heated up to a specified temperature, held at that temperature for a certain amount of time, and then cooled. While it’s hot, the metal’s physical structure, also called the microstructure, changes, ultimately resulting in its physical properties being changed. Heat treatment can be used at different stages in the manufacturing process to change certain properties of that metal or alloy. For example, heat treatment can be used to make a metal stronger, harder, more durable, or more ductile, depending on what the material needs in order to perform properly.

There are four common types of heat treating methods: annealing, hardening, quenching, and stress relieving. Annealing is used to soften metal to improve formability, while hardening heat treatments are used to enhance the hardness of the metal’s surface through heating and rapid cooling. Quenching is a process that involves the addition of carbon atoms to the surface and sub-surface of steel to improve its surface hardness. Stress relieving is a process that is used to reduce internal stresses in metals that have been s...