Heat treatment is the process of heating and cooling metals to achieve desired physical and mechanical properties through modification of their crystalline structure. The temperature, length of time, and rate of cooling after heat treatment will all impact properties dramatically. The most common reasons to heat treat include increasing strenght or hardness, increasing toughness, improving ductility and maximizing corrosion resistance.

1. Annealing

Annealing is a rather generalized term. Annealing consists of heating a metal to a specific temperature and then cooling at a rate that will produce a refined microstructure. The rate of colling is generally slow. Annealing is most often used to soften a metal for cold working, to improve machinability, or to enhance properties like electrical conductivity.

2. Normalizing

Normalizing is a technique used to provide uniformity in grain size and composition throughout an alloy. The term is often used for ferrous alloys that have been austenitized and then cooled in open air. Normalizing not only produces pearlite, but also bainited sometimes martensite, which gives harder and stronger steel , but with less ductility for the same composition than full annealing.

3. Stress relieving

Stress relieving is a technique to remove or reduce the internal stresses created in a metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly.

4.Quenching

Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often produce a harder metal, while non-ferrous alloys will usually become softer than normal.

5. Tempering

Untempered martensitic steel, while very hard, is too brittle to be useful for most applications. A method for alleviation this problem is called tempering. Most applications require that quenched part be tempered. Tempering consists of heating steel below the lower critical temperature,  (often from 400 to 1105° F or 205 to 595°C, depending on the desired results), to impart some toughness. Higher tempering temperatures are sometimes used to impart further ductility, although some yield strength is lost.