Heat treatment is used to improve the mechanical properties of metals. Desirable qualities improved by heat treating include strength, hardness, ductility, toughness, and wear resistance.
Annealing is used to soften metals such as iron, steel, copper, brass, and silver. It involves heating the metal and then allowing it to cool slowly at a controlled rate. Annealing changes the physical and chemical properties of the metal to increase ductility and reduce hardness. This allows the metal to be shaped, stamped, or formed while also enhancing the electrical conductivity.
Normalizing is used on allows to provide uniformity in grain size and composition. The metal is heated to a specific temperature and then cooled by air. This gives the metal greater strength and hardness. Normalising is typically performed on metals that are going to be machined.
During hardening, steel and other alloys are heated at a high temperature that is maintained until a proportion of carbon is dissolved. The metal is then rapidly cooled in oil or water. Hardening gives the metal high strength and wear resistance. However, it also makes the metal more brittle which is not good for certain applications.
Tempering is applied to steel when ductility is wanted. It is a low temperature heat treatment that is usually used in combination with hardening. Tempering after hardening creates a good hardness/toughness ratio. Once it is heated, the metal is cooled in still air to achieve a tougher and less brittle steel.