Heat treatment is a controlled process used to alter the microstructure of metals and alloys, like steel and aluminium. The ultimate aim of heat treatment is to impart properties that benefit the working life of a component, such as increased surface hardness, temperature resistance, ductility and strength.
The following are common heat treatments for metals:
Annealing is a heat treatment process that involves heating to and holding at a suitable temperature, followed by cooling at a suitable rate. It is primarily used to soften metals and produce desired changes in other properties or in microstructures. Annealing can improve machinability, facilitate cold work, improve mechanical or electrical properties and increase dimensional stability.
Austempering is applied to ferrous metals, notably steel and ductile iron. When used as a steel heat treatment it produces a bainite microstructure, whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce distortion.
Deep freezing can be used to enhance the transformation of austenite to martensite in case hardening, and to improve the stress relief of castings and machined parts. The optimum cold treatment temperature is -84°C (-120°F). In contrast, cryogenic treatment at around -190°C (-310°F) improves certain properties beyond the capability of deep freezing.
Martempering is a heat treatment for steel involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the martensite start (Ms) point.
Normalising is a homogenising or grain refining treatment that involves an austenitising heating cycle, followed by cooling in still or slightly agitated air. The actual temperature used depends on the composition of the steel, but the usual temperature is around 870°C (1600°F)
Process, or sub-critical, annealing is used to counteract the hardening effects of a cold-working operation on a metal. After significant cold work, a piece may become too brittle to safely continue the working process. It is then heated to a temperature anywhere below its austenitizing temperature, until stresses have been removed, and then slowly cooled to avoid new stresses.
Quenching is when steel parts are rapidly cooled from the austenitising or solution treating temperature. Stainless and high-alloy steels may be quenched to minimise grain boundary carbides or to improve the ferrite distribution. Most steels – including carbon, low-alloy, and tool steels – are quenched to produce controlled amounts of martensite in the microstructure.
Spheroidizing is a form of heat treatment for alloys (commonly carbon steels) to convert them into machinable and ductile alloys. It is conducted at temperatures slightly below the eutectoid temperature (temperature at which the solution is a solid solution rather than liquid), followed by a slow cooling process. The resulting spheroidite structure, known as the most ductile and machinable form of steel, is a microstructure that contains sphere-like cementite particles.
With tempering a previously hardened or normalised steel is heated to a temperature below the lower critical temperature and cooled at a suitable rate. This increases ductility and toughness, but also increases grain size. Most steels are heated to a temperature of 205-595°C (400-1105°F) and held at temperature for an hour or more. Higher temperatures increase toughness and resistance to shock, but at the expense of lower hardness and strength.
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