Heat treatment for metals refers to its heating and cooling to bring certain changes in its physical and mechanical properties, without altering the shape. A heat treatment can also be a method for strengthening materials and altering mechanical properties such as improving formability and machining.
The most common application of heat treatment is metallurgical, but it can also be used for manufacturing glass, aluminium, steel, and other materials.
A heat treatment process can take the following forms:
1) Hardening
Hardening involves heating the steel and letting it settle at an appropriate temperature until all pearlite is transformed into austenite. Once this happens, the metal is rapidly quenched in water or oil. The temperature at which austenite is formed depends on the carbon content present in steel. Heating time should be increased to ensure that even the core is transformed into austenite.
2) Tempering
This process involves heating steel which has been quenched and hardened for a sufficient amount of time so that metal can be equilibrated. The hardness and strength of a metal depends on the temperature at which tempering is carried out. Higher temperatures produce metals that are high in ductility, and low in strength and hardness. Lower tempering temperatures create metals that are low in ductility, but high in strength and hardness.
3) Annealing
In this heat treatment process, steel is treated at high temperatures and then cooled very slowly to room temperature. Annealing results in a microstructure that has high ductility and toughness, but low hardness. It is performed by heating a metal to an appropriate temperature, soaking it at that temperature, and shutting off the furnace while the metal is in it. Steel is annealed before it is processed by cold forming so that the requirements for load and energy are reduced..
4) Normalising
In normalising, steel is heated and then kept at that temperature for some time. It is then left to cool. The microstructure that forms, as a result, is a mixture of ferrite and cementite which have higher strength and hardness, but low ductility. This process is performed on structures and structural components that are subject to machining because it improves the machinability of carbon steels.
5) Carburization
This is a heat treatment process during which steel or iron is heated to a temperature, that is below its melting point, in the presence of a liquid, solid, or gaseous material which decomposes to release carbon when heated to that particular temperature. The outer surface of the metal will have carbon content that is greater than the primary material. Once the steel is cooled by quenching, the high carbon content on the outside surface becomes hard, while keeping the core tight and soft.