Heat treatment is a controlled process in which a steel is heated and cooled. A heat treatment process is carried out for the following reasons;
- To change the physical properties of a steel.
- To change the mechanical properties of a steel.
Heat treatment processes can offer the following;
- An increase in the strength of a steel.
- Improvements in machinability.
- Improvements in formability.
- Restore ductility (after cold forming).
- Relieve stresses within a steel.
Normalizing and annealing are used to soften a steel.
Quenching and tempering are used to harden a steel.
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Cooling and structural change
The poperties of heat-treated steel change according to the temperatures reached during heating, the length of time sustained at certain temperatures and the speed at which it is cooled.
Steel that has been heated to a sufficient temperature and maintained at that level for a certain period of time, and steel that has not been heat-treated have different structures. However, the speed at which the steel is cooled has a great effect on the structure of the steel. This structural change is called transformation.
When steel is cooled gradually over a long period of time, the structure of the steel is transformed to a structure with a chemical equilibrium. Meanwhile, when the cooling is quickened, a structure without equilibrium, such as a semi-stabilized phase, is formed due to the super cooling or the delayed spreading of carbon, i.e., not returned to ferrite or cementite but it stays as austenite. Then over time it changes gradually to martensite. Since martensite has a lower density than austenite, such a change or ageing, as it is called, often results in an increase in volume or growth.
Heat treatment is a method used to change the qualities of steel according to its required use by combining the different structural properties seen during the heat treatment process.