Manner of Functioning
Annealing of steel and other metals involves migration of atoms in the crystal lattice caused by slowly heating a material above recrystallization temperature. The material is to be kept at this constant temperature for a specific amount of time. This way, irregularities within a crystal structure (also known as dislocations) are reduced, leading to increased ductility and malleability.
Following annealing, the material is to be cooled. Additional heat treatments can achieve properties that are required of the material. Depending on the alloys, and the heating and cooling rate, the ultimate material properties will differ.
Reasons for Annealing
Materials are subject to annealing to improve following properties:
- Reduction of hardness for cold working: When it comes to cold working, one of the biggest challenges is stress cracking. Therefore, some ductility has to be restored again, which can be provided by annealing.
- Reduction of residual stress: A side effect of annealing can be the reduction of residual stress. Heating materials causes yield strength to be reduced. Stress-locations within would deform as a result, releasing internal stress. The proper method to de-stress locations within materials is called stress relief annealing. This method is aimed at reduction of residual stress only as opposed to changing mechanical properties, which is the case of annealing.
- Improvement in machinability: The reduced hardness results in improved malleability and machinability.
The most common type of material that is used in the annealing process is of course steel. However, brass, copper, and silver are also suitable. They are heated and then cooled down to room temperature. Unlike steel, which is cooled relatively slowly in still air, these metals can also be quenched in water. Steel is not cooled down quickly because annealing enables it to be processed more easily. After being processed, steel may be subject to further heat treatment that involves quenching.