There are two specific reasons to use the annealing process on any type of metal. The first is to increase ductility or to reduce the tendency of the metal to be deformed under stress. The second is to reduce the hardness of the metal. Both of these outcomes create a more workable type of metal that is suitable for a wider range of applications and uses.
General Annealing
For all types of metals, heat is used to change the physical properties through the annealing process. It may also impact the chemical properties in some metals and based on the specific annealing method used.
The process starts with actually heating the metal to a specific temperature. The temperature will vary based on the metal, and it is always above the recrystallization temperature. Then, the metal is held at this temperature for a set time and finally it is cooled.
The specifics of the copper annealing process are slightly different that with other metals. Copper, along with silver and brass, can be cooled by air or by quenching. Metals such as steel or other ferrous metals can only be cooled using air. Fast quenching, or using a cold water bath for fast cooling, is not possible with these metals.
The Purpose
While copper is often seen as a soft metal, in its natural state and before copper annealing is actually brittle. Without the annealing process, it would not be possible to bend and form copper tubing or copper components. It would also be impossible to use copper for flexible types of wiring.
It is also important to keep in mind that copper annealing can be used to address work hardening in copper components and wire. This work hardening, which is sometimes called strain hardening, occurs when copper is deformed through bending and working.
Once the copper has become work hardened, it will not be possible to work with the piece any longer. By annealing the copper, or heating it and cooling it quickly, the strain can be released and the copper will become highly workable again.
It is important before annealing copper to remove any surface materials, including any oxides on the surface. Failing to do this will result in those oxides actually embedding into the surface of the copper component, which can not only impact the look of the copper but also lead to issues with surface strength over time.