There are many different reasons that a component or part may need to go through the annealing process. There are both visual as well as structural issues that make metal annealing an important part of any OEM or part production process.
A Quick Overview
The process of metal annealing allowed to cool over a very specific time period. This allows the grains or granules within the structure of the metal to align correctly, creating a softer, more malleable metal while also providing a consistent surface appearance.
There are many different alloys or base metals that can benefit from metal annealing. These include the mild steels, ferrous alloys, nickel alloys and the 300 and 400 serious of stainless steels. The result, of the process, especially with bright annealing on stainless steel alloys, is to create a corrosion-free surface that is bright, uniform and maintains its surface even with years of exposure in potentially corrosive environments.
The Stages
During the metal annealing process, there are three distinct stages. The first occurs as the metal is brought up the desired heat. In this process, known as the recovery stage, the metal softens and the internal structure is able to move back into the correct alignment. Through hot working and even cold working these structure can be disrupted, creating stress in the metal and increasing the risk of cracking or failure.
The second stage is known as recrystallization. In this stage the damaged grains that were distorted by strain and stress are replaced by new grains that form in the correct alignment, restoring the natural strength of the metal. In most cases this is where the annealing process is stopped and the selective cooling beings.
In some situations, the third stage, known as grain growth, can occur with continued heating. This allows those internal grains to become larger and more developed, sometimes referred to as becoming coarse. This actually weakens the final metal, but it can then go through another fast heating and fast cooling after the metal annealing process to address this issue.
During the process time, temperature and the atmosphere are very carefully controlled. With metal annealing, hydrogen and nitrogen combinations are typically used in the furnace to prevent oxidation and to leave the surface bright, shiny and free from stains.
