Annealing is a process that used with a wide number of different materials. It is a heat treatment that is used to alter specific characteristics and properties of steel on a molecular level.
In a more detailed explanation, annealing steel allows for atoms to diffuse throughout a material, allowing the material to naturally move towards a state of equilibrium. This occurs as heat is able to increase energy levels in molecular structures, allowing for bonds to break and reform.
Stages of The Annealing Process
There are three different stages or steps in annealing steel. The first stage occurs as the metal is heated to a specific temperature and allows for recovery. This recovery results in a softening of the metal through the removal of internal stress. This then frees up the molecules to reform again in the next stage, which is known as recrystallization.
In recrystallization, the free grains of the metal, now without the internal stress, reform and grow to fill in the spaces where grains that were damaged by internal stress previously existing. The third stage, which is not always completed in the process, is known as grain growth. This actually creates a thickening or coarsening of the molecular structure that can cause decreases in overall strength.
Within the process of annealing steel, there are several important controls that must be maintained. The steel has to be heated to the correct temperature based on the alloy. The heating also has to continue until the recrystallization phase is continued.
Finally, in the grain growth stage, the cooling has to be done slowly to prevent the hardening that can decrease the strength. Additionally, throughout the process, the atmosphere has to be carefully controlled. Exposure to oxygen at this time will result in oxidization of the surface, while in a controlled atmosphere the surface will remain bright and free from any oxidation.