Annealing / Annealing under protective atmosphere
Annealing process is the heating of a material to a predetermined temperature for a while and then slowly cooling to the room temperature. The metal quality is high when the material is held for a longer time at the predetermined temperature and then cooled to room temperature. Since the process is carried at sub critical temperature, the cooling rate is most important. Annealing is carried out to relieve internal stresses due to solidification, forging, machining, rolling and welding. It is also done to restore toughness and ductility. Applications of annealing includes treatment of sheets and wires.
Castings in the as-cast condition are annealed to relieve internal stresses and reduce growth. Although annealing does not generally produce any extensive changes in mechanical properties, certain alloys may show slightly reduced tensile strength, while ductility may increase. The usual annealing treatment consists of heating the material at a temperature of 350oC for about 2hours. However, another less common method of annealing involves heating the castings up to their solution temperature for a short period, followed by furnace cooling to room temperature.