This is a heat treatment mostly to soften the steels. The cooling and heating both at controlled rate are presented in a furnace. Hypo-eutectoid steels are heated above the upper critical temperature as A3 line whereas hypereutectoid steels are heated merely above the lower critical temperature as A1 line. The cooling is so completed that g-to-a transformation goes to completion to every temperature. The resulting structure comprises of large grains of ferrite with coarse pearlite whether thick plates of ferrite and carbide are illustrated. This is softest probable structure and is conceptual point for starting mechanical working of steel. Low tensile strength and yield strength are connected with such treatment.
Various purposes that are achieved via annealing are listed below as:
(a) The relief of each internal stresses inside the metal.
(b) The production of uniform grain structure by the metal.
(c) The softening of the metal.
Such processes are classified either as full annealing or process annealing. Full annealing is fundamentally the procedure explained above. Though, cooling may be complete in furnace, in ashes of sand or in particularly built cooling pits lines along with covered and refractory along with refractory lid. Whether heated to moreover high a temperature or soaked for too long a time the austenitic phase undergoes grain growth resultant in coarse peralite grains. This structure is known as "overheated" and exhibits low mechanical strength.
One major problem to overcome during annealing procedure is the decarburization and oxidation on the surface. Packing the steel in unique boxes are employed a neutral atmosphere in the furnace might overcome such problem. For instance, low carbon steel parts could be packed in boxes filled along with sand, line, ground mica or cast iron swarf while higher carbon components are frequently packed into charcoal and other carbonaceous materials.
Full annealing is not generally desirables as this result into considerable loss of mechanical strength. Further this is too costly and slow process.
Process annealing, also called as commercial annealing or concerned with stress relieving is performed via heating steel to a predetermined temperature that is below the A1 temperature. The metal is air-cooled or quenched in a appropriate pickling bath. Mild steels or hypo-eutectoid steel containing less than 0.3% C after comprising undergone the mechanical treatment are softened via this process by heating to a temperature between 550oC and 650oC. The distorted grains of ferrites in steel are completely recrystallized via the process. The pearlite grains are not affected with process annealing hence the structure comprises of stress free ferrite matrix along with distorted pearlite.