Annealing
It is a heat treatment basically to soften the steels. The heating and cooling both at controlled rate are performed in a furnace. Hypoeutectoid steels are heated up above the upper critical temperature (A₃ line) while hypereutectoid steels are heated only above the lower critical temperature (A₁ line). The cooling is so done that γ-to-α transformation goes to completion to each temperature. The resulting structure contains large grains of ferrite with coarse pearlite in which thick plates of ferrite and carbide are present. This is softest possible structure and is ideal point for starting mechanical working of steel. Low yield strength and tensile strength are associated with this treatment.

Different reason which are attained through annealing are listed below :

1.    the relief of all internal stresses within the metal.
2.    the production of uniform grain structure throughout the metal.
3.    the softening of the metal.

The annealing processes are classify either as full annealing or process annealing.

In essence full annealing is the process described above. However, cooling might be done in furnace, in ashes of sand or in specially built cooling pits lines with refractory & covered with refractory lid. If heated up too high a temperature or soaked for too long a time the austenitic phase undergoes grain increase resulting into coarse peralite grains. Such a structure is termed “overheated” & exhibits low mechanical strength.

One major problem to overcome throughout annealing process is the decarburisation and oxidation on the surface. Packing the steel in special boxes are utilized a neutral atmosphere in the furnace might overcome this problem. For instance, low carbon steel parts could be packed into boxes filled with line, sand ground mica or cast iron swarf while higher carbon components are generally packed into charcoal and other carbonaceous materials.

Full annealing is not generally desirables as it results into considerable loss of mechanical strength. Further it is too slow and expensive process.

Process annealing, also known as commercial annealing or referred as stress relieving is performed by heating steel to a predetermined temperature that is below the A1 temperature. The metal is air-cooled or quenched in appropriate pickling bath. Mild steels (or hypoeutectoid steel containing less than 0.3% C) after having undergone the mechanical treatment are softened by this procedure by heating to a temperature between 550^oC and 650^oC. In steel, the distorted grains of ferrites are fully recrystallised by the procedure. The pearlite grains are not affected by procedure annealing so that the structure consists of stress free ferrite matrix with distorted pearlite.