ANNEALING

The purposes of annealing are: (a) to soften the metal for easy machining, (b) to remove internal stress caused by working, (c) to increase ductility, (d) to refine grain size, and (e) to modify electrical and magnetic properties. Normalized steel is less ductile and have more yield point and tensile strength than the annealed steel. There are two types of annealing: process annealing and full annealing.

Process Annealing: This is a process of heating the metal below or very close to lower critical temperature, i.e., 650°C for steel and slow cooling to form new grain structure. The purposes of the process are: (a) to increase ductility of cold worked metal, and (b) to remove internal stress. This is frequently used in wire drawing to increase the plasticity of the metal.

Full Annealing: The purposes of full annealing are: (a) to soften the steel, and (b) to refine grain structure above upper critical limit by 20–30°C for 0.9% C-steel and by the same amount below the critical point for high carbon steel. Carbon-steel is cooled 100–200°C per hour. It is essential that the steel should not hold less than 4–8 min for heating. To prevent the steel for carburization and oxidization workpiece is closed in metal box and put into the furnace. Austenite changes to pearlite and mixture of pearlite and ferrite.