Various treatment of metal

1.     Hardening: When heating a metal to a specified temp. and then quenched into water / oil or air, there is a structural change in metal. It become more strong and brittle. This is called hardening or molter sitting. 

2.     Tempering: Due to hardening metal becomes very brittle. To reduce brittleness and increase ductility, tempering is done. The metal is heated up to below critical temp. at about 250°C ~ 300°C. This temp. retained for some time which depends upon on mass and degree of toughness and then cooled in air. 

3.     Annealing: Heating above the critical temp. of a metal and then cooling it in the furnace at a controlled rate.

4.     Normalizing: Heating above the critical temp. of a metal and then cooling it in still air (out of furnace).

5.     Work hardening: For improving the hardness of steel, mechanical work is done in cold state, work force causes dislocations to be set up in crystal lattice work (internal structure) of steel. In order to remove these dislocations, considerable force is required. This force is called work hardening.

6.     Case hardening: In a rich carbon environment, low carbon steels are boxed by high carbon casing and heated up to 900°C. For that, a layer of carbon deposited on the surface of low carbon steel. This process is called case hardening.