There are different heat treatment method.Heat treatment change the crystalline structure and hence change the property of material.Hardness, Strength and ductility can be controlled by heat treatment process.
- Annealing: It is the process of heating the material above re crystallization temperature followed by very slow cooling at the furnace. Annealing is done to increase the toughness.However, hardness is reduced in the process.Following are the significance of annealing
It increases the ductility of the material
It improves the grain size of crystal structure
It helps to relieves internal stress - Quenching: It is the process of heating the material above re crystallization temperature (to red hot) followed by immediate cooling at brine solution. Quenching increase hardness of the material.However, the material becomes brittle after quenching
- Tempering: Tempering is usually done after quenching(hardening) to reduce the brittleness.In tempering, the metal is heated below the critical point for some time and then allowing it to cool in still air.
- Normalizing: Normalization is the process of heating the metal to upper critical temperature and then cooling in still air.Tempering provides a way to carefully decrease the hardness of steel, thereby increasing the toughness to more desirable point. Normalizing increases the ductility and help to relieve the internal stress.
If you find difficult to visualize these processes the following video might be useful
https://www.youtube.com/watch?v=fLvZkZxiXnE&list=LLaXo5zVPTkRjLHl4CHv9Vbw&index=2
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