Heat treatment (heat treating) is a process in which the desire mechanical properties of steel are achieved by heating it to a different temperature. There are many types of heat treatment processes for steel.
Types of Heat Treatment
Annealing is a process of heating steel in which temperature generally above the critical temperature, holding it for a definite period of time and cooling it at a slow rate. While cooling the steel embed inside the ash, lime or sand-like non-conducting materials.
Types of Annealing:
- Full annealing:- In this process, the metal is first heated up to 500 degrees Celcius above the critical temperature and hold it for one to two hours. Then the steel is cooled slowly to obtain the desired micro-structure.
- Isothermal annealing:- In isothermal annealing process steel is heated as in full annealing and allowed to rapid cool up to the temperature at which the transformation is desired. Then the steel is held at the temperature necessary to complete the transformation.
- Sub critical annealing:- This type of annealing first steel is heated below the critical temperature and allowing it to cool after holding it for 2 to 4 hours. This is also called the process annealing.
- Spheroidization:- Steel is heated at or just below the lower critical temperature and holds for a sufficient period of time to form small spheroids, then cooling very slowly.
Importance or uses of Annealing
- To improve machine ability.
- To soften the steel by re-crystallization and refine the grain size.
- For the improve ductility and malleability.
- To relieve the internal stresses produced during forging, pressing, etc.
- To improve the electrical and magnetic properties of steel.
- For removing the entrapped air, blow holes, pipes, etc. formed during casting.
- To overcome other imperfection produced during plastic deformation.
Normalizing is the process of heat treatment in which steel is heated up to 50-degree temperature above the critical temperature and allowed to cool in still air. This process is used to improve the quality of cast, welded or forged steel. It is also used to improve the quality of over heated or non-uniformity heated steel.
Importance or Use of Normalizing:
- To end the coarse-grained structure obtained during forging, rolling, extrusion, etc.
- For the improvement of the strength of medium carbon steel.
- To improve the machinability of steel.
- To reduce the internal stresses etc.
Read this: Detail about steel bar and its composition
C) Quenching/hardening of steel
In Quenching or hardening of steel, the steel is heated above its critical temperature and then allowed to cool rapidly by plunging it into the liquid bath. This form the extremely hard needle-like structure known as martensite. Clearwater, salty water or oil is used as quenching media to obtain desired hardness. It not only increases the hardness and resistance against wearing but also decreases the toughness and increases brittleness.
Tempering is the process of heating quenched steel to some predetermined temperature between room temperature and critical temperature, holding it for the time sufficient to transformation and then cooling it at a slow rate in air.
Steel, when allowed for quenching produces a highly hard structure known as martensite. Martensite is not only hard but also brittle and thus decreases toughness.
Importance or uses of tempering
- To increase the toughness and ductility.
- To decrease the hardness.
- To relieve the stress produced during quenching.
E) Surface hardening
Surface hardening is the process in which the outer surface of the steel is hardened and the core of the steel is only refined.
Different methods of surface hardening are
- Case hardening: It is the process at which steel is heated in presence of solid, liquid or gas which is rich in carbon content in order to enable the surface to be hardened while retaining a tough ductile core.
- Different types or methods of Case hardening
- Pack hardening: In this process steel is kept inside a box and surrounded by carbon-rich compound and heated to a temperature of 9000C to 9200C, depending upon the composition of the steel. Then the steel is allowed to cool and removed from the box. Now, it is quenched in water or other suitable media.
- Liquid carburizing: In this process, the steel is plunged to molten salt containing carbon such as sodium cyanide. This produces a thin layer of hardened steel. Then the steel is quenched as in the case of pack hardening.
- Gas carburizing: This process is carried out by heating the steel in a furnace into which the gas which is rich in carbon such as methane, propane, etc are introduced. There should be a continuous flow of carburizing gas.
- Different types or methods of Case hardening
- Nitriding: This process of surface hardening is used for only special alloy steel and not for plain carbon steel. In this process, no carbon is used for hardening, but nitrogen gas is used. Nitrogen is introduced to the steel by passing ammonia gas through the furnace containing steel and heated to a temperature between 4800C to 5400C.
- Cyaniding: In this process, steel is heated in molten cyanide salt at about 8500C followed by quenching. Both carbon and nitrogen are absorbed in this process.
- Flame hardening: In this process, the part of the surface to be hardened is heated above the critical temperature by flame and quenched rapidly. It is used to harden the more wearing parts of the machine such as gear teeth.
In this way, the heat treatment of steel is done.
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