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Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming.

· Heat treatment consists of Heating-Up and Cooling-Down process.

· Heating up the steel will change the microstructure to Austenite.

· Cooling down the steel at different cooling rates will change the microstructure from Austenite to different structures correspondingly.

· Change in Microstructures result in change in mechanical properties.

· By heat treatment, we can change the mechanical properties of moulds and machine components to our desired state.

Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability and restore ductility after a cold working operation. Thus, it is a very enabling manufacturing process that can, not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Steel is the most common metal being treated. It accounts for more than 80% of all metals.

The Heat Treatment Process

Basically, Heat Treatment just consists of heating up and cooling down process. This process can be further divided into four steps -

1. Construction of Continuous Cooling Transformations Diagram (CCT) and Time-Temperature-Transformation Diagram (TTT).

2. The second step is a heating operation intended to produce an elevated temperature homogeneous single-phase solid solution. The heating should not exceed the eutectic temperature or there might be melting if a cored structure is present.

3. After soaking to assure a uniform chemistry single phase, the alloy is cooled. The cooling rate of the alloy depends on the property of metal required.

4. The heat-treated material is then left for diffusion. Diffusion is necessary to convert the unstable supersaturated solution into the stable structure.

Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching.

· Hardening process -This process is intended to produce thorough hardened structure by quench-hardening. This is the most common heat treatment practice for mould making industry. Hardening increases wear resistance and strength of material and provides toughness after Tempering, so it is widely used for increasing the life of moulds as well as mechanical parts of machinery. However, hardening often results in turning the structure of the work brittle. Besides, internal stress increases tremendously while machinability and ductility of the metal decrease.

· Softening process - These processes are intended primarily to soften the material, such as Annealing; also those intended primarily to remove stresses either inherent or consequent upon prior operations, but generally resulting in a softer structure. The latter processes include stress relieving and process annealing.

· Toughening process- This process is intended to produce a structure possessing good strength and ductility in steels by means of Normalizing. Improved machinability, grain structure refinement, homogenization and modification of residual stresses are among the reasons for which normalizing is done.

· Casehardening process - This process is employed to produce a 'case' or surface layer considerably harder than the interior or core of the work piece. They include carburizing, nitriding and induction hardening.

Heat treatment process is carried out by using following few methods -

* Electrical Resistance Heating
* Temporary Oil Fired Furnace