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Heat treatment is the process of modifying physical and mechanical properties of metals. Metals are heated up to a specific temperature lower then the melting point and near about the re-crystallization temperature. After heating up its cooled down. There are different methods of heating and cooling. After the process the Physical and mechanical properties of metals are modified. There are many methods for heat treatment like Annealing, Normalizing, Quenching etc. Every method have different effect on the metallic properties.

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treating is a group

of industrial and

metalworking processes

used to alter the physical,

and sometimes chemical ,

properties of a material.

The most common

application is

metallurgical. Heat

treatments are also used

in the manufacture of

many other materials,

such as glass. Heat

treatment involves the

use of heating or chilling,

normally to extreme

temperatures, to achieve a

desired result such as

hardening or softening of

a material. Heat

treatment techniques

include annealing, case

hardening , precipitation

strengthening , tempering

and quenching. It is

noteworthy that while the

term heat treatment

applies only to processes

where the heating and

cooling are done for the

specific purpose of

altering properties

intentionally, heating and

cooling often occur

incidentally during other

manufacturing processes

such as hot forming or

welding.

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heat treatment is an operation or combination of operation involving heating and cooling of metals or alloys in its solid state in order to obtain certain physical and mechanical properties.

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In the welding and fabrication industry I use heat treatment to remove internal stresses in the weld and the heat affected zone (HAZ). I also use heat treatment to make a metal either softer or harder (tempering colours) depending on what is required.

I have seen very large furnaces that take fabricated components and heat treat them over a period of time to remove the stresses that are built up in fabrications Eg: Chassis , excavator booms etc. This increases component life.

I do understand the metallurgical changes that do occur, but this has been discussed above.

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For commonly used steels. HT does one or more of the following :

1.Improve Machinability :ISO Annesling

2.Harden case .toughen core: Carburising and Quenching and Carbo Nitriding and quenching(lower case depth) 

3.Relieve Brittleness : Tempering

4.Relieve internal stresses :Normalising

 

 

 

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In steel it changes the face centered cubic steel molecules to body centered cubic grains. In crude terms picture a box with a iron atom at each corner and a carbon atom in the center of each face, face centered cubic. Heat treating causes the molecule to relax and some of the carbon slips inside the box, body centered cubic. When the molecule is quenched read cooled the carbon atom is now trapped inside the box which puts the eight iron atoms around it in much higher tension. As you may or may not know iron has great affinity for itself so is very strong in tension. Carbon has great strength against crushing or compression so holds the now cooler iron atoms away from each other and thus in greater tension. The iron atoms are now prestressed so now a portion of the elasticity is removed and they are less able to be deformed and are thus harder. Take a rubber band if you will and bump it against something when it is loose. See how easily it deforms? Now stretch the band some and bump it against something. See how it is less likely to deform?  That is what is happening in heat treating it is stretching the iron atoms in the steel molecule a little and makes it less likely they will deform..
aluminum is altogether different and has to deal with grain growth of copper within the aluminum.

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