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Engineering properties are modified by heat treatment processes so that structural components are able withstand specified

operating conditions and have desired useful life.

Heat treatment is the heating and cooling of metals to change their physical and mechanical properties, without

letting it change its shape. Heat treatment could be said to be a method for

strengthening materials but could also be used to alter some mechanical properties such as improving formability,

machining, etc. The most common application is metallurgical but heat treatment can also be used in manufacture

of glass, aluminum, steel and many more materials. The process of heat treatment involves the use of heating or

cooling, usually to extreme temperatures to achieve the wanted result. It is very important manufacturing processes

that can not only help manufacturing process but can also improve product, its performance, and its characteristicsin many ways.

Heat Treatment ProcessesHardening

Hardening involves heating of steel, keeping it at an appropriate temperature until all pearlite is transformed into austenite,

and then quenching it rapidly in water or oil. The temperature at which austentizing rapidly takes place depends upon the

carbon content in the steel used. The heating time should be increased ensuring that the core will also be fully transformed

into austenite. The microstructure of a hardened steel part is ferrite, martensite, or cementite.

Tempering

Tempering involves heating steel that has been quenched and hardened for an adequate period of time so that the metal can

be equilibrated. The hardness and strength obtained depend upon the temperature at which tempering is carried out. Higher

temperatures will result into high ductility, but low strength and hardness. Low tempering temperatures will produce low

ductility, but high strength and hardness. In practice, appropriate tempering temperatures are selected that will produce the

desired level of hardness and strength. This operation is performed on all carbon steels that have been hardened, in order to

reduce their brittleness, so that they can be used effectively in desired applications.

Annealing

Annealing involves treating steel up to a high temperature, and then cooling it very slowly to room temperature, so that the

resulting microstructure will possess high ductility and toughness, but low hardness. Annealing is performed by heating a

component to the appropriate temperature, soaking it at that temperature, and then shutting off the furnace while the piece is

in it. Steel is annealed before being processed by cold forming, to reduce the requirements of load and energy, and to enable

the metal to undergo large strains without failure.Normalizing

Normalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air.

The resulting microstructure is a mixture of ferrite and cementite which has a higher strength and hardness, but lower

ductility. Normalizing is performed on structures and structural components that will be subjected to machining, because it

improves the machinability of carbon steels.

Carburization

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It is a heat treatment process in which steel or iron is heated to a temperature, below the melting point, in the presence of a

liquid, solid, or gaseous material which decomposes so as to release carbon when heated to

the temperature used. The outer case or surface will have higher carbon content than the primary material. When the steel or

iron is rapidly cooled by quenching, the higher carbon content on the outer surface becomes hard, while the core remains

tough and soft.

Surface Hardening

In many engineering applications, it is necessary to have the surface of the component hard enough to resist wear and

erosion, while maintaining ductility and toughness, to withstand impact and shock loading. This can be achieved by local

austentitizing and quenching, and diffusion of hardening elements like carbon or nitrogen into the surface. Processes

involved for this purpose are known as flame hardening, induction hardening, nitriding and carbonitriding.