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Heat Treatment Process-Definition,Purpose And Types

Heat Treatment 

Heat Treatment Process

Definition

  • Heat treatment process is defined as an operation involving the heating of sample to suitable temperature, followed by cooling at different rate(fast ,moderate & slow).
  • Heat treatment process is a series of operations involving the Heating and Cooling of metals in the solid state.
  • It is very important manufacturing process that can not only help the manufacturing process but can also improve the product, it’s performance , and it’s characteristics .

Purpose of Heat Treatment

  • Increase the mechanical properties of materials.
  • Relief the internal stress.
  • To change the structure and size of grain.
  • Improve mach-inability.

Types of Heat Treatment Process

  1. Annealing (Furnace cool)
  2. Normalising(Air cool)
  3. Hardening
  4. Tempering
  5. Case Hardening

1. Annealing (Furnace cool)

  • Annealing is a heat treatment process which involves the heating of steel sample above critical temperature(727 degree) and holding it there for about 1 hours so that their complete austenization take place.
  • Then, it allow to cool slowly in the furnace.
  • It is also known as Furnace cooling process.

Benefits of Annealing

  1. To improve the ductility of the sample.
  2. It enhances the mach-inability of the sample.
  3. It reduces the internal resting force (internal stress).

2. Normalising

  • Normalising is a process which is similar to annealing process. Here ,the specimen is heating beyond the upper -critical limit temperature(227-912)degree while lower critical temperature(227)degree and cool in open air.
  • In this process,the rate of cooling will be more as compare to annealing process.
  • The strength of sample as well as its mach inability is enhance.

3. Hardening

  • In this process, steel is cooling in water and oil baths.
  • The rate of cooling is very fast as compare to annealing and normalising process.
  • The hardness of a metal is directly proportional to the uni-axial yield stress at the location of the imposed strain.
  • It reduces the mach inability of the sample.

4. Tempering

  • This process is using to increase the mach inability of harden steel.
  • In this process,sample is heating below the lower critical temperature(227)degree and holding it for some time and then cooling it slowly.
  • The process has the effect of toughening by lessening brittleness and reducing internal stresses.

5. Case Hardening

  • It is also known as chemical heat treatment process.
  • In this process,steel surface is processing by the addition of carbon.

    Types of Case Hardening

  1. Carburizing
  2. Nitriding
  3. Cyaniding
  4. Induction hardening
  5. Flame hardening

A. Carburizing

  • In this process,steel is heating in contact with charcoal,wood.
  • After heating it will be followed by rapid cooling.

B. Nitriding

  • Nitriding is a heat treatment process in which steel will heating in atmosphere of ammonia gas(NH3) at the temperature(1|2-1|3) carburizing process.
  • At this temperature NH3 gas dissociate in nascent nitrogen and hydrogen.
  • After this nitrogen will get diffused into steel in form of nit-rides.

Advantages of Nitriding

  • High fatigue life.
  • High hardness than carburised and hardened components.
  • Better corrosion resistance than carburised and hardened components.

Disadvantages of Nitriding

  • No heat treatment can be done after nitriding.
  • White layer.
  • Thin case depth.

C. Cyaniding

  • Cyaniding is a process in which steel sample is heated in atmosphere of sodium cyanide.
  • Both carbon and nitrogen will get diffused into steel sample.
  •  The nitrides are form during heating and cyanides are form during cooling.

D. Induction Hardening

  • This process is applicable to high carbon steel.
  • In this process,steel sample is heating to red hot condition by induction hardening.
  • Then steel sample is quenching into water.

Advantages of Induction Hardening

  • Increase fatigue strength.
  • Localized areas can be heat treated.
  • Low operating costs.
  • May be incorporate into cell manufacture.

Disadvantages of Induction Hardening

  • High capital investment.
  • Only certain steels can be induction hardened.

D. Flame Hardening

  • Flame hardening is a surface hardening method that involves heating a metal with a high temperature flame,followed by quenching.
  • It is used on medium carbon, mild or alloy steels or cast iron to produce a hard, wear-resistant surface.

 Benefits of Flame hardening

  • Increase wear resistance.
  • Reduce processing time.
  • To reduce cost by hardening only selective areas.
  • Less distortion.
  • Ability to use low to medium carbon steels.

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