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Heat Treatment | Definition, Objective, Stages, Structure of Steel, Methods

Heat Treatment 

Heat Treatment ProcessFig. Heat Treatment

Definition 

  • Heat Treatment is defined as a controlled operation that involves the heating and cooling of a metal or alloy in the solid state for the purpose of obtaining certain desirable shape of metal.
  • The process of converting the solid metal into desired shape by heating is known as heat treatment process.
  • The properties of a metal depend upon its structure.

Objectives of Heat Treatment

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

Stages of Heat Treatment

  • There are following Stages in heat treatment:

Stage 1

  • Heating the metal slowly to ensure a uniform temperature.

Stage 2

  • Soaking ( Holding ) the metal at a given temperature for a given period of time.
  1.   In Soaking, internal structure changes take place.
  2.   Soaking period depends on the chemical analysis of the metal and the mass of the part.      

Stage 3

  • Cooling the metal to room temperature.
  1. Cool the metal in direct contact with a Cooling Medium composed of a gas, liquid, solid, or combination of these.

Heat Treatment of Steel

  • Steel can be heat treated to produce a great variety of micro structures and properties hence, it is a most popular solid metal.
  • It is an alloy of iron and carbon, which may contribute upto 2.1 %  of its weight. Steels are a large family of metals.
  • All of them are alloys in which, iron is mixed with carbon and other element.
  • Steel is fundamentally an alloy of iron and carbon, with the the carbon content varying up to 1.5 %.
  • Steels are of different types as follows.

A. Dead mild steel

B. Mild steel

C. Medium carbon steel

D. High carbon steel

Structure of steel

  • The shape of steel depends upon the structure of iron or different types of molecules and quantity of carbon in it.

There are many types of steel structure are as follows.

1. Ferrite

  • It is a chemical compound of ceramic materials with iron i.e., Fe 2 O 3.
  • It is almost pure state of an iron due to the presence of 0% carbon in it.
  • They can’t be hardened by rapid cooling.
  • It is soft, ductile and comparatively weak.
  • It has more magnetic property.

Heat Treatment | Definition, Objective, Stages, Structure of Steel, Methods

 

 

2. Cementite

  • When carbon exists in steel as a chemical compound it is called iron carbide ( Fe 3 C ) Or Cementite.
  • This alloy is very hard and brittle but it is not strong.
  • It increases with more percentage of carbon.
  • It shows magnetic property below 250 ° C.

Cementite

 

 

3. Eutectoid / Pearlite steel

  • It is the mechanical aggregate of ferrite and cementite in equilibrium containing 0.88% carbon.
  • It is made up of alternate white and dark bonds.
  • The size of the pearlite depends upon rate of cooling.
  • The rapid cooling will lead to the formation of the pear lite.
  • The strength of the iron and steel is due to pearlite.
  • It is much stronger than ferrite or cementite.

Heat Treatment | Definition, Objective, Stages, Structure of Steel, Methods

 

 

 

4. Hypoeutectoid Steel

  • Hypoeutectoid steel consists of pearlite and cementite at room temperature.
  • It contains carbon more than 0.8%.
  • When slowly cooling a hypoeutectoid steel, the cementite will begin to crystallise first.
  • When the steel remaining becomes eutectoid in composition, it will crystallise into pearlite. Since, cementite is much harder than pearlite.

Heat Treatment | Definition, Objective, Stages, Structure of Steel, Methods

 

Effect of Heat on Internal Structure of Steel
  • To change the internal structure of steel, applying the heat treatment in a furnace.
  • The internal changes depend upon the heating temperature conditions.
  • If a specimen of the steel is heated, then its temperature increases continuously upto 723°C  but after reaching the temperature 723°C, it remains same for some time.

Effect of Heat on internal structure of steel

  • Thereafter, it increases slowly and reached at certain temperature, then it again increases at the same rate as before.
  • After reaching the temperature of 723°C , the heat energy consumption by specimen of steel becomes maximum which is further used in changing the internal structure of the steel and remaining part of the heat energy is used to increase the temperature of the steel.
  •  In this process, the lower critical point ( LCP ) of all carbon quantities has same temperature i.e., at 723°C but its Upper Critical Point ( UCP ) always vary.

Critical Temperature

  • The critical temperature of a substance is the temperature at and above which vapour of the substance cannot be liquefied.
  • There are mainly two states of the critical temperature, which are as follows

     i. Lower critical point temperature

  • The temperature at which the structure of steel to be an austenite ( i.e., at 723°C ), is called the lower critical temperature for all plain carbon steels.
  • Normally, the value of lower critical temperature for all types of steel are 723°C but upper critical temperature is changeable.

    ii. Upper critical point temperature

  • The temperature at which the structure of steel completely changes to austenite is called the upper critical temperature.
  • This varies depending on the percentages of carbon in steel.

Critical Temperatures effect on Iron

 

Methods of Heat Treatment 
  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.

Purposes 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 ).
  4. To reduce or eliminate structural in homogeneity.

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.

Normalizing process

      Graph.1 Normalising process

 

  • 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.
  • It is applicable for ferrous metals only.

Purposes of Normalising

  • To improve structure of weld.
  • To soften metal.
  • Refine grain structure.
  • To prepare steel for sub heat treatment.

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.

Purpose of Tempering

  • To relieve the steel from internal stresses and strains.
  • To regulate the hardness and toughness.
  • Reduce the brittleness.
  • To restore some ductility.
  • To reduce shock resistance.

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. Carburising

  • 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.

9 thoughts on “Heat Treatment | Definition, Objective, Stages, Structure of Steel, Methods

  • September 5, 2019 at 1:18 pm
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    I read your notes.
    It is wonderful.

    Reply
  • September 14, 2019 at 1:51 pm
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    very helpfull notes sir

    Reply
    • September 16, 2019 at 12:35 pm
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      Thanks…..lot

      Reply
  • September 14, 2019 at 2:01 pm
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    Very good post …

    Reply
    • September 16, 2019 at 12:34 pm
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      Thanks

      Reply
  • September 27, 2019 at 4:50 am
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    Found this article curious, added to jumptags

    Reply
  • October 11, 2019 at 11:26 am
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    Good info. Lucky me I reach on your website by accident, I bookmarked it.

    Reply
    • October 11, 2019 at 3:51 pm
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      Thanks…..

      Reply

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