Heat treatment of steel: An overview of processes


It is the process of heating and cooling of metals and alloys in solid state to obtain the following properties-
a.) To relief the internal or residual stress present inside the metal.
b.) To change the atomic structure and grain size of element.
c.) To improve the mechanical properties like strength, machinability, hardness, toughness, ductility etc.

Basic process of heat treatment

There are basically three steps in a heat treatment process –
1.} Heating
2.} Holding at constant temperature
3.} Cooling as per requirement

Different types of heat treatment process

1.) Annealing-
a} Full Annealing
b} Process Annealing
c} Spherodising
*Special type-Diffusion Annealing
2.) Normalising
3.) Hardening
4.) Case hardening-
a} Carburising
b} Nitriding
c} Cyaniding
5.) Surface hardening-
a} Induction hardening
b} Flame hardening
6.) Tempering
Let’s discuss each process in details.

1.) Annealing:

It is generally done for medium carbon steel.
It is a primary heat treatment process which is performed to introduce softness in the steel and to remove the internal or residual stress due to cold working.
At first steel is heated above the upper critical temperature and held there for sufficient time say 30 to 60 minutes and finally cooled slowly in the furnace itself.
      Here in case of hypo eutectoid steel,it
converted into ferrite and pearlite.But in case of hyper eutectoid steel it changed in the form of pearlite and cementite.
1.) Residual stress is completely removed.
2.) Ductility increases.
1.) Hardness decreases
2.) Grain size becomes coarser that’s why strength decreases.
A.) Full Annealing-
It is usually done for high carbon steel.
In this process,in case of hypo eutectoid steel it is heated about 50 to 60°C above the upper critical temperature and in case of hyper eutectoid steel it is heated by the same temperature range above lower critical temperature.After that it held there for sufficient amount of time and finally cooled very slowly in furnace.
Due to full annealing metals becomes soft grain refine and internal stress removed.
During annealing Cristal transformation takes place from B.C.C. to F.C.C. and again from F.C.C. to B.C.C.
B.) Process Annealing- 
It is usually done for low carbon steel.It is also called sub critical Annealing or intermediate Annealing.In this process metal is heated first below the lower critical temperature and held there for sometime and then cooled slowly.
Here grain growth is restricted by sandwich between two cold work material.It’s temperature shouldn’t increase above 723°C.
This process is performed to provide softness during deformation process so that metal can’t be deform further.It also increases ductility.
Note- In case of process Annealing we heat the metal below critical temperature and so there is no change in crystal structure.Thus it remains B.C.C. throughout the process.
C.) Spherodising- 
It is usually done for high carbon steel.
It is mainly used to produce rounded or globular form.
In this process steel is heated between upper and lower critical temperature and held there for sometime and then cooled slowly in the furnace till 200°C and finally cooled in air.
During this process cementite is converted into spherical form such that hardness decreases.
* Diffusion Annealing- 
It is performed on heavy casting to make homogeneous grain.Here the job is heated about 100 to 200°C above the upper critical temperature and then held there about 10 hours and then cooled slowly in the furnace.
Due to above process we get homogeneous grain and homogeneous property of material.

2.) Normalising:

It is generally done for medium carbon steel.It involves heating the material to an elevated temperature and held it for 30 to 60 minutes and then allowing it to cooled in air up to room temperature.
It is generally performed after annealing or any other process.
1.) Residual stress partially removed.
2.) Ductility decreases as compare to annealing but overall ductility increases.
3.) Grain size gets finer.
4.) Machinability increases.
5.) Toughness increases.

3.) Hardening:

It is generally done for high carbon steel.
Here steel is heated above upper critical temperature up to 1100°C and held there for sufficient time and finally cooled by water quenching.The rate of cooling is so fast that austenite is converted into martensite which provides hardness in the steel.
         Based on the cooling medium we get different types of products.
A.) If the cooling medium is still air then we get ‘sorbite’.
B.) If the cooling medium is flowing air then we get ‘Banite’.
C.) If the cooling medium is oil then we get ‘troostite’.
D.) If the cooling medium is furnace then we get pearlite.
E.) If the cooling medium is water or brine then we get martensite.

4.) Case hardening:

A.) Carburising-
It is usually done for low carbon steel.
Here carbon content is increased.
Here carbon is added in ferrite region ( 800- 900°C).
In this process low carbon steel is placed in surrounding of activated charcoal where steel is at temperature 800- 900°C which catches fire to activated charcoal and then it is quinched i.e. fast cooled and we will get harden surface.
This process is used to produce the steel which is tough from inside but hard from outside such as gears,pins.
Here the carbon particle i.e. charcoal gets penetrated into the surface of steel to make it hard.
A.) If the carbon rich substance is solid then it is called solid or pack carburising.
B.) If the carbon rich substance is liquid then it is called liquid carburising.
C.) If the carbon rich substance is gas then it is called gaseous carburising.
* Shocking- It is the process of absorbing carbon particles on the surface of steel.
B.) Cyaniding- 
In this process steel absorb the carbon as well as nitrogen on its surface.
Here steel is dipped in a liquid bath of sodium cyanide( NaCN) at about 400°C and kept there for sufficient time for shocking and finally quenched into oil or water.
Here temperature always below 800°C.
NOTE: By cyaniding a case of depth 1 mm is possible.
C.) Nitriding- 
In this process pure Ammonia is placed outside of the material where Ammonia decomposes into nitrogen and hydrogen.Further nitrogen reacts with iron to form nitrate which makes the surface hard.
           N2+ 6Fe→ 2Fe3N{ Nitrate}
It’s temperature range is 500 to 600°C.
The main objective of nitriding is to increase surface wear resistance.
A.) BHN(Brinells hardness number) of 1000 can be achieved by nitriding process.
B.) It is basically performed on dies,mandrel and in the many parts of I.C. engine and pump.
*Maximum surface hardness condition-
   Nitriding > Cyaniding > Carburising

5.) Surface hardening:

It is done for quick and fast hardening.It is generally done for any type of carbon steel.
A.) Induction hardening-
It is used to make the outer surface hard with tough inner core.
Here we use a electrical copper coil to heat the component above 800°C and then water is sprayed to get hard outer surface due to formation of martensite.
B.) Flame hardening-
Here heat is applied to the part being surface harden by oxyacetylene flame.
In this process surface is heated up to upper critical temperature and quinched by spray of water.
Generally this process is used for machine tool guide,gear tooth and cutting parts.
This is a low cost process.

6.) Tempering:

It is basically a secondary heat treatment process.It becomes essential after hardening process because after hardening materials becomes too hard and brittle and it is difficult to use them.Hence to minimize the brittleness and to increase toughness slightly we perform tempering.
In tempering we reheat the material below lower critical temperature then we perform slow cooling by virtue of which some of the martensite is converted into pearlite or tempered martensite.
1.) Lowered hardness.
2.) Residual or internal stress decreases.
3.) Ductility increases.
4.) Toughness increases.

Special heat treatment processes:

1.) Austempering-
It is usually done for medium and high carbon steel.
It is basically a hardening process.It is also known as isothermal quinching.
In this process steel is heated above upper critical and then it is quinched to a molten salt bath to a temperature 500°C and kept there for long and finally cooled at room temperature in air.
As a result of this neither pearlite nor martensite formation takes place but we get a needle like structure called ‘ Banite’.
2.) Martempering-
This process is also known as step quinching process or interrupted quinching.
In this process martensite formation takes place in a controlled manner.
First of all,steel is heated above the critical temperature then it is quinched in molten salt having temperature in between 150 to 300°C and after sometime it is quinched in water and thus we get martensite with less cracking.
* Note: The above processes are used for ferrous metals only.But in case of non ferrous metals we use a different technique of heat treatment called ‘Age hardening or precipitation hardening’.
That was all about the Overview of heat treatment of steel. Hope you all like this article. If this was helpful to you then share this article with your well wishers. If you have any doubts regarding this or any mechanical engineering topics then let us know in comment section or contact us through email.
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