Heat Treatment Process-Annealing, Normalizing, Hardening, Tempering, Nitriding, Cyaniding Process, PDF

Heat Treatment:

In very simple words I am going to explain the heat treatment process consists of a succession of heating and cooling cycles applied to a metal or alloy in order to obtain the desired properties, such as hardness, ductility, tensile strength, toughness, grain size, etc.

Heat treatment involves heating of metal in the solid-state and then subsequently cooled at varied cooling rates.

It is very important manufacturing process that can not only help the manufacturing process but can also improve the product, its performance, and its characteristics in many ways. 

By Heat Treatment process, Example: The plain carbon steel. The following changes may be achieved:

  • The hardness of Steel may be increased or decreased.
  • Internal stresses that are set up due to cold or hot working may be relieved.
  • The machinability of Steel may be enhanced.
  • The mechanical properties like tensile strength the Talati shock resistance toughness etc may be improved.
  • The resistance to corrosion and wear may be increased. 
  • The cutting properties of the tool may be increased. 
  • The Grain structure of Steels may be refined. 
  • The electrical and magnetic properties may be improved.

Below we have discussed all the Heat treatment process and also answered some of the Question which will help you to better understand. Please leave feedback.

Heat Treatment Process:

1. Annealing:

The Steel parts produced by mechanical operation process such as casting, rolling or drawing, extruding, etc. develop internal stresses and change their internal structure. This renders them hard and brittle.

Annealing is carried out for such parts to remove the internal stresses and make them more ductile and less brittle.

Annealing consists of heating of steel parts to a temperature at or near the critical temperature 900 degree Celsius hold it at that temperature for a suitable time and when allowed to cool slowly in the Furnace itself.

The heating done during annealing affects the metal in two stages of recovery and recrystallization.

Recovery occurs as the temperature of the metal is gradually raised.
Internal stresses are relieved as the atom in the metal rearrange themselves into the position that there occupied before subjected to mechanical operation.

Recrystallization occurs as the temperature of the metal is rise further and nuclei for the growth of new stress-free crystal begin to form.

As these nuclei form, the Express free crystals exhibit most of the original properties of the metal. Gradual slow cooling ensures the retaining of restored properties of the metal.

Annealing is carried out for accomplishing one or more of the following:

  • Softening of a metal or alloy. This may be done due to improving machinability.
  • Relieving internal residual stresses caused by the various manufacturing process.
  • Refining the grain size of the metal or alloy.
  • Increasing the ductility and reducing brittleness.
  • Homogenizing the distribution of constituents.

Two types of annealing carried out are:
1. Process annealing.
2. Full annealing.

1. Process annealing:

It consists of heating the Steel to a temperature little below the critical range and then cooling it slowly. This causes complete recrystallization in steel to form New grain structure. This will release the internal stresses previously the strip in the steel and improve the machinability.

2. Full annealing:

It consists of heating the Steel temperature at or near the critical point holding there for a suitable time and then allowing it cools slowly in the Furnace itself. This courses wipes out all traces of the previous structure and define the crystalline structure in addition to the softening of the metal. It also removes internal stresses. Now,

2. Normalizing:

Normalizing is a heat treatment process similar to annealing in which the Steel is heated to about 50 degree Celsius above the upper critical temperature followed by air cooling.

This results in a softer state which will be lesser soft than that produced by annealing.

This heat treatment process is usually carried for low and medium carbon steel as well as alloy steel to make the grain structure more uniform and relieve the internal stresses.

Normalizing carried for accomplishing one or more of the following:

  • To refine the grain size.
  • Reduce or remove internal stresses.
  • Improve the machinability of low carbon steel.
  • Increase the strength of medium carbon steel.
  • And also To improve the mechanical properties of the medium Carbon Steel.

Differences between Annealing and Normalizing:

Sl NoAnnealingNormalizing
1.Steel parts are gradually cooled in a furnace. Steel parts are gradually cool in still air
2.Comparatively lower yield point, Ultimate Tensile strength, and impact strength. The comparatively higher yield point ultimate tensile strength and impact strength.
3.Comparatively soft and easily machinable. Comparatively less soft.
4.Low hardness. BHN ranges from 125 to 220 BHN. Relatively more hard. Bhn ranges from 140 to 245 BHN.
5.Highly ductile and percentage of elongation is more. The less ductile and relative percentage of elongation is less

3. Hardening:

Hardening is a heat treatment process carried out to increase the hardness of Steel.

It consists of heating Steel components to the temperature within or above its critical range. Held at this temperature for a considerable time to ensure thorough penetration of heat at this temperature well inside the component and then allowed to cool separately by quenching in water oil or brine solution.

This kind of heat treatment produced a small grain size in the metal.

The strength and hardness of the Steel are increased but makes it more brittle since ductility is reduced.

Hardening is carried to accomplish the following:

  • To reduce the grain size.
  • Obtain maximum hardness.
  • Reduce ductility to the minimum.
  • To increase the wear resistance of Steel.
  • Improve the magnetizing properties.

4. Tempering:

This heat treatment process carried out for steel parts which have been already hardened, in order to reduce bitterness and unequal stress develop as a result of hardening.

This process reduces brittleness and hardness but improves the tensile strength of Steel.
It increases the toughness of Steel at the expenses of loss of some hardness.

Almost all the cutting tools need a hard cutting edge while at the same time they are required to be tough and strong so as not to break when subjected to shock or fatigue.

Sampling is done by reheating the hardened Steel to some temperature below the lower critical temperature and then quenching in oil or salt bath.

Reheating of Steel during tempering temperature decreases the hardness Somewhat But improve the toughness.

This process is carried out to accomplish the following:

  • To increase the toughness.
  • Also, to decrease the hardness.
  • Relieve the internal stresses.
  • And to reduce the brittleness.

5. Nitriding:

Nitriding is a process of surface hardening in which nitrogen gas is used to obtain a hard surface for the Steel. In this process, the Steel parts are heated in an atmosphere of ammonia (NH3 ) for a prolonged period and then cooled slowly.

The heating temperature for nitriding Ranges from 480 degree Celsius to 550 degree Celsius.

During this process, when Ammonia comes in contact with steel is diffuses into nascent hydrogen and nascent nitrogen.

This nascent nitrogen so produced diffuses into the surface of the workpiece forming hard nitrites which increase surface hardness.

Beside increasing surface hardness and wear resistance nitriding provides good resistance to corrosion due to water, air, and steam.

Nitriding is generally employed to Steel parts which are moving like engine parts such a cylinder, crankshaft, etc.

6. Cyaniding:

Cyaniding is also a surface hardening process in which the heated parts to be surface hardened are immersed in a bath of molten sodium or potassium cyanide.

The immersed Steel parts are left in the molten cyanide bath for about 15 to 20 minutes.

Then the parts are taken out of the bath and Queens in lime water to neutralize the particles of Cyanide salt sticking to the surface of the steel parts.

The cyanide yield carbon monoxide and nitrogen both of which behaves as active carburizing agents in hardening the surface of the Steel.

This surface hardening is particularly suitable for small parts like a small gear, Bush pins, screws pins, and small hand tool which required thin and Hard-wear resisting surface.

Some Question Answer:

1. What is Quenching?

Quenching is the rapid cooling of a workpiece in water, oil or air to obtain certain material properties.

2. What is oil quenching vs water quenching?

When hardness can be sacrificed, mineral oils are often used. These oil-based fluids often oxidize and form sludge during quenching, which consequently lowers the efficiency of the process. The quenching velocity (cooling rate) of oil is much less than water.

3. What is heat treatment furnaces?

Heat treatment furnaces are used to achieve the processes that involve extreme heating or cooling to achieve their desired reaction.

4. What is solution heat treatment?

Solution Heat Treatment is a treatment in which an aluminum alloy is heated to a suitable temperature and held at this temperature for a sufficient length of time to allow the desired constituent to enter into the solid solution, followed by rapid cooling to hold the constituent in the solution..

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Anup Kumar Dey

A Mechanical Engineer with 19 years of working experience in various renowned MNCs.

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