Master Heat Treatment 6 Process-Annealing, Normalizing, Hardening, Tempering, Nitriding, Cyaniding Process [PDF]

In this article, we will be learning about various Heat Treatment processes like Annealing, Normalizing, Hardening, Tempering, Nitriding, and Cyaniding. We have also provided a PDF for the same.

Heat Treatment

Heat Treatment

The heat treatment process consists of a succession of heating and cooling cycles applied to a metal or alloy to obtain the desired properties, such as hardness, ductility, tensile strength, toughness, grain size, etc. Heat treatment involves heating metal in the solid state and then subsequently cooling it at various cooling rates.

It is a very important manufacturing practice that can not only help the manufacturing process but can also improve the product, its performance, and its characteristics in several ways. By Heat Treatment process, for example, Plain carbon steel. The following changes can be achieved

Heat Treatment
  • 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 Steel may be refined. 
  • The electrical and magnetic properties may be improved.

Heat Treatment Process

Annealing

The Steel parts produced by mechanical operation processes 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 steel parts to a temperature at or near the critical temperature of 900 degrees Celsius holding 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 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 being subjected to mechanical operation.

Heat Treatment Annealing

Recrystallization occurs as the temperature of the metal rises further and nuclei for the growth of new stress-free crystals 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 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 are carried out which are as follows

  • Process annealing.
  • Full annealing.

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 a New grain structure. This will release the internal stresses previously the strip in the steel and improve the machinability.

Full annealing

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

Normalizing

Normalizing is a heat treatment process similar to annealing in which the Steel is heated to about 50 degrees 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 internal stresses.

Normalizing process of steel
Photo By Harchis Heat Treatment

Normalizing carried for accomplishing 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

AnnealingNormalizing
Steel parts are cooled in a furnace.Steel parts are cooled in still air
This has a lower yield point, Ultimate Tensile strength, and impact strength.They possess higher yield points ultimate tensile strength and impact strength.
It is soft and easily machinable. Comparatively less soft and less machinable.
Low hardness. BHN ranges from 125 to 220 BHN.Relatively harder. BHN ranges from 140 to 245 BHN.
Highly ductile and the percentage of elongation is more.The less ductile and relative percentage of elongation is less

Hardening

Hardening is a heat treatment process carried out to increase the hardness of Steel. It consists of heating Steel components to a temperature within or above their 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 make it more brittle since ductility is reduced.

Hardening process of steel

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.

Tempering

This heat treatment process is carried out for steel parts that have been already hardened, to reduce bitterness and unequal stress that 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 expense of the loss of some hardness.

Almost all cutting tools types 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 it in an oil or salt bath. Reheating Steel during tempering temperature decreases the hardness Somewhat But improves the toughness.

Tempering process of steel
Photo By Dmitriy Rukhmalev

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 brittleness.

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 degrees Celsius to 550 degrees Celsius. During this process, when Ammonia comes in contact with steel it diffuses into nascent hydrogen and nascent nitrogen.

Nitriding process of steel
Photo By Thermal Processing Magazine

This nascent nitrogen so produced diffuses into the surface of the workpiece forming hard nitrites which increase surface hardness. Besides increasing surface hardness and wear resistance nitriding provides good resistance to corrosion due to water, air, and steam. Nitriding is generally employed for Steel parts that are moving like engine parts such as a cylinder, crankshafts, etc.

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 quenched 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 small gear, Bush pins, screws pins, and small hand tools which require thin and Hard-wear resisting surfaces.

FAQ

What is Quenching?

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

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.

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.

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.

Conclusion

The heat treatment process is a crucial metallurgical process that can significantly alter the properties of metal alloys. It involves various techniques such as annealing, normalizing, hardening, tempering, nitriding, and cyaniding, each with a specific purpose and benefits. For instance, annealing is used to improve ductility, while hardening increases hardness and strength.

Proper execution of these processes can result in stronger, more durable, and better-performing metal alloys. Choosing the appropriate heat treatment process depends on the specific application and desired properties of the metal alloy. It is important to follow safety protocols and work with experienced professionals to ensure successful and safe heat treatment processes.

references

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