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What is Heat Treatment Process? Types & Applications [PDF]

In this article, you learn what is the heat treatment process. Its methods, types, purpose, procedure, and application of heat treatment.

What is the Heat Treatment Process?

Heat treatment is an operation involving heating and cooling a metal or an alloy in the solid state to obtain certain desirable properties without changing composition. It is an essential operation in the manufacturing process of machine parts and tools.

The process of heat treatment is carried out to change the grain size, modify the structure of the material, and relieve the stresses set up by the material after hot or cold work.

  • The heat treatment is done to improve the machinability.
  • To improve magnetic and electrical properties.
  • To increase resistance to wear, heat, corrosion, and much more reason.

Heat treatment consists of heating the metal near or above its critical temperature, held for a particular time at finally cooling the metal in some medium which may be air, water, brine, or molten salts. The heat treatment process includes annealing, case hardening, tempering, normalizing and quenching, nitriding, cyaniding, etc.

Checkout: The Complete List of Mechanical Properties of Materials

Steps of Heat Treatment Process

There are three fundamental steps in the heat treatment process. All of these heat-treating processes include heating and cooling steel.

However, there are differences in the procedures in terms of the outcomes, the cooling rates, and the heating temperatures used.

Steel is Heated to a Specific Temperature

A lot of heat treatments start with the steel being heated to a set temperature, known as the austenitizing temperature, which is determined by the composition of the steel, and the heat treatment is applied to the steel, such as annealing, normalizing, hardening, etc.

The type of heat treatment that is used will depend on the desired properties of the steel.


It keeps the steel at the austenitizing temperature for a set period while preventing grain growth. In soaking time, the pre-existing phases transform into fine-grained, homogeneous austenite without grain development throughout the entire component (including the center).

Cooling at a Set Rate

The fixed cooling rate of a component is determined by:

  • Heat treatment is applied to the alloy fixed according to what properties the steel will finally gain.
  • Steel composition.
  • The thickness of the part.

Types of Heat Treatment Processes

Following are the different types of heat treatment processes:

  1. Annealing
  2. Normalizing
  3. hardening
  4. Tempering
  5. Nitriding
  6. Cyaniding
  7. Induction Hardening
  8. Flame Hardening

Read Also: What is the Metal Casting Process? A Complete Guide

#1 Annealing

Annealing is one of the most important processes of heat treatment. It is one of the most widely used operations in the heat treatment of iron and steel and is defined as the softening process.

Heating from 30 – 50°C above the upper critical temperature and cooling it at a very slow rate by seeking it the furnace. The main aim of annealing is to make steel more ductile and malleable and to remove internal stresses. This process makes the steel soft so that it can be easily machined.

1.1 Purpose of Annealing

  • It softens steel and improves its machinability.
  • To refine grain size and remove gases.
  • It removes the internal stresses developed during the previous process.
  • To obtain desired ductility, malleability, and toughness.
  • It modifies the electrical and magnetic properties.

1.2 Procedure for Annealing

Depending on the carbon content, the steel is heated to a temperature of about 50° to 55°C above its critical temperature range. It is held at this temperature for a definite period depending on the type of furnace and nature of work. The steel is then allowed to cool inside the furnace constantly.

1.3 Application of annealing

It is applied to castings and forgings.

#2 Normalizing

Normalizing: The main aim of normalizing is to remove the internal stresses developed after the cold working process. In this, steel is heated 30 –  50°C above its upper critical temperature and cooled in the air.

It improves mechanical and electrical properties, machinability & tensile strength. Normalizing is the process of heat treatment carried out to restore the structure of normal conditions.

2.1 Purpose of Normalizing

  • Promote uniformity of structure.
  • To secure grain refinement.
  • To bring about desirable changes in the properties of steel.

2.2 Procedure for Normalizing

The steel is heated to a temperature of about 40° to 50°C above its upper critical temperature. It is held at this temperature for a short duration. The steel is then allowed to cool in still air at room temperature, which is known as air quenching.

2.3 Application of Normalizing

  • It is applied castings and forgings to refine grain structure and to relieve stresses.
  • It is applied after cold work such as rolling, stamping, and hammering.

Read Also: Different Types of Metals and Their Properties [Complete Guide]

#3 Hardening

Hardening: The main aim of the hardening process is to make steel hard. In this process, steel is heated 30° – 40°C above the upper critical temperature and then followed by continued cooling to room temperature by quenching in water or oil. It is the opposite process of annealing.

3.1 Purpose of Hardening

  • By hardening, it increases the hardness of steel.
  • To resist to wear
  • Allows the steel to cut other metals

3.2 Procedure for Hardening

The steel is heated above its critical temperature range. It is held at that temperature for a definite period. The steel is then rapidly cooled in a medium of quenching.

The quenching medium is selected according to the degree of hardness desired. The air, water, brine, oils, and molten salts are used as quenching mediums. A thin section such knife blades are cooled in the air. Water is a widely used medium but it results in the formation of bubbles on the surface of the metal.

Hence brine solution is used to prevent this. Oil is used when there is a risk of distortion on cracks and is suitable for alloy steels. The molten salts are used to cool thin sections to obtain crack-free and impact-resistant products.

3.3 Application of Hardening

It is applied for chisels, sledgehammers, hand hammers, center punches, taps, dies, milling cutters, knife blades, and gears.

#4 Tempering

Tempering: When the hardening process hardens a steel specimen, it becomes brittle and has high residual stress. It is an operation used to modify the properties of steel hardened by quenching to increase its usefulness.

Tempering or drawing results in a reduction of brittleness and removal of internal strains caused during hardening. Steel must be tempered after the hardening process.

The tempering is divided into three categories according to the usefulness of steel required.

  • Low-temperature tempering.
  • Medium temperature tempering.
  • High-temperature tempering.

4.1 Purpose of Tempering

  • To relieve internal stress caused by hardening.
  • To reduce brittleness.
  • Improve ductility, strength, and toughness.
  • To increase wear resistance.
  • To obtain desired mechanical properties.

4.2 Procedure for Tempering

The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100°C to 700°C.

The reheating is done in a bath of oil molten lead or molten salt. The specimen is held in the bath for some time till attains the temperature evenly, the time depends on the composition and desired quality of the steel. Now the specimen is removed from the bath and allowed to cool slowly in still air.

4.3 Application of Tempering

It is applied to cutting tools, tools, and gears, which are hardened by the hardening process.

Read Also: Types of Sheet Metal Operations with Diagrams [Explained]

#5 Nitriding

Nitriding is the process of the case or surface hardening in which nitrogen gas is employed to obtain the hard skin of the metal. In this process, steel is heated in the presence of an ammonia environment.

Due to this, a nitrogen atom is deposited, making the material hard. Induction-hardening and flame-hardening objects are heated by an oxy-acetylene flame.

5.1 Purpose of Nitriding

  • To harden the surface of the steel to a certain depth.
  • Increase resistance to wear and fatigue.
  • To increase corrosion resistance.

5.2 Procedure for Nitriding

It is done in the electric furnace where a temperature varying between 450° and 510°C is maintained. The part is well machined and finished and placed in an airtight container provided with outlet and inlet tubes through which ammonia gas is circulated.

The container with the part is placed in the furnace and ammonia gas is passed through it while the furnace is heated.

During the process of heating nitrogen gas is released from ammonia in the form of atomic nitrogen, which reacts with the surface of the part, and forms iron nitrate.

The depth of entrance depends upon the length of time spent at the nitriding temperature. The part is taken out and it does not require any quenching or further heat treatment.

5.3 Application of Nitriding

  • It is applied for hardening the surface of medium carbon alloy steels.

#6 Cyaniding

In this process, steel is heated in the presence of a sodium cyanide environment. Due to this, carbon and nitrogen atoms are deposited on the surface of steel and make it hard.

6.1 Purpose of Cyaniding

  • This method is effective for increasing the fatigue limit of medium and small-sized parts such as gears, shafts, wrist pins, etc.
  • To increase surface hardness.
  • increase wear resistance.
  • To give a clean, bright, and pleasing appearance to the hardened surface.

6.2 Produce for Cyaniding

The parts to be treated are dipped in a molten cyanide salt bath maintained at a temperature of 950°C. The molten salts used are sodium chloride, sodium carbonate, sodium cyanide, and soda ash.

The immersed article is left in the molten cyanide salt at a temperature of 950°C for about 15 to 20 minutes. The decomposition of sodium cyanide yields nitrogen and carbon from carbon monoxide, which is diffused into the surface resulting in hardening the surface. The part is then taken out of the bath and quenched in water or oil.

6.3 Application of Cyaniding

  • It is applied to small articles like gears, bushing, screws, pins, and small hand tools, which require a thin and hard wear-resisting surface.

Read Also: Gear Cutting: Its Types, Advantages, Disadvantages

#7 Carburising

Carburising: In this process, steel is heated in a carbon environment. Due to this carbon atoms are deposited on the metal surface and make it hard.

#8 Case Hardening or Surface Hardening

The main aim of this process is to make the only external surface of steel hard and the inner core soft. It is the process of carburisation i.e., saturating the surface layer of steel with carbon, or some other process by which case is hardened and the core remains soft.

8.1 Purpose of Case Hardening

  • To obtain a hard and wear resistance to machine parts.
  • By case hardening, it obtains a tough core.
  • To obtain a higher fatigue limit and high mechanical properties in the core.

Purpose of Heat Treatment of Steel

The following are the purposes of heat treatment.

  • Improve mechanical properties such as tensile strength, hardness, ductility, shock resistance, and corrosion resistance.
  • Improve machinability.
  • To relieve the internal stresses of the metal induced during cold or hot working.
  • To change or refine grain size.
  • Improve magnetic and electric properties.
  • Increase resistance to wear, and corrosion.
  • To increase the surface hardness.

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Closing It Up

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What is the heat treatment process?

Heat treatment is a combination of heating and cooling that produces desired properties in metals or alloys in their solid state.

What are the 4 common types of heat treatment processes?

There are several common types of heat treatment processes, each of which achieves a different result, such as annealing, hardening, quenching, and stress relieving.

What is the temperature of annealing?

A partial transformation to austenite occurs during intermediate annealing, which is done at 649°C to 760°C / 1200°F to 1400°F. Full annealing, on the other hand, includes fully austenitizing the work at 816°C to 927°C / 1500°F to 1700°F.

What is the quenching process?

In quenching, a metal object is quickly cooled from the high temperature at which it has been shaped, such as by immersion in oil or water.

About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

22 thoughts on “What is Heat Treatment Process? Types & Applications [PDF]”

  1. Does annealing weaken steel, or does it improve its ductility? Because in the process of annealing, steel becomes quite soft?

    • Annealing can improve the ductility of steel by making it softer, but it doesn’t necessarily weaken the steel.


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