What Is Heat Treatment Process?

Heat Treatment Process or simply called as Heat Treatment. It is an essential operation in Manufacturing of ferrous and non-ferrous materials; the purpose of the heat treatment process is to change the mechanical properties of metal or steel, decrease hardness, and increase the ductility of the metal according to the requirement. As we know, the study of metals, their properties, physical and chemical behavior of metals by heating process is called Metallurgy or Metallurgical Science.

The heating treatment of materials includes heating and cooling both, whereas the various stages involved in it are

• Introducing the material into the furnace (consisting high-temperature heat) for a certain period to soften the material.
• Hammering or Forging (to change the shape)
• Re-Crystallizing (which changes the mechanical properties), and
• Quenching the heated metal.

This process is applicable for both ferrous and non-ferrous materials. The Ferrous materials that undergo heat treatment are Cast Iron, Steel and different types of steel (mainly), etc. Besides ferrous materials, Glass is one of the non-ferrous materials. Even though welding and hot forming involve heat treatments in their process, they are not categorized as heat treatment processes.

What is Quenching?

Quenching is a process of decreasing the temperature of any metal after heating it to a temperature (usually heated by introducing it into the Furnace). This is done by keeping the heated metal in chilled or room temperature water so, the metal will lose the heat to water and comes to room temperature.

The sudden cooling of heated metal will result in an increase of the Metal hardness. The quenching may be performed by using any one of the following

• Air
• Water or
• Any other type of oils

Types of Heat Treatment Process:

The Heat Treatment looks simple but requires a strict and step by step process. It will not be same for all materials or metals, and at the same time, it varies according to the application of the material too. Due to this variation, the material will acquire different mechanical properties like hardness, ductility, etc.

According to it, there are 5 types of heat treatment process, they are

• Annealing Process
• Normalizing Process
• Hardening Process
• Tempering Process
• Case Hardening Process

Annealing Process:

The Annealing Process is a type of heat treatment that reduces the dislocations of the crystal structure. The Annealing is mainly applied for steel materials where the steel is heated to Austenite Temperature (above 1000 K or 727 degrees centigrade). Further, the heated steel is cooled slowly to achieve the required properties.

Purpose of Annealing Heat Treatment Process:

• To increase Ductility
• To decrease the Hardness
• To improve metal Machinability
• To refine Grain Size
• To relieve the metal from Internal Stresses

To achieve required physical and chemical properties using the annealing, the process should be done in a particular sequence, and the sequence can be called Stage.

Stages in Annealing Process:

There are three stages in Annealing Heat Treatment Process, and they are as follows.

• Recovery Stage – The recovery stage aims to relieve the material from internal stress so, the material is heated up to a temperature of 1000 K or 727 degrees centigrade.
• Re-crystallization Stage – Once the material is relieved from internal stress, the material is further heated up to re-crystallizing temperature (the temperature below the melting point), so the new crystal grains will grow without any internal stress and defect. The new crystal grains will nucleate by consuming the old crystal grains.
• Grain Growth Stage – The growth of crystals continues until it has been stopped. In this stage, the crystal grain growth is stopped or controlled by cooling the material slowly, thus resulting in Ductility and less Hardness.

Types of Annealing Process:

1. Full Annealing Process:

The Full Annealing is a type of Annealing where the material is heated more (mostly 50-70 degrees more) than critical temperature (the temperature at which the metal changes its phase) and cooled slowly.

There are two types of steels Hypo-Eutectoid and Hyper-Eutectoid Steels.

• Hypo-Eutectoid Steel – The material is heated 50-75 degrees centigrade above the higher critical temperature, and when it cooled slowly, the Austenite structure converts into coarse pearlite and ferrite.
• Hyper-Eutectoid Steel – The material is heated 50-75 degrees centigrade above the lower critical temperature, and when it cooled slowly, the Austenite converts into pearlite and cementite.

2. Process Annealing:

The Process Annealing is another type of Annealing Heat Treatment Process where the low carbon steels are processed primarily. While cold working, the materials gain many defects like internal stress, lack of plasticity, etc., So, to relieve the material from all its defects, the process annealing is used.

The material is heated below the lower critical temperature and cooled further. Process Annealing will increase the plasticity and removes the internal stress. The temperature, period of cooling depends upon the material size and its properties.

3. Spheroidise Annealing Process:

In this process, the Steel is heated near to the lower critical temperature (around 730-770 degrees centigrade) and cooled slowly to a temperature of 600 degrees centigrade.

This Spheroidise Annealing in heat treatment is mainly used for the high carbon steels that are hard to machine using machine tools. The process develops sphere-type granule carbides in the material. The rate of cooling is also reflect in the material standard.

4. Diffusion Annealing Process:

The Diffusion Annealing is another type of Annealing Process, which is also called as Homogenizing Process. The steel is heated up to 1150 degrees centigrade and cooled up to 800 degrees centigrade which results in diffusion in the material. This process is mainly applicable for Alloy Steels ingots and complex castings.

Normalizing Process:

The Normalizing Process is a type of heat treatment process where the material is treated as its final stage. To relieve the steel from high stresses, normalizing is used. The steel or metal is heated more than 50 degrees centigrade above the Austenite Temperature and hold it for a period, then cooled in air (which we called as Air Quenching).

Due to this process, the microstructures of ferrite with pearlite for hypo-eutectic and pearlite with cementite for hypereutectoid steels will be developed.

Hardening Process:

The Hardening Process is a type of heat treatment where the material is heated up to Austenite Temperature and cooled in Water Oil or Salt Baths. The materials treated with the Hardening Heat Treatment Process are not ideal for practical applications due to their rapid cooling and converting into Martensite (which is brittle and hard in properties), which further requires Tempering Process to use.

In the Hardening Process, the materials like Hypo-eutectoid steels are heated up to 50 degrees above the Upper Critical Temperature whereas; the Hyper-Eutectoid Steels are heated above the Lower Critical Temperatures.

Tempering Process:

The Tempering Process is the Final Process in the Heat Treatment Process where the hardened material is further treated to relieve it from residual stresses and further used for different purposes. The hardened material is treated below the lower critical temperature for a certain period and then cooled slowly, which helps it to gain required properties and remove internal stresses.

Case Hardening Process:

The Case Hardening Process is usually used to convert the soft cores into hard surfaces. There are few materials like Mild Steel, which is impossible to increase its strength using the hardening process but possible using Case Hardening. There are different techniques or processes involved in Case Hardening Heat Treatment Process, which depends upon the type of material. The techniques are

• Carburizing
  • Pack Carburizing
  • Liquid Carburizing
  • Gas Carburizing
• Cyaniding
• Carbo-nitriding or Gas Cyaniding
• Deep Cyaniding or High Temperature Cyaniding
• Induction Hardening
• Flame Hardening

Note: All these types of Case Hardening Techniques will be learned in upcoming posts

Advantages of Heat Treatment Process:

The advantages of Annealing Heat Treatment Process are

• The Full Annealing will decrease the Hardness and increase the Ductility.
• The Process Annealing will increase the Plastic Deformation of low carbon steels.
• Spheroidal Annealing will increase the Ductility and Machinability of the material.
• The Diffusion Annealing will help to homogeneous the whole steel material.

Advantages of Other Types of Heat Treatment Processes:

• The Normalizing Process upsurge the Ultimate Tensile Strength and Impact Strength in Steels.
• The Hardening Process increases the brittleness and Hardness of materials.
• The Tempering Process will relieve the stresses in materials and increases the ductility and toughness both.

Disadvantages of Heat Treatment Process:

The Disadvantages of Heat Treatment Process are

• Not every material is treated with similar process, which require standard process.
• Heating the materials more than the specified temperature will result in failure of materials or decrease its life span.
• In Hardening Process, the conversion of material into martensite results in high hardness and brittleness, which is not suitable for applications. Further it requires tempering process, to relieve its internal stresses. So, the hardening followed by tempering process is must.
• The heat treatment process in industries involves high risk management and require high skilled labours and high safety precautions.

Applications of Heat Treatment Process:

The Applications of Heat Treatment Process are

• As the Steel is main source for many components, the heat treatment process is used for all Steel Materials.
• Heat Treatment Materials are used in Automobiles, Aerospace, Constructing Machineries, etc.
• Moreover, all the heat treated materials are used in Domestic Purposes too.

Conclusion:

This is being the basic and in-built process in manufacturing the components. The process will decide the application of the material. The scope of the heat treatment is prominent and necessary in all fields. For few applications like the Automobile and Aerospace sectors, the body needs to be constructed in such a way that the material should withstand high winds and temperatures, which can be achieved by heat treatment process.

Frequently Asked Questions:

1. What is Hardness?

The Hardness is a type of material property. A material is said to be hard whenever it is able to resist plastic deformation.

2. What is Toughness?

The Toughness is another type of material property that is able to resist fracture or breakage. The material that possesses toughness is called as Tough Material.

3. What is Ductility?

Ductility is a type of material property where the material possesses ductility can be drawn into wires or thin sheets.

4. Why the Heat Treatment is so Important?

The Heat Treatment Process is as important as the heart to the body. It is the process that makes the material tough, or brittle, or ductile, or high tensile, or high compressive, etc. according to the requirement of the application.

5. What are the types of Heat Treatment Process?

The Types of Heat Treatment Process are

• Annealing Process
• Normalizing Process
• Hardening Process
• Tempering Process
• Case Hardening Process