Difference Between Heat Treatment and Annealing

Main Difference – Heat Treatment vs Annealing

Heat treatment is the use of heat to modify the properties of a material, especially in metallurgy. It is a type of industrial process involved in altering the properties of metals. There are four major types of heat treatment methods as annealing, tempering, hardening and normalizing. Annealing is the heating of a material and allowing it to cool slowly. Annealing is useful in increasing the ductility of a metal and reducing its hardness. The main difference between heat treatment and annealing is that heat treatment is used to obtain different desired properties (ex: increased strength, increased hardness, impact resistance, softening, increased ductility, etc.) whereas annealing is mainly done to soften a metal.

Key Areas Covered

1. What is Heat Treatment
     – Definition, Annealing, Tempering, Hardening and Normalizing
2. What is Annealing
     – Definition, Process, Purposes of Annealing
3. What is the Difference Between Heat Treatment and Annealing
     – Comparison of Key Differences

Key Terms: Annealing, Ductility, Hardening, Heat Treatment, Normalizing, Tempering

Difference Between Heat Treatment and Annealing - Comparison Summary

What is Heat Treatment

Heat treatment is the use of heat to modify the properties of a material, especially in metallurgy. Heat treatment is done via heating and cooling of substances. It is used to alter one or more chemical and physical properties in order to obtain desirable properties such as increased strength, increased hardness, resistance to impact, softening, increase ductility, etc.

Main Difference - Heat Treatment vs Annealing

Figure 1: A Heat Treating Furnace

There are four heat treatment methods:

  • Annealing
  • Tempering
  • Hardening
  • Normalizing

Annealing

Annealing is a heat treatment process used to soften materials or to obtain other desired properties such as machinability, electrical properties, dimensional stability, etc. Annealing has been discussed below in detail.

Tempering

Tempering is the process of heating a substance to a temperature below its critical range, holding and then cooling. This is done to obtain desirable properties. Tempering is often carried out for previously quenched or normalized steel. Tempering process is useful in reducing the brittleness of quenched steel. The temperature to which the tempering is done, directly affect the hardness of the material. Higher temperatures lower the hardness.

Hardening

Hardening or quenching is the process of increasing the hardness of a material. Hardening increases the strength of the material. In the hardening process of a metal, the metal is heated into austenitic crystal phase and then quickly cooled. There are two major types of hardening processes such as,

  • Surface hardening – increase the hardness of the outer surface while the core remains soft.
  • Case hardening – increasing the hardness of the surface by infusing elements into the surface of the material, forming a thin layer of harder alloy.

Normalizing

This method is only applicable for ferrous metals. Here, the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose is to the internal stresses induced by other factors such as heating, welding, etc.

What is Annealing

Annealing is the process of softening a material to obtain desired chemical and physical properties. Some of these desirable properties include machinability, weldability, dimensional stability, etc. It is a type of heat treatment.

The annealing process involves the heating of a metal to or near the critical temperature (critical temperature is the temperature at which crystalline phase of metal changes). After heating, the metal should be slowly cooled to room temperature. This can be done in an oven.

Difference Between Heat Treatment and Annealing

Figure 2: Annealing a Silver Strip

The slow cooling of metal produces a refined microstructure. This may partially or completely separate constituents. Annealing treatment process can be used for pure metals and alloys as well. According to the process, ferrous metals are categorized as below.

  • Full Annealed Ferrous Alloys (use very slow cooling process)
  • Process Annealed Ferrous Alloys (cooling rate may be faster)

Other metals such as brass, silver, copper can be fully annealed but are quickly cooled. This can be done by quenching in water.

Difference Between Heat Treatment and Annealing

Definition

Heat Treatment: Heat treatment is the use of heat to modify the properties of a material, especially in metallurgy.

Annealing: Annealing is the process of softening a material to obtain desired chemical and physical properties.

Method

Heat Treatment: Heat treatment can be done in different methods such as annealing, tempering, hardening and normalizing.

Annealing: Annealing is done by heating a metal to a high temperature beyond the critical point and letting it cool very slowly.

Purpose

Heat Treatment: Heat treatments are used to obtain desirable chemical and physical properties (ex: increased strength, increased hardness, impact resistance, softening, increased ductility, etc.).

Annealing: Annealing is used to soften metals and to obtain desired chemical and physical properties such as dimensional stability, machinability, etc.

Conclusion

Heat treatment is the process of using heat to obtain or alter desired properties in metals (in metallurgy). There are four methods of doing heat treatment: annealing, tempering, hardening and normalizing. The main difference between heat treatment and annealing is that heat treatment is useful to obtain different desired properties (such as increasing strength, increasing hardness, resistance to impact, softening, increase ductility, etc.) whereas annealing is mainly done to soften a metal.

Reference:

1. Himanshu Verma, “Heat Treatment Processes.” LinkedIn SlideShare, 4 May 2017, Available here.
2. Wojes, Ryan. “What Is Annealing in Metallurgy?” The Balance, Available here.
3. “Annealing (Metallurgy).” Wikipedia, Wikimedia Foundation, 23 Dec. 2017, Available here.

Image Courtesy:

1. “Heat-Treating-Furnace” By Ichudov – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Annealing a silver strip” By Mauro Cateb – Own work (CC BY-SA 3.0) via Commons Wikimedia

About the Author: Madhusha

Madhusha is a BSc (Hons) graduate in the field of Biological Sciences and is currently pursuing for her Masters in Industrial and Environmental Chemistry. Her interest areas for writing and research include Biochemistry and Environmental Chemistry.

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