Main Difference – Annealing vs Hardening vs Tempering

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 chemical and physical properties of metals and metal alloys. There are four major types of heat treatment methods as annealing, tempering, hardening and normalizing. Annealing is a heat treatment process used to soften materials or to obtain other desired properties such as machinability, electrical properties, dimensional stability, etc. Hardening or quenching is the process of increasing the hardness of a metal. Tempering is the process of heating a substance to a temperature below its critical range, holding and then cooling. The main difference between annealing hardening and tempering is that annealing is done to soften a metal or an alloy and hardening is done to increase the hardness of a metal or alloy whereas tempering is done to reduce the brittleness of quenched metal or alloy.

Key Areas Covered

1. What is Annealing
     – Definition, Process, Purposes of Annealing
2. What is Hardening
     – Definition, Process, Types of Hardening Processes
3. What is Tempering
     – Definition, Process, Austempering
4. What is the Difference Between Annealing Hardening and Tempering
     – Comparison of Key Differences

Key Terms: Alloy, Annealing, Austempering, Carburizing, Flame Hardening, Hardening, Induction Hardening, Metal, Metallurgy, Nitriding, Normalizing, Quenching, Surface Hardening, Tempering

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 critical temperature (critical temperature is the temperature at which crystalline phase of metal changes). Heating to such a high temperature makes it suitable to fabricate. After heating, the metal should be cooled to room temperature. This can be done in an oven.

Figure 1: 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.

What is Hardening

Hardening is the process of increasing the hardness of a material. Hardening increases the strength of the material. Hardening is often done by quenching. In quenching process of a metal, the metal is heated into austenitic crystal phase and then quickly cooled. Cooling can be done with either forced air, other gases such as nitrogen, oil, brine, etc. (chosen depending on the type of alloy and its constituents).

Hardening process increases the strength and wear properties of the metal. But the presence of sufficient carbon and alloy content is a prerequisite for hardening. Hardening can be done for metal alloys such as steel. However, the hardening in this way makes the metal become brittle. Therefore, the tempering process is typically done followed by the hardening process.

There are two major types of hardening processes; surface hardening and case hardening.

Surface Hardening

Surface hardening increases the hardness of the outer surface while the core remains soft. Surface hardening can be done in several methods such as carburizing, nitriding and flame hardening/ induction hardening.

• In carburizing, the metal alloy is placed at a high temperature for several hours in a carbonaceous environment.
• Nitriding utilizes nitrogen and heat. This is usually used for fuel injection pumps.
• In flame hardening/ induction hardening, heat is applied for a short time period in the form of a flame and the metal is immediately quenched.

Figure 2: A Fuel Injection Pump

Case Hardening

Case hardening increases the hardness of the surface by infusing elements into the surface of the material, and forming a thin layer of harder alloy. Case hardening increases the wear resistance of equipment without altering the interior parts.

What is 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 affects the hardness of the material. Higher temperatures lower the hardness.

Figure 3: Tempering Colors of Steel

Tempering is done by re-heating the metal alloy to a temperature lower than the critical temperature (critical temperature is the temperature at which crystalline phase of metal changes). Then the material is held at that temperature for some time, followed by cooling. The cooling can be either a quenching or an air cooling operation.

A sub-category of tempering is austempering. It is mainly applied to ferrous metals such as steel and ductile iron. It is used to improve mechanical properties of metal alloys by reducing or eliminating distortion.

Difference Between Annealing Hardening and Tempering

Definition

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

Hardening: Hardening or quenching is the process of increasing the hardness of a material.

Tempering: Tempering is the process of heating a substance to a temperature below its critical range, holding and then cooling.

Process

Annealing: Annealing process involves the heating of a metal to or near the critical temperature followed by cooling to room temperature very slowly in an oven.

Hardening: In hardening process, the metal is heated into austenitic crystal phase and then quickly cooled.

Tempering:  Tempering is done by re-heating the metal alloy to a temperature lower than the critical temperature, holding for some time and cooling.

Purpose

Annealing: Annealing softens materials.

Hardening: Hardening increases the hardness and strength of materials such as metal alloys.

Tempering: Tempering reduces the brittleness of metals.

Applications

Annealing: Annealing is used for metals and metal alloys.

Hardening: Hardening is used for metal alloys containing sufficient carbon and alloy content.

Tempering: Tempering is used mainly for steel.

Conclusion

Annealing, hardening and tempering are heat treatment processes. The main difference between annealing hardening and tempering is that annealing is done to soften a metal or an alloy and hardening is done to increase the hardness of a metal or alloy whereas tempering is done to reduce the brittleness of quenched metal or alloy.

Reference:

1. Himanshu Verma. “Heat Treatment Processes.” LinkedIn SlideShare, 4 May 2017, Available here.
2. “Metal Hardening / Metal Quenching / Metal Tempering.” Hardening, Quenching, Tempering at Metlab of Wyndmoor PA., Available here.
3. Gulfam Hussain, Material Engineer Follow. “Hardening (Heat treatment) Quenching.” LinkedIn SlideShare, 28 Mar. 2016, Available here.

Image Courtesy:

1. “Annealing a silver strip” By Mauro Cateb – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Simms fuel injection pump, Fordson tractor, Cophill Farm vintage rally 2012” By Andy Dingley – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “Tempering standards used in blacksmithing” By Zaereth – Own work (CC0) via Commons Wikimedia

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