The key difference between quenching and tempering is that the quenching is rapid cooling of a workpiece, whereas tempering is heat-treating a workpiece.
Quenching and tempering are important processes that are used to strengthen and harden materials like steel and other iron-based alloys. These processes involve the rapid heating and cooling to set the components in a particular position immediately. Moreover, these processes have to strictly controlled.
What is Quenching?
Quenching is the process of rapid cooling after the heat treatment of a workpiece. We can do this using water, oil or air. Quenching is important to obtain material properties of the workpiece. In this process, the undesired low-temperature processes do not occur, i.e. phase transformations. Moreover, quenching can reduce the crystal grain size of materials, such as metallic object and plastic materials, to increase the hardness. Further, this process is mainly applied for hardening steel.
Usually, cast steel has a uniform, soft crystal grain structure that we call “pearlitic grain structure”. Since it is soft, it is not useful in industrial applications; thus, we can convert this structure into “martensitic grain structure”, which has high strength and therefore, highly resistant to deformation. So, we use the process of quenching for this purpose.
What is Tempering?
Tempering is a process that involves heat treating to increase the toughness of iron-based alloys. Also, this process is very important in removing some of the excessive hardness of steel. In this process, first we need to heat the metal to a temperature below the critical point for some time, and then we need to allow the object to cool in still air. The temperature determines the amount of hardness we can remove from the steel. However, the temperature at which we are going to heat the metal depends on the composition of metal or alloy and the properties of desire. For example, low temperatures are favorable for very hard tools, but soft tools such as springs require high temperatures.
In the above figure, the various colors indicate the temperature to which the steel was heated. Light-straw indicates 204 °C (399 °F) and light blue indicates 337 °C (639 °F).
Usually, in industries, we perform the tempering step after quenching. Therefore, the workpiece of the tempering process is the quenched object, and we need to heat the object with control to a certain temperature that is below the lower critical point of the object. During this heating, the grain structures of the object (ferrite and cementite) tend to convert into an austenite grain structure. It is a single-phase solid solution.
What is the Difference Between Quenching and Tempering?
Quenching is the process of rapid cooling after heat treatment of a workpiece, while tempering is a process that involves heat treating to increase the toughness of iron-based alloys. So, the key difference between quenching and tempering is that quenching is the rapid cooling of a workpiece, whereas tempering is heat-treating a workpiece.
Moreover, a further difference between quenching and tempering is that we perform quenching to increase resistance to deformation, while tempering can remove some of the excessive hardness of steel.
Below infographic shows more facts on the difference between quenching and tempering.
Summary – Quenching vs Tempering
Quenching is the process of rapid cooling after heat treatment of a workpiece, while tempering is a process which involves heat treating to increase the toughness of iron-based alloys. So, the key difference between quenching and tempering is that the quenching is rapid cooling of a workpiece, whereas tempering is heat-treating a workpiece.
1. “What Is Quenched and Tempered Steel?” ShapeCUT, 30 May 2019, Available here.
1. “ArthurSiegelcoke1” By Arthur S. Siegel – available from the United States Library of Congress’s Prints and Photographs (Public Domain) via Commons Wikimedia
2. “Tempering colors in steel” By Zaereth – Own work (CC0) via Commons Wikimedia