What is the Difference Between Hardening and Quenching

The key difference between hardening and quenching is that hardening is the increase of the hardness of a material, whereas quenching is the heating of a metal into an austenitic crystal phase which is then quickly cooled.

Often, the hardening process is done through the quenching step. These two processes together can help to increase the strength of a particular material.

1. Overview and Key Difference
2. What is Hardening
3. What is Quenching
4. Hardening vs Quenching in Tabular Form
5. Summary – Hardening vs Quenching

What is Hardening?

Hardening is the metallurgical metalworking process that helps to increase the hardness of a metal. This process typically starts with the heating of a metal at a critical transformation temperature. The most important types of hardening processes are case hardening and flame hardening.

The case hardening process is the hardening of a metal surface that allows the metal to remain soft deep underneath while forming a thin layer of harder metal at the surface. There are different forms of case hardening processes, including cyaniding, carbonitriding, carburizing, nitriding, flame or induction hardening, and ferric nitrocarburizing.

Figure 01: A Furnace for Nitriding and Carburizing Steel

Cyaniding is a type of case-hardening process that uses sodium cyanide. This is a very fast and efficient process that is mainly useful on low-carbon steel. In this process, we need to heat the metal object or part of it at a high temperature in a bath of sodium cyanide. Thereafter, we need to quench the metal part, followed by rinsing it in water or oil, in order to remove any remaining sodium cyanide on the metal surface.

Flame hardening is a process of heat treatment where oxyfuel gas flames are directly impinged onto the gear-tooth surface area to be hardened, which is then subjected to quenching. This process can result in a hard surface layer of martensite over a softer interior core. It has a cost that is considerably less than induction hardening.

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 the 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 objects and plastic materials, to increase their hardness. Further, this process is mainly applied for hardening steel.

Hardening and Quenching - Side by Side Comparison

Figure 02: Coke being Pushed into a Quenching Car

Generally, 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 a “martensitic grain structure,” which has high strength and, therefore, is highly resistant to deformation. So, we use the process of quenching for this purpose.

What is the Difference Between Hardening and Quenching?

Hardening and quenching are two important industrial processes having many significant applications in different industries. The key difference between hardening and quenching is that hardening is the increase of the hardness of a material, whereas quenching is the heating of a metal into an austenitic crystal phase, which is then quickly cooled.

The below infographic presents the differences between hardening and quenching in tabular form for side-by-side comparison.

Summary – Hardening vs Quenching

Hardening is the metallurgical metalworking process that is useful in increasing the hardness of a metal, while quenching is the process of rapid cooling after the heat treatment of a workpiece. The key difference between hardening and quenching is that hardening is the increase of the hardness of a material, whereas quenching is the heating of a metal into an austenitic crystal phase which is then quickly cooled.

Reference:

1. Brandon. “What Is Quenching?” Materials Science & Engineering Student, 26 July 2020.

Image Courtesy:

1. “Computerised Heat Treatment Furnance” By S zillayali – Own work (CC BY 3.0) via Commons Wikimedia
2. “ArthurSiegelcoke1” By Arthur S. Siegel – the United States Library of Congress (Public Domain) via Commons Wikimedia