Quenching – Definition and Quenching Media

Quenching is the process of rapid cooling of a metal, such as steel, after heat treatment by means of quenching materials. Common quenching agents are oil, water and air. It is used to achieve certain material characteristics and is mainly used to improve the mechanical properties of steel. Usually, the process serves the purpose of hardening or strengthening steel and preventing undesired low-temperature processes like phase transformations.

How does quenching work?

Before the process of hardening, most steels display a pearlite structure, which consists of several layers of ferrite and cementite. Due to its unfavorable characteristics, the phase mixture is treated in different steps in order to reduce the pearlitic microstructure. First, the material is heated above its eutectoid transition temperature, transforming the pearlite structure into austenite. Once the phase transformation is completed to the desired degree, the work piece is ready to be hardened by rapid cooling with a quenching media to room temperature. The choice of the agent can have a severe influence on the characteristics of the material. Through this treatment, a part of the crystal structure of the workpiece is transformed to martensitic microstructure, which is much harder than austenite. The process can be extended so that an even temperature is distributed through the whole workpiece.

Quenching Media

As previously mentioned, the cooling process can be carried out with different media, which have its own unique properties and quenching speed. The most common agents are water, oil and air.

  • Water: Water is the most common quenching medium and is able to quench heated metal in a very quick manner and is often used if the metal has to reach maximum hardness. It is very popular as a cooling medium because it is practical and cheap. Furthermore, water is reusable and non-flammable, which makes it a very reliable agent. During the process, the heated metal is lowered into a tank full of water. Because of the rapid cooling characteristics of water, it can lead to some undesired brittleness, distortion, and small cracks.
  • Oil: Similar to cooling with water, a tank is filled with oil and the workpiece is submerged in it. Another similarity to quenching with water is that oil can be flushed through the workpiece. Because of the varying rates of cooling, different oils are applied depending on the application. Oil is usually used for materials that do not require extreme hardening.
  • Air: Another widely used technique is through air or gases. The popularity of air quenching is based on the affordability and availability of air. It can be said that every workpiece which is cooled down to room temperature simply by letting it alone has been air quenched, although if air is used for slow cooling, the process is usually called normalizing. Furthermore, cooling at air can be performed more intentionally when the air is forced on and around the metal.
  • Brine: Brine, a solution of salt in water, is another common quenching agent. It cools faster than pure water due to the presence of salt, leading to harder but potentially more brittle steel.
  • Polymer Solutions: These are water-based solutions with polymers added to control the cooling rate. They offer a cooling rate between water and oil, reducing the risk of distortion and cracking.

Alternatives for stainless steel hardening

An excellent way to harden materials without quenching is the BORINOX® process. With the process, you can harden a wide range of different materials. In addition, the corrosion resistance of your workpieces remains after the treatment.