Heat treatment is a popular way to alter the mechanical properties of certain metals. One of the best ways to adapt a metal to the requirements of a given environment or application is to be able to modify its strength, toughness, and hardness without significantly changing its chemical composition. Metal can be heat treated in a variety of ways, but quenching is one of the most widely used techniques.

What is Quenching?

Quenching is a step in many heat treatment processes that involves heating the part to the required temperature and immersing it in a quenching medium to quickly cool it. Other heat treatment processes often follow quenching, such as aging, tempering, or annealing to achieve the desired results. By rapidly cooling the metal, you can prevent or encourage the formation of different microstructures within the steel to achieve the properties you need for the intended application.

The various types of crystalline microstructures include:

·Austenite: Austenite forms when iron is heated during metalworking. It is a tough yet formable microstructure that is excellent for welding. Austenitic steel has a variety of applications, but it is often further worked to create martensitic steel.

· Martensite: Martensitic transformation is what happens when you rapidly cool heated metal. While martensite steel is hard, it is also very brittle. Working the metal after quenching helps to restore some ductility to it and relieve internal stresses.

· Ferrite: Ferrite is a soft, thermodynamically stable microstructure that forms before transforming into austenite. It mostly occurs in low-carbon steel.

· Cementite: Cementite is a thermodynamically unstable microstructure that contains iron and carbon. Its rigid microstructure results in a very hard yet brittle metal.

· Pearlite: Slow cooling results in a pearlite microstructure, which is composed of alternating layers of ferrite and cementite. This structure is strong and lightweight with high wear resistance.

· Bainite: Bainite is a hard, brittle microstructure that forms when a steel component is cooled faster than the cooling rate of pearlite but slower than martensite.

Type of Quenching Media

The quenching process can be carried out using a range of quenching media. Every medium has special quenching qualities of its own. Quenching speed, quenching environmental problems, quenching media replacement, and quenching media cost are factors to take into account while determining the type of media use.Here are the main types of quenching media:

• Air
• Air is a popular quenching media used to cool metals for quenching. Affordability is one of the main benefits of air; its affordability is a result of its profusion on earth. In fact, any material that is heated and then allowed to cool to room temperature simply by being left alone is considered to have been air quenched. Air quenching is also more intentionally performed when it is compressed and forced around the metal being quenched. This cools the part more rapidly than still air, although even compressed air may still cool many metals too slowly to alter the mechanical properties.
• Water 
• Water is able to quench heated metals rapidly as well. Steel may be quickly cooled by quenching it in water, which is great for reaching the highest possible hardness in some applications. However, because of internal tension buildup, its cooling speed can also cause distortions and cracking. Consequently, additional metallurgical procedures are needed to complete the component.
• Water, like many gases, is easily obtained and has no effect on the environment, making it a sustainable quenching medium. Steam pockets can impede the process of quenching with water; if this happens, the smith must swirl the component in the medium to keep wet air pockets from forming and introducing contaminants.
• Oil 
• Oil is able to quench heated metals much more rapidly than compressed air. An element that has been heated is dropped into an oil-filled tank to be quenched. The part can also be flushed with oil. Because different types of oil have different cooling speeds and flash points, they are frequently employed in different applications.
• Brine 
• Brine, or salt water, makes an excellent quenching medium for rapid cooling. It cools faster than air, water, and oil. This is because, when the mixture of salt and water comes into contact with a hot metal, it prevents the creation of air globules. As a result, the liquid will cover a larger portion of the metal’s surface than air bubbles will.

Quench Hardening Steel

Steel deserves a special mention when the quenching process is being discussed because its mechanical properties are very sensitive to quenching. Through a quenching process known as quench hardening, steel is raised to a temperature above its recrystallization temperature and rapidly cooled via the quenching process. The rapid quenching changes the crystal structure of the steel, compared with a slow cooling. Depending on the carbon content and alloying elements of the steel, it can get left with a harder, more brittle microstructure, such as martensite or bainite, when it undergoes the quench hardening process. These microstructures result in increased strength and hardness for the steel. However, they do leave the steel vulnerable to cracking, with a large reduction in ductility. For this reason, some steels are annealed or normalized following the quench hardening process.