Heat treatment is a popular way to alter the mechanical properties of certain metals. Being able to change the hardness, toughness, and strength of a metal while keeping its chemical composition intact and virtually unaltered is a great way to tailor a metal to the needs of the environment and the demands of the job in which the metal is being used. There are many different ways to heat treat metal, one of the most popular ways is through a method known as quenching.
What is Quenching?
Quenching is a type of metal heat treatment process. Quenching involves the rapid cooling of a metal to adjust the mechanical properties of its original state. To perform the quenching process, a metal is heated to a temperature greater than that of normal conditions, typically somewhere above its recrystallisation temperature but below its melting temperature. The metal may be held at this temperature for a set time in order for the heat to “soak” the material. Once the metal has been held at the desired temperature, it is quenched in a medium until it returns to room temperature. The metal also may be quenched for an extended period of time so that the coolness from the quenching process is distributed throughout the thickness of the material.
There are a variety of quenching media available that can perform the quenching process. Each media has its own unique quenching properties. Considerations for the type of media use include quenching speed, quenching media environmental concerns, quenching media replacement, and quenching media cost. Here are the main types of quenching media:
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.
Oil is able to quench heated metals much more rapidly than compressed air. To quench with oil, a heated part is lowered into a tank that is filled with some type of oil. The oil can also be flushed through the part. Different types of oil are often used depending on the application because of their varying cooling rates and flash points.
Water is able to quench heated metals rapidly as well. It can cool a metal even faster than oil. In a fashion similar to oil quenching, a tank is filled with water and the heated metal is submerged in it. It can also be flushed through a part. One benefit of water is that flammability of the media is not a concern.
Brine is a mixture of water and salt. Brine cools faster than air, water, and oil. The reason for this is that the salt and water mixture discourages the formation of air globules when it is placed in contact with a heated metal. This means that more of the surface area of the metal will be covered with the liquid, as opposed to air bubbles.
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 recrystallisation 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 and with a large reduction in ductility. For this reason, some steels are annealed or normalised following the quench hardening process.
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