Without metal heat treatment, many industries could not operate. Metals like iron, steel, and aluminum are given special properties for hardness, corrosion resistance, wear resistance, malleability and other things. An important part of the heat treatment process is quenching or rapid cooling off. In fact, the way a metal is cooled has a huge effect on how it turns out as a finished product. Many methods employ water, oil quenching and other processes, but which one is best? Here is a look at some of the methods to help you decide.
Why douse metals in water or oil to cool them off quickly? After all, you could simply let them set and cool off naturally. Although this method is sometimes used, it is not feasible for many types of metal. For example, hardened steel cannot be created unless it is quickly cooled. This keeps the steel from transforming back to its original condition before the process can be completed. When steel is heated it reaches a state called martensite, in which the steel is chemically unstable and this is where the hardening process must take place. In some cases very rapid cooling helps to prevent crystal formation which is undesirable for many materials.
Quenching with oil is often used to achieve a slower cooling rate. This helps to keep metal from cracking during the hardening process. However, it can produce fumes. Carburized parts like gears are oil quenched.
Water quenching is achieved by placing very hot steel into water. This causes the water to evaporate and since evaporating gas cools, it removes a great deal of heat from the metal within a short amount of time. This is a rapid cooling process and is sometimes used for aluminum alloys.
When salt is added to quenching water it increases water’s ability to cool metal. In addition, it helps to remove scale. Nonferrous metal is not quenched in salt water because of the potential for corrosion. However, it is commonly used for carbon steel.
Also known as caustic soda or lye, it is a much faster method than water, brine, or oil quenching. In fact, it is a very rapid process and is used to prevent distortion or cracking in metals which require very fast cooling.
Air Cooling or Quenching
Air quenching is a cost effective method as the metal is simply allowed to cool down on its own, once removed from the heat. This method is used for high alloy steel (high percentage of alloyed metal). A good example of high alloy steel is stainless steel with its high chromium content of about twelve percent.
Which is Best?
Whether you choose water, air or oil quenching, it depends on the type of metal you are treating. One of the easiest and most efficient methods involves a process with a vacuum furnace containing quenching chambers.