From very early times in working with metals, craftsman came to see that specific processes, including the use of heating and rapid cooling, brought about different changes in the hardness and strength of metals.
While early metal workers did not have the benefit of the science of today, they recognized that an effective way to make metal stronger was to use rapid cooling in water or oil, which is known as quenching.
The Science
Quenching heat treatment, also known as quench hardening, is well understood today. It is the process of heating steels to a specific point and then cooling them rapidly to prevent molecular changes and phase transformation which reduces and controls internal grain size in the metal part or component.
This is important as the new, smaller crystal grain structure is a martensite structure. In steel that is not quench treated, there is a pearlitic grain structure, which is softer and not as durable or strong. By heating and rapid cooling, the pearlite grains are dramatically reduced and replaced with the stronger, more durable martensite structures.
The Process Used
As with all heat treatments used today, there is a great deal of precision in the specific heat, duration of heat and the quenching heat treatment. The heating has to be uniform across the part, and it must be raised above and maintained at the specific temperature required to cause the pearlitic structure to transition.
The quenching or cooling process can be done in a liquid, vacuum or an air furnace. During this short soaking process, temperature also has to be carefully maintained.
After the soaking, the final stage of the quenching heat treatment is to cool the metal through the quenching stage. This can include water, oil or different types of gases. Usually, to reduce the brittleness developed with this type of heat treatment, the part is later tempered to increase ductility.