As I attempted to explain in my previous blog, carbon, an element composed of one of the smallest atoms, is capable of moving freely throughout the atomic structure which is mainly made up of iron, if we give it enough thermal energy and a little time. When rapidly plunged into water or oil, the carbon atoms partially reposition themselves while simultaneously affecting of the treated steel.
The most commonly used quenching practice involves oil, which is not as dramatic as quenching in water, especially when the steel is strongly carbon-based. As for air quenching a steel part, the desired effects are moderate, rendering the part a bit more flexible but less resistant to erosion.
On the other hand, quenching by plunging or pulverizing oil on the metal heated to more than 900°C (1650°F) is never performed without adding another heat treatment, because the part becomes a bit too fragile for normal usage. The carbon must therefore be redistributed, because it can remain stuck in the grain boundaries, these small pieces that make up the solid structure of our steel and whose size is determined by the heating and cooling temps. To avoid this issue, the part is heated to between 200 and 700°C (400 and 1300°F) without triggering atomic alterations. When the part is air-cooled, the carbon atoms have just enough energy and time to transport into the grains, resulting in a part with greater flexibility and less chance of breakage under sudden shock. This also means a more linear stretch, which is so important for springs.
Good balance between hardness, toughness and corrosion resistance
Choosing the right tempering process guarantees optimal performance and a longer service life. Unfortunately, this particular treatment is only reserved for steels. That said, while it could be very useful for other metals, their atomic structure would react differently. Each metal has its own recipe to change its properties.