From Citizendium, the Citizens' Compendium
It involves heating the surface to be hardened with a high temperature oxygen-acetylene flame through a nozzle until it is austenitized. Water, diluted oil, polymer or compressed air is used to quench the heated area and form martensite, thus hardening the surface. This martensite is then tempered using another flame.
Flame hardening is often automated by rotating the item to be hardened is first exposed to the austenitizing flame, then a quenchant nozzle, then the tempering flame - all three occurring simultaneously on the same item.
Flame hardening is fast and inexpensive relative to other hardening methods, such as induction hardening or nitriding, but only certain types of steel can be hardened effectively, including carbon steels containing 0.35 - 0.55% carbon and many low alloy steels. Additionally, control over the area to be hardened is impossible.
The depth and value of the hardness increase obtained depends on many variables, including metal composition, flame temperature and the cycle time for each operation.
- Dufour, Jim. "An Introduction to Metallurgy." 5th Edition. 2006.