Steel is flame-hardened in the same way as many other ferrous metals—a process also known as "induction hardening," in which a metal is heated by a powerful flame and then "quenched," or rapidly cooled. The rapid cooling of the heated metal results in a final product that is harder and more brittle than the original metal.
Flame hardening is a very popular method of creating stiffer, more brittle products out of ferrous metals. It can also be selectively applied to certain parts of structures to only increase the hardness of certain parts of a finished product. Flame hardening is usually done through the usage of a burning mixture of oxygen and acetylene, such as with a blow torch. This allows the metal to be heated in its entirety or in very small concentrated areas with relative ease.
Quenching the metal after it has been heated is what gives it the properties of being harder and more brittle. There are different methods for quenching heated metals because the only requirement is a fast drop in the temperature of the object. The most common and inexpensive way to quench a metal that is undergoing the flame hardening process is to submerge it in water.