Flame hardening is a process used to harden steel or other ferrous metals. In this process, an oxyacetylene torch is used to heat the surface of the material to a high temperature, then a rapid quench is administered to produce a hard martensitic surface.

Among the advantages of flame hardening are:

(1) The hardness may be limited to a comparatively thin casing, leaving the balance of the metal unaffected by the heat of the torch. This makes it possible to heat treat a casting or forging for desired core properties, such as ductility, toughness, and resistance to impact, and subsequently harden the surfaces to be exposed to wear.

(2) Because carbon is the principal hardening agent in steel, it is possible in many applications to use a flame-hardened plain carbon steel instead of an expensive alloy.

(3) Flame hardening is done on finished surfaces after all machining has been completed, thus saving the difficulty and expense of machining a hardened metal.

Technique. Flame hardening is done in two stages: heating and quenching. First an oxyacetylene flame is used to raise the surface temperature of the area to be hardened to just over the Ac3 critical point. Then a rapid quench traps the iron carbide existing in solid solution in the austenite to produce a martensitic structure of high hardness. The degree of hardness produced will depend on three factors: (1) the constitution of the steel before hardening, (2) the rate of cooling, and (3) the temperature of the surface at the moment quenching begins.

X