Flame hardening is just that, a high temp flame is used to heat the steel then quenched in water or oil. Which liquid used for quenching used is determined by the carbon content of the steel and the desired hardness. The flame is either an oxyacetylene or oxyhydrogen one resulting in a deep hard martesite layer over a soft inner core. The flame can be applied via a single head torch or a custom design predicated on the shape of the object to be hardened.
Cyanide hardening produces a shallow hardness layer of .5 to 5 mils in depth with a hardness of 50 to 65 HRC. Pack hardening is the type used which we are generally familiar yields a case depth of 5 to 60 mils with a hardness of 55 to 65 HRC. Flame hardening yields a case depth of .5 to 8 mils with a hardness range of 45 to 70 HRC.
Large parts such a some gears and machine toolways are flame hardened.
