Flame Hardening

Flame hardening is a surface hardening process in which the surface of a steel part is heated very rapidly to a high temperature, either by direct application of an oxy-gas flame or by induction heating, and then cooled rapidly, usually with water, producing a hardened case of martensite on the surface while the interior of the part remains tougher and less brittle. It is typically applied to higher-carbon steels and is commonly used on components that need a hard, wear resistant surface combined with a tough core, such as lock shackles and mechanical gears. Flame hardening developed as one of several surface hardening techniques that emerged alongside the broader, much older tradition of case hardening, but the source consulted for this entry names no single inventor or exact date for when flame hardening itself was first developed, so none is recorded here.