Editing Case-hardening (section)

=== Flame or induction hardening ===
{{Main|Differential heat treatment}}
[[File:Flame hardened sprocket.jpg|thumb|A flame-hardened sprocket. The discoloration around the teeth delineates the area that was rapidly heated and then quenched.]]

Flame or [[induction hardening]] are processes in which the surface of the steel is heated very rapidly to high temperatures (by direct application of an [[oxy-gas]] flame, or by [[induction heating]]) then cooled rapidly, generally using water; this creates a "case" of [[martensite]] on the surface. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Unlike other methods, flame or induction hardening does not change chemical composition of the material. Because it is merely a localized heat-treatment process, they are typically only useful on high-carbon steels that will respond sufficiently to quench hardening.

Typical uses are for the shackle of a lock, where the outer layer is hardened to be file resistant, and mechanical gears, where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure.
Flame hardening uses direct impingement of an oxy-gas flame onto a defined surface area. The result of the hardening process is controlled by four factors:

* Design of the flame head
* Duration of heating
* Target temperature to be reached
* Composition of the metal being treated