Flame hardening

In Flame hardening, the material to be hardened is heated with an oxy-acetylene torch to a high temperature till the surface has become austenitic.  The material is then quenched in a spray of water.  The austenite is converted to martensite and the surface is hardened. 

It is difficult to control depth of hardening in flame hardening. Along with the surface getting hardened, other properties such as bending and torsional strength and fatigue resistance are also improved.

Flame hardening can be used to harden the surface of any metal.