Flame hardening is often used to harden only a portion of an object, by quickly heating it with a very hot flame in a localized area, and then quenching the steel. This turns the heated portion into very hard martensite, but leaves the rest unchanged. Usually, an oxy-gas torch is used to provide such high temperatures.
Flame hardening is a very common surface hardening technique, which is often used to provide a very wear-resistant surface.
Here the application is heated by flame and quickly quenched in a bath which imparts only hardness to surface, common example is gear tooth which are flame burned and hardened for resistance to wear and tear.
4. Benefits are:
Rapid annealing of desired material areas
Minimal heating of surrounding material with acetylene as fuel gas
Quench hardening upon effective cooling
5. Limitations are
There is a possibility of overheating and thus damaging the part
It is difficult to produced hardened zone less than 1.5mm in depth.
Success of process depends on skill of operator
Not so economical, hardly used for mass production hardening