There are a number of different heat treatment processes that can be used to change the hardness of steels.

1. Heat treatment such as an increased austenitising temperature to increase the prior austenite grain size before cooling (quenching). An increased austenite grain size increases the steels hardenability thereby promoting the formation of harder transformation products such as bainite and / or martensite.
2. Induction hardening: this involves selective heating of the surface region of the gear using induction coils. Induction heating heats the surface layers of the steel into the austenite region and, on subsequent cooling (quenching), higher hardness structures such as bainite and / or martensite are formed. Induction hardening is capable of producing a range of case depths (depth to which the material is hardened) by using a range of heat inputs. This technique can be applied to a range of compositions and produces a limited amount of distortion.
3. Flame hardening: this is a similar process to induction hardening but uses an oxy-acetylene flame to heat the sample. It is a less well controlled technique compared to induction hardening.

For all these processes the maximum surface hardness that can be achieved depends upon the steel composition, in particular the carbon content (since carbon has the largest effect on the hardness of martensite).