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