Heat Treatment of Gears
Heat treatment of gears is predominantly an operation or to put it
precisely a combination of operations. In this process there is involvement
of a controlled heating process and subsequent cooling of a metal used for
gear in its solid state. This heat treatment is done for obtaining desired
properties. Today there are a lot of very advanced types of heat treating
processes are made available that fulfill the requirements of hardness and
mechanical properties. Heat treating plays a crucial role in the production
process of a gear. It imparts qualities like strength, hardness, and
toughness.
The Heat treatment though consists of a variety of steps. But essentially
the parts are heated and then quenched in an atmosphere of controlled
carbon. Care is taken with regard to heating the part to a particular
temperature and then holding for a fixed time. This temperature changes
induce the desired physical and mechanical properties in a gear. However,
attention should be given to the temperature is the cause of thermal change
leading to warpage and a change of teeth surface.
Checking the hardness of Gears
The best way to know whether the gears that have been given heat treatment
have been done to the desired specification or not is through resting. Such
testing can be done in the form of inspecting the hardness of a given gear
part that has been hardened to predefined targets set before the heat
treating. Hardness are typically rated on different scales. In the U.S.A,
Brinnell and Rockwell scales are popular. Gears are generally hardened
anywhere between Rockwell C 30- 65, but this depends upon the material,
type, size, application and lastly the hardening method.
Heat Treating Methods of gears
Fundamentally the heat treating process consists of changing the micro
structure. There are following methods of Heat Treating available for gears.
- Carburzing of Gears
- Induction Hardening of Gears
- Carbonitriding Of Gears
- Ferritic Nitrocarburizing of Gears
- Through-Hardening Gears
- Quenching of Gears
- Flame Hardening of Gears
Carbonitriding of Gears
Carbonitriding is essentially a modified version of gas carburizing. The
modification consists of bringing in ammonia into the gas carburizing
atmosphere. This adds nitrogen to the carburized case. Nascent form of
nitrogen is produced at the work surface by the disintegration of ammonia in
the heat of furnace atmosphere. The nitrogen gets diffused simultaneously
into the steel along with carbon. Generally a carbonitriding process is done
at a lower temperature and for a quicker span of time than that of gas
carburizing. Carbonitriding is one of most common type of case hardening
processes for gears. Case depths are generally restricted to 0. 03 inches.
Carbonitriding is designed for only specific types of steels. Carbonitriding
is preferred choice for plain carbon and mild steels.
Benefits of Carbonitriding
Carbonitriding case-hardening treatment provides a way to enhance the wear
and strength of relatively inexpensive and easily worked parts. Typically it
is applied to near-finished components. Along with providing a substantial
sufficient depth it also improves fatigue strength.
Ferritic Nitrocarburizing
Ferritic nitrocarburizing is now gaining prominence as a preferred choice
of heat treatment process for many a gear manufacturer. The primary reasons
being for this are:
- Ferritic Nitrocarburizing process is done at very low temperatures,
which is less than critical temperatures.
- Ferritic nitrocarburizing can replace methods of carburizing,
carbonitriding,
- Can establish a proper gradient base hardnesses, i.e. eliminating
egg shell effect.
Benefit of Ferritic
Nitrocarburizing on Gears
It has been found that after Ferritic Nitrocarburizing of Gears there has
been considerable improvement in useful life for many of the gears.
Advantages for gears include temper resistance. Heightened lubricity during
cold start. Better strength of the base metal in terms of tensile, yield and
fatigue. An increase in throughput. Also elimination of problems like quench
cracking.
