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.