The large gears found in mining, steel, construction, off-road, marine and energy applications—massive and robust in nature—need to tackle the greatest production demands. This, in turn, means that a special emphasis must be put on the heat treating methods used to increase the wear resistance and strength properties of gears this size.

The October 2011 issue of Gear Technology featured the article “Low-Distortion Heat Treatment of Transmission Components,” which covered the combination of low-pressure carburizing and high pressure gas quenching in an automotive environment. Here, heat treating expert Dan Herring explains why oil quenching is an appropriate choice for many applications.

This paper presents how low pressure carburizing and high pressure gas quenching processes are successfully applied on internal ring gears for a six-speed automatic transmission. The specific challenge in the heat treat process was to reduce distortion in such a way that subsequent machining operations are entirely eliminated.

In recent years, there has been significant interest in expanding the use of induction hardening in gear manufacturing operations. Over the past several years, many of the limits to induction hardening have shrunk, thanks to recent advances in technology, materials and processing techniques.

Heat treat alternative offers advantages over conventional methods.

Bob McCulley of Comprehensive Heat Treat Specialists describes how even the most energy intensive industrial processes can be made "green."

Often, the required hardness qualities of parts manufactured from steel can only be obtained through suitable heat treatment. In transmission manufacturing, the case hardening process is commonly used to produce parts with a hard and wear-resistant surface and an adequate toughness in the core. A tremendous potential for rationalization, which is only partially used, becomes available if the treatment time of the case hardening process is reduced. Low pressure carburizing (LPC) offers a reduction of treatment time in comparison to conventional gas carburizing because of the high carbon mass flow inherent to the process (Ref. 1).

Part I, which was published in the September/October 2008 issue, covered gear materials, desired microstructure, coil design and tooth-by-tooth induction hardening. Part II covers spin hardening and various heating concepts used with it.

Induction hardening is a heat treating technique that can be used to selectively harden portions of a gear, such as the flanks, roots and tips of teeth, providing improved hardness, wear resistance, and contact fatigue strength without affecting the metallurgy of the core and other parts of the component that don’t require change. This article provides an overview of the process and special considerations for heat treating gears. Part I covers gear materials, desired microsctructure, coil design and tooth-by-tooth induction hardening.

Open any heat treating journal today and you’re certain to find multiple references (articles, technical papers and/or advertisements) promoting low-pressure carburizing (LPC). The uninformed might breeze by these references thinking it’s the next flash-in-the-pan, but unlike in the past, this time the process has legs.