Some day soon, you may be using precision gears that have been cryogenically treated to increase their wear and strength characteristics. Why? The machine tool industry uses this super- cold treatment to increase wear resistance of cutting tools. Researchers believe this technology can carry over to gears, with benefits of higher strength and better dimensional stability in addition to increased wear resistance.

INFAC project

Based on favorable reports from the cryogenic industry, the Instrumented Factory for precision gears (INFAC) recently began studying the potential of cryogenics for increasing the fatigue life, load capacity, and wear resistance of helicopter gears. Additionally, the project is intended to determine a carburizing process that, in conjunction with cryogenic treatment, optimizes these characteristics. Sponsored by the U.S. Army’s Aviation & Troop Command (ATCOM), INFAC is conducting the work at their Heat Treatment Center (HTC) in Melrose Park, Ill., a facility dedicated to developing advanced heat treating techniques and extending these technologies to industry.

Led by INFAC’s Michael Skrzypchak, a team of researchers are testing the effect of cryogenics on steel microstructure, retained austenite, and surface hardness, as well as the effect of carbon level and temperature in the carburizing process on retained austenite. With this information, they plan to establish an optimal treatment that increases wear resistance and bending fatigue life of gear materials while minimizing residual stress and distortion.

Helicopter gears

The INFAC project focuses on the needs of helicopter transmissions, which require high-capacity gears with long fatigue life. These gears must also be dimensionally stable so they don’t affect meshing of the gear set, which causes noisy operation. The aerospace industry has long used cryogenic treatment (–110 F) to stabilize the dimensions of these precision gears.

Research shows that a certain minimum amount of retained austenite (about 20%) in steel parts gives higher fatigue life. For example, one test indicated 1.66 times longer gear life when retained austenite was increased from 17% to 40%. However, this benefit comes at a loss of dimensional stability and increase in drive train noise, an unacceptable condition for helicopter gears. Therefore, researchers must carefully control the amount of retained austenite to balance the conflicting needs of dimensional stability and fatigue life.

Noise is usually a lesser problem in automotive and agricultural applications. However, as the need for quieter transmissions in cars and trucks increases, the amount of retained austenite may need to decrease in order to reduce dimensional change and backlash.

New helicopters and modified weapons systems continually tax the life of gear materials, so that designers increasingly turn to so-called “new” alloy steels. These “new” steels have been around for 5 to 25 years, but most have not been used in production transmissions. AMS 6265H (SAE 9310), Pyrowear 53, and Vasco X2 are the three most commonly used alloy steels in helicopter transmissions. In the initial tests, INFAC is investigating only the AMS 6265H material.

As mentioned earlier, previous cryogenic treatment focused mainly on tool and die materials. Gears were neglected because they typically require only a –110 F treatment to achieve the desired amounts of martensite and austenite. (Why was –110 F chosen as the de facto temperature for low temperature treating? Apparently this goes back several decades to when parts were packed in dry ice, which transforms between solid and gas at –109.6 F.)

On the other hand, researchers believe that deep cryogenic treatment (at temperatures lower than –110 F) may produce superior gear performance. One approach uses liquid nitrogen and modern control systems to slowly reduce the temperature of treated parts to –300 F. INFAC is also studying the Nu-Bit Process (NBP), a newer technique in which parts are immersed in liquid nitrogen at –320 F so they quickly reach the same temperature as the nitrogen. This process has a potential drawback: it may impart thermal shock to the parts, thereby causing cracking.

Initial findings

Though still in the early stages, INFAC tests have already produced some results. Initial tests were conducted on samples from 11 carburizing cycles of different temperatures, carbon levels, and atmospheres, using different cycle times to obtain a 0.032 to 0.042 in. case depth. Samples received either –110 F or –320 F (NBP) cryogenic treatment.

Researchers are analyzing the effect of these cycle variations on:

• Microstructure of AMS 6265 steel, especially the transformation of retained austenite to martensite in the carburized case. This transformation increases strength and fatigue life.

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