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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• Increased surface hardness, which improves wear life.
• Compressive residual stresses, which cancel tensile residual stresses, thereby increasing load capacity or fatigue life.
• Dimensional distortion due to quenching after heat treatment. Only the first two items were analyzed to date. Results show a wide range of surface hardness and retained austenite with only a small variation in case depth.
Specific conclusions:
• Cryogenic treatment reduced the amount of retained austenite from a range of 29.5 to 83.2% to a range of 7.7 to 40.9% for the lowest temperature (–320 F), Figure 1. Cryogenic treatment also increased the surface hardness from a range of 55 to 62.4 HRC to a range of 62.4 to 68 HRC (before temper) for the lowest temperature.
• Increased carbon in the carburizing process caused higher retained austenite as expected, except for the 1,600 F runs. The highest retained austenite levels were obtained with high carbon (1.2 to 1.4%) samples and with one sample where ammonia was added to the carburizing atmosphere. For example, Figure 2 shows the amount of austenite for runs E through K, with carbon levels ranging from 0.8 to 1.4%.
• Though increasing the carburizing temperature shortened cycle time, it did not lead to higher retained austenite, contrary to common belief. No trends were evident, perhaps because the carburized parts were all quenched from 1,550 F. If they had been quenched from the carburizing temperatures (1,600 to 1,800 F), one would expect more austenite for the higher temperature runs.
• The tests also did not confirm the general belief that surface hardness is a function of retained austenite. Hardness varied from 58 to 68 HRC for amounts of retained austenite less than 30%, and from 48.9 to 65.6 HRC for amounts higher than 30%. The maximum hardness resulted from –320 F treatment for both high and low extremes of retained austenite.
Recent research by the Iron and Steel Institute of Japan (ISIJ), offers a clue to this contradictory finding. Their study indicates that material performance can be improved (without reducing retained austenite) through martensitic decomposition, formation of beneficial h-carbides, reduction of detrimental commonly formed ε-carbides, and a finer martensitic structure. The beneficial hcarbides formed only when parts were treated at temperatures below –110 F.
In general, these initial tests show how processing variables affect the microstructure of the material. By feeding this information into the ongoing study, researchers will try to determine the exact cryogenic treatment needed to obtain the desired microstructure and performance.
What’s next?
INFAC is testing 77 test coupons fabricated from AMS 6265H steel to evaluate the material at various steps of the carburizing process. Researchers have cut gear teeth on each coupon to simulate real world conditions, and tempered the coupons after deep freeze to lock in the properties until performance testing could proceed.
In the next test series, they will study the effect of deep cryogenic treatment on wear and single tooth bending fatigue properties, which are important to gear performance. This will be a big step in determining if deep cryogenic treatment imparts the expected benefits to gears. Future experiments may include other gear materials such as Vasco, a modified tool steel (H13) that was developed by Boeing and Teledyne for high hot hardness and the ability to be carburized. Though the full extent of the test program is yet to be finalized, researchers hope to complete these tests within approximately 1 year. What is cryogenics?
A cryogenic treatment exposes steel parts to sub-zero temperatures for a few minutes to 24 hr or longer. Methods include wet or dry. In the wet method, parts are immersed in a cooling medium, usually liquid nitrogen. In a dry treatment, the parts don’t contact the cooling medium.
Types of treatments include:
• Dry treatment at about –110 F, with a cycle time of less than 12 hr. This “shallow” cooling process enhances the wear resistance of steel cutting tools. But studies by the cryogenic industry indicate that lower temperatures (–120 to –320 F) improve wear life even more, so tooling and die materials last two to six times longer.
• Dry treatment in the range of –120 to –320 F. This “deep” cooling process uses a cycle time of 24 hr or longer.
• Wet treatment at –320 F. This deep cooling process uses a cycle time of only a few minutes.
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