Vacuum (Low-Pressure) Carburizing: A User's Perspective
Transmission manufacturers continually are being driven to increase the horsepower rating on existing products or develop new smaller units to handle higher horsepower. Manufacturers also are striving to reduce processing costs by reducing distortion from carburizing. Gear producers know that gear fatigue strength can be increased by shot peening to increase compressive stresses, but this adds cost to the gears.
Twin Disc was introduced to vacuum carburizing in the early 1990s as a way to reduce distortion in gears, and test results showed that this was true. Also, reports from major aerospace and auto manufacturers noted higher bending fatigue experienced using this process. Since that time, Twin Disc has used commercial heat treaters to vacuum carburize its gears and found that low-pressure carburizing is the most direct way to minimize gear distortion in heat-treated gears and to improve fatigue life behavior.
In atmosphere carburizing, parts are heated in a carburizing gas atmosphere. The smaller section at the top of the gear tooth heats faster and reaches the austenitizing temperature sooner, so it begins taking in carbon before the heavier section at the root. This results in an uneven case that is deeper at the top of the tooth then at the root. Also, there typically is some intergranular oxidation at the surface due to the "water/gas" reaction. (Fig. 1)
One long boost cycle followed by a long diffuse cycle makes controlling the carbon deeper into the case more difficult. The carbon content usually is much higher at the surface and tapers off sharply farther into the case.
In low-pressure carburizing, parts are heated to the austenitizing temperature before the carburizing gas is added, which results in a very consistent case depth in all areas of the gear tooth and very close case depth control. There is no intergranular oxidation since the parts are run in vacuum (Fig. 2). Numerous alternating boost and diffuse cycles (pulsing the carburizing gas) allows a higher carbon level to be achieved deeper into the case.
Effects of process differences
The deeper case at the top of atmosphere-carburized gear teeth causes the addendum of the gear tooth to swell, or grow, changing the precision of shaped or hobbed gear geometry. If the gear tooth is not subsequently ground (shaped gears typically are not), it can force the use of higher cost special shaping cutters or additional shaving operations to compensate for the change. (Fig. 3)
It is well known that higher surface compressive stresses yield higher fatigue strength. A study by Twin Disc in 2000 of the differences in compressive stress between atmosphere-carburized and vacuum-carburized gears showed that stress is higher on low-pressure carburized test coupons. This is one explanation for higher bending fatigue reported by many users of low-pressure carburizing. In a recent study by a large company, numerous sets of gears were run under high loading until failure. A long-term Weibull analysis to predict gear life comparing the two processes showed a significant improvement (using low-pressure carburizing) of about 20,000 cycles. One furnace manufacturer claims a 30% increase in bending fatigue, which might be conservative.
Because of the better carbon control using low-pressure carburizing, much higher carbon levels (to the desired carbon content) can be attained deeper into the case. This results in a greater depth of hardness, or high hardness range (HRC 58 and higher). An extensive evaluation of many atmosphere-carburized and low pressure-carburized gears shows that low-pressure carburizing produces a high hardness range about twice that of atmosphere carburized gears. For a 1.5 mm (0.06 in.) effective case depth gear, the depth normally is about 0.4 mm (0.015 in.) for atmosphere-carburized gears and about 0.9 mm (0.035 in.) for low-pressure carburized gears (AISI 8620RH steel).
More of this "best" part of the case remains on a gear that is later ground. For example, if grinding 0.13 mm (0.005 in.) from a tooth face, an atmosphere-carburized gear with a high hardness depth of 0.38 mm (0.015 in.) leaves 0.25 mm (0.01 in.). By comparison, a low-pressure carburized gear with a high hardness depth of 0.89 mm (0.035 in.) leaves 0.76 mm (0.03 in.), which improves bending fatigue.
Low-pressure carburizing offers an advantage of not producing intergranular oxidation at the surface, which can reduce bending fatigue. There is no advantage for gears that are ground after carburizing, because the oxidation layer is removed.
Because of reduced distortion on ground gears, grinding passes have been reduced from four to three on most gears, which translates to quite a reduction in cycle time and lowers processing cost. For example, it required 16 hours to finish machine 10 parts after atmosphere carburizing compared with five hours to run 20 vacuum-carburized parts-a dramatic savings in machining time.
In atmosphere carburizing, the common quenching method is oil, and sometimes Gleason press quenching is used. Liquid quenching forms a vapor layer that insulates the part from the quenchant, slowing the cooling rate and creating differences in the cooling rate in different areas of the part. Oil agitation helps remove the vapor layer, but the vapor layer is harder to remove in pockets such as at the root of a gear tooth. Differences in cooling rate create differences in the time for the formation of compressive stresses, yielding more distortion. The combination of lower total case depth in the tooth root and the quenching affect normally results in the effective case depth at the root of atmosphere-carburized, oil-quenched gears approximately 50% of the effective case depth at the pitch line (Fig. 4).
Oil quenching also is used in vacuum carburizing, but quenching under a vacuum has an increased effect of removing the vapor layer. This results in deeper effective case depths at the root of low-pressure carburized and oil-quenched gears than in atmosphere-carburized and oil-quenched gear teeth. Effective case depth at the root equals approximately 70% of the effective case depth at the pitch line on low-pressure carburized and oil-quenched gear teeth. This also could be due to the deeper total case at the root noted on low-pressure carburized gears (Fig. 5)
High-pressure (20 bar) gas quenching (nitrogen, hydrogen or helium) in a separate cold chamber is also used in vacuum carburizing, which has yielded the least amount of distortion and highest effective case depth at the root of the gear tooth, almost equal to the effective case depth at the pitch line of the tooth. Measured effective case depth at the root is approximately 90% of the effective case depth at the pitch line on low-pressure carburized and high-pressure gas-quenched gear teeth. In addition, processed gears are very clean when they exit the furnace.
The gear tooth size, section thickness and/or material hardenability determine if gas quenching can be used on large parts. On smaller (or thin section) parts, the cooling rate can be easily adjusted to match the part size. You can also select higher hardenability materials to enable using gas quenching on larger parts. This is commonly being done in Europe. In oil quenching, a fixed-speed oil might have the correct cooling rate for a large part, but faster than needed for a small part. There is no flexibility unless there is a multiple selection of quench oils available, which is rarely the case. By comparison, with gas quenching you can change the pressure, quenching gas and gas velocity to match it to the required cooling rate without being too severe for the part geometry. This method yields the least amount of distortion possible in quenching (Figs. 6 and 7).
Tests at Twin Disc show that the low-pressure carburizing process yields improved case properties (and control), reduces distortion, is repeatable and is environmentally friendly. Along with these positive characteristics, there also are also some things to be aware of, such as the high importance of putting very clean parts into the furnace.
This was discovered during initial processing by finding processed gears having soft spots (Fig. 8). One cause was carbon stop-off paint popped off due to the vacuum and settled onto other areas of the part. Make sure to use a stop-off paint that lends itself to low-pressure carburizing. Another cause was water-soluble, boron-containing machining coolant residue on the parts. Boron, if allowed to dry on the parts, acts as a "stop off" in vacuum carburizing and is very difficult to remove. This coolant was not washed off after our final machining operation before being sent to the commercial heat treater. The heat treater washed the parts in a vacuum degreasing washer using trichlorethylene, but if the solvent is not closely monitored for cleanliness, residue can be put back onto the parts. The cleaning process, equipment, and process maintenance is extremely important.
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