Fig. 1. Vacuum carburized 8620 gear


Once reserved for aerospace or nitch applications in other industries, vacuum carburizing has found its way into general specifications for automotive, gearing, bearing, oil field and heavy-equipment products. Why has this technology taken off at this point in time, whereas earlier interests in vacuum carburizing seemed to die off?

Before looking deeper into the metallurgy of a vacuum-processed component, the bright, clean surface appearance is what is noticed first (Fig. 1). This photo represents an 8620 gear vacuum carburized at 1750°F using cyclo-hexane at 10-torr pressure. Parts are typically clean, bright and have a silver luster finish.

High-Pressure Gas Quenching

Many customers prefer the associated high-pressure gas quench technology employed by most vacuum-carburizing systems. Even though capital equipment costs are higher for gas quenching over conventional oil or polymer quenching, the elimination of post washing of parts, bright surface finish and reduced distortion have been the main driving factors in favor of vacuum carburizing. Elimination of quench oil also improves the working environment. In many industries, post heat-treat machining steps have been reduced or even completely eliminated. The reduction in processing steps allows for a reduction in capital equipment required along with factory floor space and labor savings. Even though overall heat-treating capital costs may be higher, the total capital equipment required is often less. Savings are also reported in the time required to process parts from raw material to final product readiness.

There has also been a move to higher hardenability materials over conventional grades such as 1018 or 8620. This allows larger section parts to be gas quenched. There are also many new alloys available that are earmarked for high-pressure gas quenching and/or higher carburizing temperatures. Many of these new materials have already been introduced to the marketplace and are cost-effective alternatives for many applications.

Fig. 2. Root-to-pitch hardness profile of 5130 automotive gear processed at 1700°F, gas quenched and tempered at 350°F

Process Benefits of Vacuum Carburizing

Vacuum processing allows the heat-treating equipment to be located in clean room environments or in the area adjacent to production machining equipment. Atmosphere equipment is typically isolated in an area designated strictly for heat treating. With more and more manufacturing areas now being air-conditioned, the cool nature of vacuum equipment fits right in (see sidebar Fig. B).

Vacuum carburizing has also been popular in the gear industry, including those used in high-production automotive and truck transmission gearing as well as final drive gears. Root-to-pitch hardness profiles are typically in the 90-percentile range, whereas atmosphere car-burizing typically yields profiles in the 60s to 70s (Fig. 2).



Fig. 3. Lower distortion resulting from more uniform case

Many industries require low distortion. As seen in Figure 3, vacuum carburizing typically results in a highly uniform case depth throughout the part. This improvement in uniformity can be attributed to the reduced distortion often experienced. Better uniformity of part case depths typically equates to more uniform compressive stresses. Gas quenching can often further reduce distortion since the traditional vapor phase that occurs in liquid quenching is eliminated.

Fig. 4. Higher carburizing temperatures are easily obtained due to vacuum-furnace construction

High-Pressure Gas Quenching

Many customers prefer the associated high-pressure gas quench technology employed by most vacuum-carburizing systems. Even though capital equipment costs are higher for gas quenching over conventional oil or polymer quenching, the elimination of post washing of parts, bright surface finish and reduced distortion have been the main driving factors in favor of vacuum carburizing. Elimination of quench oil also improves the working environment. In many industries, post heat-treat machining steps have been reduced or even completely eliminated. The reduction in processing steps allows for a reduction in capital equipment required along with factory floor space and labor savings. Even though overall heat-treating capital costs may be higher, the total capital equipment required is often less. Savings are also reported in the time required to process parts from raw material to final product readiness.

There has also been a move to higher hardenability materials over conventional grades such as 1018 or 8620. This allows larger section parts to be gas quenched. There are also many new alloys available that are earmarked for high-pressure gas quenching and/or higher carburizing temperatures. Many of these new materials have already been introduced to the marketplace and are cost-effective alternatives for many applications.

Fig. 5. Cyclohexane vacuum carburizing repeatability tests (1625°F)

Most applications require carbon levels in the 0.75% to 1.05% range on the part surface. These levels are easily accomplished by boost and diffuse timing relationships. There are, however, some unique applications where extensive carbides are desired in the case. This results in higher surface hardness to stand up to fluid erosion and, in general, to enhance wear properties. For these applications, operating the major-ity or even the entire cycle at saturation is easy to do with vacuum processing. Most normal applications use a short boost time, running at saturation, followed by a longer diffuse cycle, which allows the surface carbon to fall to the desired final surface-carbon level. The boost and diffuse times are a function of temperature and directly related to carbon saturation but, most importantly, to the final desired surface carbon level (Fig. 5).

Fig. 6B. Sensor to detect hydrocarbon and hydrogen levels

Since the process easily obtains carbon saturation directly associated to temperature, vacuum-carburizing cycles are extremely repeatable (Figs. 5, 7 and 8). This demonstrates the repeatability of different runs conducted on different days. All runs were also high surface-area loads. It is important that the carbon-bearing atmosphere have sufficient carbon available to satisfy the demands of these loads. Cyclohexane (C6H12) atmospheres provide high levels of carbon since each molecule contains six carbon atoms. The hydrogen-to-carbon ratio for cyclohexane is good at two to one. Abundant levels of hydrogen can lower, or dilute, the potency of a carburizing hydrocarbon. Surface Combustion has also developed several direct-reading sensors for assuring satisfactory hydrocarbon flow rates. This technology assures flow rates are sufficient by actually sensing hydrocarbon levels and/or hydrogen levels in the vacuum chamber (Fig. 6).

Fig. 7. Cyclohexane vacuum carburizing repeatability tests (1700°F)

Reproducibility

Vacuum carburizing is very predictable. In conventional atmosphere carburizing, both temperature and carbon potential control case depth as well as final surface carbon levels. Both must be strictly controlled, but often carbon potential is harder to maintain. In addition, atmosphere carburizing is subject to incoming gas conditions, CO levels, etc. With vacuum carburizing, the temperature is easy to control (as with all furnaces operating in these temperature ranges), but carbon potential automatically goes to saturation (or boost). This inherent process characteristic eliminates carbon control from the vacuum-carburizing process. For this reason, the process is easily repeatable and can be mathematically predicted based on temperature along with the boost and diffusion times involved in the cycle. Another positive aspect of vacuum carburizing is that most cycles allow the load to soak out at carburizing temperature before the carburizing gas is introduced. By doing so, the load is at temperature throughout assuring that parts that may be farther from the radiant tubes or heating source are at temperature before carburizing begins. Load case depth uniformity benefits from this factor.

Fig. 8. Cyclohexane vacuum carburizing repeatability tests (1750°F)

The predictable nature of the vacuum-carburizing process also applies to different operating temperatures. Comparing the 1750°F and 1625°F carbon-gradient graphs as shown in Figures 8 and 5, we can see that the two cycles are very similar even though saturation levels and diffusion rates are substantially different for the two temperatures used.

Fig. 9. VringCARB® vacuum-carburizing technology

System Design Considerations

Surface Combustion has recently sold multiple vacuum-carburizing systems using the liquid fuel-injector technology built around a readily available, high-density, high-purity, cyclohexane hydrocarbon. One such system (Fig. 9) features individual heating zones that can be isolated from each other via integrated tight-sealing vacuum doors.

The tight-sealing vacuum doors provide process isolation from each cell or chamber. This allows for processing different items at the same time, such as hardening with nitrogen or argon partial pressure in one cell while carburizing under cyclohexane or diffusing under a hard vacuum in another cell. This isolation not only improves processing but also greatly simplifies leak testing or servicing of any given hot zone or cell.

In addition to the tight-sealing vacuum doors, each chamber has a service door. The service doors are also provided for the high-pressure gas quench, oil quench and the transfer mechanism. Any chamber can easily be isolated from production and accessed after it has cooled down and vented to atmosphere. Individual vacuum pumps are also provided to improve the overall integrity and reliability of the system.

The system shown has been provided with three vacuum-carburizing chambers and one high-temperature non-carburizing chamber intended for high-temperature processing of tool steels and stainless steels. This chamber can operate under partial pressure or hard vacuum.

Gas-fired radiant tubes utilizing pulse-firing technology heat the three carburizing chambers. The radiant tubes utilized are silicon carbide, using technology previously supplied by the manufacturer on another gas-fired vacuum furnace at an industrial-tool manufacturer located in Elyria, Ohio. The referenced gas-fired vacuum furnace has been in operation since 1997, processing at temperatures up to 1975°F.

Gas quenching is accomplished with 20-bar nitrogen-backfill capability, recirculated with an internal 400-horsepower cooling fan. Oil quenching is also provided in a separate chamber using quench oil formulated for vacuum service designed for operation between 160°F to 220°F.

An Allen Bradley PLC and a PC running Iconics Genesis 32 provide control of the total furnace system, storage of recipes, data acquisition and alarm monitoring. IH

VringCARB®, VacuDraw®, Uni-DRAW® are all registered trademarks of Surface Combustion, Inc. and PURIFIRE® & TELALERT® are registered trade-marks of Air Products & Chemicals, Inc.

For more information: Ralph Poor is Director, Standard Heat Treat Products, Surface Combustion, Inc., 1700 Indian Wood Circle, Maumee, Ohio 43537; tel: 419-891-7150; fax: 419-891-7151; e-mail: info@surfacecombustion.com; web: www.surfacecombustion.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: vacuum carburizing, low-pressure carburizing, high-pressure gas quenching, boost, diffuse