By Dan Herring

Fig 1. Dual partial-pressure circuitry on the side of a vertical vacuum furnace (photograph courtesy of VAC AERO International Inc.)

This is the 10th in a series of articles in our Vacuum Heat-Treatment Series. This part continues a discussion begun in Part Seven (Vapor Pressure) and focuses on the use of partial pressure and related areas necessary to control vaporization and prevent damage to both the parts and the equipment .

One of our goals in vacuum furnace processing is to minimize both alloy depletion from the part surface and subsequent hot zone contamination. Many of the materials we run are processed at temperatures and pressures at which individual elements can volatilize (leave the part surface). Partial pressure systems (Fig. 1) are designed to prevent this from happening by establishing a combination of pressure-temperature-time that minimizes the vaporization of the more volatile alloy constituents.

Figure 1 key

What is Partial Pressure?

We know from our school days that all gases are made up of molecules that are in constant (and random) motion. The Kinetic Theory of Gases tells us that a gas exerts a pressure on the walls of its enclosure because of the impact of these molecules on the vessel walls. These impacts tend to be elastic in nature, hurling the gas molecules back from the wall with the same speed with which they struck. The direction of the rebound, however, is independent of the direction of the impact; the angular direction of which can be expressed by Knudsen’s cosine law (the intensity of the reflected molecular beam is proportional to the cosine of the angle between the molecule’s path and the direction normal to the surface).

In simplest terms, therefore, the partial pressure of a gas introduced into a vacuum furnace is the force exerted by the gas (or gases) constrained in the vacuum vessel. If only a single gas is present, the partial pressure of the system is the same as the total pressure. Air is a good example of a multi-gas system. At sea level the atmospheric pressure is 760 torr (760 mm Hg) while at an altitude of 3,657 m (12,000 ft) is only 483 torr (Table 1).

In vacuum systems, partial pressure typical indicates the operation of a vacuum furnace at or above 0.10 torr (100 microns). The chamber is usually evacuated to a higher vacuum level; commonly between 10-3 torr (0.1 micron) and 10-5 torr (0.01 microns) then an inert gas is introduced at a controlled rate to a fixed partial pressure range and then controlled within this range. The high vacuum portion of the pumping system is usually isolated and bypass circuitry employed using the mechanical pump so that a continuous flow of gas is introduced equal to the pumping capacity (throughput) at the required operating pressure.

Mean Free Path

In Part One of this series we touched upon a related topic, that of the mean free path of a gas (Table 2). The mean free path is a function of the gas pressure in the vacuum vessel.

The mean free path is the average distance a molecule travels between successive collisions with other molecules in a gaseous state (Table 3). The mean free path establishes the type of gas flow that will occur in the system. The mean free path is given by the following formula (Equation 1):

(E1) L=0.0086 n/p √ T/M

Knowing the mean free path allows one to know, or at least estimate, the type of flow that is occurring (Table 4). A useful formula for the mean free path in air at room temperature is (Equation 2):

(E2) L=5/p

Practical Partial-Pressure Ranges for Common Steels

What follows is a general guideline for partial-pressure settings used in industry for processing some of the more common materials. From a practical standpoint, there are two process considerations for determining partial pressure. The first is the metal-oxide reduction partial pressure. The partial pressure of oxygen at a given temperature determines the direction of the reaction and consequently whether the part is "bright" or “discolored” (oxidized). These values are typically in the range of 10-6 torr to 10-2 torr. This is why materials such as titanium alloys and superalloys must be processed at extremely low vacuum levels. The second consideration is the vaporization of metal at high temperature and hard vacuum. The metal solid to vapor partial pressures require higher pressures to avoid alloy depletion. These higher pressures often produce sufficient dilutions of contaminants to drive the reaction to be reducing.

What is often overlooked or misunderstood is that higher levels of partial pressure "dilute" any oxygen or water vapor partial pressure and can produce oxide-free "bright" parts at higher pressures. (This dilution also occurs when a retort is purged with nitrogen or argon to achieve clean processing.) The oxygen partial pressure is reduced by dilution rather than by vacuum. In addition, it can't be overstated that the oxidation on parts from exposure to the atmosphere and moisture absorbed by the furnace lining when the door is open are critical in the processing. Oxidation occurs on heat-up and when the temperature is high enough to reverse the oxidation reaction the parts will clean up. This is why it is harder to bright temper than to bright harden. In batch vacuum furnaces, combination hardening and tempering cycles are used to take advantage of not removing the parts from the furnace. Oftentimes the same parts will discolor if tempered in the same furnace after they have been removed and the furnace exposed to air.

Also, a thorough understanding of the required component properties and material characteristics (e.g. alloy composition, grain size, hardenability response) is needed to design the final vacuum heat-treat cycles and select the final partial-pressure settings.

For example, stainless steels, tool steels and more exotic alloys are run in a vacuum furnace that will benefit from the use of partial-pressure atmospheres. For example, chromium present in these materials evaporates noticeably at temperatures and pressures within normal heat-treatment ranges. Processed above 990°C (1800°F), chromium will vaporize as a function of both vacuum level and time; a vacuum level of no better than 1 x 10-4 torr (0.1 microns) being typical. Thus, the practical operating vacuum level for most materials is significantly above the equilibrium vapor pressure (Table 5). It is also helpful to know the temperature at which individual elements exceed a critical (10-6 g/cm2-s) vaporization rate (Table 6).

In addition, heat treaters often observe a greenish discoloration (chromium oxide) on the interior of their vacuum furnaces, the result of chromium vapor reacting with air leaking into the hot zone. Otherwise, the evaporation deposit is bright and mirror-like. To avoid this, an operating partial pressure between 3 x 10-1 torr and 5 torr (300 microns to 5,000 microns) is typical for many chromium bearing parts.

Stainless Steels

Fig. 2. 410 stainless steel housings (photo courtesy of VAC AERO)

Common vacuum-processed stainless steels include the 300 series (austenitic grades), which are annealed or subjected to a stress-relief cycle after hardening by cold working, and the 400 series (martensitic grades), which are vacuum hardened (Fig. 2). For either process (except low carbon and stabilized grades), a rapid gas quench is necessary to prevent carbide precipitation that will lead to a loss of corrosion resistance. Quenching with nitrogen at pressures of 2-6 bar is typical for these alloys.

Superalloys

Superalloys fall into two general categories, nickel-based and cobalt-based. Most of these alloys are hardened by solution treating in vacuum and then subjected to age hardening. Solution treating involves austenitizing at high temperature followed by gas quenching. In most cases, a very fast quench speed is not required due to the large amount of alloying present, and gas quenching with nitrogen at pressures of 2 bar or less is usually sufficient. This is followed by reheating to an intermediate temperature for extended periods of time. Certain superalloys require other special treatments to develop required properties.

High-Strength Steels

Fig. 3. Knee implants (cobalt-chromium-molybdenum alloy) vacuum heat treated under an argon partial pressure at 1 torr

High-strength, low-alloy steels in the 41xx and 43xx series are hardened by either oil quenching or high-pressure gas quenching. Modified versions of 4340, up to a critical section size of approximately 65 mm (2.5 inches), can be hardened by gas quenching with nitrogen at pressures in the range of 6-12 bar. 41xx steels having lower hardenability are usually oil quenched if maximum mechanical properties are required.

Tool and High-Speed Steels

Most tool and high-speed steels are hardened in vacuum furnaces and gas quenched. The cooling rate is dependent on the grade. Generally, the air-hardening grades (A, D, S and H series) can be consistently hardened by nitrogen gas quenching at pressures in the 2-10 bar range. Other steels (O and W series) are typically oil quenched.

High-speed steels (M and T series) usually require high-pressure quenching in the 5-12 bar range to develop full properties.

Titanium Alloys

Vacuum heat treating of titanium alloys is usually limited to age hardening and stress relief. The solution treatments that many titanium alloys require involve fast cooling rates only achievable by liquid quenching. Because of their propensity to react with contaminants, the heat treating of titanium alloys requires tight control of cleanliness and high vacuum levels. No partial-pressure processing is allowed. Titanium alloys will react with residual water, oxygen, hydrogen, nitrogen and carbon dioxide, producing brittle surface conditions such as alpha case that must be removed before the part goes into service. Precleaning of titanium is also critically important (even fingerprints!) before heat treatment.

Other Materials

Fig. 4. Brass coils annealed under a nitrogen partial pressure at approximately 800 torr

Many other materials can be vacuum heat treated. Even materials such as beryllium copper and brass (Fig. 4) can be heat treated in vacuum or in equipment employing a vacuum purge and positive pressure, + 0.5 psi (0.035 bar) provided the partial pressure suppresses zinc vaporization. (Zinc has a very high vapor pressure and is susceptible to vaporization near atmospheric pressure.) Surfaces of these materials may turn from a shiny appearance to a dull finish indicative of some zinc depletion.

Brazing

Fig. 5. Typical vacuum brazing cycle

For vacuum brazing (silver, copper, nickel), depletion of the filler metal alloy can be avoided by raising the pressure in the furnace to a level above the vapor pressure of the alloy at brazing temperature. For example, copper (Fig. 3) having an equilibrium vapor pressure at 1120°C (2050°F) of 1 x 10-3 torr (0.1 microns) is usually run at a partial pressure between 1 and 10 torr (1,000-10,000 microns). Nickel brazing normally is done in the 1 x 10-3 torr to 1 x 10-4 torr (0.1 micron to 1 micron) range since in the 1 x 10-5 torr to 1 x 10-6 torr (0.001 micron to 0.01 micron) range you run the risk losing some of the nickel which has an equilibrium vapor pressure of 1 x 10-4 torr (0.1 micron) at 1190°C (2175°F).

In brazing, the quality (dew point) of the gas and leak rate of the furnace (typically 10-20 microns per hour or less) is important variables to control.

Partial-Pressure Gases

Argon, hydrogen and nitrogen are the most common partial-pressure gases. Often, argon is preferred as it tends to “sweep” the hot zone. That is, the heavy molecule tends to reduce evaporation as compared to nitrogen or hydrogen. Specialized applications such as those in the electronics industry may use helium or even neon (if an ionizing gas is needed). Gases with a minimum purity and a dew point (Table 15) should be specified.

Certain cautions are in order. For example, nitrogen may react with certain stainless steels or titanium-bearing materials, resulting in surface nitriding. In the case of hydrogen, the normally near-neutral vacuum atmosphere can be sharply shifted to a reducing atmosphere to prevent oxidation of sensitive process work or for furnace/fixture bakeout/cleanup cycles. Embrittlement by hydrogen is a concern for certain materials (e.g. Ti, Ta).

Measurement and Control

Fig. 6. Partial-pressure control; yellow band (photo courtesy of VAC AERO)

It is critical to know the exact pressure, flow and type of gas being injected into the vacuum furnace (Fig. 6) so that the process being run is under control. Thermocouple gauges typically found on vacuum furnaces are affected by gas species (since they are calibrated for air). It is not uncommon to believe, for example, that you are running an argon partial pressure at 1 torr when in reality you are running at 0.4 torr, or with hydrogen (or helium) that you are at 10 torr when you are really at 1 torr. Absolute pressure gauges should be used to determine precise partial-pressure values.

For flow accuracy, flowmeters should have a micrometer needle valve installed in the downstream line. On many units the gas is pulsed in using a solenoid valve and setpoint control on the vacuum gauge, akin to continuous flow with a needle valve installed. Also, it is extremely important to inject the partial-pressure gas directly into the hot zone so that the gas does not short-circuit the work area.

Equipment Limitations

It is worth noting that there are some constraints to the partial-pressure setpoints due to furnace design. Some semi-continuous furnaces may require limits on the partial-pressure setpoints since, for example, load transfer doors must equalize at around 0.5 torr (500 microns). If a partial pressure greater than this is used, it can affect the function of the furnace’s load transfer system.

Another constraint on a single-chamber furnace may be the ability to measure the partial pressure if it exceeds 1 torr (1,000 microns) for, say, a copper brazing application. Many of the gauges found on these types of furnaces are calibrated in the range of 1 x 10-3 to 1 torr (1-1,000 microns), so that when you try to control in a setpoint range of, say, 0.9-1.2 torr (900-1200 microns), the gauge will flash “999," meaning you are out of the calibrated range. Many companies, therefore (in a copper brazing application), end up using setpoints of 0.7-0.9 torr (700-900 microns) so that values are recorded on the gauge.

Conclusion

Partial-pressure atmospheres are required in many heat-treating and brazing operations to achieve the results we expect. They are necessary from both a cost and quality standpoint and are easily controlled using today’s modern technology.

Next time: Part 11 of this series discusses vacuum valves, penetrations, feedthrus and flanges; where they are used, how they operate and how they should be maintained.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. Mr. Richard Houghton, Hayes Heat Treating, private correspondence.
2. Mr. Norman Sousa, The Sousa Corporation (www.sousacorp.com), private correspondence.
3. Herring. D. H., "Using Partial Pressure in Vacuum Furnaces," Industrial Heating, November 2005.
4. The Nature of Vacuum, SECO/WARWICK Corporation (www.secowarwick.com)
5. O’Hanlon, John F., A User’s Guide to Vacuum Terminology, John Wiley & Sons, New York, 1980.
6. Ipsen, Harold N., Fundamentals of Vacuum Heat Treating, 3rd Edition, 1974
7. AMS 2769 Rev B (12/2009), Heat Treatment of Parts in a Vacuum.
8. Jones, William R., "Partial Pressure Vacuum Processing – Part I and II," Industrial Heating, September/October 1997.
9. Danielson, Phil, "Understanding Pressure and Measurement," R&D Magazine, February 2003.
10. Fabian, Roger, ed., Vacuum Technology: Practical Heat Treating and Brazing, ASM International, 1993.
11. Training Manual, Vacuum Brazing and Heat Treating, VAC AERO International, Inc.
12. Mr. Dan Kay, Kay & Associates (www.kaybrazing.com), private correspondence.