By Dan Herring

This is the seventh in a series of articles in our Vacuum Heat-Treatment Series. Here we talk about vapor pressure and how it is influenced by the various materials of construction used in a typical vacuum furnace as well as the materials we process.

All solids and liquids have a tendency to evaporate into gaseous form, and all gases have a tendency to condense back into their liquid or solid form. In other words, all materials have a characteristic vapor pressure that varies with temperature. Formally, vapor pressure is the pressure of a vapor in (thermodynamic) equilibrium with its condensed phase(s) in a closed container or vessel.

Why is Vapor Pressure Important in Vacuum Systems?

In a vacuum system, we must make sure that all component parts to be heat treated, fixtures and furnace materials of construction subjected to a vacuum environment will not experience significant evaporation or volatilization (“boiling away”) of their elemental constituents at the operating temperature and pressure of the process (or at the bake-out temperature). Materials of construction for vacuum systems must, therefore, be carefully chosen to avoid this condition, as any breakdown of these materials will have serious consequences to the products being run. Affected parts may find their surfaces, and in some cases their chemical composition, altered. This is of special importance in brazing, where the base or filler metal may be affected, changing the braze characteristics.

Materials of Construction

Material that we readily associate with having low boiling points (e.g., water, oils, greases) can be expected to give us trouble in a vacuum environment. Surprisingly, other materials, including most forms of rubber, plastics and certain insulating materials, can also readily break down. Tables 1 and 2 show the vapor-pressure characteristics of a number of fairly common materials used in and around vacuum systems.

Fig. 1. Vapor pressure versus temperature for selected materials[1]

Vapor Pressure – Theory

The vapor pressure of a material is the partial pressure present in the atmosphere that surrounds it. In other words, the vapor pressure tells us how much vapor a material will produce. A high vapor pressure means that the material will readily evaporate. Every material has a characteristic vapor pressure associated with it that varies with temperature. As the temperature increases, the vapor pressure increases.

All metals evaporate as a function of temperature (first-order effect) and vacuum level (second-order effect). Equation 1 allows us to determine the evaporation rate, Q, and shows us that the vapor pressure/temperature relationship is nearly logarithmic.

(1) Qmax = 0.058Pv √M/T

Fig. 2. Vapor pressure versus temperature for selected materials[1]

Relationships such as Equation 1 allow us to create vapor-pressure-vs.-temperature curves (Figs. 1–2) for a number of common elements (metals). For example, processing aluminum, cadmium, magnesium, manganese and zinc or their alloys at temperatures as low as 400°C (752°F) may be marginally or totally impractical. This is why processing brass (a mixture of copper and zinc) is normally not done in vacuum systems, or if it is, at partial pressures near atmospheric pressure. As the temperature increases, fewer and fewer materials can be run without being affected. These curves tell us that materials such as carbon, niobium (columbium), molybdenum, tantalum and tungsten are preferred for interior hot-zone components on high-temperature vacuum furnaces operating up to 3000ºC (5432ºF).

It is important to note that alloys do not behave precisely in accordance with the curves for pure metals. The tendency in these types of systems is for the lower vapor pressure elements (those that are more volatile) to vaporize out and deposit on components at lower temperature (cold walls, element terminals, etc.).

How Can We Prevent Vaporization from Happening?

One way to overcome the problem of vaporization is to introduce a gas partial pressure in excess of the material’s vapor pressure. Different gas choices, introduction methods and controls are possible. The natural questions are how and when should they be used?

In vacuum furnaces, metals tend to volatize at temperatures below their melting points. Table 3 shows this relationship for a number of common metals. The longer parts are held at the temperature and at the vacuum level shown, the greater the loss of the metallic element by evaporation. As noted above, where the element is part of a metal-alloy system, the vapor-pressure relationship will change. The total vapor pressure of the alloy is said to be the sum of the vapor pressures of each constituent times the percentage in the alloy – although this relationship has been debated by those knowledgeable in the field.

Chromium is an example of an element that will vaporize in an intermediate vacuum level during the processing of stainless and tool steels (or more exotic alloys). In vacuum, the chromium present in these materials evaporates noticeably at temperatures and pressures within normal heat-treatment ranges. Processed above 990°C (1815°F), chromium will vaporize if the vacuum level is less 1 x 10-4 torr and parts are held for a prolonged time. 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 0.3 and 5 torr is typical for most chromium-bearing parts.

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 having an equilibrium vapor pressure at 1120°C (2050°F) of 1 x 10-3 torr is usually run at a partial pressure between 1 and 10 torr. Nickel brazing normally is done in the 10-3 to 10-4 range. However, in the 10-5 to 10-6 torr regime you run the risk of losing some of the nickel, which has an equilibrium vapor pressure of 1 x 10-4 torr at 1190°C (2175°F).

When vacuum furnaces are used for brazing operations, even at intermediate temperatures, selective volatilization must be taken into account. Besides copper, manganese and lithium (often added as a savaging element) are particularly troublesome. In vacuum, lithium should be avoided or the system should be run at high partial pressures in the order of 1 x 10-2 to 2.5 x 10-2 torr.

Vacuum acts as a reducing agent, as shown in Table 5, so that many of he common oxides present break down without the use of a reducing gas such as hydrogen (Figs. 3, 4). For example, iron and copper oxides break down at relatively low combinations of pressure and temperature while other oxides such as those of aluminum, calcium, magnesium and silicon will not break down at all under any reasonable combination of pressure and temperature.

Which Gases Can We Use?

Fig. 3. Dissociation of selected metal oxides (based on free energy)[1]

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 of 99.99% and a dew point of -60°C or lower 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).

Fig. 4. Dissociation of selected metal oxides (based on free energy)[1]

Measurement and Control

It is critical to know the exact pressure, flow and type of gas being injected into the vacuum furnace 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 such as the MKS Baratron® should be used to determine precise partial-pressure values.

For flow accuracy, flow meters 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.

Summing Up

Partial pressure atmospheres are required in many heat-treating and brazing operations to achieve the results we expect. Introduction of the partial-pressure gas into the furnace hot zone at one or more locations and controlling the partial-pressure injection gas stream as a continuous flow rather than trying to operate at a specific pressure are critical considerations. The choice of partial-pressure gas is also important both from a cost and quality standpoint.

Next time, we will begin a discussion of the interior construction of vacuum furnaces by considering hot-zone designs.

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.

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