Nicrobraz Technical Articles Library
Vacuum Furnace Pressure Considerations
Watch the Effects of Changing Vapor Pressures!
More and more brazing shops are using vacuum furnaces. These furnaces are quite complex, offering more options for heating, cooling, partial pressure, or multi-bar pressure quenching (pressure capabilities above one atmosphere). The overall effectiveness of the equipment, however, still lies with the people who program and run the furnaces.
Figure 3(1) shows a series of vapor pressure curves for many metallic elements. Each curve shows the melting point of the element (indicated by a circle along the curve) and its boiling point at one standard atmosphere (the top end of each curve). When the atmospheric pressure on an element is reduced (as in a vacuum furnace), the temperature required to begin to vaporize it decreases. For example, the curve for zinc ("Zn" near the top left) identifies its melting point at 787° F (420° C), and its vaporization point at 1663° F (906° C). However, never vacuum braze anything (base metal parts or brazing filler metal) containing elements such as zinc or cadmium! Why? Because, as you can see from the chart, the vaporization point for zinc drops to only about 570° F (300° C) at 1 micron (10-3 Torr).
By the way, what is a "Torr" and a "micron"? Figure 4* shows the various operating levels for a vacuum furnace and the terminology used to describe them. A Torr (properly identified with a capital 'T') is a measure of vacuum equal to 1/760th of atmospheric pressure. The term Torr is named after Evangelista Torricelli (1608-1647), an Italian mathematician and physicist. In 1643, Torricelli invented the mercury barometer to measure atmospheric pressure. This barometer was a long glass tube sealed at one end, filled with mercury, and then inverted in a dish of mercury. The weight of the mercury in the tube causes it to descend until the outside air pressure on the mercury in the dish balances with the weight of the mercury still in the tube. The height of the mercury in the glass tube, at sea level (at 0° C), is called "one standard atmosphere", and stands 29.90 inches (760 mm) high. This height will vary in either direction as atmospheric pressure varies.
Since we are dealing with vacuum furnaces, and very small fractions of air pressure, we will need terms to describe these minute pressures. A convenient starting point is a pressure level of only 1 mm of mercury, or 1 Torr (instead of the full 760 mm at one standard atmosphere).
However, even 1 Torr is too high for meaningful vacuum brazing operations (unless you are backfilling the furnace with a partial pressure of argon or nitrogen). Thus the micron is used to indicate levels of vacuum equal to 1/1000th of a Torr, or 1 x 10-3 Torr (equal to approx. one millionth of an atmosphere!).
Other terms used by some brazers to describe levels of vacuum are "rough," "soft" or "hard" vacuums. These descriptive terms give qualitative meaning to vacuum levels; however, they lack specific quantitative values because each term covers a broad spectrum of the vacuum chart
We recommend that vacuum brazing operations be run in the 1-0.1 micron range. Even though some brazers may evacuate their furnaces to 1/100 of a micron or below (the "hard vacuum" range), this is not necessary (except in aluminum brazing) and, in some cases, can cause problems with the furnace brazing load!
Back to our discussion of vaporization of metallic elements when vacuum levels are made "harder". If you were to use a pure copper filler metal to braze steel parts at 2025° F (1107° C) at 1 x 10-4 Torr, you can also see from the chart that you would lose some of the copper to vaporization. Even chromium (which is found in many superalloys and high temperature nickel-based brazing filler metals) could vaporize, as in chrome plated parts, if they were brazed at 2150° F (1175° C) at half a micron (1 x 10-4 Torr). Similarly, those who brag about running brazing cycles in the 10-5 to 10-6 Torr regime could risk losing some of the nickel plating from parts that are run at such temperatures.
The curves in Figure 3 are time/ temperature dependent. The longer that parts are held at the temperatures and vacuum levels shown, the greater the loss of the metallic element by vaporization. Where the element is part of a metal alloy, these curves will not specifically apply. However, they should raise a caution flag.
It is critical to ensure that as little oxygen as possible is allowed to come in contact with the parts being brazed. Small leaks in your furnace allow air (and water vapor) to enter the furnace and oxidize parts during heating, irrespective of the "vacuum level." Your best tools to ensure a high "atmosphere quality level" are a helium leak detector to aid in FLUR measurement (request our data sheet on measuring FLUR; see reply card), and the stainless T-specimen, your on-site atmosphere QC inspector. -D. Kay
*Note: For a free copy of a detailed Pressure Conversion Table (showing vacuum pressure values in atmospheres, millibars, Torr, microns, Pascal, inches of mercury, and volume and dewpoint of water vapor at reduced pressure), please call us today.
(1) AWS, Brazing Manual, 3rd ed. (Miami: AWS, 1976), p. 15, fig.2.1.
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