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Brazing Furnaces: Vacuum vs. Continuous-Belt

By Dan Kay

vacuum furnace 1
Fig. 1 -- Typical vacuum brazing furnace (front-loading).

People sometimes ask me to help them determine if it is better for them to purchase a vacuum furnace or a continuous-belt furnace for their particular brazing needs. This important decision (for any brazing company) should not be a difficult question for them to answer for themselves, and involves understanding primarily three (3) key factors about their production: what is the quantity of brazed components that they need to produce, what is the sensitivity to oxygen of any of those base metals that they are planning to braze, and thirdly, do any of those base metals contain elements that will easily and readily outgas when heated.

Figures 1 and 2 show what a typical vacuum furnace and a continuous-belt furnace look like. These are just examples, and many modifications to each of these is available.

One of the first things to note about a vacuum furnace, as shown in the photograph in Fig., 1, is that it is essentially a batch-type furnace, i.e., it processes one load of parts at a time. Parts are loaded into the furnace, the door is closed and the load is heated up to brazing temperature. When the brazing cycle is completed, the load is cooled to room temperature, the door is opened, the load is removed, and the process then repeats itself for the next load of parts.

cont belt 1
Fig. 2 -- This continuous-belt furnace is known as a “hump-back” continuous-belt furnace due to the elevated heating chamber (hot zone) shown in the photograph.

By comparison, a continuous-belt furnace, as its name implies, is a furnace, open at both ends, in which a long, metallic-mesh belt is constantly moving though it in a continuous, endless loop. Parts to be brazed are loaded onto the belt at the front of the furnace, then proceed into a heating chamber (hot zone) on a steady, non-stop basis. The belt has a speed-adjustment, which can be varied, so that the belt can carry parts through the furnace at different speeds, as needed. The brazed components are carried from the hot-zone through a cooling zone, and when the belt exits the back end of the furnace the brazed components are removed from the moving belt. The belt then loops around a large-diameter roll at the end of the furnace and heads back toward the front of the furnace, where it will begin its next run through the furnace.

Let’s examine further the two factors that need to be considered when evaluating continuous-belt processing, versus vacuum-furnace brazing.

1. Quantity of brazed parts needed. Probably the most important consideration in deciding between a vacuum-furnace (which is a batch-furnace) or a continuous furnace is the volume of parts you will need to produce each day. If you need to produce 1000 parts per month, then you may find batch processing quite adequate for your needs, since that would only represent about 50-parts per day produced, assuming a one-shift, 5-day work-week. But, if you need to produce 5000 parts per day, then you would probably opt for a continuous belt furnace, since that represents about 625 parts per hour, again assuming only a one-shift operation per day. Continuous-belt furnaces are designed to primarily handle very-high production volumes each day. 

2. Base metal sensitivity to oxidation. The second major concern in evaluating continuous-furnace vs. vacuum is the reactivity to oxygen of the base-metals being joined. It must be remembered that most continuous-belt furnaces utilize a dry gaseous atmosphere, such as pure, dry hydrogen, or nitrogen, argon, or a blend of nitrogen and hydrogen, etc. The degree of “dryness” or “wetness” of any atmosphere used for atmosphere-brazing is determined by the dewpoint of that gas, and should always be measured at the furnace (not back in its storage tank). All gaseous atmospheres used for brazing will, in fact, contain some moisture (as indicated by the dewpoint of that gas), and moisture, of course, also represents the presence of oxygen in that atmosphere. The presence of oxygen will encourage the formation of oxides on metals whenever those metals are heated to elevated temperatures.

Example: Titanium is a metal that will quickly oxidize when heated in any kind of gaseous atmosphere, and these tenacious titanium-oxides, once formed, cannot be “reduced” (eliminated) in that furnace atmosphere by any known means, and will thus render the surface of the titanium parts to be non-brazeable. Thus, it is not wise to attempt to braze titanium in a gaseous atmosphere, and therefore, not in a continuous-belt atmosphere furnace.

REMEMBER: Brazing filler metals will not bond to, or flow over, oils, lubricants, dirts, or any oxides on the metal surface!

Thus, if any metal surface that you want to braze is covered with oxide, it will NOT braze! The oxides must be kept from forming in the first place.

3. Outgassing potential of metallic elements. The third major concern relates to the chemistry of the base-metals and the brazing filler metals (BFMs) that will be used in a brazing furnace. Is there any cadmium, zinc, or lead in any of the metals being introduced into the brazing furnace? If so, any of these three elements will easily and readily outgas when heated up toward brazing temps, contaminating the inside of the furnace, and in the case of a vacuum furnace, the complete pumping system as well. Outgassing of these volatile elements can also significantly increase the void content in the braze joints as those volatile elements form gas bubbles in the liquid BFM.

In the past, a number of steels used lead as an ingredient for free-machining capabilities. Lead is rarely used any more, but it should be noted as a dangerous element in brazing. Zinc and cadmium are metals added to many silver-based BFMs to help lower their melting point and increase their flowability. Both of these ingrediaents readily outgas, and althrough their fumes might be swept aside in a continuous-belt atmosphere furnace, they can seriously contaminate and injure any vacuum furnace, and should never, ever be used in vacuum-brazing.

Many copper alloys, such as brass and bronze, are commonly brazed in atmosphere belt-furnaces, but because brass typically contains zinc, brass should never be brazed in any vacuum furnace.

I’ve previously written about using partial-pressures in a vacuum furnace to suppress the outgassing of certain metals (such as copper, silver, etc.), but please understand --- and note --- that zinc (as well as cadmium) have no place in any vacuum brazing process under any circumstance whatsoever!

Chart comparing atmosphere qualities

An interesting comparison of the amount of moisture (and therefore oxygen) in a furnace atmosphere and in a vacuum furnace is shown in Fig. 3. Notice the vertical dewpoint scale on the left side of the chart, and the vacuum level shown on the right vertical axis of the chart.

MetalOxide Chart sm
Fig. 3. Metal/Metal-oxide chart showing effect of atmospheres, or vacuum, on the formation or reduction of metal oxides.

It is interesting to note that the dewpoint of a high-quality, dry gaseous atmosphere in a continuous-belt furnace might typically be about -75°F (-60°C) inside the furnace. When a horizontal line is drawn across that chart in Fig. 3 from the -75°F dewpoint line all the way over to the vacuum scale, it’s approximately equivalent to about a vacuum level of 10-2 Torr! Alternatively, look at where a standard vacuum level of 10-4 Torr crosses the dewpoint line when extended from the right axis over to the left dewpoint axis: it’s approximately equivalent in atmosphere “dryness” to a dewpoint of -130°F (-95°C).

It should therefore become evident that most clean vacuum furnaces, when operated correctly (with a low leak-up rate) will have an atmosphere quality far superior to that which is possible in any atmosphere continuous-belt furnace.

Based on the information provided above, the reader can understand why titanium brazing is typically done in a vacuum furnace rather than in a continuous-belt atmosphere furnace.

The reader is advised to consider the oxidation potential of any metal they wish to braze, and determine if that kind of metal can be successfully brazed in gaseous atmospheres as well as in a vacuum furnace, or if the nature of their oxides is such that that metal should only be brazed in a vacuum. Carbon steels, and many stainless and high-temp super-alloys can be brazed in both vacuum or in a continuous-belt atmosphere furnace, but some metals (such as titanium) cannot.

CONCLUSION: Determining what kind of furnace to use in your brazing shop should not be too difficult a matter when you take into account the factors I’ve described in this article: production quantity needs, the sensitivity of the metals to oxygen and oxidation, and the outgassing potential of any of the elements in the metals you desire to braze. Happy brazing!


DAN KAY BRAZING SEMINARS: 3-Day seminars provide intensive training in "The Fundamentals of Brazing", covering the brazing of materials ranging from aluminum to titanium to ceramics! FIND OUT MORE

Dan Kay - Tel: 860-651-5595: - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/

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