Typical Bottom-Loading Vacuum Furnace
We’ve been looking at the leak-up rate of vacuum furnaces in my last two articles, and this month’s article looks at another type of vacuum leak, that isn’t really a leak at all!
This kind of “leak that isn’t a real leak” is actually known as a “virtual leak”, and represents the outgassing of substances that have condensed onto the inner walls of the vacuum furnace during prior furnace runs. Then, when those walls get very hot during subsequent furnace runs, the condensed substances on the walls may volatilize and outgas once again into the vacuum chamber, the effects of which often appear to be very similar to an actual furnace leak.
When parts are brazed in a vacuum, anything that can volatilize easily from those parts, such as oils and lubricants from parts that have not been adequately cleaned prior to assembly for brazing, or perhaps brazing filler metal (BFM) binders, or easily outgassed metals such as zinc, copper, silver, or even magnesium (when aluminum brazing) — any of these materials may indeed outgas, and this outgassing can literally form a gaseous field in the vacuum chamber. Although much of it may be swept out of the chamber via the vacuum furnace pumping system, a portion of that gaseous field may react with other metals in the vacuum chamber that are being brazed, or may merely condense onto the furnace walls. The outgassing can be more apparent as stronger vacuums are pulled (i.e., as the vacuum gets “harder”). Thus one might expect to see more outgassing when vacuum brazing at 10-6 Torr, than when one is brazing at 10-4 Torr.
In a virtual leak, some (or all) of the condensed materials on the inside furnace walls will outgas when the furnace is heated up to brazing temperature, and will then interact with the parts that you are trying to braze. This interaction may cause unwanted oxidation of brazing surfaces, or perhaps the formation of compounds on those metal surfaces that can deter brazing. As an example of the latter, a number of years ago someone accidentally vaporized some aluminum during a high-temp aerospace brazing run, believing that the aluminum tag on the part was actually a stainless-steel tag. The parts did not braze well, and the aluminum vapors also condensed onto the furnace walls and then ruined a subsequent load of parts being brazed before it was determined that the entire hot-zone of the vacuum furnace had to be cleaned to remove all the outgassed materials that were coating the walls and hot zone components.
Remember, in summary, most vacuum furnaces today are known as “cold-wall vacuum furnaces” because their outer walls are water cooled. Thus, any materials that outgas during a brazing operation may condense onto the relatively cooler inside furnace-wall surfaces. Then, when the furnace is heated up again during the next furnace run, it is quite possible for those condensed products to re-outgas from the furnace walls, causing the pressure in the furnace to change (as measured by the vacuum instruments), giving readings similar to that exhibited by an actual furnace leak. This gives rise to the term “virtual leak”, since it SEEMS to be a leak, but actually is merely the outgassing of condensed products from the furnace walls inside the vacuum chamber.
Thus, it behooves brazing shop personnel to control (and prevent) any “virtual leaks” by keeping their furnaces clean, by regularly removing condensed contaminants from the inside walls of their vacuum furnaces, and most important, by only putting clean parts into the furnace for brazing. None of the readers have ever heard the phrase “Oh, don’t worry about cleaning those parts. The furnace will clean them up!” Or have you??