vav4872 wsWe’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. by Dan kay

vsmr151 wsAs mentioned in last months article, all vacuum furnaces will leak air into their vacuum chambers over time, causing the pressure in the vacuum chamber to go back up. Thus the term “leak-up rate” is used to determine just how fast that vacuum chamber is allowing air back in.

Of course, air represents the presence of oxygen entering the furnace, which is not good for brazing. The air will also contain moisture, which then becomes another source of oxygen. Moisture itself can become an adherent problem on the metal surfaces being brazed, with adsorbed moisture often being very difficult to drive off the metal surface (can be quite a problem in aluminum vacuum brazing). by Dan kay

high-vac wsAll vacuum furnaces are leaky! That’s right, all vacuum furnaces can leak outside-air into the vacuum chamber, which could potentially cause some problems with your brazing, since an air-leak represents an influx of oxygen into the furnace.

Fortunately, the problem is usually quite controllable, and regular furnace inspections can usually keep those leaks completely under control. Leaks most typically occur through some of the sealing-surfaces in the furnace, the most common leak-source being the O-ring seal-surface in the furnace door itself. As the door is opened and closed everyday, the light coating of vacuum-grease on the door seal and on the O-ring can pick up dust and dirt, which, if not properly removed regularly, might begin to initiate small holes/cracks in the O-ring seal, which can eventually open up enough to allow air to start to leak into the furnace during furnace operation. by Dan kay

brazing-symbols wsThe first, and obvious, point to make about dimensioning braze fillets on drawings is that once a braze-fillet size is specified on a drawing, it must be inspected and measured to verify compliance. This can be a meaningless and very expensive waste of time for a manufacturer when it comes to braze fillets.

The American Welding Society (AWS) document that is most commonly used for specifying how to properly place such requirements on a drawing is AWS A2.4 "Standard Symbols for Welding, Brazing and Nondestructive Examination." That document includes numerous illustrations about how to use so-called "welding-arrow symbols" to specify fillet sizes on drawings. Be very careful. by Dan kay

brazing-fluxOver the years, a strange question has sometimes been raised by people who are trying to optimize their vacuum brazing procedures. The question usually sounds something like this:

“If vacuum is good for brazing by pumping out the air from the furnace (thus, also removing a lot of the oxygen from the brazing zone that could hurt brazing), and if paste-fluxes are very good at helping to remove any residual oxides that can get onto metal surfaces that are being heated, then, isn’t it reasonable to consider that the combination of brazing in a vacuum along with the use of a paste-flux on the metal surfaces to be brazed would be highly effective for optimal brazing?”. by Dan kay

imagesIt is erroneously believed by many that brazing filler metal (BFM) pastes need to be refrigerated prior to use, or for long-term storage. This has led to misunderstandings and difficulties in the handling and use of BFM pastes in many shops, and to incorrect, misleading product labels.

Having been involved in the manufacture and testing of brazing pastes for many years, I will categorically state that there is nothing inherent in the chemistry of brazing pastes that requires their refrigeration! Some industry specs (and some cartridge labels) still state: “This paste must be refrigerated prior to use”. This very misleading statement has caused some people to believe that the paste must be cold when used in order to perform properly. This is completely wrong! Cold brazing-paste is much stiffer and harder to extrude (which can also be dangerous), and serves no useful purpose. by Dan kay

figure-1-wcMost brazing shops today will occasionally be given “rush” jobs by their good customers, and these brazing shops will usually do their best to respond as fast as they can, so that the customer will remain one of their best customers! No-one wants to do anything that would turn a good customer into an unhappy customer! by Dan kay

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