Remember, good surface-prep prior to brazing requires that any residual oxidation and/or contamination on the component surfaces has to be removed before placing the assembly into the vacuum furnace! Thus, parts going into any brazing furnace should have clean braze-surfaces right from the start, and the use of a proper furnace atmosphere (gaseous or vacuum) will then keep them clean throughout the entire brazing cycle. Thus, some people ask: “If fluxes keep oxygen away from base metals, and a vacuum-furnace pumps air out of the furnace so that oxygen will not be present to react with the base metals, wouldn’t it make sense to combine those two processes in order to take advantage of the oxygen-absorption/oxide-prevention capability of both?” The answer is a resounding “NO” !! In a similar manner, a vacuum furnace used for brazing has an effective pumping system that removes the air from the vacuum chamber, thereby keeping oxygen away from the base metals that are to be brazed. This is necessary since molten brazing filler metal (BFM) will not bond to oxidized surfaces. 1 - Photo of a typical plastic brazing flux jar containing paste flux for brazingīrazing pastes that are used for torch or induction brazing usually contain fluorides (an acid) and borates so that the flux paste will be effective at reacting with, and absorbing, oxygen, thereby preventing that oxygen from reacting with the base metals being brazed, and thus preventing the formation of oxides on those base metals. 1, are thick paste products, with the consistency of thick mayonnaise, and the paste is either white or black in color. The brazing fluxes that people usually refer to when asking about the use of “a flux” in a vacuum furnace are the same fluxes that they use when hand-brazing with a torch or induction coil out in the open air. It is time to once again bring up that subject for today’s brazing community, in order to be sure that everyone understands that you should NEVER put a brazing flux in a vacuum furnace! A number of years ago I wrote an article for this website about the use of a brazing flux in a vacuum furnace, in answer to an inquiry I had received about the advantages or disadvantages of using a paste flux to enhance the brazeability of components in a vacuum brazing furnace.
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