Question: We’re silver brazing a 304L stainless component in air using BAg-7 filler metal, a torch and some black brazing flux. Although the results are good, we’re having some trouble removing the flux in certain hidden parts in the assembly. So, we thought we might get better results if we vacuum brazed the part instead using the BAg-8b filler so that flux wouldn’t be needed.
The vacuum level in the furnace was held at about 10-4 torr, and we brazed at 1650°F. But the results in the vacuum were poor, the stainless looked a bit discolored, and the silver filler metal didn’t really stick well at all. What went wrong?
Answer: As far as the BAg-7 silver-based brazing filler metal (BFM) is concerned, it is a very good BFM that can join 304L stainless quite well when torch brazing with a flux. The problem you are facing is that you decided to do this low-temperature brazing in a vacuum (thus eliminating the need for flux).
It works very nicely in air with brazing flux because the flux reacts with the chromium-oxide layer on the stainless to remove the oxide layer and keep the surface free from any new oxides that might want to form. By so doing, the molten BFM can readily alloy with and effectively join the stainless components together.
In a vacuum furnace, however, because you do not use a flux (nor should a flux ever be used in a vacuum furnace), the chromium-oxide layer on the stainless remains in place and actually gets thicker and darker as it is heated (there is still a lot of oxygen molecules in a vacuum furnace operating at 10-4 torr.) This oxide layer does not start to dissociate until you are above about 1950°F (1065°C) in that level of vacuum. Because the vacuum was being operated at about 1650°F (900°C), the molten BFM will be trying to bond to a heavily oxidized surface, which it cannot do.
To solve this problem, you should electrolytically nickel plate the stainless steel surfaces that you wish to braze (prior to brazing) so that you will be bonding to an oxide-free, pure-nickel layer at brazing temperature. Please be aware that because you are brazing in a vacuum, the silver and copper in the BFM may start to outgas, thus possibly “plating” the insulators holding the hot zone in place as well as causing voids (bubbles) in the brazed joint.
Therefore, a partial-pressure atmosphere would be recommended in your vacuum furnace to prevent this outgassing of silver and copper.