Question: We are vacuum brazing 304L stainless steel and have been trying to braze it in vacuum with a BVAg-6b filler alloy, but it doesn’t seem to want to braze. The filler metal balls up on the surface no matter how carefully we clean the stainless ahead of time, including chem-etching the surface. Our vacuum furnace appears to be clean also, and it has a leak-up rate of only about 8-10 microns per hour. The vacuum is being run at about 10e-4 or so. What are we doing wrong?
Answer: The problem you are facing has to do with trying to use a low-melting silver-based brazing filler metal (BFM) to braze stainless steel in an atmosphere where no brazing flux will be allowed, since no one should ever, ever use flux in a vacuum furnace (it will destroy the inside of the vacuum furnace).
Additionally, the furnace environment is not hot enough to effectively remove, or dissociate, the chromium-oxide layer that is always on the stainless steel. This, coupled with the fact that you cannot use a brazing flux to keep that oxide layer from growing even thicker during your brazing run in the vacuum furnace, creates a situation where trying to get molten BFM to “wet” a stainless surface will not occur. BFMs cannot penetrate through the chromium oxide, and you cannot braze to the chromium-oxide layer itself.
Trying to clean the furnace ahead of time is good from a general brazing practice point of view, but it will not help your situation of trying to braze to the surface of the 304L stainless steel. Chem-etching of the stainless ahead of time will not have a positive effect on brazing to the surface of the stainless since the chromium-oxide layer is going to steadily form and get thicker as you heat the stainless steel in the vacuum furnace up to brazing temperature.
This is something that many people do not realize (i.e., that the oxide layer on metals actually gets thicker as you heat it up to brazing temp). Many people think that once they close the door of their furnace the oxide layer can no longer form and the degree of oxide layer that exists at the time they introduce it into the furnace will remain that way throughout the entire brazing cycle. WRONG!
There will always be a lot of air molecules present in a vacuum furnace, and oxygen will be available to react with the metal as it is heated. ALL metals will react with oxygen as they are heated and will form a thicker and thicker oxide layer on their surface as the furnace gets hotter and hotter and the cycle gets longer and longer. You mention that you are brazing in a vacuum of approximately 10e-4 level. That can be OK for brazing some metals that don’t oxidize too readily, such as pure nickel, because there are lots of oxygen molecules running around in the furnace at that vacuum level and any metal that will form an oxide at that temperature will, in fact, do so. Pure nickel is one of the metals that does not form an oxide at any of the temperatures used in typical furnace brazing – low or high.
The vacuum you are using is basically just a reduced-pressure atmosphere and certainly not a “perfect vacuum.” It still contains lots and lots of oxygen molecules, which will readily react with the chromium in the stainless to form chrome oxides. The longer the time in the furnace at elevated temperatures, the thicker that oxide layer will become. Thus, there is no way you can directly braze to that stainless surface.
Instead, it is very necessary to put a non-oxidizable layer on top of the stainless to which the BFM can alloy. Pure nickel is just such a material that can be plated onto the stainless steel after the stainless has been thoroughly cleaned and prepped. Tthat job is the responsibility of the company doing the nickel plating.
Electrolytic nickel is my recommendation for you to use in your situation where you want to silver braze the 304L in a vacuum furnace. Approximately 0.0006-0.001inch (0.15-0.25 mm) of plating should be quite adequate for your needs.
Important Additional Point: In a vacuum furnace, you run the risk of volatilizing silver and copper from the BFM when you heat it in a reduced-pressure vacuum atmosphere. Therefore, it will be VERY important to also use a dry back-fill pressure of inert atmosphere in the vacuum chamber (approximately 50-100 microns of back-fill pressure) so that the silver and copper will not volatilize out of the molten BFM at brazing temperature.