We silver brazed what we thought was a so-called “oxygen-free” grade of copper (C10200) in our hydrogen brazing furnace at 1500°F (815°C) – using BAg-8 filler metal – and the parts did not braze well at all. The parts came out of the furnace with blisters and cracks on their surfaces. When we analyzed the now-ruined copper, we found it to actually be Copper-110 (UNS C11000), i.e. tough-pitch copper, rather than an oxygen-free grade. The parts came with a certification that it was C10200! Why does this blistering/cracking occur with C110, and what can we do to prevent it from happening again in the future? What about the certification?
Copper-110, also specified as C110 or UNS C11000 and known as electrolytic tough pitch (ETP) copper, has about 0.02-0.06% oxygen in its matrix as cuprous-oxide, primarily along the grain boundaries. Because hydrogen atoms are so small, they can easily penetrate deeply into any metal matrix when that metal is heated in a hydrogen-containing atmosphere.
In your case, you heated the parts to 1500°F (about 815°C) and the hydrogen atoms penetrated deep into the C110 matrix. It reacted with the oxygen encountered to form H2O molecules, which, under the high heat of your brazing process, then formed steam. The expanding steam can form blisters on the metal and crack the grain boundaries, etc. This is a nonreversible reaction and is a good example of what we call “hydrogen embrittlement” of the metal.
To prevent this problem, C110 (ETP) copper should never be brazed in hydrogen but only in a nonreactive gas such as argon or nitrogen. It can also be brazed in a vacuum atmosphere if precautions are taken to perhaps use a partial pressure of argon in the furnace to prevent any outgassing of the copper itself. In fact, it’s probably wise to braze ALL copper parts in such nonreactive atmospheres, just to be on the safe side, since there is usually some tiny amount of oxygen present in all grades of copper.
Now, as far as the certification you received with the copper sent to you for brazing, please understand that such mistakes are rare. The vast majority of certs sent to folks are good and accurate representations of what you have received. Having said that, however, there are occasions (though rare) when the certification does not match the material. Someone at the material supplier selected the wrong material by mistake and for whatever reason failed to see that he/she was sending the incorrect material with the certification. Again, this is rare. But IF this happens, then that certification suddenly takes on new value, since it is a legal document. You can now use that to go back to your supplier to seek restitution for the mistake they made.
A word of caution about such restitution ... In this litigious society, I personally recommend some compassion. Everyone makes mistakes. May I suggest that you follow the motto “Do unto others as you would have them do to you.” If you had made that mistake of sending out the wrong material, how would you want your customer to respond to you? You should do likewise.