Question:
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?
Answer:
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.
