On Sun, 22 Jan 2012 16:34:53 -0500, Ed Huntress
wrote:

On Sun, 22 Jan 2012 15:17:16 -0600, Jon Elson
wrote:

Ignoramus6092 wrote:

I have a couple of medium voltage switchgear contacts, with seemingly
silver buttons brazed to them. I have a little heat treating oven. How
can I use it to unbraze the buttons, to what temperature should I set
it?
Man! Without knowing a bunch about the silver alloy and the brazing
alloy, it is pretty hard to know what would be the magic temperature.
If you had a barrel of them, you could do a couple tests to find
the temp.

Jon

Pure silver melts at just under 1,600 deg. F. All, or nearly all, of
the silver binary alloys melt at lower temperatures, down to around
800 deg. F.

The most commonly used silver alloy fror ordinary electrical contacts
is coin silver, which is a binary with 10% Cu. It melts at around
1,430 deg. F.

Silver brazing alloys melt at somewhat lower temps. So, disregarding
the unusual low-temp silver binaries, I'd start at around 900 deg. F
and start raising the temp until you can pop the contacts off of the
copper.

There's a slim chance that they aren't brazed on at all, but just
diffusion-bonded in a furnace. To do that, you'd clean and flux the
contact area between the silver and copper, put them in an oven, and
soak them at around 1,500 deg. until the silver and copper mutually
diffuse and form a thin layer of silver/copper binary alloy. To get
them to separate, you'd want to soak them at the same temperature
again, or slightly higher, and pop off the silver as quickly as you
can, before a bigger area diffuses out.

Correction: The 1,430 deg. F melting temp for silver/copper is for the
eutectic, which is 28% copper. Coin silver, with 10% copper, melts at
a somewhat higher temperature.

The process would be the same, but it requires a higher temperture to
separate a brazed or diffusion-bonded contact. On the other hand, the
1,500 degrees I mentioned would be too high. Probably between 1,450
and 1,500 should do it, if the bond is diffused. But try lower first,
around 1,400, on the stronger possibility that the joint is brazed
with a lower-temp alloy.

If you soak it too long, you'll create new intermetallic compounds and
you may have a b&%ch of a time getting them apart. Work as fast as you
can.

--
Ed Huntress