How to unbraze silver buttons from copper switchgear contacts

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?
Thanks
Reply to
Ignoramus6092
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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
Reply to
Jon Elson
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.
Reply to
Ed Huntress
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.
Reply to
Ed Huntress
I only have two, I would say I can make 1-1.5 ounce of silver from the contacts.
i
Reply to
Ignoramus6092
In that case, it may be possible to saw off the back part, and then grind a bit until you hit the silver layer from the back. That will get the silver out no matter how the thing was made.
Jon
Reply to
Jon Elson
I just use a Presto-lite torch and replace with either Sterling or Fine silver plate stock I get from a jewelry making supply store. Use their soft sheet solder and spray flux.
Reply to
Tom Gardner
I just use a Presto-lite torch and replace with either Sterling or Fine silver plate stock I get from a jewelry making supply store. Use their soft sheet solder and spray flux. My contacts outlast factory contacts many times over.
Reply to
Tom Gardner
So it costs you 10 bucks in electric current and an hour or so of time (mine runs $35.00/hr for odd work) To make about 5 bucks?
How about a simpler solution? Take the contacts, clamp them so the buttons are hanging down. Then hit the back side of the contacts with a MAPP torch and let the contacts fall off. Might take 5 minutes to do.
Reply to
Steve W.
About 10 degrees below the melting point of the silver???
Reply to
clare
But which alloy is he dealing with? Most electrical contacts are made of coin silver, which melts at a lower temperature.
Reply to
Ed Huntress
Grinding copper is mnot exactly fun, do not ask me how I know.
i
Reply to
Ignoramus6092
Thanks. I may want to just scrap them as is.
i
Reply to
Ignoramus6092
Of course you do not want to grind copper or other soft metals; they will just mess up your stone. But sanding with a belt sander works pretty well if you do not try to remove material too quickly (otherwise the paper heats up too much.)
Reply to
anorton
It may not be worth trying to get them apart, but, for the record, what I would do is this: Heat them to around 1,200 def. F in a heat treating furnace, or just heat them good with a torch, to a low red color (maybe 1,200 = 1,300 deg. F). If they're brazed, they'll pop apart. If they don't, they aren't worth screwing with. I'd just scrap them as is at that point.
Reply to
Ed Huntress
I will do just that, perhaps with O/A instead.
i
Reply to
Ignoramus6092
I'd just heat the copper backing with my torch until the braze lets go or the terminal melts
Reply to
clare
Yeah, as a practical matter, I agree. And if it all just goes "schlump," then don't do it again.
Brass goes "schlump" very easily when you silver-braze it, if the braze melts at too high a temperature. But copper does not.
Reply to
Ed Huntress
Some things you need to know.
Some silver contacts contain cadmium. If the contact does not, the solder may. Don't do anything with them without a breeze coming from the side, to carry fumes away from you.
Some silver contacts are not silver along, but sintered tungsten, filled with silver. They will have a waffle pattern on the back side, obvious when you remove the contact from the buss. The silver can be leached by prolonged heating in distilled water and nitric acid. Keep the container (beaker) covered with a watch glass to prevent losses of values.
Contacts are easily removed by heating with a torch. I've done, literally, hundreds of pounds of them through my many years of refining precious metals. Want to get them off easily? Hold the buss with a pliers, while heating the contact. Apply heat directly on top, and keep watch on the solder, below. When you see it's molten, rap the buss on the edge of a coffee can. That dislodges the contact and the bulk of the solder, both of which will end up in the coffee can. Done.
Don't toss the silver with the base metal. Silver is now over $30 troy ounce.
Harold
Reply to
Harold & Susan Vordos
Nope. I never encountered coin silver in contacts, not ever.
Harold
Reply to
Harold & Susan Vordos

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