Hard to unsolder copper piping? Here's why...

I ran across this exchange on the rec.crafts.metalworking newsgroup.
I'd never known the reason why it can be a tough job to pull apart an old soldered copper joint, even when you've heated it well above soldering temperatures. Turns out it's a metalurgy thing.
Enjoy.....
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I'm not likely ever to encounter gold wire in my soldering experiences. <g> Try it with some copper water tubing that was a bit overheated when the original joint was made, and almost everyone is surprised. It can be a b*tch to pull apart; it squeeks from the friction of the hard intermetallics; and it just may freeze up on you and refuse to come apart until you nearly melt the parent metal.
Most common metals that solder can wet will alloy to some degree with the solder. The result is a tutti-fruiti mess of intermetallic compounds. They embrittle the joint in many cases; they lead to unpredictable freezing and crystalization; and they can either strengthen or weaken the joint depending on what's mixed and at what temperatures. Usually, the result is bad.
But the phenomenon can be carefully engineered to produce a stronger, higher-temperature joint, when it's done intentionally.
The extreme example of this actually is a brazing technique, in which a small percentage of bismuth is added to what actually is a parent-metal alloy, and the joint is soaked at a slowly increasing temperature. The end result is a joint that melts at the same temperature as the parent metal.
Anyway, the most common way most of us run into this is when we try to disassemble old soldered copper or brass plumbing. It can be very surprising if the original joint was overheated.
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Happy New Year,
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone
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This is Turtle.
All here just might be true as can be but watch out for a hvac man coming along and use Braze-zone on the joint and your going to just about make the copper glow before it will turn loose. It can resemble the Metalbolic Steroids or what ever it was you was speaking about.
TURTLE
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Thanks Jeff for the interesting information. I've had the odd one that squeaked during removal! Makes me now think twice about 'reusing' one side of an unsoldered joint; except when I have too! Regards for 2004.
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I've found that with water copper (like AC/R joints) that it's a lot easier to use new fittings, and cut back to fresh pipe when you can. This thought on metal mixing sure makes sense.
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Christopher A. Young
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