Welded copper pipe from too much heat when desoldering?

I am looking at a couple copper pipe joints where a temporary copper cap was soft soldered in the normal way over hard copper water line. From the coloration the joints have obviously been overheated in the process of trying to desolder them. They swear that they used ordinary plumbing solder to solder them together and a third joint they had soldered at the same time with the same solder but did not do anything in the way of desoldering to, desoldered very easily with only moderate heat when I tried. It slid right apart as soon as it got hot enough for the solder to flow just like it's supposed to, but the other two (the overheated ones) will NOT come apart. I have tried grabbing the very end of the cap with pliers and pulling twisting etc and it will not budge. I am positive they have been heated enough that there is no way that soft solder by itself is capable of holding them together. Solder touched to the pipe next to the joint readily flows but the joint stays rock solid. The pipe is not distorted so it is not a crimp type effect jamming them together. It is obviously plain copper pipe there are no threads. The only thing around the joints is soft plumbing solder residue, which easily melted when the joints were heated. No sign at all of any hard solder/brazing and they say they just used ordinary plumbing solder. Could they have been over heated enough with MAPP gas to effectively weld the copper? I don't want to heat them much beyond the flow point of plumbing solder if I can help it because they are pretty close to the wall, and I don't want a fire or destroyed solder joint in the wall. Anybody seen this before?
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DB writes:

No, not with MAPP + air. MAPP + oxygen, possibly.
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To make sure you are not fighting water/steam, use a nail or screw and put a hole in the cap. Sometimes fittings get a tenacious hold, it is almost more like they are in a physical bind than the solder holding them, that might require a swat with a hammer while it is hot. I know you don't have enough hands and this will require some creativity or an assistant. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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the caps off you might have to peal them apart . Are you sure they are not brazed ?
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I thought that you weren't supposed to desolder copper pipes??? It's not like electrical where you can easily desolder stuff.
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On Mar 27, 9:03?am, snipped-for-privacy@gmail.com wrote:

its easier to start new but desoldering is possible and the trace solder if cleaned well helps adhesion of new solder. some new fittings come pre soldered, a form of this.
some bozo hammered the cap on:(
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Once you get it apart all you is heat the pipe end up while hot wipe the solider of with a rag a little sand cloth as good as new. But I do think trying to use fitting over is a waste of time
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No, they aren't welded. Somebody probably drove the cap on over a burr formed when the tube was cut, or the fitting was on the tight side.
I've seen larger sizes (1-1/2, 2") that have been in service for a while stick-sometimes that's a case of not enough solder to fill the joint and corrosion setting in...
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It finally came apart but the caps had to be cut lengthwise with a die grinder and then pried away with a screwdriver while hot. Even at that there were several small spots where copper adhered and would not come off no matter how much direct heat was applied. It just tore away as the rest of the cap was forced off and stayed stuck to the pipe. Even red hot (wet towels wrapped around the pipes at the wall to heat sink heat away from the joints in the wall). Those places had to be filed down. A few small test holes in the wall verified that the joints in the wall were still intact and not leaking so I guess that's the good news. Thanks for the responses.
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time is that the molten solder actually dissolves some of the copper. This alloy has a higher melting point than the solder and each time it is heated more copper will dissolve, raising the melting point even more. This dissolving is also what causes copper soldering tips to pit and erode away. I heard of someone who used copper to make a soldering pot and the bottom dissolved out of it. Your experience is normal and quite common. Working quickly with a minimum of heat will reduce the problem.
Don Young
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