The basic concept is temperature.
Most good copper solders melt under 500 F degrees.
Brazing occurs over 840 degrees.
Welding occurs at around the melting point of copper, over 1900 degrees.
Soldering: The goal of sweating copper pipe is to heat the pipe/fitting enough to melt the solder but not so much as to anneal and soften the pipe….also over 800 degrees you can burn off all your flux. When the solder melts it seeps into the space between the copper pipe and the fitting cup creating a very strong joint. Strength of the silver solder is Tensile Strength – 5,000 PSI (at 250°), higher than the hard pipe (1250psi) and higher than the 95/5 solder (1000 psi). The solder must fill at over 70% of the void space between the pipe and fitting to meet piping code standards. Silver solder ‘flows’ much better and should be used. The extra cost will be covered by not having failed fittings. It sucks having to take the system all apart just for one failed joint…so the extra cost of the solver solder pays for itself pretty quickly.
Figure 2 Brazing Copper Pipe
Brazing: usually occurs between soldering and the copper melting temperatures. An interesting thing happens when copper is brazed or welded. Copper anneals at around 700 degrees and loses strength when this happens…up to half its strength. So the act of brazing produces a system where the pipe is the weak point and the joints the strongest. However, this system is tougher and better fit for vibration like a mechanical system(refrigeration). That’s why most refrigeration systems are brazed. However, be sure to use the annealed copper charts when looking to meet the system strength requirements, not the hardened copper. Brazing has some conditions: there should be a fillet buildup as well as penetrating the brazing material into the pipe fitting Joint(minimum penetration is three times the heaver wall thickness)….vs a soldered joint does not need the fillet but the penetration is about 10x the wall thickness. Brazing is done with MAPP gas or oxygen/acetylene.
Welding occurs at higher temperatures. For example most of the filler metals melt between 1100-1600 degrees depending on composition. Using rods containing silver yield great results. There is no penetration into the socket, the pipe and fitting are actually fused and heated together along with the filler metal. The oxygen/acetylene torch or welder create a copper puddle and then the rod is fed into it in a swirling pattern to mix the two parts. Copper is tricky though, heat it too much and it separates and beads like water on a waxed surface…poking a nice big hole in your work. Practice on scraps before welding copper pipes.