Handbook-Brazing & Soldering

11 Some Additional Pointers When the tube size is 2 in. or greater, it is almost impossible to bring the entire joint up to brazing temperature at one time, even with a larger welding head. The trick here is to try to bring about two inches of the circumference of the joint (in the case of 2-in. tube, that’s about one-third of the joint) up to brazing temperature, then start to apply the brazing alloy, and finally work your way around the fitting, applying heat to both tube and fitting ahead of the brazing alloy. On very large joints (4-in. or more) it speeds up the job to use two torches. A helper uses one to preheat the joint, while the principal operator brings the joint to brazing temperature and applies the filler metal. Soldering Copper Tube Fittings As pointed out earlier in this chapter, the only essential difference between brazing and soldering lies in the melting points of the alloys used. Because soldering alloys all melt at temperatures much below 427⁰C, to use an oxy- acetylene flame for a soldering operation is not only pointless, but difficult, since that flame is so hot that overheating of the metal is hard to avoid. A variety of heat sources are available for the soldering of copper tubing systems. Most widely used are air-acetylene and air-propane torches. (Some of these can also be used for brazing of joints, although the work will go more slowly than it will with an oxy-acetylene flame). The relatively new ”swirl” tips for air-acetylene and air-propane torches are strongly recommended, since they produce short flames which are little disturbed by breezes, and allow the operator to put the heat right where he wants it. In making up soldered joints in copper tubing, follow the six steps previously given for brazing operations. Do not fail to bottom the tube all the way in the fitting, since the mechanical strength of a soldered joint is substantially less than that of a brazed joint. If you are allowed to choose your own solder (in many cases, the type of solder to be used in making up a system is covered by specification, and you have no choice) the so-called ”50-50” tin-lead solder (about half lead, half tin) will prove the easiest to work with, since it has a melting range of more than 30⁰C. Some of the other widely-used solders melt completely at a specific temperature, and require more precise heating if the solder is to be completely and evenly dispersed in the joint.