Joining Methods of Dissimilar Metals
The American Welding Society defines Soldering as metal coalescence below 800F (Brazing being above 800F.) Both processes bond adjoining metal surfaces by completely wetting the surfaces with molten filler metal and maintaining that bond upon solidification. The bond created is only as strong as the filler metal used. Soldering is often most useful for joining pipes and small fittings. Brazing is useful for making high strength bonds between dissimilar metals. Stainless steel is difficult for solders and braze filler metals to wet. The surface oxides that protect it from corrosion also prevent the filler metals from wetting the surface. Special fluxes are necessary to eat through these stainless oxides. The silver solder commonly sold for home plumbing with copper pipe will work on stainless but a different flux is necessary. Use a flux containing hydrochloric acid or one that says it is for fluxing nickel alloys or stainless steel.
Getting the steel hot enough is also a problem. A propane torch can be used, but the flame needs to be slightly reducing in nature to prevent the re-formation of surface oxides. The best method for soldering a copper or brass fitting onto a stainless steel pipe is to “tin” the fitting first with the solder. Flux is then applied to the stainless pipe and the two pieces are fitted together. Heat is applied to the joint, and more solder is fed into the joint once it is hot. This way the steel surface is protected from the air until it is hot enough to be wetted by the solder. Silver-based brazing alloys have lower melting temperatures than Copper/Zinc brazing alloys, so the silver-based alloys are the more practical choice. Keep in mind, that the braze must be conducted efficiently to ensure that the time limit for the onset of diffusion is not exceeded. Acetylene and propane are two of the most common gases used for Torch Brazing. Use a slightly reducing flame and AWS type 3A flux. This flux has the higher useful temperature range needed for brazing (1050-1600F). Both surfaces must be cleaned and fluxed for best results. As in soldering, it is a good idea to pre-braze the fitting, since it has the higher thermal mass in the localized area. Pre-heating the fitting will help decrease the amount of time that heat is applied to the joint.
Most USCS systems are shipped with accessories attached. There are times with special, custom, or back orders a switch or valve will need to be connected prior to or during installation. For any additional information on this or other installation topics call Dan on the Tech Line 562-728-5774