Handbook-Brazing & Soldering

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BRAZING AND SOLDERING Brazing is a process which differs from braze welding in one very important way: In brazing, the filler metal is drawn into the joint by capillary attraction, rather than deposited in the joint in somewhat the same fashion as in oxy-acetylene fusion welding. While the majority of industrial brazing operations do not involve the oxy- acetylene fusion welding, in one field – the fabrication and installation of copper-tube piping systems – the oxy-acetylene torch is frequently employed. We shall cover this application shortly. First, let’s take a general look at brazing, an extremely important process in metal fabrication. By American Welding Society definition, brazing is a welding process in which the filler metal has a melting point higher than 800⁰F (425⁰C) but lower than that of the metal of metals being joined, and in which the filler metal is drawn into the joint by capillary attraction. What do we mean by ”capillary attraction”? To put it in very simple terms, it is the ability of a liquid to rise into a narrow gap or passage against the force of gravity. You can demonstrate capillary attraction with two thin pieces of clean glass, as shown in Fig. 19-1. Or you can use two short pieces of glass tubing, one with a very small diameter bore (1 mm or less) one with an inside diameter of 5-6 mm. The water will rise appreciably in the smaller tube, little in the larger tube. (Not all liquids behave in this fashion; many different factors are involved. With mercury, for example, the level in a small diameter tube inserted into a pool of the liquid will actually fall below the level of the pool surface.) The process known as soldering is generally similar to brazing except that the filler metals used melt at temperatures below 427⁰C (800⁰F). In actual practice, most brazing alloys melt at temperatures well above 427⁰C, most solders at temperatures well below 427⁰C. Many of the brazing alloys based on silver (all of which melt above 600⁰C) were formerly termed ”silver solders”. Avoid that term, and its relative, ”silver soldering”. Even the term ”silver brazing” is sometimes misleading, since some brazing applications for which silver alloys are generally used can also be handled with alloys which contain no silver.