Brazing and Soldering

Everything that Piping & Fabrication try to explain is only for the information to all of us, and hope that will help us to have more knowledge about piping system. and now we will talk about Brazing and Soldering.

Brazing. For services involving the ASME Boiler and Pressure Vessel Code or the B31 Code for Pressure Piping, brazing procedures and brazers must be qualified in accordance with ASME Section IX similar to welding procedures and welders. See the section ‘‘Procedure and Personnel Qualification.’’ There are a great many types of brazing processes. In establishing a brazing procedure, consideration must be given to the ability of the filler metal to produce suitable physical properties, its melting point and wettability, possible base metal and filler metal interactions, loss of base metal properties, increased sensitization to corrosion, increased hardness in the base metal due to brazing temperature, and the need for postbrazing heat treatments.

Since most piping materials can be welded, the use of brazing for joining is rather limited. It is most often used for joining coppers and for combinations of metals which cannot be welded. Brazing is a process wherein the base metals do not melt, the filler metal has a liquidus above 840°F (450°C), and the filler metal wets the base metal and is drawn into the joint by capillary action. Although butt or scarf joints can be used, a lapped joint with an overlap of 3 times the thickness of the thinner member gives the best joint efficiency and ease of fabrication. It should be noted that typical copper or brass fittings have a depth of socket based on the strength of tin-lead solders. When brazing is used, only a small percentage of that depth is needed. Required clearance between the faying surfaces usually vary from 0.001 to 0.010 in (0.025 to 0.25 mm) depending on the filler and flux combination used during the operation. The flux melts upon application of heat and is displaced by the molten filler metal. Flux residue should be removed after the operation is complete. Silver, copper-phosphorus, and copper-zinc filler metals are most often used for copper brazing.

Torch brazing is commonly used for fabrication and installation of copper piping systems. For torch brazing, the type of fuel gas selected is a function of the melting temperature required to melt the filler metal. For piping joints NPS 2 (DN 50) and larger, use of a second torch to preheat may be desirable. In brazing metals with differing coefficients of expansion, it is preferable that the metal with the higher expansion coefficient form the socket and the metal of the lower expansion coefficient form the pipe or tube. Clearance between the parts at room temperature must be adjusted so there will be a suitable clearance at brazing temperature. On cooling, the greater contraction of the socket will put the joint in a compressive stress state.

Soldering. Unlike welding and brazing, ASME Section IX has no requirements for qualification of soldering procedures or personnel. Soldering is much like brazing in that the base metals are not melted, the faying surfaces are wetted by the filler, and the filler is drawn into the joint by capillary action. However, the melting point of the filler metal is lower than 840°F (450°C) usually between 450 and 500°F (230 and 260°C). Since the strength of soldering filler metals is considerably less than that of brazing fillers, a longer overlap is required to develop a joint equal to base metal strength. A clearance of about 0.003 in is preferred for optimum strength. A good soldered joint depends again on the cleanliness of the faying surfaces. Fluxes are used to assist in the wetting action by removing tarnish films and to prevent oxidation. Rosin fluxes and organic fluxes are used for most materials. Inorganic fluxes may be required for certain other materials that can be soldered, while in some cases precoating of the material with a surface that can be soldered may be required. Most piping applications use tin-lead solders. These range in composition from 5 percent tin, 95 percent lead to 70 percent tin, 30 percent lead, with 50 percent tin, 50 percent lead the most common. Tin-antimony and tinsilver solders are also frequently used. For soldering aluminum, tin-zinc and zincaluminum are used.