Manual Brazing of Pipe and Tube Fig. 1 — Manual brazing of pipe using oxyfuel gas and facefed brazing filler metal. Before any attempt is made to braze a joint in pipe or tubing, all the preliminary steps required to prepare the joint surfaces must be completed. Since pipe and tubing assemblies are normally brazed with oxyfuel gas torches, the heating of joints using torches fueled with acetylene, propane, natural gas, or other gases will be emphasized here. Figure 1 illustrates this operation. The amount of heat required for the size of the pipe or tubing being joined governs the size of the torch tip. To heat the joint rapidly and evenly, the selected tip should provide a lowvelocity, bulbous flame. The flame then should be adjusted to be neutral or slightly reducing. A neutral flame has a smooth and even inner cone, without a feathered appearance. To adjust an acetylene flame to its neutral state, a flame that has an excess of acetylene is lit. The acetylene flow is then decreased, or the oxygen flow is increased very slowly until the feather just disappears. The flame should be directed so that it is perpendicular to the pipe or fittings. If the pipe is larger than 4 in. (101.6 mm), multiple tip torches or additional heating torches may be required. The pipe or tube is usually heated before the fittings are. After a short heating period, the flame is directed alternately from the pipe to the fitting. This procedure should be used to heat the fittings and pipe to the same temperature. Holding the flame too long at one location on the fitting or pipe may result in localized overheating, excessive drying of the flux, distortion of the pipe, and possible cracking of the fitting. Overheating is the most common cause of poorly brazed joints. The flame’s inner cone should not directly impinge on the tube or the shoulder of the fitting or into the fitting socket. Keep the flame in motion so the heat is distributed evenly on the joint. You can test whether the temperature for fabricating brazed pipe joints is appropriate by touching the brazing filler metal to the heated junction between the pipe and the fitting, and then observing its flow. The flux should be very fluid on both fitting and pipe. Fluxes usually melt into a clear liquid at the brazing temperature. A common mistake is to begin feeding the brazing filler metal into the top of the joint when brazing a horizontal joint. The appropriate procedure — for both small- and large-diameter pipes — is to begin heating on the tube or pipe to start the conduction of heat into the fitting socket. Both the fitting and the tube are then heated as the torch is moved slowly back and forth to keep the temperature uniform. After the tube and fitting are uniformly heated, the torch is moved to the lower quarter of the tube to heat the joint to the melting temperature of the brazing filler metal. When the filler metal begins to melt, capillarity draws it into the joint clearance. At the same time, the torch is moved around toward the top of the fitting with the brazing filler metal being fed to the joint just behind the flame path. For large diameters, a second torch may be used to simultaneously braze the opposite side from the bottom to the top of the joint. Do not directly apply the heat to the filler metal. If the torch flames are directed to the heel or bottom of the fitting socket, the filler metal will flow throughout the joint. Opinions differ as to whether or not a fillet of brazing filler metal is necessary or desirable on pipe joints. Often, a full annular fillet indicates good brazing filler metal flow and distribution in the joint. However, generous fillets at the bottom of the joint may be caused by accumulation of brazing filler metal that has solidified after flowing over relatively cold metal. Heating from the bottom of the fitting generally prevents this condition. Filler Metals The brazing filler metals used to braze pipe and tubing must be metallurgically compatible with the base metals they are intended to join. In addition, they must provide adequate strength to the joints in service. When brazing copper, the BCuP brazing filler metals are normally good general-purpose filler metals. They should not, however, be used on copper-nickel alloys with a nickel content of more than 10%. Brazing filler metals with a broad melting range should be selected for brazing pipe joints because of the difficulty in maintaining the assemblies in a narrow temperature range. Postbraze Cleaning After the brazing filler metal solidifies, the remaining flux and residues can be removed by wiping with a wet cloth or by wet brushing. Cast fittings should be allowed to cool slowly to below 300°F (150°C) before applying swabs to the joints. Since most brazing fluxes are hygroscopic and may contain active corrosive agents, they should be removed. It is essential to remove flux residues when aluminum alloys are brazed. WELDING WORKBOOK 94 WELDING JOURNAL / JUNE 2016 WJ DATASHEET 365 Excerpted from the Brazing Handbook, Fifth Edition, published by the American Welding Society, Miami, Fla.
Welding Journal | June 2016
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