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  Soldering is a process that joins base metals using a filler metal (solder) which melts at a lower temperature than the base metals. Most soldering is done with solders that melt at temperatures ranging from approximately 175oC (35oF) to 290oC (550oF) In order to consistently make satisfactory joints, the sequence of operations presented in ASTM Standard Practice B 828, Making Capillary Joints by Soldering of Copper and Copper Alloy Tube & Fittings, should be followed. It should also be noted that Canadian codes prohibit the use of solders containing more than 0.2% lead in potable water systems. There is a wide variety of solders available which can be used in place of the once standard 50% tin-50% lead solder, commonly called 50-50. They melt at slightly higher temperatures and may exhibit different flow characteristics. A suitable flux must be used when making a solder joint. Flux acts as a cleaning and wetting agent, and when properly applied, permits uniform spreading of the molten solder over the surfaces to be joined. Flux is a chemically active substance, and only enough should be applied to remove and exclude oxides from the joint area during heating and to ensure that the melted solder will wet the surfaces to be joined. Do not overflux! ASTM Standard B 813, Liquid and Paste Fluxes for Soldering Applications of Copper and Copper Alloy Tube, covers the requirements and test methods for liquid and paste fluxes for soldering copper-base materials. Measuring Accurately measure the length of each piece of tube needed. (Figure 3) If the tube is too short, it will not reach all the way into the cup of the fitting and a good, strong joint cannot be made. If the tube is too long, strain may be introduced into the system.  Figure 3Cutting Cut the tube to the measured length. It can be cut with a disc-type tube cutter (Figure 4), a hacksaw, an abrasive wheel, or with a stationary or portable bandsaw. Care must be taken that the tube is not deformed while being cut. Regardless of the method used, the cut must be square in order that the tube will seat properly in the fitting cup.  Figure 4Reaming All cut tube-ends must be reamed to the full inside diameter of the tube, to remove the small burr created during cutting. If this rough inside edge is not removed by reaming, erosion corrosion may occur due to localized turbulence and high flow velocity near the joint. A properly reamed tube-end provides a smooth surface for optimum flow. Also remove any burrs on the outside of the tube-ends, to ensure proper insertion of the tube into the cup of the fitting. Tools which may be used to ream tube-ends include the reaming blade on a tube cutter, half-round or round files, a pocket knife, or a special deburring tool. (Figure 5)  Figure 5With soft temper tube, care must be taken not to deform the tube-end by applying too much pressure. If soft temper tube is deformed, it can be brought back to roundness with a sizing tool, which consists of a plug and sizing ring. Cleaning Removal of all oxides and surface soil from the tube-ends and fitting cups is essential for the proper flow of solder into the joint. Failure to remove such oxides can interfere with capillary action and may reduce the strength of the joint and cause failure. Lightly abrade (clean) tube-ends using sand cloth (Figure 6) or nylon abrasive pads for a distance slightly more than the depth of the fitting cups. Clean fitting cups by using abrasive cloth, abrasive pads, or a properly sized fitting brush.  Figure 6The capillary space between the tube and fitting is most effective from 0.002 to 0.005 in. (0.05 to 0.13 mm), but may be up to 0.010 in. (0.25 mm). Solder fills this gap by capillary action. This spacing is critical for the solder to flow into the gap and form a strong joint. A certain amount of looseness of fit can be tolerated, but too loose a fit can cause difficulties, particularly with large size fittings. It may also allow too much solder to be fed into the joint, resulting in a blob forming inside the fitting. Chemical cleaning may be used if the tube-ends and fittings are thoroughly rinsed after cleaning according to the procedure furnished by the cleaner manufacturer. Do not touch the cleaned surface with bare hands or oily gloves. Skin oils, lubricating oils and grease impair the soldering operation. Applying Flux Use a flux that will dissolve and remove traces of oxide from the cleaned surfaces, protect the cleaned surfaces from reoxidation during heating, and promote wetting of the surfaces by the solder, as recommended in the general requirements of ASTM B 813. Using a brush, apply a thin, even coating of flux to both the tube and fittings as soon as possible after cleaning. (Figures 7 and 8)  Figure 7  Figure 8WARNING: Do not apply flux with fingers. Chemicals in the flux can be harmful if carried to the eyes, mouth, or open cuts. Use care in applying flux. Careless workmanship can cause problems long after the system has been installed. If excessive amounts of flux are used, the flux residue may cause corrosion. In extreme cases, such flux corrosion can perforate the tube and/or fitting. Assembly and Support Insert the tube-end into the fitting cup, making sure that the tube is seated against the base of the fitting cup. (Figure 9)  Figure 9A slight twisting motion ensures even coverage of the flux. Remove excess flux from the exterior of the joint with a cotton rag. (Figure 10)  Figure 10If possible, support the tube and fitting assembly to ensure a uniform capillary space around the circumference of the joint. Uniformity of the space will ensure good capillary flow of the molten solder. Excessive joint clearance can result in the solder cracking under conditions of stress or vibration. The joint is now ready for soldering. Joints prepared and ready for soldering must be completed the same day and should not be left unfinished overnight. Valves When joining copper tube to valves with solder cups, follow the manufacturer's instructions. The valve should be in the full-open position before applying heat, and the heat should be applied primarily to the tube. Disassembly of the valve may be required if there is a chance that non-metal components could be damaged. WARNING: When dealing with an open flame, high temperatures and flammable gases, safety precautions must be observed. Heating Heat is usually applied with an air-fuel torch. Such torches use acetylene or propane gas. Electric resistance soldering tools can also be used. They employ heating electrodes and should be considered when an open flame is a concern. Begin heating with the flame perpendicular to the tube. (Figure 11)  Figure 11The copper tube conducts the initial heat into the fitting cup for even distribution of heat in the joint. The extent of preheating depends upon the size of the joint, and experience will indicate the amount of time needed. Then move the flame onto the fitting cup. (Figure 12)  Figure 12Alternate the flame from the fitting cup back onto the tube a distance equal to the depth of the fitting cup. With the torch at the base of the fitting cup, touch the solder to the joint. If the solder does not melt, remove it and continue heating. CAUTION: Do not overheat the joint or direct the flame into the face of the fitting cup. Overheating could burn the flux, which will destroy its effectiveness, and the solder will not flow into the joint properly. When the solder melts when touched to the joint, apply heat to the base of the fitting cup, to aid capillary action in drawing the molten solder into the joint. Applying Solder Solder joints depend on capillary action to draw free-flowing, molten solder into the narrow space between the fitting and the tube. Capillary action takes place regardless of whether the solder flow is up, down or horizontal. For horizontal joints, start applying the solder metal slightly off-centre at the bottom of the joint. (Figure 13)  Figure 13Proceed across the bottom of the fitting and up to the top-centre position. Return to the starting point, overlap it, and then move up the incompleted side to the top, again overlapping the solder. For joints in the vertical position, use a similar sequence of overlapping passes, starting wherever it is convenient. Cooling and Cleaning Allow the completed joint to cool naturally. Shock cooling with water may stress the joint. When cool, clean off any remaining flux residue with a wet rag. (Figure 14)  Figure 14Whenever possible, based on end use, completed systems should be flushed to remove excess flux and debris. Testing Test all completed assemblies for joint integrity. Follow the test procedure required by codes applicable to the service application. Estimating The amount of solder consumed when adequately filling the capillary space between the tube and the fitting may be estimated from Table 12. The flux needed is about 2 ounces per pound of solder.  Brazing is another joining process for connecting copper tube and fittings. However, it involves filler metals that melt at temperatures ranging from 590oC (1,100oF) to 815oC (1,500oF), which are much higher than the solders covered in the previous section. The temperature at which a filler metal starts to melt on heating is the solidus temperature; the liquidus temperature is a higher temperature at which the filler metal is completely melted. The liquidus temperature is the minimum temperature at which brazing will take place. Brazing filler metals for joining copper tube are divided into two classes: BCuP alloys which contain phosphorus, and the BAg alloys which have a high silver content. Brazing filler metals are sometimes referred to as "silver solders" or "hard solders", but these confusing terms should be avoided. The fluxes used for brazing are different in composition from soldering fluxes, and they cannot be used interchangeably. Brazing fluxes are water based, while most soldering fluxes are petrolatum based. Like soldering fluxes, brazing fluxes dissolve and remove residual oxides from the metal surface, protect the metal from reoxidation during heating, and promote wetting of the surfaces to be joined. They also provide an indication of the metal temperature during heating. (Figure 17)  Figure 17Fluxes suitable for brazing copper and copper alloy tube should meet AWS Classification FB3-A or FB3-C, as listed in the American Welding Society's Brazing Handbook. It should be noted that a brazing flux may not always be required. When using copper tube, wrought copper fittings and BCuP filler metal, fluxing is optional due to the self-fluxing action of the phosphorus. Preparation Like soldering, the preparations for making a brazed joint consist of measuring, cutting, reaming and cleaning. (Figures 3 to 6) Fluxing: Apply the brazing flux to both the tube end (Figures 15 - 16) and the inside of the fitting. (Figure 18)  Figure 15  Figure 16 Figure 18Heating and Brazing Apply heat, preferably with an oxy-fuel flame; air-fuel is sometimes used on smaller sizes. A neutral flame should be used. Heat the tube first, beginning about one inch from the edge of the fitting and sweep the flame around the tube in short strokes at right angles to the axis of the tube. (Figure 19)  Figure 19It is very important that the flame be in motion continuously, and it should not remain on any one point long enough to damage the tube. The flux may be used as a guide as to how long to heat the tube; continue heating it until the flux becomes quiet and transparent like clear water. Then switch the flame to the fitting at the base of the cup. (Figure 20)  Figure 20Heat uniformly, sweeping the flame from the fitting to the tube until the flux on the fitting becomes quiet. Avoid excessive heating of cast fittings. When the flux appears liquid and transparent on both the tube and fitting, start sweeping the flame back and forth along the axis of the joint to maintain heat on the parts to be joined, especially toward the base of the cup of the fitting. The flame must be kept moving to avoid melting the tube or fitting. Apply the brazing filler metal at a point where the tube enters the socket of the fitting. (Figure 21)  Figure 21When the proper temperature is reached, the filler metal will flow readily into the space between the tube and fitting socket, drawn in by capillary action. Keep the flame away from the filler metal itself as it is fed into the joint. The temperature of the tube and fitting at the joint should be high enough to melt the filler metal. Maintain the heat by moving the flame back and forth between the tube and fitting as the filler metal is drawn into the joint. When the joint is properly made, a continuous fillet of filler metal will be visible completely around the joint. Stop feeding as soon as you see the fillet. For 1-in. tube and larger it may be difficult to bring the entire joint up to heat at once. It frequently will be found desirable to use a multiple-orifice torch tip to maintain a proper temperature over large areas. A mild preheating of the whole fitting is recommended for larger sizes. Heating then can proceed as outlined in the above steps. When brazing horizontal joints, it is preferable to first apply the filler metal at the bottom, then the two sides, and finally the top, making sure the operations overlap. On vertical joints it is immaterial where the start is made. If the opening of the socket is pointing down, care should be taken to avoid overheating the tube, since this may cause the brazing filler metal to run down the outside of the tube. If this happens, take the heat away and allow the filler metal to set. Then reheat the cup of the fitting to draw up the filler metal. Cooling and Cleaning: After the brazed joint has cooled, the flux residue should be removed with a clean cloth, brush or swab, using warm water. Remove all flux residue to avoid the risk of the hardened flux temporarily retaining pressure and masking an imperfectly brazed joint. Wrought fittings may be cooled more readily than cast fittings, but all fittings should be allowed to air cool before wetting. Troubleshooting If the filler metal fails to flow or has a tendency to ball up, it indicates oxidation on the metal surfaces or insufficient heat on the parts to be joined. If the tube or fitting start to oxidize during heating there is too little flux. If the filler metal does not enter the joint and tends to flow over the outside of either member of the joint, it indicates that one member is overheated or the other is underheated. Testing Test all completed assemblies for joint integrity. Follow the test procedure required by codes applicable to the service application. Estimating A general guide to estimating how much brazing filler metal will be consumed is provided in Table 12.  Brazing a 5-inch Type K copper vacuum line in a hospital.  In addition to gas-fueled torches for soldering and brazing, electric resistance hand tools may be used. They consist of tong-like heating electrodes. With the power on, the tongs are clamped around the fitting and held in place until the filler metal melts when touched to the capillary gap between the tube and fitting. Joint preparation is the same as for the gas torch method. Lightweight electric resistance tools may be preferred in new and retrofit installations where an open flame would be a concern. Another technology uses a tee-pulling tool to drill into a section of tube and pull out a collar for a tee connection. A branch line is then brazed into the raised collar; soldering cannot be used. This method is popular for fabricating manifolds and in copper fire sprinkler installations, since it reduces the number of tee fittings used and thereby the number of brazed joints.  In natural gas systems, copper can be added to existing steel pipe systems using a flared fitting. Grooved-end pipe and fittings have been used for many years to join iron and steel pipe in a variety of systems. This method of mechanical joining is now available for copper tube in sizes from 2 to 6 inches. It uses a clamping ring with a gasket to hold together the butt ends of a tube-to-tube or tube-to-fitting joint. A roll-formed groove near the end of the tube or fitting permits the clamp to firmly grasp the two components of a joint. Pre-grooved couplings, elbows, tees and flanges are available from the manufacturers. Flared joints are commonly used to join soft temper copper tube. The joint consists of three components: the flare fitting, the flared end of the copper tube, and the threaded flare nut which holds the joint together. This type of joint is commonly used for natural gas or propane distribution systems. Flare fittings are also used for underground services, but in recent years compression fittings have become most popular for this purpose. Epoxy bonded joints are a relatively recent development. A two part, fast-curing, epoxy-based adhesive is used to join copper tube and capillary fittings for water distribution systems. It may also be used in copper fire sprinkler systems (excluding dry systems), or installations where an open flame may not be appropriate.
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