Brazing Copper Tube
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 Copper Tube
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)
Fluxes 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.
Fluxing:
Apply the brazing flux to both the tube end (Figure 16) and the inside of the fitting. (Figure 18)
Heating 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)
It 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) Heat 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) When 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.
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