Apply heat to the parts to be joined, preferably with an oxy-fuel torch with a neutral flame. Air-fuel is sometimes used on smaller sizes. Heat the tube first, beginning about one inch from the edge of the fitting, sweeping the flame around the tube in short strokes at right angles to the axis of the tube (Figure 7.18, position 1).
It is very important that the flame be in motion and 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. The behavior of flux during the brazing cycle is described in Figure 14.6.
Switch the flame to the fitting at the base of the cup (Figure 7.18, position 2). 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, due to the possibility of cracking.
When the flux appears liquid and transparent, 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 (Figure 7.18, position 3). The flame must be kept moving to avoid melting the tube or fitting.
For 1-inch tube and larger, it may be difficult to bring the whole joint up to temperature at one time. It frequently will be found desirable to use an oxy-fuel, multiple-orifice heating tip to maintain a more uniform temperature over large areas. A mild preheating of the entire fitting is recommended for larger sizes, and the use of a second torch to retain a uniform preheating of the entire fitting assembly may be necessary in larger diameters. Heating can then proceed as outlined in the steps above.
Apply the brazing filler metal at a point where the tube enters the socket of the fitting. When the proper temperature is reached, the filler metal will flow readily into the space between the tube and fitting socket, drawn in by the natural force of 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.
Keep both the fitting and tube heated by moving the flame back and forth from one to the other as the filler metal is drawn into the joint.
When the joint is properly made, filler metal will be drawn into the fitting socket by capillary action, and a continuous fillet of filler metal will be visible completely around the joint. To aid in the development of this fillet during brazing, the flame should be kept slightly ahead of the point of filler metal application.