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BACKFIRE AND FLASHBACK.
-Improper
handling of the torch may cause the flame
to backfire or, in very rare cases,
to flashback. A backfire is a momentary
backward flow of the gases at the torch tip, causing
the flame to go out. Sometimes the flame may immediately
come on again, but a backfire is always accompanied
by a snapping or popping noise. A backfire may
be caused by touching the tip against the work, by overheating
the tip, by operating the torch at other than recommended
pressures, by a loose tip or head, or by
Figure 15-30.-Welding light gauge metals. dirt or slag in the end of the tip. A backfire is rarely dangerous, but the molten metal may be splattered when the flame pops. A flashback is the burning of the gases within the torch, and it is dangerous. It is usually caused by loose connections, improper pressures, or overheating of the torch. A shrill hissing or squealing noise accompanies a flashback; and unless the gases are turned off immediately, the flame may burn back through the hose and regulators and cause great damage. The cause of a flashback should always be determined, and the trouble remedied before relighting the torch. Fundamental Welding Techniques The composition, thickness, shape, and position of the metal to be welded govern the techniques to be used. The fundamental techniques that apply to different thicknesses, shapes, and positions of the metal to be welded are discussed in the following paragraphs.HOLDING THE TORCH. -The proper method to use in holding the torch depends upon the thickness of the metal being welded. For light gauge metal, hold the torch as shown in figure 15-30, with the hose draped over the wrist. For heavier work, hold the torch as shown in figure 15-31.
Figure 15-31.-Welding heavy plate. Hold the torch so that the tip is in line with the joint to be welded, and inclined between 30 and 60 from the perpendicular. The exact angle depends upon the type of weld to be made, the amount of preheating necessary, and the thickness and type of metal. The thicker the metal, the more vertical the torch must be for proper heat penetration. The white cone of the flame should be held about 1/8 inch from the surface of the base metal.If the torch is held in the correct position, a small puddle of molten metal will form. The puddle should be composed of equal parts of the two pieces being welded. After the puddle appears, begin the movement of the tip in a semicircular or circular motion. This movement assures an even distribution of heat on both pieces of metal. The speed and motion of the torch are learned only by practice and experience. |
 
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