Handbook-Braze Welding

In all cases, that part of the base metal on which tinning action is to take place must always be cleaned thoroughly

of all rust, scale, paint, oil or grease, or any other foreign substance. Clean the metal back from the edge of the joint for at least a half-inch if possible. Welding Technique. For braze welding steel or cast iron, select a welding tip at least one size smaller than you would use for fusion welding steel of equivalent thickness. Adjust the flame to be slightly oxidizing. (First, arrive at a neutral flame which is relatively soft; that is, less than maximum size. Then throttle the acetylene flow just enough to cause a perceptible shortening of the flame’s inner cone.) Heat the base metal, moving the flame back and forth over a generous area, until it just starts to glow. While you are doing this, heat the end of the filler rod and dip it in the flux to get it well-coated. When you think the base metal is hot enough concentrate the flame on the rod until a bit of it melts off onto the hot base metal. If the melted bronze flows evenly and smoothly on the surface of the metal, spreading over a surprisingly large area, you have hit the right temperature. If it merely rests on the surface as a large drop, the base metal temperature is too low. If the bronze spreads, but seems to bubble and separate into small drops, the base metal temperature is too high. (It is never necessary to raise the base metal temperature above the melting temperature of the bronze. ) Once good tinning action has been established, completing the weld should not be difficult. You must be careful not to melt rod into the puddle faster than the base metal ahead of the puddle has reached tinning temperature. You must avoid overheating the base metal or the puddle. You must learn to move the rod and flame in such a fashion as to secure adequate reinforcement and a uniform ripple effect. It is suggested that you start braze welding practice by merely laying down a bead on the surface of a piece of steel plate. Then try making a braze weld between two pieces of 6 mm (1/4-in.) steel plate, each piece bevelled to a 45 deg. angle. Tackweld the pieces, as you did in preparation for fusion welding; however, spacing at the root of the weld, after the tackwelds have been made, need be no more than 2 mm (1/16 in.)