7-18. BRASS AND BRONZE WELDING
a. General. Brass and bronze are alloys of copper. Brass has zinc, and bronze has tin as the major alloying elements. However, some bronze metals contain more zinc than tin, and some contain zinc and no tin at all. High brasses contain from 20 to 45 percent zinc. Tensile strength, hardness, and ductility increase as the percentage of zinc increases. These metals are suitable for both hot and cold working.
b. Metal-Arc Welding. Brasses and bronzes can be successfully welded by the metal-arc process. The electrode used should be of the shielded arc type with straight polarity (electrode positive). Brasses can be welded with phosphor bronze, aluminum bronze, or silicon bronze electrodes, depending on the base metal composition and the service required. Backing plates of matching metal or copper should be used. High welding current should not be used for welding copper-zinc alloys (brasses), otherwise the zinc content will be volatilized. All welding should be done in the flat position. If possible, the weld metal should be deposited with a weave approximately three times the width of the electrode.
c. Carbon-Arc Welding. This method can be used to weld brasses and bronzes with filler reds of approximately the same composition as the base metal. In this process, welding is accomplished in much the same way the bronze is bonded to steel. The metal in the carbon arc is superheated, and this very hot metal is alloyed to the base metal in the joint.
d. Oxyacetylene Welding. The low brasses are readily jointed by oxyacetylene welding. This process is particularly suited for piping because it can be done in all welding positions. Silicon copper welding rods or one of the brass welding rods may be used. For oxyacetylene welding of the high brasses, low-fuming welding rods are used. These low-fuming rods have composition similar to many of the high brasses. A flux is required, and the torch flame should be adjusted to a slightly oxidizing flame to assist in controlling fuming. Preheating and an auxiliary heat source may also be necessary. The welding procedures for copper are also suitable for the brasses.
e. Gas Metal Arc Welding. Gas metal arc welding is recommended for joining large phosphor bronze fabrications and thick sections. Direct current, electrode positive, and argon shielding are normally used. The molten weld pool should be kept small and the travel speed rather high. Stringer beads should be used. Hot peening of each layer will reduce welding stresses and the likelihood of cracking.
f. Gas Tungsten Arc Welding. Gas tungsten arc welding is used primarily for repair of castings and joining of phosphor bronze sheet. As with gas metal arc welding, hot peening of each layer of weld metal is beneficial. Either stabilized ac or direct current, electrode negative can be used with helium or argon shielding. The metal should be preheated to the 350 to 400°F (177 to 204°C) range, and the travel speed should be as fast as practical.
g. Shielded Metal Arc Welding. Phosphor bronze covered electrodes are available for joining bronzes of similar compositions. These electrodes are designed for use with direct current, electrode positive. Filler metal should be deposited as stringer beads for best weld joint mechanical properties. Postweld annealing at 900°F (482°C) is not always necessary, but is desirable for maximum ductility, particularly if the weld metal is to be cold worked. Moisture, both on the work and in the electrode coverings, must be strictly avoided. Baking the electrodes at 250 to 300°F (121 to 149°C) before use may be necessary to reduce moisture in the covering to an acceptable level.No tags for this post.