The deposition of a thin layer of bronze on cast iron or steel to provide corrosion resistance or to reduce sliding friction. Bronze is used to build up worn surfaces to restore them to original size. Bronze surfacing is frequently used to repair pistons, valves, and other sliding surfaces on pumps, engines, and machines.
Many bronze alloys are available. Those of the softer type (phosphor bronzes) are intended to wear more than the mating surface, and the harder type (hard aluminum bronzes) are intended to wear less than the mating surface.
Bronze surfaces can be deposited with the oxyacetylene process; however, most surfaces are deposited by shielded metal arc welding and submerged arc welding. Shielded metal arc welding electrodes are available for use with direct current, electrode positive (DCEP) and alternating current. These are basically flat-position electrodes, but can be used in the vertical
position by skilled welders. Electrodes and wires available for these applications are listed in ANSV AWS A5.13, Specification for Solid Surfacing Welding Rods and Electrodes.
Base Metal Preparation
It is important that the base metal and filler metal are thoroughly clean before overlay welding is started. All foreign matter, such as grease, rust, oxides, paint, and other impurities should be completely removed. Both should be free of moisture to prevent porosity in the deposit of aluminum bronze and other bronzes.
Minimum Penetration
Penetration must be minimized when overlaying iron base metals. Excessive dilution of the overlay with base metal, particularly cast iron, may result in a deposit so hard that it is essentially unmachineable.
Deposition by stringer bead is not recommended, because this technique tends to increase penetration and dilution of the deposit with base metal. In general, the largest size electrode consistent with the mass and thickness of the base metal should be used. The electrode should be manipulated in a rapid weaving motion to deposit a bead width three to five times the
diameter of the electrode. In controlling the weave, the operator should take particular care that the molten edges of the bead do not solidify in a way that causes undercutting and entraps slag. This can be avoided by hesitating slightly at the edges of the bead and keeping the frequency of the weave fast enough to maintain a completely molten pool of deposited weld metal. This relatively wide weaving motion also enables the operator to control the molten metal to minimize slag inclusions, impurities and porosity. While the angle of the electrode in relation to the work surface is not critical, inclining the electrode slightly back in the direction of welding will float the molten slag back over the deposited weld metal. This will protect the deposit from oxidation while cooling and prevent the slag from rolling ahead of the weld metal as it fuses with the base metal. It minimizes any tendency for slag inclusions and poor fusion.
Slag Cleaning
All slag should be completely removed from the surface of deposited metal, especially between beads of overlay work and layers of multi-layer deposits. Generally, the deposit should be wire-brushed thoroughly after removing the slag and before making the next deposit. Careful attention to cleaning can avoid slag inclusions and porosity in the weld deposit.