May 11, 2011
Flux-cored welding is growing in popularity, yet its dependence on high concentrations of CO2 has prevented fabrication shops that employ a bulk gas delivery system from using the process. Recently wires have been developed that utilize a higher content of argon and a lower content of CO2. The result is a wire that provides low spatter, less welding fumes, and higher out-of-position deposition rates, as well as excellent weld characteristics in the vertical and overhead positions.
Flux-cored arc welding (FCAW) is capturing a greater segment of the welding fabrication market in many regions around the world. When compared to shielded metal arc welding (SMAW), the flux-cored wire process is capable of higher deposition rates, increased operating efficiency (welder arc-on time), and improved deposition efficiency (that is, no stub loss). FCAW is also more forgiving of differences in welder skill than its solid-wire counterpart, gas metal arc welding (GMAW).
FCAW allows you to join various materials in many thicknesses; reduces the need for plate preparation before welding; provides good tolerance for contaminants on the base material; and yields high-quality weld metal with excellent bead shape.
The majority of flux-cored wires are designed to work with either 100 percent CO2 or an argon/25 percent CO2 mixture. While these wires have performed well in many applications, they generally are unusable in environments where the shielding gas composition is fixed, such as in fabrication facilities where bulk gas is supplied via a pipeline system for multiple welding processes. Also, flux-cored wires typically have not been considered suitable for use where welding fume levels must be reduced.
As flux-cored wires have become more popular, manufacturers have worked to develop a wire that can operate with more shielding gas blends. The new family of E71T-1 electrodes can be used with argon-rich blends containing 5 to 15 percent CO2, as well as in specially developed, three-component shielding gases containing argon with additions of helium and CO2.
Where you may have grown accustomed to using either solid or metal-cored wires with shielding gas blends like argon and 10 to 15 percent CO2, you now have the option to use a flux-cored wire for vertical welding with that same gas blend. This increases your welding productivity without increasing your shielding gas cost because only one blend is required for all operations. Heavy-equipment fabrication and railcar assembly operations have benefited from these combined solid, metal-cored, and flux-cored wire products that are used with one gas blend.
Figure 1 illustrates the wider operating range and higher deposition rates possible with such flux-cored wires used in the vertical welding position. The operating characteristics that are possible with two-part gas blends such as argon/8 to 15 percent CO2 and special-purpose three-part blends containing argon/helium/CO2 produce out-of-position deposition rates up to 11 lbs. per hour—a nearly 20 percent increase when compared with the typical E71T-1 wires shielded with argon/25 percent CO2.
The deposition efficiency of these wires—the amount of product that actually becomes deposited weld metal is also improved when compared to typical E71T-1 wires. The high-argon-content shielded wires are 3 to 7 percent more efficient than the typical CO2shielded wires and 2 to 4 percent more efficient than these same wires shielded with argon/25 percent CO2. This increased deposition efficiency helps to reduce welding costs.
The new wires generate less welding fumes than regular flux-cored wires, with rates that are nearly as low as solid or metal-cored wires. Figure 2 depicts fume generation at approximately the same deposition rate for two of the most common E71T-1 flux-cored wires used today. It compares these rates to a new high-argon shielded flux-cored wire.
For conventional flux-cored wires that require CO2 or argon/25 percent CO2 shielding blends, welding fume is reduced by 20 to 25 percent. Even lower levels of CO2 (8 percent, in this case) reduce fumes even more.
These wires have performance characteristics that will allow you to run vertical-up stringer beads at high deposition rates without the need for weaving. The fast-freezing slag permits easy control of the weld pool for a variety of out-of-position welding needs. The stable, consistent arc produces little or no spatter and excellent bead shape with only a low to moderate level of operator skill necessary. Since these wires are less sensitive to parameter variations than traditional flux-cored wires, you don't have to be an experienced welder to produce high-quality welds.
Figure 3 shows a vertical-up weld using argon/10 percent CO2 with one of these new 1⁄16-in.-dia. wires. A conventional, CV power supply was used with no manipulation or oscillation of the welding torch. The results confirm the excellent bead profile and minimal spatter with little potential for undercut. In addition, the slag was found to be self-releasing or very easily removed.
While lower levels of CO2 in the shielding gas typically are associated with decreased weld penetration, these wires are manufactured to produce a higher-current-density arc that still produces significant penetration with low CO2 levels. An additional benefit of the higher-current density is good low-current arc stability and a higher wire burnoff rate at current levels equivalent to a conventional cored wire.
Be sure to exercise caution when welding in-position with this new generation of consumables, as the fast-freezing slag may be more prone to entrapment. The low amount of deoxidizers in these products also makes them better-suited for applications in which only light mill scale and surface oxides are present.
Kevin Lyttle is a development scientist and Philip Miller is a development specialist at the Praxair Technology Center, 175 E. Park Drive, Tonawanda, NY 14150, 716-879-7290, www.praxair.com.
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