Gas tungsten arc welding (GTAW) has become an indispensable welding method for many industries because it produces high quality welds and keeps equipment costs low.
In the power generation industry, GTAW with cold filler wire is a method often used to weld tubes in the butt joint position in heat exchangers and pipelines. Standard benefits of the GTAW with filler wire include high quality spatter free welds, as well as 100 percent penetration with independent control of the heat source for the root pass. The addition of cold filler wire and multiple passes to fill joints results in a consistent and clean cap weld.
Weld quality in the final welds in power plant applications is critical; therefore the welds must be free of imperfections. Heat exchanger components in boilers, pipes and/or tubes are often exposed to extreme conditions such as open flame heat sources, as well as environmental and dynamic stresses due to weight or pressurization of the product within.
The GTAW method has consistently produced high-quality welds that have worked well in power plants and manufacturing applications. However, the cycle time for joining tube welded products is becoming unacceptable because industries are seeing significant increases in demand. Manufacturers are being pressured to develop more efficient manufacturing techniques through lean principles and cost reductions.
Plasma Laser Technologies (Yokneam, Israel) developed a new hybrid plasma/MIG welding process that is helping meet the challenges of increased demand, faster cycle times and more efficient manufacturing for tube joining applications. The new welding process called Super-MIG, which was recently validated as a viable alternative to GTAW in tube joining applications, delivers high quality and consistent welds, significantly increases weld processing speed, reduces the weld cycle time and lowers costs.
Plasma Laser Technologies and Welding Solutions Inc., the North American distributor of Super-MIG, worked jointly to develop a welding solution that can be implemented within an automated welding system. The new system (Figure 1) includes:
- Super-MIG interface, torch and cleaning station
- ABB robot
- Electric motor and chuck collet
- Pre-tacked assembled tubes
- Backing gas delivery
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New Process Validation
The new system was proven in a tube-to-tube butt weld joint. Tubes made of 1010 mild steel with 0.25-inch thick walls were welded together in a butt-weld joint using an argon backing gas with a flow rate of two liters per minute. This same procedure is commonly used with a TIG type machine to produce heat exchangers for the boiler industry. The filler wire, ER70S-6, with the diameter of .045 inches, was used for MIG with shielding gas of 80 percent argon and 20 percent carbon dioxide. Two weld passes were performed to ensure solid and consistent root penetration (Figure 2).
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The root weld was performed in a plasma mode alone using the Super-MIG unit with no filler wire. The second pass was finished with combined plasma-MIG using the Super-MIG unit. This procedure was possible due to the Super-MIG’s software and was performed “on the fly” without hesitation during the weld cycle.
Figures 3 and 4 show edge preparation of the tubes for butt welding. Figure 5 shows the final weld. Table 1 summarizes the welding parameters for the root and final welds.
 Figure 3 Tubes prepared for butt welding |
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 Figure 4 Edge preparation and torch positioning for tube joining S=Plasma stand-off; W=Opening; F=Face thickness; L=Initial torch off-set; α=traveling angle |
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 Figure 5 Super-MIG final weld |
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The tube-to-tube butt-weld joint demonstration was not only successful, but was performed in much less time than the traditional GTAW welding procedure. The arc-on to arc-off weld time was decreased from 12 minutes to 55 seconds, a 92 percent decrease. In addition, by using a single pass cap weld with filler, as opposed to a GTAW multiple pass with filler, the filler wire consumption decreased.
The weld quality was acceptable and was validated via radiographic inspections in accordance with ASME Section V, Article 2 following procedure SP-RT-1, Revision 10.
Other benefits of the Super-MIG process were displayed, including the ability to maintain higher welding speeds under variable gap conditions and reduction in part distortion due to a narrower heat affected zone. The Super-MIG process produces higher quality welds for the same range of weldable materials (MIG/TIG/Plasma) and results in a significant reduction in weld spatter. It can also be easily integrated into standard GTAW applications
The demonstration shows that the Super-MIG process can be a viable replacement to GTAW for orbital welding of pipe and tubing.