TIG welding or Gas Tungsten Arc Welding (GTAW) is commonly used for high quality welding. An arc is formed between a non-consumable tungsten electrode and the metal being welded. Gas is fed through the torch to shield the electrode and molten weld pool. Filler rod can then be added to the weld pool as necessary. TIG welding requires a highly skilled operator who precisely controls the current using a foot petal or hand control to continuously adjust the amount of current necessary during every second of the weld process. This process is characterized as a much slower deposition rate, but results in higher quality welds without spatter.

There is a revolutionary new process developed and patented by the Lincoln Electric Company called Surface Tension Transfer or STT. This process could be called an intelligent TIG welding process for MIG welding! The weld current (waveform) is modified hundreds of times per second to transfer each droplet of molten metal when the electrode is shorted, so there is no volatile explosion as in the MIG process, thus eliminating spatter. The STT process was named after the way this technology monitors and controls the surface tension of the weld droplet as it adheres to the weld puddle. It does this through a high speed inverter that precisely adjusts the output current waveform during the entire shorting cycle. In principle, it’s a power source capable of delivering and changing the electrode current in the order of microseconds. It’s also designed for semiautomatic applications, where rate of travel and electrode extension lengths will vary. The STT process also makes it possible to complete open root welds three to four times faster that GTAW, with low heat input and no lack of fusion. It makes it easier to perform open gap root pass welds with better back bead and edge fusion, yet produces consistent X-ray quality welds.

While there are many positive advantages to the STT welding process, there are a few disadvantages. First the power supply is more expensive that a constant voltage or MIG power source. However, the cost savings are recognized through its benefits. Secondly, there will be additional training required to operate the STT power source. Thirdly, the STT process cannot perform aluminum welding at this time.

The implications of this emerging technology are far reaching as the automotive, petroleum, chemical, and structural steel industries. It has caused manufacturers to grapple with a paradigm shift in their traditional application of short arc welding.

This process could be used in conjunction with another labor saving welding process called Submerged Arc Welding or (SAW), which is a high quality, very high deposition rate welding process. Originally developed by Linde Union Carbide, it requires a continuously fed consumable flux cored electrode. Molten arc and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux. When molten, the flux becomes conductive and provides a current path between the electrode and the work. The thick layer of flux completely covers the molten metal thus preventing spatter as well as suppressing the intense ultraviolet radiation and fumes that are part of the SMAW process. Thereby, exposure of the eyes to arc flash is minimized. However, this doesn’t downplay good safety practices to assure protection of the eyes and face.

The Submerged Arc Machine is normally operated in the automatic or mechanized mode; however, semi-automatic guns are available. Deposition rates of 100 lb/hr have been reported using this process. Deep penetration in some cases may eliminate joint preparation as well. A burnback control is beneficial to both automatic and semi-automatic systems to prevent the wire from sticking to the weld puddle at the end of the process. Material applications include carbon steel, low alloy steel, stainless steel, nickel based alloys, and surface applications such as wear facing or buildup as well as cladding processes.

While there are many positive advantages to the Submerged Arc welding process, there are a few disadvantages. The lack of adaptability to thin materials poses a problem as welding operators find it difficult to travel fast and steady enough to avoid melt-through. Also the flux pile has a tendency to hide the weld joint making it difficult for the welder operator to follow the joint. Another disadvantage may be the difficulty in recovering unfused flux for reuse.

The STT process would be used for the root pass and then transferred to the Submerged Arc Machine for the final cover pass(s). Gaps of any type between the two pieces being welded increase penetration and can result in burn-through or flux drop through. Therefore, the need to weld the root pass using a process such as the STT is important. Both processes would typically utilize a turntable, where the piece to be welded is affixed using a jig, then rotated by the operator, keeping the weld puddle on top in the vertical position. The Submerged Arc Machine utilizes a track system, like a railroad, to move the welding head along the pipe. It is most effective to have the track systems mounted on the floor between two turntables. The first turntable would have a piece affixed with the Submerged Arc Machine welding away utilizing its high deposition rate. While the machine is welding, another piece can simultaneously be affixed to the second turntable ready to go once the machine is finished welding the first piece. Once completed, the Submerged Arc weld head is swung around to the second turntable and the weld process is then started. The finished piece from the first turntable is then removed and the whole cycle starts all over again.

Now is the time to maximize your profits, and prefabrication is one avenue to do so. Once management has supported the idea, then it’s time to think of the tools and equipment necessary to perform the tasks. The STT welding process used in conjunction with the Submerged Arc process could be a great choice to obtain your goals. Happy welding!

Illustration by Angelo Katsaros