September 15, 2009
Gas tungsten arc welding (GTAW or TIG), a popular process for high-quality manual welding, has its limitations and requires highly skilled operators. A process used in Europe addresses those limitations, enhances productivity and weld quality, and reduces the skill level required to GTAW.
For at least six decades, traditional gas tungsten arc welding (GTAW or TIG) has been considered the process of choice for attaining high-quality welds in any metal application. However, this process has certain drawbacks, such as the weld energy limitation influenced by the weld pool dynamics and typically slow manual wire feed rates. Manual GTAW requires highly skilled operators who possess the dexterity necessary to feed the wire. Manual GTAW techniques vary, and the weld-wire-to-arc and weld puddle placement are inconsistent.
A process*, however, has emerged that could change how people think about GTAW. This welding process, developed in Europe, is a manual and automated GTAW wire feed control combined with a hot-wire power source that operates with any GTAW or plasma welding power source. Suitable for all-position welding on materials of any thickness, the process addresses traditional GTAW limitations and can enhance both manual and automated TIG weld quality and productivity.
The wire feeder feeds a weld wire from a typical 30- to 50-lb. gas metal arc welding (GMAW or MIG) wire reel at a constant linear forward motion while applying a "mechanical action" to the wire. The wire entering the weld pool is superimposed with a high-frequency, dynamic, linear forward - backward motion, which agitates the weld pool and generates kinetic energy into the puddle.
The weld wire motion associated with this process eliminates the surface tension present in traditional GTAW weld pools, which changes the weld dynamics and creates a highly mobile molten poolthat is receptive to higher-than-normal wire filler rates associated with the conventional and hot and cold GTAW processes. Increased wire feed rates enable higher weld current per application, which reduces the likelihood of internal weld defects and weld fusion.
The weld pool agitation and increased weld energy from the higher weld current allow the pool to remain molten longer. Weld fluidity is especially important with sluggish alloys like duplex, chrome, and nickel alloys.
This agitation-energy combination enables a greater release of weld gas oxide reactions and contaminants and reduces weld stresses.
This process's wire feed rates facilitate typical weld deposition rates in the range of 2 to 10 lbs./hr. On most applications previously welded by traditional GTAW or the shielded metal arc welding (SMAW) process, the higher weld deposition of the new approach enables higher weld speeds, which lowers the weld heat and reduces weld heat-affected zones (HAZ). For example, heat-sensitive P91 pipe weld joints typically are made with GTAW for the root and SMAW for the fill passes.. Normal GTAW on the fill passes was considered too slow, providing too much weld heat for the P91 parts. In Europe many P91 applications now are welded with this process, which is used for both the P91 pipe and valve root and fill passes (Figure 1).
This process requires fewer welding skills than traditional TIG, SMAW, and pulsed MIG. With this process, welders can put both hands on the lightweight water-cooled torch, which can improve weld control. Constant, automatic wire feed reduces stops and starts.
It also does not require argon/helium or argon/hydrogen mixes for most alloy applications. Because the process welds faster than traditional manual GTAW, gas consumption can be reduced by 50 to 80 percent.
Examples of welders using this process are shown in Figure 2 and Figure 3. In Figure 2, the welder is holding the torch with one hand and using his other hand to steady himself as he performs an overhead pipe weld.
In Figure 3, the welder is depositing a GTAW weld with GMAW weld deposition rates on an INCONEL® alloy screw.
In the past achieving the best weld quality pm a common application like the stainless cooling tubes in Figure 4 would involve conventional manual GTAW process and orbital welding. If you used the manual pulsed GMAW process, you might produce welds in which the tie-ins and fusion occasionally would be suspect and the weld appearance would be irregular.
GTAW of this application would require high skills, and the inconsistency of the slow, manual wire feed rates and the varying manual techniques, such as the different wire-to-work placement from each GTAW welder, typically would be indicated in an irregular external weld appearance and in the internal weld quality.
Figure 5 shows a close look at the "manual" stainless tube welds done with the new process. Made by the welder shown in Figure 4, these welds were accomplished three times faster than using conventional GTAW. Because of the higher weld energy; the controlled dynamic wire feed; the exact wire-to-tungsten and wire-to-arc placement; and fewer arc starts and stops, these manual welds look like orbital welds. Note the weld cleanliness and HAZs.
This new welding approach can benefit several industries: power plants; oil platforms; shipyards; liquefied natural gas pipeline construction; naval vessels and submarines; space and aircraft; subsea components; petrochemical; refining; waste-to-energy; industrial processing; pulp and paper; military equipment; medical equipment; and food and beverage equipment manufacturers.
This process is a modified hot wire GTAW process that is accepted by most code bodies.
*This process is called TIP TIG and TIP Plasma process and was invented in Austria by Ing. Siegfried Plasch. Extensive information on this process is available at www.weldreality.com.