November 8, 2011
A process called TIP TIG uses a constant-feed hot-wire that imparts additional energy to the weld. The wire also is superimposed by a secondary high-speed oscillation. The hot-wire’s current combines with the weld-pool agitation to disrupt the surface tension and change the weld dynamics.
For those industries requiring code-quality pipe and fabrication welds, challenges mount with the increased use of new, heat-sensitive alloys, as well as the power and oil industries’ push for more stringent weld standards, including those that demand fewer weld defects. In fact, one major global oil corporation now demands that its welding and cladding operations achieve zero defects.
For decades the process of choice has been gas tungsten arc welding (GTAW, also called TIG). While many equipment and consumable advances have been with semiautomatic gas metal arc welding (GMAW) and flux-cored arc welding (FCAW), these processes have never consistently matched the quality manual GTAW offers, especially on alloy pipes in 5G (vertical-up and overhead).
Producing a steel or alloy pipe weld (5G position) to consistently meet stringent code, radiograph, ultrasonic, and metallurgical requirements has remained a significant challenge. When choosing a process, be aware of its influence on the potential for weld defects. It’s also important to understand the relationship among all the primary process variables inherent in pipe welding, such as the weld energy, shielding gas reactivity, polarity, deposition rate, speed, weld mass influence on the weld fusion, and porosity.
Pipe shops may use semiautomatic processes such as FCAW and GMAW. Depending on the weld position, other processes such as gas-shielded fluxed-cored, spray-transfer GMAW, or pulsed GMAW may be suitable only for the pipe fill passes.
Reverse-polarity pulsed GMAW and FCAW enable much higher weld deposition rates for all-position pipe welds. In contrast to GTAW, these processes do produce a lower-temperature arc, but they also produce weld heat that is highly localized in the arc zone, which is beneficial in melting the rapid, constant-feed welding wire. The concentrated reverse-polarity weld heat also causes the typical large heat-affected zones (HAZ), which can be a concern with alloy pipe welds and other heat-sensitive applications.
Both FCAW and GMAW provide 10 times the weld deposition attained with GTAW, which results in much higher welding speeds. From a productivity perspective, this can be beneficial—but from a quality perspective, this can be detrimental. Depending on the pipe alloy and thickness, GMAW and FCAW may produce insufficient weld energy for the speed and weld mass delivered, especially in the first two 5G groove weld passes over the pipe root. When that weld energy is lacking, nondestructive examination will reveal incomplete fusion and excessive weld porosity.
Pulsed GMAW also provides healthy weld deposition rates, but 50 percent of that pulsed weld current typically is less than 100 amps. In addition, FCAW and pulsed GMAW energy is influenced by the wire stickout (WSO) variations, which is especially relevant with groove applications in pipe welding. Small WSO changes in manual operations can dramatically influence the amperages delivered, again negatively affecting the weld energy generated as well as the amount of fusion and porosity attained. For steel and stainless steels, pulsed GMAW requires reactive gas mixes that contain CO2 or oxygen. Reactive gas mixes increase the potential for weld porosity.
The FCAW rutile wires used for 5G and vertical-up welds provide a fast-freezing slag. The slag is designed to mold and control the bead during all-position welding. This process also uses a reactive gas mix and enables slightly higher manual 5G weld deposition rates than those attained with pulsed GMAW. With FCAW, incomplete fusion defects are common, and the frequently trapped slag adds to the lack of fusion. Worm tracks (elongated porosity), slag inclusions, and excess single-pore porosity also are common defects.
Mechanizing the pulsed GMAW or FCAW processes can better control some of the variables and increase the weld quality. But despite advancements in semiautomatic GMAW equipment and in FCAW consumables for all-position welding, manual GTAW provides a more desirable ratio of high weld energy to the small amount of weld deposited. This is why GTAW frequently is the pipe process of choice.
With requirements to deftly feed a welding wire into the small arc zone and the frequent use of a foot amperage control, manual GTAW demands the highest skills for 5G pipe welds. The low weld deposition rates of conventional, manual GTAW influence the slow weld speeds, which often result in excess heat input. This is a frequent concern with heat-sensitive alloys.
During 5G pipe welding, the low- to moderate-current GTAW process produces a small, rapid freeze weld in which a portion of the arc plasma energy is directed at the weld, and the other portion melts the tip of the large weld wire (see Figure 1). The gas tungsten arc welder must manually direct the wire into the optimal arc zone position at the correct moment. The welder also must rapidly direct the torch to move the small, fluid weld pool over the desired area.
The large-diameter wire and small, fluid weld zone dramatically limit the manual GTAW wire feed rate. The narrow GTAW plasma and small weld zone are sensitive to minor tungsten-to-work-distance variations. The gas tungsten arc welder frequently uses a foot control to make large current changes necessary to deal with rapid changes in the weld arc and pool. GTAW demands highly skilled welders with good eye-hand-foot coordination. These are skills that take a long time to acquire and may be difficult to maintain as the welder gets older.
Conventional manual GTAW uses large-diameter wires; 1⁄16 and 1⁄8 in. (1.6 and 3.2 mm) are typical. The large wire diameters have more in common with the consumables used for high-current submerged arc welding applications. Smaller wires like 0.045 or 0.035 in. (0.9 or 1.2 mm) would be more logical to use with the low- to moderate-current GTAW applications. However, with small-diameter wires the welder simply could not feed the wire at the required higher feed rates. Also, the rapid-freeze GTAW weld pool dramatically restricts the wire feed deposition rate potential to typically less than a pound an hour.
As they use large wire diameters, manual GTAW operators rapidly move the wire in and out of the weld pool. The wire dipping or delivery technique is frequently inconsistent, and the wire placement may not be consistently in the optimal arc zone sweet spot. The bottom line: The few pipe weld defects that occur with regular manual GTAW frequently happen because of irregularities in the weld wire delivery, along with numerous arc starts and stops that create weld tie-in issues. Evident on large-diameter pipe welds, numerous manual weld starts and stops hinder the weld’s uniformity and continuity.
GTAW’s electrode-negative polarity puts the majority of weld heat into the part to be welded, which enables rapid heat dissipation away from the weld. Unfortunately, GTAW’s slow speed means that significant heat still transfers into the weld joint—a particular concern with many multipass and alloy applications.
To overcome manual GTAW’s shortcomings, some pipe welding and cladding operations have adopted automated hot-wire (HW) GTAW, which has a wire feed unit and a separate HW power source that typically delivers 50 to 100 amps to the constant-feed welding wire. The HW current adds more energy to the small, rapidly freezing weld pool. These automated welding and cladding operations typically reap excellent productivity benefits, especially when the welds are made in the flat position and the current is more than 250 amps, which provides a large plasma as well as a large fluid weld area.
At less than 250 amps, however, the automated HW process produces a smaller plasma and molten pool. With the lower current, the arc becomes sensitive to minor tungsten-to-work (arc length) variations. In this situation, an automated arc voltage control is necessary. The arc length monitoring equipment is the reason that the HW process has never been suitable as a manual GTAW process.
When it comes to pipe weld defects, some managers will point their finger at human error and may consider automation to eliminate that human element. It’s worth noting that a frequent root cause of manual weld defects is the process and consumables utilized, and automating the process may not completely resolve the problems.
Siegfried Plasch, an Austrian weld engineer, invented and patented a process called TIP TIG, an approach that changes the welding dynamics of conventional GTAW. First introduced in Europe, TIP TIG uses a constant-feed hot wire that imparts additional energy to the weld. The wire also is superimposed by a secondary high-speed oscillation. The oscillation is generated by a unique four-roll drive plate. This mechanical action creates a vibration on the wire that is transferred into the weld, agitating the molten weld pool.
The hot wire’s current combines with the weld pool agitation to disrupt the surface tension and change the weld dynamics. The altered dynamics increases the weld’s receptivity for faster wire feed and increased deposition rate potential. Increased wire feed speeds enable the use of higher current, adding more energy to the weld.
The improvement in the weld dynamics dramatically increases the wire feed rate; a 200 to 400 percent increase over conventional GTAW pipe fill pass deposition rates is typical. The higher deposition enables much faster manual or automated gas tungsten arc welding.
While many weld shops are not concerned with gas tungsten arc weld speeds, it’s important to remember that with TIP TIG’s electrode-negative polarity and faster weld speeds, the rapid weld heat dissipation now enables low weld heat input and, therefore, produces all alloy welds with the smallest possible HAZ. The helps attain optimal mechanical and corrosion properties and reduce crack sensitivity. The heat reduction also minimizes fumes (see Figure 2).
The altered weld dynamics improves and prolongs the weld fluidity, slowing the weld solidification. This is especially beneficial in improving pipe sidewall fusion and reducing pore inclusion defects. Improving the fluidity eliminates sluggish weld concerns common with many alloys, such as duplex, chrome, nickel, and INCONEL®. As the process offers faster speeds than conventional GTAW and uses an inert gas, you can anticipate low weld oxidation and high weld cleanliness (see Figure 3 and Figure 4).
The higher weld speeds, low heat generated by these speeds, and electrode-negative attributes dramatically reduce the weld oxidation potential. This helps reduce the extensive cleaning and grinding so typical with alloy and multipass welds.
Using this process, manual welders do not need to feed a wire into the weld, and a foot control is not necessary. The welders can get into a more comfortable position and use one hand to support their body, or use two hands on the torch and focus on the weld arc zone (see Figure 5).
The common denominator of most metallurgical weld problems is weld heat. With this new process, many titanium parts can be welded without a gas trailing shield; duplex alloys can be welded without concerns for attaining optimum ferrite-austenite levels; impact properties should never be a concern; and crack-sensitive alloys should have less crack sensitivity. TIP TIG also reduces the need for the multiple processes frequently used on pipe welds.
Many managers talk about pipe weld rework costs, but not necessarily the GTAW and SMAW costs associated with low deposition rates, slow weld speeds, and poor weld duty cycles. But consider one North American pipe welding operation in which the welders used conventional GTAW with a 3⁄32-in. (2.4-mm) carbon steel or stainless steel wire. In this application, welders delivered 8 to 10 in. of wire per minute on a 5G pipe fill pass using conventional GTAW. When you convert 3⁄32-in. wire (at 8-10 IPM) to an 0.035-in. (0.9-mm) GMAW wire commonly used with TIP TIG, you end up with 56 to 70 IPM of 0.035-in. wire, which equals 0.8 in. to 1 lb. per hour. This deposition rate enabled typical manual GTAW speeds of 2 to 8 IPM.
A typical gas tungsten arc welder usually achieves about 20 minutes of arc-on time every hour—and GTAW steel or stainless steel wire delivers 0.8 to 1 lb. of weld metal per hour. Considering a typical 20-minute arc-on time, a welder using conventional GTAW for a 5G fill pass can deposit 0.27 to 0.33 lbs. of weld metal per hour.
Now consider a typical TIP TIG application on an 8-in.- (20-cm-) diameter pipe with a 3⁄8-in. (10-mm) wall thickness, using 0.035-in. wire deposited at 2 to 3 lbs. per hour. With this new process, welders could achieve about 30 minutes of arc-on time every hour. Considering this, the TIP TIG process allows workers to deposit 1 to 1.5 lbs. of weld metal per hour.
Additional productivity increases also can come from changes in weld preparation, made possible by the unique TIP TIG weld dynamics. The weld pool agitation and improved surface tension characteristics allow for narrower V- or J-groove preparations, without concern for lack of fusion. For example, a pipe application calling for a combined V-prep of 60 degrees could be reduced to a 45- or 50-degree bevel.
One of the most important considerations in controlling costs is the ability to minimize weld rework caused by common incomplete fusion and porosity defects. When managers consider the process and consumable variables and understand how they relate to the specified welds—again with a focus on the required skills, weld energy, amount of metal deposited, welding speed, and shielding gas reactivity—they should of course choose a process that is most cost-effective in attaining the desired quality for the specific weld application.
Weld skills are only a small part of the requirements necessary to attain code-quality with optimum weld productivity. Today in North America, welding professionals have another important weld process to consider.
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