Until recently, pipe fabrication shops had only a few processes to choose from when performing root passes. GTAW, with its slow travel speed (2 to 4 i.p.m.), has the potential for high quality welds but requires a highly skilled operator. SMAW has a faster travel speed of 5 to 8 i.p.m. but requires the welder to chip slag and grind between passes. Traditional short circuit GMAW is a third choice, but many pipe fabricators avoid it altogether for the root pass because of the skill required to produce code-quality welds.
|Before Farmer & Irwin switched from SMAW to a new modified short circuit GMAW process, one pipe fitter and welder could bevel and tack weld enough pipe to keep two or more welders busy. Now one welder keeps two fitters busy.
However, modified or “regulated” short circuit processes offer a viable option. A modified short circuit GMAW process can double or triple travel speed (6 to 12 i.p.m.), possibly eliminate the hot pass, is very tolerant of differences in work-to-tip distances and produces superior quality welds with a minimum of training. Plus, depending on the manufacturer, the same equipment, wire and gas may be used for the fill and cap passes using an advanced pulse process that is also very tolerant of differences in work-to-tip distances and can substantially increase travel speed and deposition rates.
Moving to MIG
Farmer & Irwin, a mechanical contractor based in Riviera Beach, Fla., fabricates carbon steel pipe from 1-1/4 in. up to 16 in., using SMAW to weld Schedule 40 carbon steel pipe in a fixed position. Joints were being completed in three passes using 1/8-in. diameter E6010 electrodes for the root pass and 1/8-in. and 3/32-in. diameter E7018 electrodes for the fill and cap pass.
“Our estimating department and the performance of our production forces brought in so much work that we had to figure out new ways to refine our [welding] processes just to keep up,” said Larry Wrye of Farmer & Irwin. Wanting to increase productivity, Wrye looked for alternatives.
“We tried [conventional short circuit GMAW] with a few different arrangements, but we could never get satisfactory results on the root pass,” explained Wrye. “We always ended up with excessive penetration, which caused keyholes and ‘stalactites’ on the inside of the pipe.” The company continued using SMAW until it tested one of the new modified short circuit GMAW processes for the root pass and advanced pulsed spray transfer process for subsequent passes.
As a result, Farmer & Irwin increased production rates by 100 percent on 3-in. diameter pipe and by up to 500 percent on 12-in. diameter pipe (the larger the diameter of the pipe, the greater the percentage of increase). Before implementing the new technology, one pipe fitter and welder could bevel and tack weld enough pipe to keep two or more welders busy. Now one welder keeps two fitters busy, according to Wrye.
Modified vs. Traditional Short Circuit MIG
In traditional GMAW, the short circuits occur at irregular intervals and are of varying intensity. This agitates the puddle, causing it to splash onto the sidewall of the pipe, which can lead to cold lap or lack of fusion as well as spatter. Thus, this process requires a high level of skill to produce code-quality welds, and many companies shy away from it.
With the new modified short circuit GMAW technology, however, the metal transfer process is precisely regulated, resulting in a uniform droplet deposition, which makes the puddle easier to control. It also creates only small ripples in the weld puddle. With a more stable puddle, it is far easier to create consistent tie-ins with the sidewall.
With older technology, changing the stick-out would affect the arc parameters and could lead to quality issues, which would necessitate reworking the weld. This modified short circuit process, however, compensates for operator differences by maintaining a consistent arc length regardless of stick-out. This is especially helpful for novices and others who have trouble maintaining a constant tip-to-work distance.
With a robust system process such as this, novice welders learn faster and quickly gain confidence in the quality of their welds. Learning to create code-quality welds typically takes two hours for an experienced welder and two days for an apprentice welder.
Because the process produces a faster-freezing, calmer weld puddle, it provides another benefit: the potential to eliminate the need for a hot pass. Typically, SMAW, GTAW and traditional GMAW create root passes 1/8 in. to 3/16 in. thick, depending on the operator. This requires a hot pass to add more metal so subsequent fill passes with FCAW or spray transfer GMAW do not blow through the root pass.
|Comparison of root passes performed by modified short circuit GMAW (left), GTAW (center) and traditional GMAW (right).
The modified short circuit process creates a thicker root pass, 3/16 in. or greater, enough to support the heat input of pulsed GMAW or FCAW fill pass. Since the weld metal in a faster freezing puddle stays where it is directed, the modified short circuit process is also more forgiving of high-low misalignment, being able to bridge gaps up to 3/16 in.
With the modified short circuit process, the software controls the electrode current during all phases of the droplet transfer. After the ball on the end of the wire wets out in the puddle, current is increased to a level sufficient to start pinching the electrode. The current is then increased until the short circuit is cleared and a pinch is detected. Once the pinch is detected the current is rapidly decreased.
Because the pinch is detected before the short clears, the inverter quickly sets the current to a low level before the circuit breaks. The current is then increased to form a ball for the next short circuit, and then decreased to allow a short circuit to occur. The current is then monitored, and if necessary, dropped even further to avoid an arc force that could agitate the puddle.
The process is fairly simple to learn, and if the operator can already weld with GMAW, he or she can become productive with the modified short circuit GMAW in two hours. Typical training times for apprentice welders is about two days.
In this process, pipes are prepared with a minimum 1/8-in gap to ensure proper root reinforcement. Lands can range from a knife edge to 3/32 in. Once the puddle is established, the electrode is positioned in the center of the weld. If the joint is misaligned, it is not necessary to favor the high side. The equipment will automatically compensate. It is effective in both the 1G rolled or 5G fixed position.
Compared to conventional short circuit GMAW, the modified short circuit GMAW
- Is designed for root pass welding
- Can replace TIG in some applications
- Good gap filling; tolerant of joints with high-low sides
- Reduces spatter
- Reduces heat input
- Allows less-skilled welders to put in x-ray quality welds
- Very tolerant of changes in tip to work distance
- Excellent fusion and reduced toe angles (smooth transition) reduce grinding on the side face of the weld
Fill It Up-Advanced Pulsed GMAW
As noted, Farmer & Irwin was able to use the same equipment for the fill and cap passes. CMN Steel Fabricators of Miami, Fla., was also able to take advantage of the advanced pulsed GMAW process. CMN fabricates pipe for the waste energy and other industries. Recently, CMN was contracted to weld 66 joints of chromium-molybdenum alloy, commonly called P11 on the New Hope Power Partnership (an expansion of the Okeelanta Cogeneration Plant). P11 was chosen because it provides the same level of pressure capabilities as carbon steel while being lighter and thinner. WPS specified TIG for the root, hot and first fill passes.
The specifications also called for an additional 15 SMAW fill passes using 1/8-in. E8018-B2 electrodes; however, by switching to the advanced pulsed GMAW process, CMN was able to fill the joint with only seven passes (< 3 lbs./hr for a 1/8-in. E7018 electrode for Stick vs. 4.5 to 14.4 lbs./hr for an .045-in. ER70S-6 electrode for Pulsed MIG).
|Rivaling GTAW in appearance, the modified short circuit GMAW process is easier to learn, more tolerant of operator differences and may eliminate the need for a hot pass.
As a result, the company finished the proj
ect in six weeks instead of eight and reduced welding time by 64 percent with each weld passing UT on the first try.
When using conventional pulsed GMAW, different arc lengths can change the overall parameters; however, the advanced pulsed GMAW process maintains the optimum arc length and weld parameters within a broad range of stickouts (up to one inch is possible) and travel speeds. This makes it far easier to train new welders and even easier for experienced welders to maintain consistently high quality welds.
It also allows the welder to hold a shorter arc length than with older technology, which required a longer arc length to prevent short circuits but could lead to spatter and undercut if travel speed was too high. The shorter arc lengths possible with the new process helps eliminate undercuts and provide good fusion.
With the advanced pulsed GMAW technology, both current and voltage stay within the optimum range for a specific wire type and diameter, wire feed speed and gas combination. Each pulse starts by ramping up the current. Once the target current is reached at the beginning of each phase, the constant current (CC) control turns off and the constant voltage (CV) control loop turns on. The CV loop modulates the current within a range that maintains the target voltage. This occurs independently of the contact tip to work distance.
As a result of faster and tighter control over parameters, this technology provides shorter arc lengths and a more focu
sed arc column. Compared to older pulsed GMAW technology, the puddle is easier to control (thus the process is easier to learn) and improves fusion at the toes of the weld. Like the modified short circuit process, the advanced pulsed GMAW process is more tolerant of contact tip-to-work-distance variation, which helps when encountering tack welds, welding in tight corners or on pipe beveled with steep angles and when training new welders.
Benefits-compared to conventional pulsed MIG
- Hold shorter arc lengths
- Better puddle control
- More tolerant of contact tip to work distance variation
- No arc wandering in tight corners
- Narrow arc plasma column
- Allows welds to fill in at toes, increasing travel speed and deposition rate
- More tolerant of poor fit-up and gaps
- Excellent out-of-position puddle control
These benefits are possible due to the fast reaction time of today’s inverters coupled with advanced software that can shape every aspect of the arc. Together, the hardware and software of the systems remove the burden of setting a complex set of parameters from the operator. The user simply enters the wire type, diameter and gas combination and the equipment selects the optimum parameters from an extensive library.
These new processes have the capability to drastically alter a pipe shop’s productivity, according to Wrye.
“We have come to the conclusion that the larger the pipe, the more efficient we are,” said Wrye, because larger diameter pipe has a greater percentage of arc-on time compared to preparation time. “I would venture to say that on 12-in. diameter, schedule 40 pipe, the new wire welding processes out-produce SMAW by a 5:1 ratio. On 3-in. diameter pipe, wire welding out-produces SMAW by 2:1.”