Examining Self-Shielded Flux Cored Welding (FCAW-S) in High-Strength Pipeline Welding Applications as an Alternative to Stick Welding (SMAW)
June 21, 2011
As pipelines transition to high-strength steels such as X70, X80, and X100, welding challenges — including matching tensile strength and avoiding hydrogen-induced cracking — have pushed the industry to consider new welding procedures. Advances in Self-Shielded Flux Cored welding (FCAW-S) wire and equipment have made it an increasingly viable option in manual pipeline welding applications compared to Stick welding (SMAW). Advantages include lower hydrogen content, increased speed and deposition efficiency, increased control of the arc and ease-of-use.
There are a couple of ways to reduce pipeline costs as it relates to the pipe itself. One is to reduce the weight of steel used in its construction. The other is to increase the pressure tolerances of that pipeline so that it may pump a greater amount of material through it. These factors have led pipelines to transition to high-strength steels such as X70, X80, and X100.
High-strength steel provides obvious advantages, such as lower cost-per-foot, increased strength with thinner wall thicknesses that require less filler metal to complete the joint (reducing welding time and consumables), and reduced hauling and fuel costs because of the lighter weight. It also made it easier to transport pipe to more remote areas, and proves better suited to areas prone to significant temperature extremes.
These high-strength steels also pose new challenges: filler metals must match or exceed the higher tensile strengths while also having lower hydrogen content—high strength steels feature a low carbon base and are therefore more prone to hydrogen-induced cracking. The cellulosic Stick electrodes previously used for welding low-strength steels possess too high of a hydrogen content (typically 16 ml per 100g) to make them viable options for welding X70 or higher. This higher hydrogen content can lead to significant cracking and expensive downtime related to rework.
To address these issues, a shift is taking place from Stick welding (SMAW) to Self-Shielded Flux Cored welding (FCAW-S). While Stick is still almost universally specified for root passes (although new Metal Cored and Miller’s Regulated Metal Deposition (RMD) processes are making gains there, too), Flux Cored welding offers substantial advantages in terms of productivity and quality. This article will look at these advantages, review some of the common Flux Cored filler metal options, and examine advances in welding technology that have made Flux Cored welding an increasingly viable option.
Advantages of Self-Shielded Flux Cored Welding
Just as with Stick electrodes, Self-Shielded Flux Cored wires provide shielding of the weld puddle, making it suitable for welding in the often windy and dirty environments encountered in pipeline welding applications. While it does require some additional equipment considerations (detailed further in the article), and may require re-qualifying certain joints and applications, the process offers noticeable advantages over Stick in a number of areas:
- Lower Hydrogen Content: Arguably the most important benefit, Self-Shielded Flux Cored wires designed for onshore pipeline applications provide the high-strength properties required in these applications while also meeting the low hydrogen content standards required to reduce hydrogen-induced cracking.
- Increase in Speed, Deposition Efficiency: While quality is paramount, Flux Cored welding typically allows for faster travel speeds and offers greater filler metal deposition efficiency as compared to Stick electrodes. A standard E6010 or E7010 Stick electrode offers 50 to 65 percent deposition efficiency (the amount of a filler metal that actually goes into the weld joint) while a E71T8-Ni1 J H8 Flux Cored wire like Hobart Brothers’ Fabshield 79T8™ offers 78 to 87 percent deposition efficiency. This greater deposition efficiency ultimately means welders will need less filler metal to get the same job done. This fact saves money and ensures welders are laying more filler metal in a shorter amount of time.
- Increased Control of Arc: With the addition of voltage sensing wire feeders and dual schedule capabilities associated with Flux Cored wire feeders and guns, the welder has more control over the precise performance of the welding arc.
- Easy to Operate: Comfort with existing technologies, and qualifying new processes, can often be a roadblock to progress. The Flux Cored process allows for relatively easy operator training. When welding higher-strength pipe, variables such as pre-heat and interpass temperatures, along with the overall welding parameters (amperage, voltage, etc.), must be carefully monitored. Those features alone make welding this material precarious, so having a welding process that is easy to use minimizes additional variables involved with training.
Self-Shielded Flux Cored Filler Metal
Filler metals used to weld high-strength pipe must meet or exceed the material’s mechanical and chemical property requirements and also provide the ductility to mitigate instances of cracking. They also need to compensate for the extreme temperatures to which pipelines are typically subject, as the impact of thermal expansion or contraction, frost and/or other environmental loadings can easily damage steel and the welds. Special pipeline and joint designs, in conjunction with proper filler metal selection, are helping to protect against such issues.
To date, several classifications of Self-Shielded Flux Cored wires have proven successful for welding high-strength pipelines worldwide. These wires have been specially formulated to generate low levels of spatter and create an easily removable slag so as to speed inter-pass and post-weld clean up. Because they require no shielding gas, they can eliminate the time and need for setting up shielding tents to protect gas coverage. In addition, these wires provide good vertical-down capabilities and offer higher deposition rates than Stick electrodes to help improve productivity.
Available Self-Shielded Flux Cored wires for welding high-strength pipelines include:
E71T8-Ni1 J H8: This all-position wire has been designed for onshore transmission pipelines composed of Grade X70 (and below) pipe and provides high-impact toughness at low temperatures. It offers a tensile strength of 78 ksi in the as-welded (AW) condition and creates welds with low diffusible hydrogen levels (5.45 ml/100 g). The wire’s unique formulation offers excellent weld puddle control, particularly when welding from the 4 o’clock to 7 o’clock position. It is especially useful for training welding operators with modest experience. It also provides good weldability and uniform weld beads when welding multiple passes on deep-groove pipe weld joints. Typical CVN impact values vary according to weld position (1G versus 3G) but fall in the range of 295 ft.lbs. at -20 degrees Fahrenheit (-29 degrees Celsius) to 135 ft.lbs. at -40 degrees Fahrenheit/Celsius.
E81T8-Ni2 J H8: This wire can be used for fill and cap passes on pipeline projects constructed from Grade X80 (and below) pipe. It provides high tensile strength (94 ksi) and low CVN impact properties (as low as 96 ft.lbs. at -40 Fahrenheit/Celsius), as well as excellent ductility. The wire’s “J” designation ensures that it meets the strict requirements for low-temperature CVN impact toughness as determined by AWS A5.29, and ensures more exacting properties than many E81T8 wires classified to only “G” designations. It can be used on fillet, lap or deep groove welds in single- or multi-pass applications, and has a fast-freezing slag that peels easily to help reduce post-weld cleaning time. E81T8-Ni2 J H8 wires can be used for welding in all positions.
AWS E91T8-G H8: In addition to offering relatively low diffusible hydrogen content (6.2 ml/100 g), this all-position wire provides a tensile strength of more than 113 ksi, as well as excellent low-temperature impact strengths (44 ft.-lb. at -40 degrees Fahrenheit/Celsius). This wire has been designed for use as an overmatch on Grade X80 pipe and also offers good ductility. It is available in 1/16-inch diameters, which tend to be easier for welders new to the process to use, and it has smooth arc characteristics, which improves operator appeal. Like other Self-Shielded Flux Cored wires for high-strength pipe, this wire provides excellent mechanical properties and generates a fast-freezing, easy-removable slag that allows for good weld puddle control and easy cleanup. It also operates similar to E71T8-Ni1 J H8 and E81T8-Ni2 J H8 wires, which makes it easier for welding operators moving between various pipeline projects to use.
AWS E111T8-G H8: Used for welding Grade X100 pipe and as an overmatch on Grade X80 steel, this wire also operates in a manner similar to the aforementioned wires but offers much higher tensile strengths. Specifically, when welded in the 1G position, the wire creates welds with 123 ksi tensile strength and provides over 120 ksi tensile strength when welded in the 3G position. It works well for oil and gas transmission pipelines and is available in 5/64-inch diameters. The hydrogen content for this wire is 6.33 ml/100 g of weldment, so it is a good option for helping reduce cracking.
Flux Cored Equipment Considerations
Stick and Flux Cored welding can be performed with the same engine-driven welding generators pipeline welders have used in the past, although there are some machines (Miller’s Big Blue® 350 PipePro® , for example), that feature special programs and enhanced outputs (350 amps at 100 percent duty cycle) that optimize performance for Flux Cored welding.
There are additional equipment considerations that have been deterrents in the past for Flux Cored welding in pipeline applications: a Flux Cored gun can be more complicated than a stinger used in Stick welding, the wire feeder provides an additional piece of equipment to own and maintain, and controlling things like amperage and wire feed speed are perceived to be more difficult than a Stick electrode.
Recognizing these beliefs while also acknowledging the advancement of Flux Cored welding in these applications, equipment manufacturers have taken steps to improve the overall quality, simplicity and productivity of these products. Advancements include:
- Wire Feeders: Once large and clunky, wire feeder designs—like that of Miller’s SuitCase® X-TREME™ 8 HD—have become more compact and reliable. Tight-sealing polypropylene cases keep out dirt and contaminants. The addition of high-torque motors handles both large and small diameter Flux Cored wires, and provide more consistent feeding. Digital meters capable of displaying both voltage and amperage while welding also help improve welding accuracy. Voltage sensing models offer precise arc control. Dual schedule capabilities (when matched with a dual schedule Flux Cored gun) allow welders to adjust wire feed speed on the fly based on their position on the pipe. More robust, reliable designs include vastly improved contactors—a former heavy maintenance item.
- Flux Cored Guns: Flux Cored guns have been made substantially lighter and more ergonomically friendly than past models. Advances to guns such as the Bernard PipePro® Dura-Flux™ gun include internal trigger leads that won’t get caught on surrounding equipment, replaceable liners, and improved visibility to the weld puddle. Most importantly, the dual schedule functionality gives welders the opportunity to adjust wire feed speed right at the gun. Consumables (contact tips, etc.) are now also much more reliable and much easier to change out than older models.
- Wireless Remote Controls: Whether Stick or Flux Cored welding, new wireless remote controls extend the work envelope up to 300 feet from the power source. Typical corded remotes only extend to 90 feet. Digital displays on wireless remote controls further give welders and welder’s helpers a precise knowledge of their machine’s settings. That, combined with the dual schedule capabilities of the gun and the voltage sensing capabilities of the feeder, gives the welder and welder’s helper greater control over arc characteristics and filler deposition than previously possible.
As for meeting the overall demands of building high-strength pipelines, it is likely that the shift to the Self-Shielded Flux Cored process will continue as contractors seek to complete projects faster and for less cost. It is equally likely that filler metal and welding equipment manufacturers will be challenged to build on this technology and offer even more solutions to meet the strict welding requirements of high-strength pipe and provide these contractors with an increasingly competitive edge.
SIDEBAR: Self-Shielded Flux-Cored Welding Improves Productivity on Ruby Pipeline Repairs
Pipeline welder Bruce Weeks, a Local Union 798 member working on the Ruby Pipeline Project on Spread 4 as it passed through Elko, Nevada, made the transition to Self-Shielded Flux Cored welding (FCAW-S) as he worked repairs along a stretch of the 42-inch X70 pipeline. Weeks had welded with the Flux Cored process in other industries, but had not worked much with it on pipeline repairs.
“They were running automatic processes on the pipeline, and some of the repairs were pretty lengthy,” says Weeks. “Anything over eight inches I repaired with the Flux Cored wire because it was quicker. We had some repairs 17- to 20-inches long and it was much quicker with the wire.“
Weeks’ rig features Miller’s PipePro™ 304 diesel engine-driven welding generator, Miller’s SuitCase® X-TREME™ 8 HD voltage sensing wire feeder, and a Bernard PipePro Dura-Flux™ gun. The wire he used along this spread of the pipeline was Hobart Brothers’ Fabshield® 79T8 Self-Shielded Flux Cored wire (E71T8-Ni1 J H8). As described in the main body of this article, E71T8-Ni1 wire is designed for all-position welding and provides high-impact toughness at low temperatures. It also offers a tensile strength of 78 ksi in the as-welded (AW) condition and creates welds with low diffusible hydrogen levels (5.45 ml/100 g). The wire’s unique formulation offers excellent weld puddle control, particularly when welding from the 4 o’clock to 7 o’clock position.
Using the dual schedule capabilities of both the wire feeder and the gun, Weeks was able to manipulate his wire feed speed (amperage) to match his position on the pipe, such as dropping amperage as he got to the bottom half of the pipe. The ability to weld downhill with the Flux Cored wire as compared to uphill with a 9018 Stick electrode made the process faster. Most importantly, it allowed him to maintain the high degree of quality demanded in this application while noticeably increasing his productivity.
“If I do a 20-inch repair with Stick, I’ll be there for an hour and a half. If I do it with this wire, I’m there 45 minutes or an hour. Overall I’d say it cuts it down by about 30 percent. That makes it a lot easier on me. And I’ve never lost a repair.”