The transition to using high-strength pipe for onshore pipeline projects continues to expand globally as companies seek to increase lay time and build in progressively more remote areas. In addition to allowing for the construction of longer pipelines with greater carrying capacity and higher operating pressures, high-strength pipe (up to X100 grade steel) can also be used in areas that are subject to significant temperature extremes. With wall thicknesses as thin as 18 millimeters, high-strength pipe also offers companies measurable cost savings, both in terms of transportation and labor. In short, the thinner materials weigh less, which reduces material haul and fuel costs, and it takes fewer passes to weld. 
That said, high-strength pipe is also much more complicated to weld than lower-strength steels associated with pipelines in past years. That fact continues to challenge filler metal manufacturers to develop new filler metal technologies capable of meeting the exacting chemical and mechanical properties of this material.

Recently, a shift in welding processes — from stick welding (SMAW) to self-shielded flux-cored welding (FCAW-S) — has emerged on pipelines across the world as one answer to these challenges. There are three key reasons for this transition. 
First, the self-shielded flux-cored wires filler-metal manufacturers have designed for onshore pipeline applications provide not only high strength properties, but they also offer low hydrogen content — typically less than 8 ml per 100 g of weldment. That is a critical feature considering that high-strength materials also possess high yield strength and a relatively low carbon base, making them particularly prone to hydrogen-induced cracking. Further, the cellulosic stick electrodes previously used for welding low-strength pipelines possess too high of hydrogen content (typically 16 ml per 100g) to make them viable options for welding Grade 70 or higher steel. Industry reports have indicated that significant cracking and expensive downtime for rework has occurred when these stick electrodes have been used on pipeline projects.

Secondly, the FCAW-S 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. 
Finally, the FCAW-S process is significantly faster than stick welding, making it a better option for completing weld passes in less time and finishing pipeline projects sooner.

The Self-Shielded Flux-Cored Solution

Filler metals used to weld high-strength pipe must meet 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. They also require no shielding gas, making them ideal for welding outdoors and eliminating 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:

AWS 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.

AWS 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 temperature 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.

In conjunction with these wires, welding equipment specifically designed to withstand the harsh environment of pipeline welding have become available. Specifically, flux-cored guns with dual-schedule features allow welding operators to change wire feed speeds and voltages easily when welding at the top of the pipe (12 o’clock to 4 o’clock positions) compared to when welding in the 4 to 6 o’clock positions. They also provide greater puddle control. Also available to help contractors are suitcase-style, voltage-sensing wire feeders that protect wires from external contaminants and adjust to changes in voltage to offer precise arc control. Some contractors have also found the addition of wireless remote hand controls adds to productivity when welding on high-strength pipe. This equipment allows welding operators to change parameters at the point of use (as opposed to at the power source location), and eliminates clutter surrounding the pipe.

As for meeting the overall demands of building high-strength pipelines, it is likely that the shift to the FCAW-S process seen thus far 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 for meeting the strict welding requirements of high-strength pipe and providing these companies with an increasingly competitive edge.