African Fusion talks to Arnold Meyer, Afrox’s applications development manager and an IIW Welding Engineer, about his determined efforts to achieve acceptance in South Africa for advanced pipe welding processes using modern GMAW solutions such as Miller’s PipeWorx welding system.
The traditional way of welding process pipe for the refinery, petrochemical, oil and gas, power, water or HVAC industries, according to Meyer, is to first put in a TIG root and then to fill and cap the joint using manual SMAW welding rods.
“This usually has to be done in the 6G or 5G position and pipe diameters are typically between 150 and 300 mm in diameter (6-12-inch) with joint thicknesses from 6.0 to 40 mm. These are often high-pressure pipes that require very high integrity welding. In the petrochemical industry, typical thicknesses are below 20 mm, but for power generation, because of the high steam pressures involved, wall thicknesses of up to 40 mm are common,” he tells African Fusion.
“Materials can range from carbon steels for lower pressure and temperature use to the low-alloy 1¼Cr-½Mo, 2¼Cr-1Mo; 9%Cr-1Mo (P91); and the host of the other low-alloy creep-resistant steels for higher temperatures and pressures,” he says, adding that austenitic stainless steels in materials such as 304L are also commonly used for pipe, which are typically welded using 308 or 316 consumables.
The weld preparations typically have 60 to 70° included angles with relatively large root gaps: between 2.5 to 4.0 mm in order to guarantee full penetration of the joint.
“The TIG/Stick processes used are tried and trusted, but the welding industry is seeking to use more efficient and productive processes that are less dependent on very high levels of skill,” Meyer continues.
“Welding ’faster and cheaper’ is becoming essential in our cost competitive markets and, since labour constitutes the largest component of welding costs, any reduction in total welding time substantially reduces the total cost of pipe welding,” he notes.
Changing to a new and faster welding processes, however, comes with significant challenges, most notably, getting the procedures approved and accepted by plant operators and end users.
TIG/Stick procedures are well established and familiar to pipe contractors and end users, while replacement procedures are seen as higher-risk with respect to quality and repair rates. “To successfully replace traditional pipe welding methodologies, we need to develop and showcase a set of robust procedure qualification records (PQRs) and welding procedure specifications (WPSs) so that people can start trusting these processes going forward.
“And we have to do this in South Africa!” he exclaims. “We know that new processes are already accepted in other parts of the world and we cannot continue to fall behind with respect to cost competitiveness and productivity,” he argues.
“Fully automated pipe welding is already routinely applied overseas. In South Africa, however, we seldom automate. Pipe installations cannot usually be rotated, so we need manual welding procedures that are so robust that they can be successfully repeated by ordinary welders – as opposed to having to train and qualify highly skilled welders or import skills from other countries.
“Training TIG/Stick pipe welders to the high levels required takes a long time, so we are seeking to develop advanced pipe welding processes and procedures that can be successfully used by regular welders. This will help to grow the local skills base more rapidly and get more of our people into jobs,” he suggests.
The Miller PipeWorx welding system is a multi-process solution capable of SMAW (stick), GTAW (TIG) as well as all of the GMAW variants – short and open-arc GMAW, pulsed-GMAW and flux-cored welding (FCAW). For modern pipe welding, however, Afrox is focusing on the use of Pipeworx’s advanced GMAW capabilities.
“For the root pass, Miller has developed a technology called RMD (Regulated Metal Deposition) to control metal transfer during short-arc GMAW welding,” Meyer explains. Instead of simply switching the current between an open arc level and a short circuit level, during the arcing phase, the current is stepped down twice before short-circuiting occurs. On short-circuiting, the molten metal droplet is first allowed to wet into the joint at a very low current level under surface tension. Then an up sloping ‘pinch’ current is initiated. On arc reignition, the current immediately ‘blinks’ low to minimise the spatter associated with reignition, before stepping up to its high open arc current level to create a droplet ‘ball’, then stepping down again in anticipation of the next short circuit.
This controlled deposition provides less chance of cold lapping or lack of fusion, less spatter and a higher quality root pass around the pipe. The stability of the process lessens the puddle manipulation required by the welder and is more tolerant to high-low conditions, reducing training requirements. Weld bead profiles are thicker than conventional TIG root welds, and this can eliminate the need for a hot pass, again improving weld productivity and reducing welding costs.
“For the root and capping pass, we are recommending the use of the ProPulse feature of the PipeWorx system… read more.