Afrox is leading the way in improving welding standards in the South African power generation and petrochemical industries, by conducting welding procedure qualification tests using Miller Electric’s advanced PipeWorx FieldPro 350 multi-process pipe welding solution.
Afrox is the exclusive local distributor of the FieldPro 350 solution, launched in 2014. Afrox business manager for manufacturing, Johan Pieterse notes that the solution is a multi-process, inverter-based welding power source, purpose-built for advanced pipe welding.
He said, “PipeWorx FieldPro 350 is well-suited to site work, as it does not require a separate control cable. It sends all communication signals through the welding cables, making the system as robust as a basic stick welder.”
As part of the company’s commitment to improving welding standards in South Africa, Pieterse reveals that Afrox has already developed a welding procedure specifications for carbon steel pipes of various sizes. “We are now moving on to qualifying procedures for more exotic steels used in the industry. This will prove to be a game changer in terms of welding quality, productivity and skills.”
According to Pieterse, pipe welding with high quality requirements is conventionally done using tungsten inert gas (TIG) welding for the weld root and the hot pass. The joint is then usually filled using stick welding, otherwise known as manual metal arc (MMA) welding. Pieterse indicates that both of these processes require high levels of skill to achieve quality, flaw-free welds.
Pieterse says that, through this process, TIG-quality welding can be achieved using the advantages of the gas metal arc welding (GMAW) process. “This effectively removes the need for high-level skills for pipe welding, while simultaneously improving quality and productivity.”
Afrox Mechanisation Development Manager Andrew Peters notes that PipeWorx offers two advanced GMAW arc welding control options to operate the system. These are; regulated metal deposition (RMD) – an electronically controlled modified dip-transfer mode output solution for root-pass welds; and ProPulse – an open arc pulsed solution for fill and capping passes.
RMD – an optimised root welding solution
The molten tip of the wire short circuits in the weld pool up to 200 times a second in a traditional short circuit cycle. RMD welding is an advanced short circuit GMAW process that produces precisely controlled metal transfer, making it easier for the welder to control the power and the weld pool, as it boasts four distinct phases that can be identified during the short circuit welding cycle.
During the first phase, the molten metal at the end of the wire touches the weld pool, causing a short circuit. In the second phase, Peters highlights the fact that resistance heating and the associated magnetic field surrounding the wire from the rising short circuit current cause the molten metal droplet to neck down or pinch at the solid/liquid interface.
“In phase three, the molten droplet detaches and is deposited into the weld puddle, breaking the short circuit and causing the arc to reignite, often explosively. In phase four, while the arc is melting the tip of the wire, the wire begins to approach the base metal again. The cycle then repeats, returning to phase one when the droplet touches the weld pool,” he continues.
Peters observes that, in traditional processes, short circuiting is erratic with varying intensity, often leading to different sized droplets and inconsistently high arc re-ignition currents, which cause spatter. “From the welder’s perceptive, this produces an agitated weld puddle that is not easy to manipulate. Molten metal from the turbulent puddle can wash up the sides of a joint, causing lack of fusion or cold lapping.”
The RMD current waveform anticipates and controls the short circuit current phase of the process to improve the consistency of the metal transfer and short circuit stability. In addition to reducing spatter, the net effect is a weld pool with less turbulence. This allows the welder to control the position of the weld pool, and avoid lapping and washing of molten metal up the side walls, thereby enabling the production of consistent quality welds in terms of both fusion and weld bead profile.
Peters reveals that a highly-skilled stick welder can achieve an acceptable quality root weld at speeds between 75 mm and 125 mm/min in the vertical up positions, and between 125 mm to 200 mm/min welding vertically down. “It is important, however, to bear in mind that starts and stops are needed to replace electrodes. Slag deposits must also be cleaned off, which affects cycle times too.”
TIG welding gives a higher quality weld root, but at lower speeds of 50 mm to 125 mm/min, and more skilled welders are required. For pipe welding, TIG welders also have to reposition themselves around the joint several times to achieve the necessary manipulation control. RMD allows a moderately skilled welder to achieve high quality root welds at 150 mm to 250 mm/min using a solid wire. On pipes over 500 mm in diameter, speeds up to 300 mm/min are possible.
Due to the fact that the wire is continuously fed, fewer starts and stops are needed. “As a result of the higher deposition rate of the process, RMD produces a throat depth of between 3 mm and 5 mm, which eliminates the need for a hot pass typically applied when using the TIG process. This also offers excellent tolerance for misalignment and gaps up to 5 mm can be bridged,” adds Peters.
Achieving faster fill and capping passes with ProPulse
ProPulse offers high speed and deposition rates and its focused arc cone, short arc lengths and low heat input result in a fast freezing puddle and good weld pool controllability. Narrow joints can be accommodated and both vertical up and vertical down welding is possible. A major benefit for the welder is the improved puddle control, when compared to traditional pulse or spray GMAW.
According to Peters, ProPulse operates on a hybrid constant peak voltage (CV)/constant current characteristic (CC) basis. “During the peak phase of the current waveform, the current is regulated to maintain a CV. The peak current, therefore, varies depending on the arc length, which stabilises stick-out and gives arc length self-regulation. The pulse frequency remains constant and the background current operates according to the CC typical of TIG and MMA processes.”
Pieterse says that RMD and ProPulse can be used to achieve quality welds on pipes from a single gas and wire. “This means greater productivity, less inventory and ease of use. For root runs, the process is four times less expensive than TIG welding per kg deposited. By adopting the system, quality and high productivity can be achieved while reducing the need for high-level skills. This results in a strong base for skills localisation, while increasing economic competitiveness,” he concludes.