African Fusion November 2015

Above: A Bug-O Piper Plus mechanised welding systems ready for use. These systems are commonly used with Outershield 91K2-HSR flux-cored welding wire for fill and cap passes. Right: Lincoln Electrics’ Power Wave S500 with the addition of an STT® Module has the fast digital communication necessary for controlling the root welding process for

consistent penetration and bead profile. of pipeline – in Mozambique, Tanzania, Nigeria, Namibia, the Congo and more – are currently being planned. “As in all parts of theworld, Africa faces shortages of skills; rising labour, material, and energy costs; a decrease in the world oil price and intense competition. In addi- tion, more stringent quality demands are being applied for the higher strength pipe steels, which demand that old con- structionpractices be updated,” he says. ”Conventional stick welding is still OK for some applications, and these processes have remained unchanged for over 40 years – but well-established practicesmust be followed and compla- cency has to be avoided!” he exclaims. Stronger pipelinematerials, such as the X70 and X80 grades are driving the industry away from the use of cellulosic stick electrodes, which are associated with high hydrogen content. “When we reach X70 properties, in terms of real strengths, then the family of low-hy- drogen stick electrodes, called Pipeliner LHD should be considered due to their low hydrogen levels (less than 5.0 ml) – and these are also specified when wall thickness is greater than 12.7 mm. “Low productivity, however, associ- ated with the use of stick electrodes, poses serious risks and challenges, which is driving a trend towards the use of semi-automatic andmechanised welding,” says Lamond. “Productivity on a pipeline project is all about operating factors, the ratio of arc time to non-arc time. When stick welding, alongwith stop/starts and stub losses, a significant amount of grinding is required between passes.

“A cellulosic root pass has a concave top surface that must be ground flat before applying the hot pass. This can result in a 1.6-2.0 mm fusion layer, and in worst-case high/low mismatch of more than 2.0mm, grinding can remove the root – and the welder has no way of knowing when this will happen. These are the productivity risks that tend to drive the adoption of the more mecha- nised processes,” he suggests, adding that the use of modern root welding processes such as STT is twice as fast as cellulosic welding, because neither grinding nor a hot pass are required. STT + flux-cored wire By adopting more modern technology, such as the use of Lincoln’s STT process followed by mechanised flux-cored welding, operating factors on pipelines can be increased to between 60 and 70 %. “You need to respect the welders to achieve these results, but high pro- ductivity levels areno longer exceptional and do not require massive levels of investment,” Lamond assures. Both of these are gas-shielded processes, so they do not like windy conditions – hence the tent. Being low hydrogen processes, gas shieldedmeth- ods can tolerate lower levels of pre-heat and the deposit generally has very good mechanical properties. “While good skills levels are required for STT root welding, fewer welders are needed and welders can easily be trained to use the process effectively,” he adds. Benefits of STT, according to La- mond, include: • Reduced training time: It is difficult

to find experienced highly skilled welders, especially for the critical root pass. STT offers ease-of-use, re- sulting in shorter training times com- pared to other welding processes. • Lower repair rates: STT minimises the most common defects. • Low smoke and spatter: STT uses high frequency inverter technology resulting in high quality welds with less spatter and fume generation. • The process makes it easier to per- formopen root welding on pipewith better back bead profiles and edge fusion: The STT process is designed to allow surface tension to ‘suck’ the weld bead into an open root gap of around 3.0 mm. The resulting weld bead has a uniform flat profile with a higher alignment measurement (4.0 mm) than standard cellulose electrodes (1.6 to 2.0 mm). The flat STT profile removes the need for grinding and no hot pass is required. This makes it possible for the root welders to move on immediately after completing a singe pass. • The thicker nugget also reduces the risk of bead shrinkage or ‘suck back’. “Semi-automatic processes place the welding controls into the welding system instead of in the hands of the welder. But skilled and knowledgeable people are still needed, to insert the root welds and to tend the mechanised processes, for example. On a pipeline, every single welding joint is different. Welders are under-rated and, because of their ability to react the variations they see, they keep many engineers out of trouble,” Lamond says.

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November 2015

AFRICAN FUSION