Double sided laser girth welding of high thickness pipe-line

The positional welding of high thickness steel pipeline girth welds (in either carbon or stainless steel) using a laser, remains limited to a value of about 15-20 mm. This is not due to the limitation of laser power but mainly due to the physical gravitational effects on the molten pool during welding. Moreover, the high level of quality demanded dictates that there is an assurance that the weld is free from significant defects and exhibits mechanical properties similar or in excess of the parent linepipe. It has been demonstrated that such a quality can be achieved for thicknesses up to 15-20 mm but is much more difficult for wall thicknesses beyond this. A potential solution to increase the thickness that can be welded by lasers is to use multipass technique whereby a single laser welding pass is welded both internally and externally of the pipe surface weld. Multipass techniques have been previously demonstrated with a root pass in keyhole welding mode and then adding additional filling passes (depositing 3 – 4mm per pass) with the addition of wire – usually as a hybrid laser combination. However welding from both sides uses the main benefit of the laser weld in producing deep penetrating and narrow weld beads: Its limits the volume of molten material in the weld pool that is essential for fast welding high wall thicknesses. Equipment developments in the pipe line industry allow such an arrangement to be considered with a laser head welding from inside the pipe and another one welding from outside. With such a solution, thicknesses up to 30 mm could be welded with a 10 kW CW laser. Further, this arrangement of having two partial penetration welds is likely to prove extremely tolerant to the inevitable variations in pipe fit-up that is experienced in the field. In this paper, the influence of various parameters (groove shape, power density, focus position, gas shielding, hybrid process or laser and cold wire) will be investigated on the overall quality of the weld from the view of welding defects that are found and the mechanical properties of the weld.