The welding processes currently used for the fabrication of steel pipelines have essentially remained the same for the last 35 years. They all consists in multi-pass arc welding with various degrees of mechanisation and electrical waveform control. To further advance automation but also improve welds quality and mechanical properties, Friction Stir Welding (FSW) has been assessed as a potential disruptive technology. It also has the potential to allow high strength steel welding for deep-water applications. It is a solid-state one-shot process using a rotating tool to generate frictional heat at the interface between the parts to be welded. Its adaptation to steel presents several challenges: High mechanical loads, high tool wear and difficulties in welding wall thicknesses representative of the offshore pipeline industry.
A combination of pre-heating and back-wall insulation was seen as a possible means to make the process work on steel.
Local preheating centred on the joint was implemented. The thermal contribution of the pre-heating was assess by detailed temperature measurements along the joint line. FSW was then combined with the pre-heating and its effects on welding loads and tool wear were quantified. Various back support materials were tested and the effect on the weld zone temperature profile and weld penetration were characterised.
Significant contribution of the pre-heating method in improving the tool durability during FSW of even high strength steels was revealed. Fully consolidated, full penetration one-sided welds could be made. Laboratory test results will be included and discussed. The process economic viability will also be outlined.
Better control of the heat distribution through thickness and tool wear are essential factors to allow the weldability of steel by FSW and to obtain full penetration one-sided welds. The results of this programme support that this has been achieved successfully.