2003 Volume 41 Issue 4 Pages 160-167
Authors have carried out investigations on multi-pass GTAW process for 9. 5mm thick Invar alloy. In the welding of Invar alloy, use of the filler wire of base metal compositions resulted in the occurrence of ductility-dip cracks in weld metal. Authors' previous study had clarified that addition of Nb and C was effective to prevent the crack. Based on that result appropriate compositions of Nb and C were investigated through welding tests and a filler wire containing 0. 8%Nb and 0. 2%C was established for multi-pass GTAW process of Invar alloy. In the welding of U-shaped groove configuration, however, higher dilution rate of base metal at weld root lowered Nb and C contents and caused cracks in that area. To keep the amount of NbC at grain boundaries beyond the necessary level for preventing cracks, new filler wire of higher Nb content (1. 6%) was applied to the deposition at the root. Finally, to complete U-shaped groove by GTAW process, two filler wires of different Nb and C contents were used properly. By applying this deposition method, crack-free GTAW joints of Invar alloy with sufficient mechanical properties especially in respect of Charpy absorbed energy were made. Moreover, abovementioned combined use of filler wires were validated also in the welding of dissimilar joint between Invar alloy and 304 stainless steel. As the result of those investigations, authors have established multi-pass GTAW process for Invar alloy LNG piping that assures crack free weld metal with sufficient low temperature toughness under wide range of welding conditions.
