Structure and Properties of X80 and X100 Pipeline Girth Welds

Abstract

This study aims to provide an understanding of the factors that control weld metal strength and toughness of mechanized field girth welds produced in X80 and X100 line pipe steels using a range of pipeline gas metal arc welding procedures. In the investigation of X80 welds, a series of experimental single and dual torch gas metal arc welds were prepared with three C-Mn-Si wires, which contained additions of Ti, Ni-Ti and Ni-Mo-Ti. The weld metal microstructures, tensile properties, notch toughness, and fracture resistance were evaluated. The results indicate that high weld metal yield strength and good toughness can be achieved. The X80 single torch welds exhibited higher yield strength but lower toughness compared to the corresponding dual torch welds. For the development and evaluation of welding procedures for mainline girth welding of X100 pipe, two narrow gap mechanized gas metal arc welding procedures were evaluated with emphasis placed on measurement of the tensile properties. The results show that dramatically different properties (strength and toughness) can be found as a result of differences in energy input, interpass temperature and weld width or offset distance. Additionally, the preliminary tensile testing, which utilized both standard round bar and modified strip tensile specimens, illustrates the potential variation that can occur when assessing all-weld-metal tensile properties of narrow gap pipeline girth welds.