Weld Metal Mechanical Properties in Hyperbaric GTAW of X70 Pipeline

In the present investigation, 2 different wires for hyperbaric (underwater) GTA (gas tungsten arc) welding of X70 pipeline have been tested with respect to their weld metal mechanical properties. Welding of full coupons at different pressures (seawater depths of 16, 75 and 200 msw) was done with subsequent weld metal chemical analyses, hardness measurements, tensile testing and Charpy V notch testing as well as microstructure characterisation.  It is shown that both wires satisfied strength requirements set to X70 grade, representing a weld metal overmatch situation. Both wires gave sufficient impact toughness, but the toughness of the Ni-Mo containing weld was reduced with increasing sea water depth. This observation was strongly linked to the positioning of the Charpy V notch, and crack growth in a brittle partially transformed region as a consequence of reheating by subsequent stringer beads. The embrittling microstructure consisted of high carbon MA (martensite-austenite constituents) decorating prior austenite grain boundaries. This microstructure was less pronounced when welding with the high Ni wire, which may explain why no similar toughness drop was found.