Abstract
In this study, the development of a high-strength, high-fracture toughness double submerged-arc weld for X-70 arctic grade line pipe is documented. The weld was made with a two-wire DC/AC tandem setup at a speed of 27.5 mm/s (65 in./min.). A fused, semi-neutral, fine grained (32 × 200) flux was used. The resulting weld metal microstructure was at least 95% acicular ferrite. The weld had excellent fracture toughness with not less than 60 joules at −60°C and 50% FATT at −50°C. The formation of acicular ferrite was interpreted as the result of suppression of proeutectoid ferrite owing to the high molybdenum content of the wire and the presence of TiO2 inclusions. The properties were consistently achieved during the manufacture of spirally welded pipes 1067 mm O.D. x 14 mm W.T.
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