Characterization of the Microstructure and Toughness of DSAW and ERW Seam Welds of Older Linepipe Steels

The aim of the present work is to quantify the seam weld properties, including both weld metal and heat affected zone regions, for a series of double-submerged-arc welded pipe, and the base metal and bondline regions for an electrical resistance welded linepipe steel. The chemical composition, microstructure, microhardness, tensile properties, Charpy impact toughness and J-integral/crack-tip opening displacement fracture resistance were characterized for linepipe produced between 1953 and 1981. The toughness results of the base metal, heat affected zone and weld metal regions of the older (higher carbon) linepipes were significantly poorer than those obtained for a more modern low-C microalloyed linepipe steel. In the latter case the base metal, HAZ and weld metal regions failed by ductile fracture at room temperature in both fracture toughness (quasi-static) and Charpy impact tests. It was possible to show that there is a linear correlation between the J-integral at 0.2 mm crack growth and the upper shelf Charpy energy. It is, however, important to note that the Charpy transition temperatures of the older pipes are considerably higher than for the modern pipe. In the case of the electrical resistance weld, very poor toughness was observed for the bondline. Fracture occurred along the bondline in a brittle mode (cleavage) that was attributed to the formation of a coarse, relatively hard microstructure and the presence of inclusions along the bondline region. In addition, it was shown using base-metal BxB and Bx2B samples that initiation toughness is a function of the remaining uncracked ligament. This emphasizes the necessity of ensuring that the crack-tip constraint in the test specimens is similar to the constraint in the crack geometry being assessed.