The modification of microstructure and mechanical properties of steels after a welding process has received considerable attention in the literature. In the case of welding HSLA steels for pipeline applications, the filler metal employed usually is overmatched (i.e. higher strength) compared to base metal to avoid fracture in this zone of the weld. For this reason, considerable work has emphasized microstructure evolution in the heat affected zone and the associated modification of mechanical properties in this region. In this study, the combined effect of microstructure/property distribution and the geometry of the weld are examined to understand where localization, necking and fracture occurs under tensile loading of a laboratory weld.

To achieve that, a series of tests were conducted on two different types of X80 submerged arc welds: Single and tandem wire welds. The tensile samples were machined transverse to the weld on plates 16 mm thick. Samples were tested where the geometry of the weld was preserved (i.e. the weld cap is left intact) and where the cap was removed in order to remove its effect. The local plastic strain during testing was determined using the Digital Image Correlation technique (DIC).

For the single wire weld, the influence of the cap geometry seems to be of second order, as the fracture location is the same with or without caps. But for the tandem wire welding, the fracture location is very different depending on the geometry: In the case where caps are kept, the fracture occurs outside the HAZ, in the base metal suggesting the important interplay between local mechanical properties and the weld geometry.

A Finite Element Model (FEM) was developed to gain insight into the geometrical effects on the local strain field distribution. The experimental strain distribution is compared to the FEM results to rationalize the effect of geometry. The model results are then used to discuss the position of fracture for the different samples, which correspond well to the experimental results.

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