The welding process is a time-consuming step in the fabrication of monopiles for off-shore wind energy turbines. The established process is multiwire submerged arc welding (MWSAW), applied to structural steel sheets in the order of up to 100 mm thickness. In an approach to drastically reduce welding time, the single-pass electron beam welding process (EBW) is under research. However, resulting joints often do not meet the minimum criteria of low temperature Charpy impact toughness. Similar problems arise when trying to increase MWSAW speed by increasing the amount of wires and energy.

A challenging aspect of weld microstructure characterization is the presence of a gradient microstructure and different zones with different thermal history and properties. In the present work, MWSAW and EBW joints in S355 structural steel are investigated. In an approach to identify the weakest area in a tensile test, wide-field SEM-based in-situ experiments with digital image correlation (DIC) were performed. A modified Beraha etching was used to obtain a well-contrasted sample surface for DIC. In addition, microhardness was measured locally over the heat affected zone in order to correlate it with the weakest area in the tensile tests to proof it the time consuming in-situ tensile approach for a first look is necessary.

It was found that in the case of the MWSAW, the elongation is localized in the transition zone from fine-grained heat affected zone to the base metal while in the case of EBW, failure occurs in the base metal. The demonstrated approach combines a time-resolved DIC-based measurement of local deformation with the benefits of a wide-field SEM for micron-range resolution imaging.