This thesis work investigated how welding speed and chemical composition influence weldgeometry, microstructure and mechanical performance in robotized GMAW of HSLA steeljoints. The study was done in collaboration with Volvo Construction Equipment and includedtwo different steel grades, S355MCD and VCE355.The experimental work was done with three different predefined welding procedurespecification (WPS) parameter sets and included analysis of weld geometry, macro- andmicrostructure, hardness measurements, tensile testing, and fractography.The result showed that the materials exhibited different responses in weld geometry atcorresponding parameter sets. VCE355 exhibited more favorable weld geometry and weldquality compared to S355MCD at higher welding speeds. The base materials (BMs) showeddifferent microstructural morphologies, where S355MCD was interpreted as fineferritic/pearlitic structure, whereas VCE355 showed a more banded and elongatedmorphology. The clearest microstructure difference between the steel grades was at the coarsegrained heat affected zone (CGHAZ), where the welded samples in the VCE355 series exhibitedharder and more transformed microstructure, interpreted as predominantly bainitic. The weldmetal showed mixed acicular ferrite and Widmanstätten ferrite features in both materialseries. Differences in hardness distribution between the materials and welded joint zones werealso observed, where S355MCD showed lower and even distributed hardness levels at HAZ. Incontrast, VCE355 showed higher peak hardness, specifically located at the CGHAZ, with avarying hardness distribution throughout the HAZ and WM. Tensile performance was alsoevaluated using welded cruciform specimens. However, all cruciform specimens failed at thebase material (BM) region including specimens with lack of fusion, porosity, and undercut. Insummary, the study shows that welding speed and chemical composition influences weldgeometry, microstructure, and mechanical performance in high-speed GMAW of HSLA steel. License number 1743284-1 for image usage from Kou (2003).