The lateral-torsional buckling of a steel welded I-section beam is highly affected by the residual stress distribution, in particular at the flange ends. These distributions are impacted by the fabrication process, such as flame- or plasma-cuts performed at the flange ends. However, few studies compared the impact of these three cases on residual stresses and stability of welded I-members. For this reason, residual stress measurements have been performed on 9 welded I-section specimens made of S355 steel to assess the impact of these fabrication processes. 3 series of 3 specimens were built-up with the same dimensions but varying the type of flange: flame-cut, plasma-cut and hot-rolled. Experimental results highlight a clear effect of the flange fabrication process on the residual stresses at the flange ends, and a small effect on the flange compressive stresses. The experimental distributions are then employed to perform a numerical sensitivity study on welded I-section beams under constant bending moment. The results demonstrate a significant favorable effect of the plasma- and flame-cuts on the lateral-torsional buckling resistance when compared to hot-rolled flanges.