Effect of welding processes on the fatigue behaviour of ultra-high strength steel butt-welded joints

In the last decades, advances in steel manufacturing made possible the use of high-strength steel (HSS) and ultra-high strength steel (UHSS) for several applications, such as bridges, cranes, offshore structures, oil pipelines and automotive parts. Capacity of withstanding loads with reduced cross-section and minimum weight could be efficiently increased. Since most structures need to be joined, welding procedures are a major issue in mechanical design of HSS elements. Particularly in construction codes and design documents, it is normally assumed that fatigue resistance of as-welded joints is independent of strength level. Nevertheless, fatigue loaded as-welded components with high quality welds or post-weld treated joints could experience benefits from the use of HSS as the base material (BM). The purpose of the present work is to analyse fatigue behaviour of ultra-high strength steel butt-welded joints, by means of experimental testing and a fracture mechanics approach. Sheets of steel S960MC and S960QL were joined with different welding techniques: Gas Metal Arc Welding (GMAW), Laser Hybrid Welding (LHW) and Electron Beam Welding (EBW). Fatigue tests were performed with stress ratio R=0.1, under four points bending loading. All specimens exhibited fatigue crack initiation and subsequent propagation from the weld toe area, near the heat affected zone (HAZ). Different SN curves were obtained for the different welding processes. The Resistance Curve methodology was employed to assess the effect of microstructure, defect size, hardness, and joint geometry resulting from each technique. This fracture mechanics approach allowed to estimate the relative influence of the different geometrical and mechanical parameters of the weld and showed that joint geometry could not explain by itself the differences in fatigue strength. It was observed that microstructure and the size of defects played an important role in early crack growth, and they can reduce the benign effect of a high-strength base material.