Properties of welded joints made in high strength steel using laser technology

. The usage of high strength steels ( HSS ) provides numerous benefits in the form of the cross--section area reduction causing weight decrease, improvement in exploitation parameters and in result positive financial effect. The most advantageous results are observed in the parts loaded by tensile stresses. The factors most influencing the fatigue strength of the metals, especially the steels of high value of yield point, are stress concentration in the originated geometrical notches, local changes of the stress state or the microstructure of paternal material. Improvement in the fatigue behaviour of HSS steels can be achieved by changing the commonly used technique for high-energy joining technology, namely laser beam welding. Relatively narrow welds and reduced heat-affected zone ( HAZ ) have minimized the residual stresses and distortions. The paper presents the results of the studies focused on mechanical properties of welded joints of high strength fine-grained steel S960QL. There was investigated the impact of heat input on the microstructure, microhardness- and residual stresses in the weld and HAZ. The crucial part is devoted to the fatigue research and fractographic investigation of the fatigue ruptures of laser-made welds. There were also found in the origins the cause of crack initiation.