Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.