In general welds have significantly lower fatigue strength in comparison to the adjacent base material. This is mainly caused by the notch effect due to the weld seam geometry. Furthermore changing material and microstructure conditions in the weld zone (metallurgical notch effect) and tensile residual stress in regions which are susceptible to cracking reduce the fatigue strength of welded joints.
In recent years High Frequency Mechanical Impact or HFMI method has achieved great significance in practice. In this method a hardened cylindrical pin with a round tip strikes on a component with high speed, respectively frequency >90 Hz, whereby the seam notch geometrically will be flattened, the surface will be strengthened and high compressive residual stresses will be induced. The effectiveness of the HFMI method to increase the service lifetime respectively the fatigue strength of welded joints as well as their applicability has been confirmed by numerous studies. By an effective post-treatment of welds not only constructive problems of new designs can be solved but also the use of high-strength steels will be facilitated.