Fatigue Strength of HFMI-treated and Stress-relief Annealed High-strength Steel Weld Joints

The fatigue strength of welded joints is generally irrespective from the base material yield strength due to notch effects at the weld toe or root, tensile residual stresses by the thermo-mechanical joining process and significant changes in local material properties within the heat-affected zone. An effective way to increase the potential of light-weight design exhibits the application of post-weld treatments. Mechanical methods as the high-frequency mechanical impact treatment (HFMI) reduces the geometrical notch, introduces compressive residual stresses and increases the local hardness in the surface layer of the post-treated weld toe area.

In this paper the fatigue strength of HFMI-treated joints superimposed by stress-relief annealing is studied. High-strength steel grades S690 and S960 are investigated in as-welded and HFMI-treated condition, both before and after stress-relief annealing. Fatigue tests at a tumescent stress ratio of R=0.1 show an essential increase of the fatigue strength due to the HFMI-treatment compared to the as-welded condition. In contrary, the superimposed post-heat treatment lowers the fatigue strength of HFMI-treated high-strength steel joints characteristically, but however, the final condition exhibits a higher fatigue strength compared to the initial as-welded state. Special focus is laid on the influence of welding distortion, whereat numerical simulations and experimental strain gauge measurements support the back tracing of the acting mechanism. The results indicate that the final local mean stress state due to clamping of the distorted specimens significantly affects the local effective stress ratio at the weld toe during testing, which basically needs to be considered for the fatigue strength evaluation.