Material characterization of high-frequency mechanical impact (HFMI)-treated high-strength steel

Fatigue strength improvement due to high-frequency mechanical impact (HFMI) is based on compressive residual stresses, improved weld toe geometry and strain hardening of the treated surface region. In order to understand the combined effects of these factors on the weld toe elastic-plastic response and residual stress stability, the material properties in the HFMI-treated weld toe region need to be determined. In this study, five different material conditions were studied: high-strength steel base material, Gleeble simulated heat-affected zone, and three HFMI-treated material conditions. The local material characteristics were determined by microhardness measurements, electron backscatter diffraction and mechanical testing. The specimens were fatigue tested under fully reversed strain-controlled loading. Surface residual stresses in the fatigue test specimens were measured with X-ray diffraction. The results showed that the heat-affected zone had lower fatigue and yield strength than the base material due to larger grain size. HFMI-treatment increased the steel yield strength up to 1-1.5 mm. This observation was supported by change in grain shape in the treated surface region. This lead to an increase in fatigue strength at low strain amplitudes and a decrease in fatigue strength at high strain amplitudes. Continuous cyclic softening was observed for all material conditions. (C) 2015 Elsevier Ltd. All rights reserved.