The influence of coverage for high frequency mechanical impact treatment of different steel grades

The High Frequency Mechanical Impact (HFMI) process is a comparably new, manually-driven machine hammer peening (MHP) process applied to increase the fatigue performance of metallic components. The HFMI-process is characterized by a large number of impacts, also when the tool was used with a high feed rate. This leads to a fast saturation and highly plastic deformation at the surface layer. Experimental investigations of the surface topography, near-surface residual stresses, and work hardening were performed on HFMI-treated, flat specimens made of S355J2, S690QL and S960QL steel grade to investigate the influence of the process parameters on the surface conditions. The residual stress state, experimentally determined by neutron diffraction, was supplemented by the numerical simulation of the HFMI-process. For this, an elasto-viscoplastic material model was implemented and calibrated with experimental test data. As a unified parameter for the process quality, a representative coverage value, defined as the number of impacts per reference surface area, was calculated for different sets of process parameters. This coverage value was also used to compare the numerical simulation and the experimental investigations by neutron diffraction techniques of the HFMI-induced residual stress field. It is shown, that high coverage has a strong effect on the depth of indentation and in the work hardening effect for mild and medium steel grade. Only slight changes of the process-induced residual stresses could be observed with an increasing coverage.