Micro-needle peening method to improve fatigue strength of arc-welded ultra-high strength steel joints

•A fatigue strength improvement method for arc-welded parts was proposed.•Peening with a small tip pin (micro-needle peening) prevented the crack-like defects associated with needle peening.•The improvement effect was comparable to that of shot peening.•The effects of both compressive residual stress and work hardening contributed to improvement of fatigue strength. A new technique called micro-needle peening was proposed to improve the fatigue strength of arc-welded ultra-high strength steel joints. Use of a small tip pin (micro needle) makes it possible to introduce compressive residual stress and work hardening effects without causing the crack-like defect which is inevitable with conventional needle peening. The fatigue strength of micro-needle peened joints was assessed in comparison with those of needle peened, shot peened and un-peened joints of ultra-high strength steel with a tensile strength grade of 980 MPa. The results showed that micro-needle peening and shot peening improved fatigue strength at 3 × 106 cycles from 275 MPa (un-peened) to 600 MPa, while the conventional needle peening improved it to 300 MPa. The fracture surfaces near the toes of the micro-needle peened and shot peened joints displayed a “smooth surface.” An electron back scattered diffraction study of the surface layers of the micro-needle peened and shot peened joints revealed that the top layer was so deformed that the crystal orientation could not be identified confidently, and the second layer was distorted and presented high kernel average misorientation and grain orientation spread values. The total depth of these layers was comparable to that of the smooth fracture surface. The effects of residual stress and work hardening induced by micro-needle peening were evaluated separately by comparing the results of different types of post-heat treatment. That study revealed that both residual stress and work hardening make comparable contributions to the improvement of fatigue strength by micro-needle peening. However, the effect of the change in weld toe geometry was negligible. The micro-needle peening method is expected to contribute to enhanced fatigue strength in welded parts.