Assessment of the Surface Hardening Effects from Hammer Peening on High Strength Steel

The effect of hammer peening on the fatigue resistance of high strength welded joints in high strength structural steel S690 has been investigated. Cyclic four points bending tests have been performed on butt weld samples, manually and automatically hammered. Microstructural and mechanical modifications induced by both methods and their subsequent influences in fatigue life have been studied using several characterization techniques. It was found that the roughness of manually hammered samples is different than that of automatically hammered samples for which a lack of correct treatment in some regions of the weld cordon has been observed. A comparison of residual stress measurements using X-ray diffraction shows also differences between both methods. On the other hand, hardness profile of the strain hardened region is very similar for both types of samples. The ultra-fine grain microstructure of the heat-affected zone (HAZ) induced by the hammer peening process has been studied by Field Emission Scanning Electron Microscopy and Electron Back Scattered Technique (EBSD). The strain-hardened depth determined by Kernel Average Misorientation (KAM), microhardness and residual stress are compared.