Abstract
Ultrasonic impact peening (UIP) is one of the most promising surface engineering techniques to enhance the durability (fatigue life, stress corrosion cracking resistance, wear resistance, etc.) of metallic materials by imparting surface hardening and compressive residual stresses. Besides these two beneficial surface modifications, the surface morphology after UIP plays another key role in determining the material’s durability. In literature, the effect of UIP on surface morphology was studied with a focus on average surface roughness (Sa). However, the surface integrity and quality after peening are determined by not only Sa, but also surface skewness (Ssk) and surface kurtosis (Sku). In the present study, surface morphological modifications of pipeline steel processed by UIP are investigated, with emphasize on Sa, Ssk and Sku. Surface morphology was characterized using a structured light scanning system. The effects of processing parameters (energy levels and peening time) are investigated. It is found that Sa increases with the increase of energy levels and peening times, whereas Ssk value remains below zero for all peening conditions, indicating the predominance of the valleys rather than peaks on the surface. In addition, the Sku value remains below three for all measurements, which ensures the absence of extreme surface peaks, indicating the removal of stress concentration points. The in-depth hardness is also tested to observe the surface strengthening effect.
