Ultrasonic nanocrystal surface modification (UNSM) of surface properties and residual stress in 300 M steels

Abstract

The effects of different processing parameters of ultrasonic nanocrystal surface modification (UNSM) on surface properties of 300 M steel were investigated in this study. Experiments were performed using different scanning speeds, scanning intervals and static loads. The surface microhardness, roughness, wettability, appearance and residual stresses were evaluated for every set of processing parameter. The results showed that trenches, visible under microscope, formed in scanning direction indicating the contribution of burnishing effect of sliding motion of tungsten carbide (WC) ball tip on the surface of the specimen. It was observed that the surface roughness increases with a decrease in scanning speed as peening effect overtake the burnishing action of the WC tip and increase in the static load intensifies the peening effects in turn increasing the surface roughness. As static load increases, the surface begins to form microcracks and at very high static load the surface experiences severe surface damage. It was observed that the hardness and surface residual stresses of the processed sample increased with an increase in static load however at high static load the surface damage causes stress relaxation as material experiences localized fracture. Wettability of the processed surfaces was observed to be independent of the processing parameters.