Positron Annihilation Lifetime Study of Steel Surface Modification by Shot Peening

Steel made machine components are often exposed to variable loads during the operation. Thus, their important characteristic is fatigue strength, which largely depends on the residual stress in the surface layer of these elements. The fatigue strength of components can be increased e.g. by exposing them to shot peening. Influence of shot peening on the defect structure in the samples observed by positron annihilation lifetime spectroscopy was studied for various steels. The differences between unprocessed and shot peened samples reflect mostly in the intensity ratio decrease of the components attributed to monovacancies and vacancy clusters. Shot peening result in uniformization of the vacancy clusters size. The carburized steel does not exhibit any changes in positron annihilation lifetime spectra caused by shot peening. On the other hand, the changes in the chromium plated steel are considerable. Their origin lies is compression of the microcracks present in the chromium layer caused by shot peening.