Focused ion beam study of the effects of shot peening on the subsurface microstructure of normalized pearlitic steel

The subsurface microstructure of machined, heat-treated and shot peened samples of pearlitic steel was studied by focused ion beam (FIB) microscopy. Samples with flat surfaces were prepared by mechanical milling and subjected to a normalizing heat treatment. Then the surface was shot peened at an inclination angle of 45° with respect to the surface normal at pressures of 2, 2.5 or 4 bar. Near-surface cross-sections of as-normalized and of shot peened samples were prepared using a focused ion beam workstation and studied using the same instrument. Results show that a nanocrystalline surface layer less than 1 μm thick is formed by the mechanical milling and the subsequent heat treatment. The effects of shot peening reach to a depth of about 20 μm. Within this layer the material is sheared parallel in one direction due to the peening inclination. The amount of shear appears to increase monotonically towards the surface. The study demonstrates that FIB preparation and microscopy techniques offer unprecedented insight into the subsurface microstructure of mechanically treated surfaces.