The fine particle peening process, which makes particles of several tens microns in diameter impact at high speed, has been adapted industrially to auto motive steel parts or tool steels because the fatigue property is superior to that of a conventional shot peening process. However, microstructural changes obtained from this process have not been fully understood.
In this study, microstructural change of JIS-SCr420 carburized steel (HV 7.55 GPa) by the fine particle peening process was investigated comparing with cases of a conventional shot peening and of softer steels. In SEM observation of the carburized steel, although plastic flow was observed in the region from the surface to a depth about 2 μm, nanocrystalline structure was not detected. TEM observation revealed that nanocrystalline layer with about 0.5 μm in thickness was produced continuously along the surface. In softer steels which hardness below HV 3.72 GPa in this study, nanocrystalline structure was observed as the folded and layered deformation region, which existed locally and discontinuously along the surface. Morphological difference of these nanocrystalline structures is considered to result from surface deformation modes by particles impact. In softer steels, the convex part in considerable uneven surface formed by the impact is folded and imposed into interior region by the repeated impact and developed to nanocrystalline structure. In carburized steel, owing to absence of the considerable unevenness, throughout surface area deformed during the impact and nanocrystalline structure is developed homogeneously and continuously along the surface. In a conventional shot peening condition, nanocrystalline structures were not formed.