Mechanical nano-structuration of a C45 steel under repeated normal impacts

Under repeated impact loadings that are encountered during peening process, surface mechanical attrition treatment or erosive wear, metals undergo severe plastic deformation which may lead to a local refinement of their microstructure in the near-surface. These mechanically-induced surface nano-structures exhibit very interesting physical properties such as high hardness and better tribological properties, ... Strong research efforts have been undertaken during the last years to understand the mechanism explaining how these nanostructures are created and grow under such loadings. It is commonly accepted that the shear stress induced by oblique impacts is the driving force for such mechanical transformations. Nevertheless, we have recently observed that normal impacts may also lead to such grain refinement. In this paper, this mechanism is investigated on a AISI1045 steel submitted to different heat treatments. A phenomenological mechanism based on a previous work is presented and shows a good efficiency on the air cooled sample. Nevertheless it failed to explain the differences observed between the different samples, showing the necessity to take into account both the material stress-strain curve and the microstructural state.