Residual stress and dislocation density development in single track deep rolled AISI 4140H steel

Abstract

Plastic deformations induced by the rolling contact in the deep rolling process are the origin of the formation of residual stress fields and increased dislocation density in the material. The near surface material modification achieved through the process, like a compressive residual stress field, improved hardness and reduced surface roughness, give rise to improved component performance during operation. For a general understanding of the generated surface-near properties, an analysis of single tracks contributing to surface integrity is required. In the present study, a detailed investigation of the achieved residual stress state distribution as well as the local dislocation density of single processed tracks was performed through high spatial resolution X-ray diffraction analyses. Using two spherical deep rolling tools of 6 mm and 13 mm diameter, single tracks were processed with different parameters on samples of steel AISI 4140H. It could be observed that residual stress fields at the surface reach much larger lateral expansion than the plastically deformed track which can be determined by optical measurements, while a correlation with the dislocation density distribution was found. For the maximum dislocation density in the tracks, a correlation with the theoretical maximum equivalent stress was determined. Furthermore, a method to connect the measured dislocation density to the localized plastic strains during processing was applied to exemplary distributions.