Non-monotonic evolution of surface roughness in a stainless steel during cold deformation

In this study, polished and unpolished sheet samples of austenitic stainless steel 316L were tensile strained to investigate the evolution of surface roughness based on Sa- and Ra characteristics. In polished steel sheets, surface roughness increases with the increase of true strain up to a maximum of e = 0.14. Thereafter, roughness decreases until e = 0.26 for approximately 25%, and then becomes independent on strain. The highest roughness levels are found to be localised primarily around surface grain boundaries. The roughness-strain correlation can be explained by grain rotation and cross-slip. Unpolished sheets demonstrate near-linear relationship between tensile strain and surface roughening due to the presence of an oxide layer. The layer has a thickness of approximately 1 μm with a morphology resembling the microstructure in the substrate. When strained, it appears to show two roughness components. First one is a shortwave component originating at the oxide grain boundaries, which is believed to be produced by the rotation of underlying grains. The second one is a longwave component, which is generated by the fracture of oxide layer due to lower ductility. The slope of roughness – true strain relationship is found to be also grain orientation-dependent.