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 S_a- and R_a 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.