Influence of microstructure on surface characteristics in cold-deformed stainless steel tubes

Sammanfattning: Cold deformation of metals is known to induce roughness on their free surface. In some of the processing equipment of Tetra Pak processing systems, tubes of austenitic stainless steel 316L are used. During the production of this processing equipment, such tubes are occasionally cold-deformed. In this work, surface and microstructure characterization of alloying type 316L austenitic stainless steel has been performed to understand how the cold deformation during production process affects the surface structure of the material. Prime focus of the study has been the relationship between tensile strain and surface roughness on polished and unpolished 316L samples. It is shown that in polished steel sheets, surface roughness increases to a maximum with the increase of engineering strain level up to 15 %. Thereafter roughness decreases slightly and settles at a level where the roughness is no longer affected by the strain. The roughness was found to be localized primarily in the vicinity of grain boundaries. Further analysis reveals that the roughness-strain correlation can be explained by grain rotation and subdivision. Unpolished sheets demonstrated approximately linear relationship between tensile strain and surface roughening. They had a passivation layer on the surface, as confirmed by SEM and EDS measurements. The passivation layer has a thickness of 1 µm and demonstrates scale-like structure with the morphology of underlying austenitic microstructure in the substrate. When strained, it appeared to inherit two roughness components. First one is a shortwave component originating at the boundaries of the scale, which is believed to be produced by the rotation of the underlying grains. The second one is a longwave component, which is generated by the cracking of scale due to the lack of ductility. The latter prevents the oxide layer to deform plastically and conformally with the macroscopic specimen strain. The slope of the roughness – true strain relationship was found to be orientation-dependent. The magnitude of the slope correlated to respective grain dimension. A model for biaxially strained steel tubes was also developed, which showed to be accurate within the dimension boundaries of interest. Techniques and instrumentation used in the thesis were tensile testing, 3D-focus varying optical microscopy, SEM, EBSD, EDS