1 Department of Management Engineering, Technical University of Denmark
Strategic stainless steel surfaces have been developed and investigated during the project in close cooperation with the Department of Chemistry, DTU with the purpose of enhancing the tribological properties. This has been achieved successfully by applying electrochemical treatments of normal as delivered stainless steel surfaces implying microstructure changes in terms of larger ratio of closed lubricant pockets due to selective grain boundary etching. Strategic surfaces have also been created by macroscopic texturing using spherical indentations having a very small edge slope in order to promote micro-plasto lubrication mechanisms. Tribological testing by the simulative strip reduction test, deep drawing and stretch forming clearly show that the developed strategic surfaces perform superior to normal surfaces tribological-wise. In case of strip reduction, 40% reduction by a strategic surface using plain mineral oil is possible without any lubricant breakdown. In deep drawing, 2mm stainless steel blanks can be drawn to drawing ratio of DR=2.0 over a die entry radius of rd=3mm again using a plain mineral oil containing no additives. In stretch forming, friction is reduced considerably by strategic surfaces in comparison to normal stainless steel surfaces implying a larger extent of bi-axial stretching. Numerical simulations have been applied in order to evaluate limits of lubrication in the simulative strip reduction based on predictions of critical parameters appearing in terms of temperature and contact pressure. The numerical models have been calibrated regarding friction and thermal contact resistance based on experimental results from actual testing conditions. It has been found that predictions of limits of lubrication are possible by numerical means and that the FE-models corresponds well to experimental results in terms of lubricant film breakdown and subsequently pick-up development. Punching and blanking have been investigated regarding tribological conditions in case of using stainless steel workpiece materials. However, this has called for development of a new test method being one of its kinds since previously no tribological test existed for the shearing process. Thus the work has involved design and construction of such new test equipment, which later was modified by a feeding system implying an automation of the entire test procedure. The test principle has been based on analysis of the appearing backstroke force, which is very sensitive to tribological changes in the punch/workpiece interface, hence to lubricant breakdown. Fundamental studies of pick-up development in punching and blanking show that cold-welding of workpiece particles initially start at the tip of punch and develop gradually up on the punch stem for increasing number of strokes. It has also been found that a retention mechanism appear in punching by which lubricant is transported by the punch topography to the contact between punch and workpiece during penetration. Tribological testing of different lubricants indicate that both commercial and prototype, environmental friendly lubricants do not perform as well as hazardous lubricants such as chlorinated paraffin oils. Dry-in lubricants and polymer-coated sheets have been found to perform very poorly in punching and blanking due to shearing of the lubricant layer. Improvements of polymer-coated sheets have however been realized by changing the punch geometry or by applying a secondary lubricant.