High Temperature Friction and Wear in Press Hardening

In the automotive industry, press hardening is usually employed to produce safety orstructural components from advanced high–strength steels. This hot forming process, andthermomechanical forming processes in general, is highly dependent on friction betweentool and workpiece as friction affects and controls the deformation of the workpiece.However, friction is also directly associated with wear of the forming tools. Tool wear isa complex system response depending on contact conditions and is a serious issue whenit comes to process economy as it reduces the service life of the tool. Therefore, it isnecessary to enhance the durability of thermomechanical forming tools by studying theinfluence of parameters such as contact pressure, cyclic thermal loading, repetitive mech-anical loading and others on tool wear. Then, computational mechanics can be utilised tonumerically simulate and optimise the thermomechanical forming process by predictingwear of the tools.Dry sliding tests were carried out on a high temperature reciprocating friction andwear tester. The aim was to identify the occurring wear mechanisms and determine thetribological behaviour of prehardened hot work tool steel when sliding against 22MnB5boron steel. A normal load of 31 N, which corresponds to a contact pressure of 10 MPa, asliding speed of 0.2 ms −1 and temperatures ranging from 40◦Cto800◦ C were employed.It was found that the coefficient of friction and the specific wear rate decreased at elevatedtemperature because of the formation of compacted wear debris layers on the interactingsurfaces.Increasing material and energy expenses, rising demands for process flexibility andstability as well as requirements for minimal trial and error have led to a growing interestin numerical simulation of wear phenomena. Finite element simulations of a strip drawingtest were conducted to explore the possibility of predicting tool wear in press hardening.The focus laid on unveiling the contact conditions on the forming tools through numericalsimulation. The influence of high temperature on wear was studied and the results wereimplemented in Archard’s wear model to introduce temperature dependence. Further-more, another wear model used for warm forging was also considered. It was found thatthe extreme contact conditions occurred at tool radii and that the different wear modelsled to similar wear depth profiles on the radii but with different orders of magnitude.Standard high temperature tribometers allow fundamental tribological studies to becarried out in order to investigate the tribological behaviour of the materials in contact.However, the conditions prevalent during the interaction of the hot workpiece and toolsurfaces in thermomechanical forming are not adequately simulated in these tribometers.A novel high temperature tribometer has been employed in order to more closely simulatethe interaction between tool and workpiece at elevated temperatures during thermomech-anical forming. It was found that a higher load led to a lower and more stable coefficien