High Temperature Friction and Wear in Press Hardening
In the automotive industry, press hardening is usually employed to produce safety or structural components from advanced high–strength steels. This hot forming process, and thermomechanical forming processes in general, is highly dependent on friction between tool 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 is a complex system response depending on contact conditions and is a serious issue when it comes to process economy as it reduces the service life of the tool. Therefore, it is necessary to enhance the durability of thermomechanical forming tools by studying the influence of parameters such as contact pressure, cyclic thermal loading, repetitive mechanical loading and others on tool wear. Then, computational mechanics can be utilised to numerically simulate and optimise the thermomechanical forming process by predicting wear of the tools.
Dry sliding tests were carried out on a high temperature reciprocating friction and wear tester. The aim was to identify the occurring wear mechanisms and determine the tribological behaviour of prehardened hot work tool steel when sliding against 22MnB5 boron steel. A normal load of 31 N, which corresponds to a contact pressure of 10 MPa, a sliding 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 elevated temperature because of the formation of compacted wear debris layers on the interacting surfaces.
Increasing material and energy expenses, rising demands for process flexibility and stability as well as requirements for minimal trial and error have led to a growing interest in numerical simulation of wear phenomena. Finite element simulations of a strip drawing test 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 numerical simulation. The influence of high temperature on wear was studied and the results were implemented in Archard’s wear model to introduce temperature dependence. Furthermore, another wear model used for warm forging was also considered. It was found that the extreme contact conditions occurred at tool radii and that the different wear models led to similar wear depth profiles on the radii but with different orders of magnitude. Standard high temperature tribometers allow fundamental tribological studies to be carried 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 tool surfaces in thermomechanical forming are not adequately simulated in these tribometers. A novel high temperature tribometer has been employed in order to more closely simulate the interaction between tool and workpiece at elevated temperatures during thermomechanical forming. It was found that a higher load led to a lower and more stable coefficient.