Friction and Wear Phenomena in Steels at Elevated Temperatures

Sammanfattning: Different grades of steels are often exposed to high temperatures whether during their shaping/forming or during their use in several applications. This exposure to high temperature has a great bearing on the resulting friction and wear phenomena in steels due to changes in their surface and near-surface properties. This means that the wear and frictional behaviour will be no longer controlled nor determined by the original properties of steels but rather by the changes in steels surfaces brought about by high temperatures. A thorough understanding of friction and wear phenomena in steels under these conditions is crucial in terms of control as well as prediction of friction and wear. This thesis has focussed on friction and wear phenomena in some selected steels suitable for working at high temperatures. The initial part of this work concentrated on investigating the effect of load and temperature on the friction and wear behaviour of tool steel sliding against boron steel in a pin-on-disc (POD) test configuration. This investigation revealed the formation of oxidised protective layers and their role in reducing wear and friction at elevated temperatures. Experimental studies in a specially designed high temperature tribometer for simulating tool-workpiece interaction in hot sheet metal forming were also carried out using similar conditions to those used in the POD tests. These studies corroborated the presence and importance of oxidised layers at elevated temperatures. However, the thickness of oxidised layers was lower compared to those on the POD specimens. The results showed a good correlation between mechanisms of wear and friction especially at 400 °C. As in the case of the POD studies, the main wear mechanisms were adhesion and three body abrasion. Further, three-body abrasive wear behaviour of different tool steels, heat treated high-Si steels and boron steel at different temperatures was also investigated. The two main wear mechanisms identified were microploughing and microcutting. The results revealed near surface modifications in steel surfaces such as work hardened layers, mechanically mixed layers and recrystallization of ferrite grains. The wear behaviour of different steels was strongly influenced by the occurrence of these transformations as well as changes in mechanical properties like hardness and toughness. Nanoindentation and multiple-pass nanoscratch tests were carried out using a high temperature nanoindenter with a view to investigate the relationship between mechanical properties measured (hardness, fracture toughness, plasticity index) and the tribological behaviour of different tool steels. Higher volume losses were obtained for tool steels with low hardness and high plasticity index values.