부산대학교 도서관

As a result of the ongoing trend towards lightweight design in various industrial sectors like the automotive industry, mechanical joining methods become increasingly important. These methods make multi-material design possible, while thermic joining techniques reach their limits. Semitubular self-piercing riveting is a mechanical joining technology which allows joining dissimilar materials by using rivets as fasteners. The manufacturing of the rivets, however, is costly and time-consuming, as the rivets conventionally have to be hardened, tempered and coated after forming, in order to achieve adequate strength and corrosion resistance. By using high nitrogen steel for the rivets, these additional process steps are not necessary anymore. Hence, the rivet production becomes more efficient. Because of the strengthening, caused by strain hardening, high-strength parts can be produced by cold bulk forming of high-strength steels. Nevertheless, pressure-nitrided steels are still rarely used in cold bulk forming due to high tool loads, which is a major challenge for the manufacturing process. Under those circumstances, there is a need for new approaches to manufacturing with the aim of reducing the tool loads and realising the production of rivets made of pressure-nitrided steel. One possibility is forming at elevated temperatures below recrystallisation temperature. For this purpose, the influence of the temperature on the forming behaviour of high nitrogen steel is investigated. In this context, possible changes in nitrogen content caused by heating of the material and thereby induced negative effects on the material properties have to be evaluated. The results of compression tests at different temperatures demonstrate the distinct reduction of yield strength at elevated temperatures. Moreover, further investigations show that the heat treatment tends to result in a change of the nitrogen content determined using energy dispersive X-ray spectroscopy. However, the changing nitrogen content does not affect the strength of the heat treated material after cooling. [ABSTRACT FROM AUTHOR]