Literaturbericht zu den Dezimalklassifikationen

Bei der Jahresveranstaltung der Arbeitsgruppe für 2005 gab zunächst Dr. Bernd Lorenz (Fachhochschule für Öffentliche Verwaltung und Rechtspflege, München) einen Überblick zur Arbeit und Entwicklung des abgelaufenen Jahres

The low-temperature behavior of the quark-meson model

We revisit the phase diagram of strong-interaction matter for the two-flavor quark-meson model using the Functional Renormalization Group. In contrast to standard mean-field calculations, an unusual phase structure is encountered at low temperatures and large quark chemical potentials. In particular, we identify a regime where the pressure decreases with increasing temperature and discuss possible reasons for this unphysical behavior.Comment: 7 pages, 7 figures, v2 corresponds to published versio

Das Dilemma mit der Glaubwürdigkeit

Die Verantwortung von Unternehmen spielt eine immer stärkere Rolle. Wenn Betriebe Verantwortung für Andere oder die Umwelt demonstrieren, wird dies in der Regel nur dann honoriert, wenn das Unternehmen eine gewisse Glaubwürdigkeit besitzt. Doch Glaubwürdigkeit aufzubauen ist ein langwieriger Prozess, und sie ist vergleichsweise schnell wieder zu verlieren

Resistance Heating by Means of Direct Current for Resource-Saving CO2-Neutral Hot Stamping

Hot stamping is a well-established and frequently used manufacturing process in automotive body construction. The number of components manufactured in this way is continuously increasing. Hot stamping is used to produce components with a completely martensitic structure, resulting in high strength and hardness. These components are mainly used in safety-relevant areas of the passenger cell, such as the A-pillar, B-pillar, tunnel and sill. For hot-stamping processes, it is necessary to austenitize the blanks. Heating the sheet metal up to 930 °C in a furnace is very energy-intensive. In large-scale industrial applications, the sheets are generally heated in gas-fired roller hearth furnaces up to 60 m long. Apart from the poor energy balance and the high CO2 emissions of such furnaces, they are associated with high investment and maintenance costs, large space requirements and a long heating time. Rapid heating by means of the Joule effect and direct current instead of alternating current offer an energy-efficient and environmentally friendly alternative for sheet metal heating. Therefore, this technology can make a major contribution to environmental protection and resource saving. Within the scope of this work, parts were rapid-heated and subsequently hot-stamped by means of a novel heating system based on direct current with energy savings of up to 80 %. Using electricity guarantees a good CO2 balance. In addition, resistance heating with a new type of DC-heating system and an adapted process chain is compared with conventional furnace heating. In thermographic images and microstructural examinations of the hot-stamped parts, it can be demonstrated that this direct-current technique is well suited for achieving homogeneous hardness and strength in the whole sheet metal. Thus, this new heating system can enhance the efficiency of the hot-stamping technology

Approach for modelling the Taylor-Quinney coefficient of high strength steels

Precise knowledge of the temperature that arises in the material during plastic forming is of crucial importance, as it has a significant influence on material behaviour and therefore on the forming process. In order to describe the amount of heat that is generated during plastic forming accurately, the Taylor-Quinney coefficient β was introduced as the ratio of dissipated heat to plastic work and generally assumed to be a constant value. However, recent studies have shown that there is a dependency on material and process-specific parameters. In this study, the Taylor-Quinney coefficient β is shown as a function of strain and being influenced by the test specific strain rate and stress state. The tested material is a dual-phase steel HCT980X. The uniaxial tensile test and the Marciniak test with different tallied specimen at forming-relevant global strain rates were investigated. By means of thermographic and optical measuring systems the temperature and local strains were recorded during the tests. Based on an approach similar to the finite volume method, both experimental setups were modelled taking heat transfer effects into account. As a result, the Taylor-Quinney coefficient is calculated by means of experimental data. It is shown that the Taylor-Quinney coefficient is a variable value depending on the flow behaviour of the steel. The local strain rate and the specimen geometries of Marciniak test have a significant influence on the arising heat conduction. The stress state, however, has minor influence on β

Influence of Cooling Parameters on the Surface Layer Structure of Hot Working Tools

The surface layer of hot working tools is subject to alternating thermo-mechanical loads during forging. It experiences a fast increase in temperature on contact with the heated billets, followed by a steep temperature decline during application of the spray-coolant. This can lead to a cyclic surface rehardening of the tool surface layer due to the formation of a martensitic structure, which can either delay or accelerate tool failure. The microstructural changes in the tool surface layer mainly depend on the thermal, mechanical and tribological loads during forging. The influence of these loads is of particular interest to understand the effect of cyclic surface rehardening. The goal of this research is to investigate the influence of cooling parameters on microstructural changes in the tool surface layer during die forging. Mechanical and tribological loads are kept constant while cooling parameters are varied. Three internal thermocouples are applied to forging tools to measure the base tool temperature. To keep the amount of lubricant in each forging cycle at constant levels, cooling and lubrication are separated by use of boron nitride as lubricant, which is applied by electrostatic adherence. For cooling, the duration of water application is varied while maintaining pressure and spray pattern. Fourtools with differenttool base temperatures are investigated and the influence of the thermal loads on the wear behaviour displayed