Markus Lengauer

Finite element modelling of the electrical impulse induced fracture of a high voltage varistor

In testing and in service, varistors are subjected to very short (μs range) high current pulses. Due to the inertia effects that appear on rapid Joule heating dynamic stress waves are generated, which can cause brittle failure. An analytical solution for the one-dimensional case was presented recently by Vojta and Clarke. In this work a full three-dimensional analysis of an axisymmetrical varistor has been performed using Finite Element Simulation. The reflections of the stress waves from the bases and the shell of a varistor and their interference are analyzed. The resulting stress field and its development with time is much more complex than in the 1D case. The aspect ratio of the varistor has been shown to have a strong influence on the amplitude of the mechanical stresses and can be varied to minimize the maximum stress level reached. Damping has been considered but found to be negligible in realistic cases.

Failure Analysis of Si3N4 Rolls for Wire Hot Rolling by Numerical Simulation of Thermal and Mechanical Stresses

Silicon nitride rolls for wire hot rolling have been tested in the rolling mill. After short employment for processing of ultra-high strength materials, cracks appeared in the roll calibre, which limited further application of the rolls. In order to find out possible causes for the damage, relevant mechanical properties of the roll have been determined and thermally and mechanically induced stresses were assessed by numerical simulation. It has been found that thermal stresses are of little relevance, whereas mechanically induced stresses are high enough to enhance subcritical crack growth resulting in the development of the macroscopic cracks, so that the observed damage of the rolls could have been adequately explained. One further result of the analysis was that flaws induced by grinding are decisive for the service time of ceramic rolls, so that special attention has to be paid to the machining of the roll calibre.

Silicon nitride tools for hot rolling of high-alloyed steel and superalloy wires : load analysis and first practical tests

Abstract For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of cemented carbides. In service they suffer from roughening of the surfaces and severe wear, which deteriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hardness and good high-temperature properties, improvements of tool behaviour can be expected by the use of ceramic. In this paper the suitability of silicon nitride as material for rolls is investigated. The thermal and mechanical loads in silicon nitride rolls during the hot rolling of steel and superalloy wires are analysed. Although the working temperature can be up to 1100 °C the tensile thermal stresses in the rolls reach only a few percent of the materials strength. But mechanical stresses due to contact stresses may become severe. When rolling wires of superalloys tensile contact stresses in the rolls can reach up to 600 MPa – about 60 % of the characteristic bending strength of the silicon nitride material. Experiments in the rolling mill of Boehler in Kapfenberg confirm these theoretical findings. When rolling high-speed tool steels the silicon nitride rolls were superior to the common hard metal rolls. But when rolling superalloys cracks in the rolls arise. For less demanding applications (driving rollers, guiding rollers) silicon nitride rolls are still routinely used by Boehler in Kapfenberg. In summary, silicon nitride ceramics are well suited as tool material for rolling steel wires, if the rolls are properly manufactured and used. For rolling superalloy wires the ceramic material is at its limit, and a safe operation can only be expected for rolls with a material-based design.

Silicon Nitride Tools for Hot Rolling of High-Alloyed Steel and Superalloy Wires

For hot rolling wires of high-alloyed steels or superalloys tools are nowadays made of ce¬mented carbides. In service they suffer from roughening of the surfaces and severe wear, which de¬teriorates the surface quality of the wires and restricts the lifetime of the tool. Due to their high hard¬ness and good high-temperature properties, improvements in tool behaviour can be expected by the use of silicon nitride tools. Experiments with several types of rollers were performed in commercial rolling mills. At modest and medium severe loaded positions (e.g. in the case of guidance rolls) silicon nitride rolls show superior performance to conventional steel or cemented carbide rolls. At the most severe loaded positions silicon nitride rolls were also superior to conventional rolls when rolling high strength steel wires. But for rolling superalloy wires, cracks, which limited further applications of the rolls, appeared in the roll surface profile (calibre). Cracks in the surface of the rollers are in general caused by Hertzian contact stresses, which can reach several hundred MPa. These cracks come into existence if a limiting load is exceeded. Then small flaws can quickly extend to a length of more then one millimetre, and then they stop again (pop in behaviour). Popped in cracks can slowly extend by cyclic fatigue up to a length where breaking out of large fragments of the rollers occurs. The critical load depends on the flow curve of the rolled materials and on the design of the rolls. For the analysed design it is exceeded when rolling superalloy wires, but it is not exceeded when rolling materials having a lower flow curve.