Papers by Author: Rainer Telle

Abstract: In this study, we carried out a sol-gel processing to synthesize two types of phenol resin and TEOS sol with fluoric acid as the catalysts, and fabricated the β-SiC powder by carbothermal reduction of the obtained precursor gels. The gels were observed to be cross linked between TEOS and phenol resins by FT-IR and TG-DTA. After pyrolyzed at 1000oC in Ar to obtain the precursor, to confirm the phase transition and morphologies by XRD, and SEM were measured. Finally, the light green colored SiC powder was obtained after carbothermal reduction process up to 1800oC. The residual condensed SiO(v) phase and free carbons as a role of impurities for final products using the solid type phenol resin was observed in the surface of powder by BSE.

Abstract: To improve the purity of SiC and the yield of high purity SiC in the conventional Acheson Process, the behaviour of metal oxide impurities (Al2O3, CaO, Fe2O3 and MgO) with/without addition of sodium chloride (NaCl) is studied with the help of FactSageTM from a thermodynamic point of view. Equilibrium phases and compositions during evaporation and gas deposition are examined in the system of SiO2 + 3C. The calculation results are compared with industrial observations and published data. Purifying mechanism is clarified for the first time since the invention of the Acheson Process in 1891.

Abstract: Isothermal semi-solid extrusion experiments using steel grade X210CrW12 as work material were performed on an industrial forging machine. An improved and up-scaled tool design was applied, based on the concept of self-heating ceramic dies tested in previous laboratory-scale studies and allowing for die preheating temperatures of up to 1400°C. Steel rods of complex cross sectional geometry were formed at low extrusion forces. Shape accuracy of as-formed rods is accurate and metallurgical examination yields no evidence of liquid phase separation. With a view on the intended industrial implementation construction of ceramic tools has to be improved in order to avoid rupture of ceramic parts due to thermo-mechanically induced stresses and to benefit from the advantages inherent to ceramics in metal forming.

Abstract: The silica-alumina sol bonding agent, prepared by the sol gel route from ethyl silicate and aluminium isopropanol, was characterized by X ray diffraction, infrared spectroscopy, thermogravimetric analysis, particle size distribution. It was utilized in the corundum based refractory castables. The properties and characters such as cold crushing strength, apparent porosity, hot modulus of rupture, pore size distribution, scanning electron microscopy, energy dispersive spectral analysis, thermal shock and slag corrosion resistance tests of the selected samples from both the sol-gel bonding castables and the cement bonding castables were determined to identify the role of sol-gel additive in castables. The results show that the sol-gel bonding castables have a lot of advantages compared with traditional cement bonding castables and are more suitable for application in the ladles.

Abstract: The rapid development of magnetic materials has witnessed a sustained consumption increase in corundum-mullite kiln furniture application, yet a comparatively short duration of them. In this view, the thermal shock resistance of sagger plate in floppy magnets sintering turns out a factor of critical importance. This paper makes a study concerning the influence of factors including the size of critical electro-melted mullite particles, the electro-melted corundum or mullite as medium particles and the addition of upon the thermal shock resistance. The result shows that, with critical particles of 2 mm, the sheet of about 10mm thickness is characterized by better performance in modulus of rapture and thermal shock resistance. The latter proves to be more outstanding in samples rather than mullite. When zircon powder is added in a two-hour sintering process at the temperature of 1550°C, a remarkable improvement is made by the sample in terms of its thermal shock resistance. SEM analysis shows the existence of micropores in the section of samples with corundum as medium particles, whereas the glasses phase in that of mullite sample, resulting in a dense structure. Finally, zircon powder added makes for the improvement of thermal shock resistance due to the formation of baddeleyite on the particle surface of the corundum.

Abstract: Zirconia based material is one of the most interesting materials for semi-solid processing of steel because of its good mechanical properties and high corrosion resistance at high temperatures. In many cases, degradation of zirconia refractories or advanced ceramics is due to unexpected tetragonal-tomonoclinic^transformation accompanied by volume expansion. But only little attention was paid to outdiffusion of stabilising additives. In present paper, zirconia based ceramics with different rare-earth oxides (Y2O3 and CeO2) are used in contact with HS6-5-2 steel at 1330°C in order to obtain a good understanding about the corrosion behaviour of high-strength zirconia tool material, especially the effect of yttria and ceria on the corrosion behaviour of zirconia. The results showed that both Y2O3 and CeO2 diffused out of the zirconia structure and formed new compounds with alloying elements of steel at the zirconia grain boundaries. Both lack of stabiliser and grain growth result in t-m transformation and cracks were formed in zirconia ceramic.

Abstract: A novel ceramic tool concept allows near-isothermal steel thixoextrusion experiments. Thermal shock impacts are successfully eliminated from the load profile of conventional semi-solid processing technologies of high melting point alloys. Thus, the application of thermal shock sensitive oxide ceramics exhibiting excellent corrosion resistance as forming dies is feasible. Extruded steel parts show high shape accuracy at very low extrusion forces.

Abstract: The design and performance of silicon nitride (Si3N4) dies for the semi-solid processing of steels is studied by the example of a punch and a lower swage in upset forging. The observed failure mechanisms and degradation effects are related to short-term effects resulting from mechanical and thermal loads and long-term effects owing to chemical and tribological attack. Results show that well-defined process conditions (cycle time, solidification time in the die) and a ceramic-suitable design are a pre-requisite for reliability and sufficient service life of ceramic dies.