The origin of the nonlinear behavior of the Young's modulus (E) of carbon‐bonded alumina at high temperatures was addressed, based on the microstructural changes observed during processing and their thermo‐mechanical properties. Impulse excitation technique, thermogravimetric analysis, porosity measurement, and scanning electron microscopy were conducted in order to highlight and explain the E behavior. The finite element model of a virtual microstructure was simulated and the results attained are in good agreement with the experimental data. The tests revealed that the Young's modulus of a cured sample heated from room temperature up to 500°C was governed by the release of volatiles. Above this temperature, the thermal expansion mismatch among alumina, graphite, and the carbon matrix is dominant resulting in an increase in the effective Young's modulus. During cooling, crack networks and gaps between alumina particles and the carbon matrix were developed. The former were induced by volatile release and by the graphite's highly anisotropic thermal expansion. The latter was derived by the thermal expansion mismatch between the alumina and the carbon matrix. The closure of the gaps and cracks governed the expansion behavior during the second heating cycle and a nonlinear effective Young's modulus increase as a function of temperature was observed.
This phenomenological study investigates the corrosion of refractories by a highly corrosive steel (1.6587, 18CrNiMo7‐6) with a high aluminum content and casting temperature of 1580 °C. The applied refractory castables with matrices based on alumina, mullite, and zirconia/titania doped alumina (AZT) are carbon free or low carbon (4 wt%) containing with and without nanoscaled additives. The corrosion is analyzed mainly by microscopy after the corrosion tests. The carbon containing samples are negligibly corroded due to inhibited wetting. The nanoadditives in the carbon containing samples show no influence on the corrosion. The carbon free AZT is attacked most strongly with a corrosion layer of about 14 mm. In the alumina (corrosion layer about 6 mm) and AZT sample, compositions corresponding to manganese aluminates form with manganese from the steel. When also silicon diffuses into the refractory, compositions referring to manganese aluminosilicates form. In the mullite matrix crucible (corrosion layer about 1 mm) compositions corresponding to manganese aluminosilicates form directly with manganese from the steel resulting in a highly viscous melt at the interface which retard the further attack. For a future final evaluation, however, also the steel quality has to be taken into account as will be studied by an Aspex‐SEM.
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