Avaliação da resistência ao choque térmico em refratários de alta alumina formados a partir de matérias - primas de alta reatividade

Abstract

The refractories of high alumina, within the alumina-zirconia-silica system, stand out because of its excellent thermomechanical properties when required operationally, particularly in the steel industry. They combine remarkable high refractivity characteristics, compressive strength and chemical stability of alumina, with the possibility of increasing the fracture toughness inherent to zirconia and, also the ability to further increase the mechanical strength for better densification of the ceramic body, relating to silica. The addition of silica also tends to favor formation of mullite, an excellent refractory phase that can check other interesting features to these products, especially high hardness, creep resistance and increased fracture toughness. In this work, formulations with varying fractions of ceramic oxides were evaluated (some high reactivity), especially alumina, in order to reach the best conditions of formation of mullite conditions in the solid state and to evaluate the influence of different heat treatments on the strength of refractories studied when subjected to the so-called thermal shock. For this study, samples were uniaxially pressed and sintered at 1400, 1500 and 1600 °C. The technological properties such as porosity, apparent density and water absorption were evaluated. Samples of several formulations were also sintered at 1600 °C for 2 and 10 hours in order to investigate how the sintering time could be decisive for the thermomechanical resistance. The specimens were subjected to thermal shock test, and with the interpretation of results of mechanical resistance tests for diametrical compression and bending at three points, it was noted the prevalence of low granulometry (and consequent increase reactivity) of the feedstock for formation of mullite and how heat treatment might optimize the final properties also evaluated with the aid of analysis by X-ray diffraction, spectroscopy by dispersive energy, optical microscopy and scanning electron microscopy. It was verified that refractory material studied obtained results with respect to damage caused by thermal stress ascribed to thermal shock, and had high mechanical strength. The longer sintering, however, was not effective in thermal shock resistance.