High temperature mechanical behavior of Al2O3-MgO-C refractories

C.A. CALAFIORE; A.L. CAVALIERI; V. MUÑOZ; A.G. TOMBA M.; P.G. GALLIANO

Congreso; 11th Biennial Worldwide Conference on Unified International Technical Conference Refractories 2009; 2009

A suitable tool to obtain a comprehensive knowledge of high temperature mechanical behavior of carbon based refractories, essential to evaluate the material performance in service, is the determination of stress-strain curves. In this case, mechanical testing has to be performed at in-service temperatures and under controlled atmosphere to prevent the chemical degradation of the specimen by oxidant gases. The information provided by these laboratory tests is essential to understand the refractory materials behavior during the vessel campaign. Furthermore, the mechanical parameters extracted from stress-strain curves are needed for any type of numerical stress calculations of both the refractory lining and vessel shell. In this work, two types of commercial bricks of Al2O3-MgO-C that are used in the steel industry were evaluated. These refractories have tabular alumina and different percentage of sintered magnesia (6 and 32 wt. %) as aggregates, similar C content (6-8 wt. %) and metal additives as antioxidant. Characterization of these materials included: qualitatively phase analysis by X-ray diffraction (XRD), pycnometric (kerosene and He) and bulk density measurements (based on DIN IN 993-1), open, closed and true porosity determination (based on DIN IN 993-1), differential thermal analysis (DTA) and thermogravimetric analysis (TGA) up to 1200°C and microstructural analysis by optical and scanning electron microscopy (SEM) with EDS. The stress-strain curves were obtained by compressive uniaxial loading on cylindrical specimens (diameter: 27 mm; height: 40 mm) obtained by cutting and machining of bricks with diamond tools. Results were obtained at room temperature and high temperatures in the range of 600°C-1400°C under controlled atmosphere (flowing N2 gas). An Instron 8501 servo-hydraulic machine was used for mechanical testing, together with an electric furnace with Si2Mo heater elements for high temperature tests. A capacitive extensometer, suitable for axial strain measurements at high temperatures (0.6m) was also used. The tests were performed in displacement control, at constant a constant rate of 0.1 mm/min until the specimen failed. From these stress-strain curves, the mechanical behavior of the materials and several quantitative mechanical parameters were determined: a) secant Young modulus (E0,001) for 0.001 of strain, b) fracture strength (R), calculated from the maximum loading and c) fracture strain (R). These parameters were dependent on the type of refractory and the test temperature. After testing, the specimens were analyzed in order to determine fracture features and main microstructural changes. Tested specimens were characterized by: weight variation, measurement of density and porosity, optical microscopy with a binocular glass (up to X150) and XRD qualitatively analysis. Experimental results were analyzed in function of the microstructural and compositional characteristics of the refractories and the testing temperature. In addition, main microstructural changes were also keeping in mind to explain mechanical behavior.