Influence of Chemical Reactions in Magnesia-Graphite Refractories: II, Effects of Aluminum and Graphite Contents in Generic Products

A group of magnesia-graphite and magnesia-graphite-aluminum materials, the compositions of which represent a wide range of graphite contents (~10-16.4 wt%), aluminum contents (0-5.2 wt%), and MgO and graphite qualities, were fabricated, using standard commercial practices. Chemical analysis and determination of room-temperature modulus of rupture (MOR) and Young's modulus, as well as a complete microstructural characterization of the as-received materials, were performed. Mechanical characterization at high temperature (1000°, 1200°, and 1450°C) was done in terms of Young's modulus and MOR in an argon atmosphere (<1000 ppm oxygen at 1000°C). Modulus-of-elasticity values ranged from 4 to 16 GPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens. A strong effect of aluminum-metal concentration on Young's modulus overrode other microstructural differences among the materials. MOR values ranged from 6 to 20 MPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens at the lower testing temperatures ( T lessthan equal to 1200°C) and by phase assemblages in the surface regions of the specimens-essentially by the presence of the dense MgO zone-at 1450°C. The thickness of the dense MgO zone in the aluminum-containing materials was determined by the amount of aluminum and the MgO aggregate size.