EFFECT OF ATMOSPHERE DURING FIRING ON THE HIGH TEMPERATURE MECHANICAL BEHAVIOR OF PRE-REACTED MAGNESIA-CHROME REFRACTORIES

The effect of atmosphere during firing on a pre-reacted 60% magnesia-40% chrome type refractory was investigated. A reducing heat-up was maintained with pure nitrogen atmosphere while variations in the percent oxygen content were made for the soak and cool-down stages of the firing cycle. High temperature strengths were determined by three techniques: (a) A hot modulus of rupture test at 1340(DEGREES)C, used presently in the refractory industry. (b) A torsion test in which a high shear stress was applied at various elevated temperatures and deflections measured as a function of time. Viscosity and "apparent" modulus of rigidity were calculated from the deflection versus time curves. (c) Thermal shock resistance in which a sample was subjected to ten cycles between room-temperature and 1200(DEGREES)C.; Results showed that the control of atmosphere during the soak and cool was important. Hot modulus of rupture and thermal shock resistance decreased with an increase in oxygen content present during firing. On the other hand, viscosity increased with this increase in percent oxygen. An oxidizing atmosphere with 20 percent oxygen or more during soak and cool-down was required to obtain high values of viscosity (low flow). High values of hot modulus of rupture and thermal shock resistance were obtained under reducing conditions with 5 percent oxygen or less during soak and cool-down.; Microstructural analysis by optical microscopy and scanning electron microscopy showed that the amount of oxygen affected the size, amount, and distribution of the various phases present. Photomicrographs clearly showed the presence of large amounts of a grain-boundary phase for firings with nitrogen and 5 percent oxygen. Large amounts of this type of phase appeared to prevent intergranular fracture at high temperatures thereby increasing the hot modulus of rupture and thermal shock resistance. The more oxidizing firings with air (20% O(,2)) and oxygen showed the presence of low amounts of the grain-boundary phase and large amounts of the euhedral phase. Low amounts of the grain-boundary phase appeared to reduce grain-boundary sliding at high temperatures and thus increased the viscosity.