Study On Hot Properties And Microstructure Of Corundum Based Castables

Using fused white corundum, ultrafine alumina (uf-Al₂O₃), ultrafine silica (uf-SiO₂) and pure calcium aluminate cement as main raw mateials, corundum based castables bonded respectively by conventional cement(sample C), low-cement (sample L), ultra-low-cement (sample U) and cement-free (sample N) were prepared.The high temperature properties, including hot modulus of rupture, creep, refractoriness under load, thermal expansion and thermal shock resistance, as well as conventional physical properties of the four kinds of corumdum-based castables were investigated. The phase compositions and the microstructures of the specimen of the castables were also analysed by XRD and SEM. The results show that:1. The phase composition and microstructure measured from the quenched samples are more accurate than that from the naturally colded ones. CA₂ and interlocking needle like CA₆ have been observed from the sample C after heated at 1500℃. A liquid phase containing CAS2 occurs in sample L above 1300℃,and there isn't any mullite phase observed. Around 1400 ℃ or so, a needle like mullite phase occurs with three dimesnsional network structure. Another kind of liquid phase can also be observed in sample U during heating, which is different from CAS2 in composition with the crystallized needle mullite.2. The hot modulus of rupture(HMOR) of the sample C decreases as the test temperature increases.It begins to creep at 1200 ℃ and creeps more fastly at higher temperature. As temperature increases, hot MOR of the sample L decreases signifcantly beyond 1200 ℃ due to the apperance of the liquid phase and creeps at much higher rates at 1400℃ and 1550℃. Hot MOR of the sample N reaches its maximum(34MPa) at 1000℃ and begins to decrease as the temperature increases,getting the bottom(2.2MPa) at 1300℃,after which it begins to increase once more due to the formation of needle mullite and gains a strength of 11MPa at 1500℃. The sample N creeps at a low value at 1400℃ and 1500℃. The hot MOR of the sample U decreases as the temperature increases especially above 1200 ℃. At 1400℃ and 1500℃ the sample U gives higher creep rates.3. The cold strength and hot strength for all of the sample C, L, U and N trend oppositely, meaning that the cold strength increases while the hot MOR decreases at elevated heating temperature.4. By using P -Al₂O₃ instead of uf-SiO₂ in the sample L, the castable sets normally and gets a higher hot MOR above 1200℃ at the same time. In the sample U, magnesia and uf-SiO₂ were used as MgO-SiO₂-H₂O bonding system to take place of cement to accelerate the setting and were endowed an increased hot MOR too.5. Hot linear expansion rates (HLER) of C, L, U and N go up linearly with the temperature increased to 1000℃. That for sample C decreases above 1200℃. At 1350℃, that for the sample L begins to decrease sharply. That for the sample N decreases at 1000 ℃ slowly until 1400 ℃, while that for the sample U decreases at 1300 ℃.6. Because the temperature differences of thermal shock show different impact on C, L, U and N, it is difficult to evaluate thermal shock resistance of corundum based castables by the measurement of residual strength or the ratio of strength before/after test.