Investigation of thermal shock resistance in alumina based refractory castables

Abstract

Refractory castables, like all ceramic materials, contain numerous structural defects in the form of flaws, pores, microcracks, and impurities. These materials are often used in conditions of exposure to sudden and cyclical temperature changes, that is, to thermal shock. Such conditions can cause the creation of cracks in the material structure. Namely, the formation of new microcracks and the growth of those that already existed take place as a consequence of thermal shock. The decrease of mechanical strength as well as a general weakening and degradation of the material are also happening. This research evaluates the effects of thermal shock on high alumina low cement castable. The synthesized samples of one castable composition, based on alumina as an aggregate and calcium-aluminate cement, with the same cure conditions, were sintered at three different temperatures (1100°C, 1300°C, and 1600°C) with a dwell time of 3 hours. A water quench test was applied as an experimental method for thermal shock resistance testing. Morphological analysis of the samples before and during the testing was done. The changes in the material during thermal shock testing were monitored by destructive and non-destructive methods. Image analysis of the photographs of the samples' surfaces and SEM microphotographs of the sample cross-sections were applied to monitor degradation at the surface and inside the bulk. Ultrasonic measurements were used for monitoring internal degradation and elastic properties. Both non-destructive methods also served for homogeneity analysis. Overall material degradation was determined by measuring the mechanical strength of the samples using a classic method according to the standardized procedure. According to the results of performed experiments and analyses, it is evident that during the thermal shock, the surface and interior of the samples sintered at 1600°C behave differently compared with those sintered at 1100°C and 1300°C. The investigations have revealed that all three series of the obtained alumina-based refractory castables displayed high resistance to thermal shock since they withstood more than 40 thermal shock cycles without cracking and with a level of degradation below 50%, even under 25%. The samples sintered at 1300°C exhibited the best thermal shock resistance with more than 110 cycles without cracking. This behavior recommends that all three series of high alumina low cement castables can be effectively used in conditions that require good thermal stability.