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Influence of the Damage Level During Quenching on Thermal Shock Behavior of Low Cement Castable | Refractory | Sintering

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  Science of Sintering, 42  (2010) 211-219   ________________________________________________________________________  _____________________________ *)  Corresponding author: s.martinovic@itnms.ac.rs doi: 10.2298/SOS100518001M UDK 622.785:621.742.48 Influence of the Damage Level during Quenching on Thermal Shock Behavior of Low Cement Castable S. Martinovi ć 1*) , J. Majstorovi ć 2 , V. Vidojkovi ć 1 , T. Volkov-Husovi ć 2 1 Institute for Technology of Nuclear and other Raw Mineral Materials, Franchet d’Esperey 86, 11000 Belgrade, Serbia 2 University of Belgrade, Faculty of Mining and Geology, Djusina 4, Belgrade, Serbia, 3 University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, POB 3503, 11000 Belgrade, Serbia  Abstract:  In the recent decades, the use of unshaped monolithic refractories has been increasing greatly because of their significant advantages over other shaped refractory bricks of the same class. A low cement high alumina castable was synthetised and sintered at 1300°C in order to investigate thermal and mechanical properties, as well as thermal shock behavior. The water quench test was applied as an experimental method for thermal stability testing. Modification of the water quench test was performed by additional monitoring of the  samples behavior during the water quench test such as implementation of image analysis and ultrasonic measurements. The image analysis program was applied on samples in order to measure the level of surface damage before and during the water quench test. Ultrasonic measurements were performed with the aim to measure the Young modulus of elasticity during the testing. Strength deterioration of the samples was calculated by the model based on ultrasonic velocity changes during the water quench test. The influence of monitoring the damage level before and during the quench experiment and its influence on thermal shock behavior will be discussed.  Keywords:  Low cement castable, Modified water quench test, Image analysis, Ultrasonic measurements, Anisotropy Introduction Among unshaped refractories, castables are used especially in critical high temperature applications for complex constructions, easy applications to thin sections and regions that are difficult to reach [1,2]. Initially, conventional castables beside aggregates contained a relatively high cement content and therefore high mixing water forming high strength bonding, high open porosity (up to 20%) and low raw density. Afterwards, research was directed to the development of low cement and ultralow cement castables due to enlarged industry requirements meaning better rheology, superior physical and mechanical properties, very high thermal shock resistance, etc . Accordingly, different fine and ultra fillers (in the form of calcined alumina, reactive alumina, microsilica) were added to the conventional castable composition with the aim to fill the open pore space between the coarse aggregates [3,4]. Additionally, their cement content and amount of mixing water can be reduced. Because of the addition of fines and reduction on mixing water, some dispersing agents must  be added to improve the rheological behavior of these castables. Addition of deffloculants and  S. Martinovic et al. /Science of Sintering, 42 (2010) 211-219  ___________________________________________________________________________   212 fillers allow simple installation of the castable with a low content of water, providing high density and low open porosity. Reduced content of cement and therefore lime induces decreased formation of low melting phases with low refractoriness. Low and ultralow cement castables have improved hot strength, higher thermal shock resistance, lower open porosity and increased corrosion resistance compared with conventional castables [1-10]. A wide range of Al 2 O 3  ceramics are commercially available with strength and temperature capability depending on the Al 2 O 3  content, which is usually in range of 85 to 99. Alumina is a ceramic material suitable for high temperature applications with good chemical resistance. Al 2 O 3  offers good corrosion resistance to many substances including inorganic and organic acids, molten and dissolved salts, weak alkali solutions, anhydrous ammonia, hydrogen sulphide, hydrocarbons, organic and inorganic sulphides, molten Sr, Ba, Na, Be, Fe, Co, P, As, Sb, and Bi, and free molecular hydrogen. Alumina is the most widely used engineering ceramic material due to properties such as high hardness (25 GPA or 9 on the Mohs scale), high melting point (2054 ºC), good electrical, and thermal insulation [11-13]. Using nondestructive testing for characterization of refractories was increasingly used in the last decade [14-24]. The goal of this paper was to implement the modified water quench test for thermal shock behavior testing. Modification of the classical test (ICS 81.080 SRPS B. D8.308 former JUS B. D8. 306) was in additional implementation of image analysis and ultrasonic measurements for sample behavior characterization. Image analysis was used for damage monitoring at the surface and inside the sample during the water quench test. Ultrasonic measurements were applied for decreasing the Young modulus of elasticity and strength degradation during testing. Material A low cement castable (LCC) was prepared by tabular alumina (T-60, Almatis) used as an aggregate with maximal particle size of 5 mm, and matrix composed of fine fractions of tabular alumina, 5 wt. % of calcium-aluminate cement (CA-270, Almatis), reactive alumina (CL-370, Almatis), and dispersing alumina (ADS-3 and ADW-1, Almatis). The castable was mixed with 4.67 wt. % of water (dry basis) dispersed with citric acid. Particle size distribution was adjusted to a theoretical curve based on a modified Andreassen’s packing model, with a distribution coefficient ( q ) of 0.375. The castable mixture was cast in steel moulds with vibration. Prepared samples were cubes of 40 mm edge length for mechanical strength and prisms of 40 mm x 40 mm x 15 mm for ultrasonic measurements. After demoulding, the samples were cured for 24 hours at room temperature and dried at 110 ˚ C / 24 h. Then, they were sintered at 1300 ˚ C and cooled down to the room temperature inside the furnace. In this paper, the behavior of the sintered samples during thermal shock will be discussed. Chemical composition of the samples is given in the Tab. I. and relevant mechanical  properties are shown in the Tab. II. Tab. I.  Chemical composition and physical properties of raw materials Chemical analysis (wt.%) Physical properties Al 2 O 3 CaO Na 2 O Bulk density (g/cm 3 ) BET (m 2 /g) Porosity (%) Tabular alumina ≥ 99.4 0.02 ≤ 0.38 3.79 - ≤  5 Reactive alumina 99.7 0.02 0.098 3.9 3 - Cement 73 24 ≤ 0.3 3 2 - *All components are from Almatis  S. Martinovic et al./Science of Sintering, 42 (2010) 211-219   ___________________________________________________________________________   213 Tab. II.  Relevant mechanical, physical and thermal properties of the reference samples The structure of samples after sintering at 1300 °C is given in the Fig. 1. Fig. 1.  Structure of a sample sintered at 1300 °C Water Quench Test Thermal shock behavior of the samples was investigated using the water quench test as the experimental method (ICS 81.080 SRPS B.D8.308 former JUS B. D8. 306). Samples were cubes 4 x 4 x 4 cm. Each thermal shock cycle consisted of several consequent steps. Slow heating up by a nominal heating speed of 10°C/min to the quench temperature set at 950 °C, holding at this temperature for 30 minutes to reach thermal equilibrium in the whole specimen volume and finally quenching into a water bath at the temperature of 23 °C. The experimental method is similar to the procedure described in PRE Refractory Materials Recommendations 1978 (PRE/R5 Part 2). The material exhibited excellent resistance to rapid temperature changes. Samples did not exhibit total destruction during the test procedure till 110 cycles. In this paper results up till 60 cycles will be presented. Property Value Compressive strength after drying on 105ºC/24h 84.37 MPa Compressive strength after sintering on 1300ºC/3h 171.55MPa Flexural strength after drying on 105ºC/24h 16.09 MPa Bulk density 3.12 g/cm 3  Water Absorption 3.2 % Apparent Porosity 9.9% Modulus of Elasticity 44.95 GPa Refractoriness SK>35 (1780°C) Refractoriness under load Ta, Te > 1780°C  S. Martinovic et al. /Science of Sintering, 42 (2010) 211-219  ___________________________________________________________________________   214 Results and discussion Image analysis Image analysis using the Image Pro Plus Program was applied to the samples for determination of the level of sample destruction. Samples were photographed before and during testing, in order to measure the level of deterioration. The results of image analysis are given in Fig. 2. as a function of the number of cycles of the water quench test. Fig. 2.  Samples during quenching
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