Thermal Stability of an Alumina-Based Refractory

Transcription

1 Thermal Stability of an Alumina-Based Refractory Thermal stability, dynamic Young s modulus of elasticity, anisotropy and damage level of a 42% alumina refractory material were investigated using a water-quench test. T. Volkov-Husović University of Belgrade, Faculty of Technology & Metallurgy, Metallurgical & Engineering Dept., Belgrade, Serbia and Montenegro J. Majstorović and M. Cvetković University of Belgrade, Faculty of Mining & Geology, Rocks & Mechanics Dept., Belgrade, Serbia and Montenegro T hermal shock resistance dictates refractory performance in many applications. A twofold approach often is used to characterize thermal shock behavior of refractories: material properties 1 4 and/or heat transfer conditions. 5 7 As an alternative approach, information about the thermal shock behavior of refractories can be obtained using experimental methods. One highly popular alternative approach is the water-quench test. Thermal quenching of refractories leads to crack nucleation and/or propagation, which result in loss of strength. Because the formation of cracks has a profound influence on ultrasonic velocity and Young s modulus of the material, measurement of either of these properties can directly monitor the development of the thermal shock damage level. Such measurement has been used here to monitor the strength degradation in samples during thermal shock. Also, ultrasonic velocity and strength change in the sample in two directions has been used to describe the anisotropy of the sample and its influence on the thermal shock resistance of the material. Materials Studied The samples used in the present study (42% alumina refractory) were used in several previous studies in which physical properties related to thermal shock behavior were measured Samples were photographed before quenching and after several quench cycles (Fig. 1). Analysis of the photographs was performed using the Image Pro Plus program. The deterioration of the samples during quench experiments was studied (Fig. 2).The The American Ceramic Society American Ceramic Society Bulletin March

2 (A) (B) (C) (D) (E) Figure 1 Samples (A) before and after (B) 3, (C) 5, (D) 9 and (E) 11 cycles of quenching. samples had some damaged areas before quenching because of the manufacturing process, chemical content of alumina and large grain size. The Experiment Thermal stability of the samples was measured using a water-quench test (JUS B-D8 319). Cylindrical specimens 5 cm in diameter and 5 cm in height were used. This experimental method was similar to the procedure described in "PRE Refractory Materials Recommendations 1978" (PRE/R5 Part 2). Ultrasonic velocity was measured (Model 5210, OYO) according to the standard JUS D-B Transducers were rigidly placed on the two parallel faces of the cylindrical samples using Vaseline grease as the coupling medium. The ultrasonic velocity then was calculated from the spacing of the transducers and the waveform time delay observed on an oscilloscope. Dynamic Young s modulus was calculated. Young Modulus and Strength Degradation The effect of the thermal shock fatigue on Young s modulus was studied (Fig. 1). The good agreement between decreased Young s modulus and number of cycles (multiple R = 0.93) suggested that thermal shock damage could be quantitatively evaluated nondestructively by measuring dynamic Young s modulus. Good results were obtained monitoring changes in ultrasonic velocities Therefore, these results were used for the correlation of strength degradation caused by thermal shock. High The American Ceramic Society American Ceramic Society Bulletin March

3 Table 1 Ultrasonic Velocities and Strength Distribution in x and y Directions N x 1 x 2 x 3 y 1 y 2 y 3 V p (m/s) σ (MPa) values of the coefficient of correlation (R = 0.988) suggested that degradation in strength during thermal stability testing was in strong correlation with the water-quench test experiment results. Similar results were obtained when strength degradation was related to damage level (P/P 0, where P is the damaged area after testing, and P 0 is the area of the sample before quenching). 15 Anisotropy of the Samples Materials used in this study were obtained from the producer. During the testing, obtained results suggested that anisotropy of the samples (which was obvious), should be investigated in more detail. Therefore, influence of the anisotropy of the samples on the thermal stability could be discussed. Measurements, which were presented in the previous section, were repeated in x and y directions. Also, various depths of the samples were of interest for both directions( x 1, x 2 and x 3 and y 1, y 2 and y 3 ). First results for ultrasonic velocities suggested different results were obtained for different directions, as well for different depths of the sample (Table 1). Results could be discussed in several ways: Differences in obtained results for x and y directions were similar, but different. Additional quench cycles resulted in increased differences in x and y directions. Depth of the material for which the measurements were performed also influenced the results. Less change was observed at x 1 (1 cm) than at x 2 (2.5 cm) and at x 3 (4.5 cm). Similar results were obtained for the y direction. When the number of quench experiments was increased, the differences in all directions were increased, because the strength degradation of the material was the result of increased stress because of temperature changes. Similar results were obtained for measured ultrasonic velocities V p and V s.because ultrasonic velocity V s was more difficult to measure, V p values were more reasonable to use for further calculations (strength degradation, Eq. (2)). Resistance Parameters Thermal shock stability of the refractories could be presented using resistance parameters. 1 4 Fracture and damage resistance parameters were calculated from thermal and mechanical properties, and the usual equations were used (R = σ(1 )/Eα and R' = kr). Both fracture The American Ceramic Society American Ceramic Society Bulletin March

4 resistance parameters described the relative resistance against the nucleation of fracture. Degradation of resistance parameters because of the degradation of strength also was determined (Figs. 3 and 4). Excellent correlation (multiple R = 0.97) between fracture resistance parameters with the number of cycles of the quench experiment and damage level was observed (Figs. 5 and 6). Conclusion Monitoring of thermal shock damage development in refractories using a nondestructive method is of industrial significance in relation to productivity and safety. Sonic measurement is a useful tool to solve this problem because of the following advantages: the instrument is small, portable and easy to use on site; results can be obtained quickly; and, because the results are related to damage, the damage level in the material can be predicted reliably. This study suggests that the results of the sonic measurements can be used for prediction of damage level in samples caused by thermal shock. Monitoring the damage level can be obtained using the decrease in the ultrasonic velocity. Strength degradation in a material caused by thermal shock also can be evaluated, and it has a strong correlation with the number of quench cycles. Prediction of strength degradation can be a useful parameter to predict the lifetime of a material. Anisotropy of the samples can be detected using the same type of measurements. Anisotropy analysis of the samples demonstrates the influence on the thermal shock behavior of the refractory sample. Therefore, detection of the level of anisotropy can be helpful in lifetime prediction of the sample. Deterioration of samples can be measured using the Image Pro Plus program for calibration of the damage level at the surface of the sample as a result of the water-quench test. Obtained results for the damage level can be correlated to the number of quench experiments and to the fracture resistance parameters. Thermal shock resistance of the specimen expressed by fracture resistance parameters also can be satisfactorily correlated to the number of quench cycles. This is the expected result from the observed strength degradation caused by thermal shock. The same observations can be made when resistance parameters are related to the damage level in the samples. Equations Equation to Calculate Young s Modulus E dyn = V p 2 ρ[(1 + µ dyn )(1 2µ dyn )/(1 µ dyn )] (1) where Vp is the ultrasonic velocity of the longitudinal waves (m/s), µ dyn the dynamic Poisson s ratio and ρ the density (kn/m3). Equation to Calculate Strength Degradation 15 σ = σ 0 (V L /V L0 ) n (2) where σ o is the compressive strength of the material before exposure of the material to thermal shock testing, V L the longitudinal ultrasonic velocity after testing, V L0 the longitudinal ultrasonic velocity before testing and n the material constant (n = 0.488; Ref. 15). The American Ceramic Society American Ceramic Society Bulletin March

5 Acknowledgement This research was sponsored by the Ministry of Science & Environment of Serbia as part of project MXT References 1 D.P.H. Hasselman, Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics, J. Am. Ceram. Soc., 52 [1] (1969). 2 D.P.H. Hasselman, Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock, J. Am. Ceram. Soc., 46 [11] (1963). 3 J. Nakayama, Direct Measurements of Fracture Energies of Brittle Heterogeneous Materials, J. Am. Ceram. Soc., 48 [11] (1965). 4 J. Nakayama and M. Ishizuka, Experimental Evidence for Thermal Shock Damage Resistance, Am. Ceram. Soc. Bull., 45 [7] (1965). 5 W.D. Kingery, Factors Affecting Thermal Stress Resistance of Ceramic Materials, J. Am. Ceram. Soc., 38 [1] 3 15 (1955). 6 S.S. Manson and R.W. Smith, Theory of Thermal Shock Resistance of Brittle Materials Based on Weibull s Statistical Theory of Strength, J. Am. Ceram. Soc., 38 [1] (1955). 7 H. Hencke, J.R. Thomas Jr. and D.P.H. Hasselman, Role of the Material Properties in the Thermal-Stress Fracture of Brittle Ceramics Subjected to Conductive Heat Transfer, J. Am. Ceram. Soc., 67 [6] (1984). 8 T. Volkov-Husović; Ph.D. Thesis. Belgrade, T. Volkov-Husović, R. Janc ić, M. Cvetkovic, D. Mitrakovic and Z. Popovć, Thermal Shock Behavior of Alumina-Based Refractories: Fracture Resistance Parameters and Water Quench Test, Mater. Lett., 38, (1999). 10 T.D. Volkov-Husović and Z.V. Popović, Resistance Parameters and Water Quench Test as Criteria of Thermal Shock Behaviour of Alumina Reafractories, Mater. Sci. Technol., 15 [10] (1999). 11 T. Volkov-Husović and R.M. Janc ić, Thermal Shock Behavior of Alumina-Based Refractories: Comparison between Damage Resistance Parameters and Water Quench test, Ind. Ceram., 20 [3] (2000). 12 T.D. Volkov-Husović, R.M. Janc ić, V. Radojević and Z. Popović, Prediction of the Thermal Shock Behavior of Alumina-Based Refractories: Temperature Difference, Damage Resistance Parameters and Water Quench Test ; p. 150 in Abstracts and Papers CD of EUROMAT 2001 (Rimini, Italy, June 10 14, 2001). 13 T.D. Volkov-Husović and R.M. Janc ić, Thermal Shock Behavior of Alumina-Based Refractories, Ind. Ceram., 21 [1] (2002). 14 T. Volkov-Husović, Monitoring the Damage Level during Thermal Stability Testing of Refractory Sample Using Sonic Measurement, J. Metall., 8 [3] (2002). 15 S.K. Nyiogi and A.C. Das, Prediction of the Thermal Shock Behaviour of Castable Refractories by Sonic The American Ceramic Society American Ceramic Society Bulletin March

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