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Refractories

Refractories are complex materials with a microstructure which is generally complex and heterogeneous (grains with a wide range of size and with a multiplicity of morphologies, porosity and cracks, etc). At room temperature, refractories generally exhibit microscopic cracks due to thermal expansion coefficient mismatches of their various phases caused by thermal stresses at the cooling stage of their fabrication. The analysis of their thermo-mechanical properties, such as Young’s modulus as function of temperature, is crucial for improving the high-temperature performance of refractory and castable materials.

Refractories2

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Thermal shock resistance

Application note: Damage characterization after thermal shocks
The impulse excitation technique is a non-destructive technique used to measure material properties. After the sample is mechanically excited, the resonance frequencies and internal friction are recorded. These material properties are a fingerprint of the sample and monitoring them in time gives valuable information regarding failure mechanisms. Typically, measurements are performed at room temperature after the samples are exposed to different cyclic loading schemes or specific heat treatments.

Traon et al. studied the thermal shock behavior of high aluminia castables composed of tabular alumina and partially stabilized zirconia (PSZ). They were pre-fired at 1300 °C for 6 hours at a heating and cooling rate of 1K/min. The grains of the sample had a maximum size of 3mm and the sample had a rectangular shape with a length of 160 mm, a width of 40 mm and a thickness of 40 mm.

The samples were subjected to thermal shocks in according to DIN EN993-11 at four different temperatures (750 °C, 850 °C, 950 °C and 1050 °C). After every thermal shock, the Young’s modulus and internal friction were measured according to the ASTM E1876-07 standard with the RFDA professional.

Fig. 1 shows the retained Young’s modulus values of the PSZ samples as a function of the number of thermal shock cycles at different tested temperatures (750, 850, 950 and 1050 °C). During the first thermal shock cycles, the Young’s modulus is degrading significantly. This can be explained by the crack nucleation in the grain boundaries which becomes more noticeable as the temperature changes increases.

As shown in Fig. 2, the internal friction data is highly correlated with the Young’s modulus data as the lower the Young’s modulus, the higher the internal friction due to the formation of cracks and microcracks. Damping values do not change as a two-step process. Indeed, damping increases proportionally with the damage increase. Based on the conclusions of Traon et al., the experiments conducted at a thermal shock temperature change of 1050 °C leads to the conclusion of a four-step evolution of the microstructural changes. Up to and including the second cycle, crack nucleation at the level of the grain boundaries causes the first increase of the damping measurements up to 1000 %. The second increase reaching 2000 % of the initial value occurring up to and including the fifth cycle can be explained by the propagation of the cracks within the matrix and throughout the Al2O3 aggregates. This step is followed by a steady state up to and including the eighth cycle while the cracks have to consume a lot of energy in order to bypass the ZrO2 aggregates.

Finally, the unification of cracks occurs during the ninth cycle, this network of cracks explains the depletion of the elastic properties and the further difficulties in achieving realistic damping values over 2500 % of the initial value.

Based on the obtained results, it is clear that the combination of elastic properties and damping data obtained by the impulse excitation technique leads to a better understanding of the microstructural changes within the refractory castables after progressive thermal shocks.

References

  1. N. Traon et al., Estimation of Damage in Refractory Materials after Progressive Thermal Shocks with Resonant Frequency Damping Analysis. J. Ceram. Sci. Tech., 07 (2016) 165-172
Application note: Microstructural characterization by high temperature Young’s modulus analysis

Pabst et al. studied the temperature dependence of the Young’s modulus of silica brick materials. They consist almost entirely of tridymite (64 %) and cristobalite (36 %). The rectangular shaped samples had a length of 160 mm, a width of 20 mm and a thickness of 10 mm. The Young’s modulus at elevated temperatures have been measured by the IET according to the ASTM E1876 standard in the IMCE RFDA HT1600 furnace. Resonant frequencies have been measured continuously from room temperature to 1200 °C (first heating cycle), down to room temperature (first cooling cycle) and followed by two similar heating and cooling cycles with a controlled heating and cooling rate of 5 °C/min.

Fig. 1 and 2 show the temperature dependence of the Young’s modulus of a silica brick material with 19,4 % porosity from room temperature to 1200 °C/300 °C measured with the impulse excitation technique. Based on the conclusions of Pabst et al., it is clear that the temperature dependence of the Young’s modulus during the first heating cycle is different from the subsequent cycles. In the intermediate temperature range between approximately 50 and 250 °C, these materials become very flexible with stiffness minima of around 60 % of the room temperature values. The sudden changes in this temperature regime are clearly related to phase transitions between subpolymorphs of tridymite and cristobalite.

Also a clear hysteresis effect i.e. shifts of the phase transition temperature (of around 30 °C) is shown in this temperature regime. The volume contraction opens existing microcracks and lowers the E-modulus of the material. Therefore the abrupt volume changes during heating and cooling over phase transitions are responsible for microcrack opening and closing and lead to an indirect enhancing effect. Furthermore, it is shown that the Young’s modulus at around 800 °C can be more than three times as high as the room temperature. Finally, after a complete heating-cooling cycle, the initial room temperature Young’s modulus is obtained again which means that a temperature of 1200 °C does not induce any irreversible changes in the microstructure. In other words, the elastic modulus of these materials is not a unique function of the temperature but due to the thermal memory, it is dependent on the thermal history.

From the results, it is clear that the hysteresis loops can be reproduced many times and that the microstructure affects the absolute values of the Young’s moduli, while the transition temperatures are generic features of the solid phases, essentially unaffected by microstructural details. Therefore impulse excitation measurements can be considered as a highly sensitive tool for the investigation of phase transistions.

References

  1. W. Pabst et al., Elastic anomalies in tridymite- and cristobalitebased silica materials. Ceramics International 40 (2014) 4207-4211

Publications

Temperature dependence of damping in silica refractories measured via the impulse excitation technique.

Gregorová, E., Pabst, W., Diblíková, P., & Nečina, V. (2018). Ceramics International 44(7), 8363-8373.

The potential usage of waste foundry sand from investment casting in refractory industry.
Xiang, R., Li, Y., Li, S., Xue, Z., He, Z., Ouyang, S., & Xu, N. (2019). Journal of Cleaner Production 211, 1322-1327.
The microstructure evolution and mechanical properties of MgO-C refractories with recycling Si/SiC solid waste from photovoltaic industry.
Zhang, Y., Chen, J., Li, N., Wei, Y., Han, B., Cao, Y., & Li, G. (2018). Ceramics International.
Elucidating the role of Ti3AlC2 in low carbon MgO-C refractories: Antioxidant or alternative carbon source?
Chen, J., Li, N., Hubálková, J., & Aneziris, C. G. (2018). Journal of the European Ceramic Society 38(9), 3387-3394.
Effect of the phase transformation on fracture behaviour of fused silica refractories.
Dai, Y., Yin, Y., Xu, X., Jin, S., Li, Y., & Harmuth, H. (2018). Journal of the European Ceramic Society 38(16), 5601-5609.
Creep testing of carbon containing refractories under reducing conditions.
Stueckelschweiger, M., Gruber, D., Jin, S., & Harmuth, H. (2019). Ceramics International.
Matrix microstructure optimization of alumina-spinel castables and its effect on high temperature properties.
Wang, Y., Li, X., Chen, P., & Zhu, B. (2018). Ceramics International, 44(1), 857-868.
Mechanical behavior and thermal shock resistance of MgO-C refractories: Influence of graphite content.
Zhu, T., Li, Y., Sang, S., & Xie, Z. (2017). Ceramics International, 43(9), 7177-7183.
Fracture behavior of low carbon MgO–C refractories using the wedge splitting test.
Zhu, T., Li, Y., Sang, S., & Xie, Z. (2017).Journal of the European Ceramic Society, 37(4), 1789-1797.
Fracture behaviour and microstructure of refractory materials for steel ladle purging plugs in the system Al 2 O 3-MgO-CaO.
Long, B., Xu, G., Buhr, A., Jin, S., & Harmuth, H. (2017). Ceramics International.
Influence of carbon sources on nitriding process, microstructures and mechanical properties of Si 3 N 4 bonded SiC refractories.
Chen, J., Li, N., Wei, Y., Han, B., & Yan, W. (2017). Journal of the European Ceramic Society, 37(4), 1821-1829.
Enhanced formation of magnesium silica hydrates (MSH) using sodium metasilicate and caustic magnesia in magnesia castables.
Zhang, Y., Li, Y., Xu, Y., Sang, S., & Jin, S. (2017).Ceramics International, 43(12), 9110-9116.
Estimation of Damage in Refractory Materials after Progressive Thermal Shocks with Resonant Frequency Damping Analysis.
Traon N., Tonnesen T., Telle R. (2016). Journal of Ceramic Science and Technology, 7(2), 165-172.
Effect of zirconia particle size on the properties of alumina-spinel castables.
Liu, Y., Han, B., Zhang, T., Yu, H., Yan, W., Wei, Y., & Li, N. (2016). Ceramics International, 42(15), 16961-16968.
Effect of ferrosilicon additive and sintering condition on microstructural evolution and mechanical properties of reaction-bonded SiC refractories.
Chen, J., Li, N., Wei, Y., Han, B., & Yan, W. (2016). Ceramics International, 42(15), 17650-17658.
The relationship between the pore size distribution and the thermo-mechanical properties of high alumina refractory castables.
Li, Y., Li, X., Zhu, B., & Chen, P. (2016). International Journal of Materials Research, 107(3), 263-268.
Processing and Characterization of Multi-Walled Carbon Nanotubes Containing Alumina-Carbon Refractories Prepared by Nanocomposite Powder Technology.
Liang, F., Li, N., Liu, B., & He, Z. (2016). Metallurgical and Materials Transactions B, 47(3), 1661-1668.
Microstructure evolution during the heating process and its effect on the elastic properties of CAC-bonded alumina castables.
Wang, Y., Li, X., Zhu, B., & Chen, P. (2016). Ceramics International, 42(9), 11355-11362.
High-temperature Young’s moduli and dilatation behavior of silica refractories.
Pabst, W., Gregorová, E., Kloužek, J., Kloužková, A., Zemenová, P., Kohoutková, M., … & Všianský, D. (2016). Journal of the European Ceramic Society, 36(1), 209-220.
Temperature dependence of Young׳ s modulus of silica refractories.
Gregorová, E., Černý, M., Pabst, W., Esposito, L., Zanelli, C., Hamáček, J., & Kutzendörfer, J. (2015). Ceramics International, 41(1), 1129-1138.
Thermoelastic properties evolution and damping phenomena of Cameroonian calcined bauxite stabilized with calcium dialuminate refractory cement.
Tchamba, A. B., Elimbi, A., Mbessa, M., Melo, U. C., & Nzegge, O. M. (2015). Ceramics International, 41(1), 53-59.
Mechanical Properties and Thermal Shock Resistance of Alumina/Hexagonal Boron Nitride Composite Refractories.
Liang, F., Xue, Z., Zhao, L., Zhang, H., & Zhang, S. (2015). Metallurgical and Materials Transactions A, 46(9), 4335-4341.
Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement–Bonded Refractories.
Alex, J., Vandeperre, L., Lee, W. E., Touzo, B., & Parr, C. (2015). Journal of the American Ceramic Society.
Simulation of refractory fracture as a tool for advanced material testing. In Advances in Science and Technology (Vol. 92, pp. 232-241).
Gruber, D., Jin, S. L., & Harmuth, H. (2014). Trans Tech Publications.
Influence of the Pore Shape on the Internal Friction of Refractory Castables. In
Traon, N., Tonnesen, T., Telle, R., Myszka, B., & Silva, R. (2014, February). Proceedings of the Unified International Technical Conference on Refractories (UNITECR 2013) (pp. 141-146). John Wiley & Sons, Inc..
Determination of Young’s modulus, fracture energy and tensile strength of refractories by inverse estimation of a wedge splitting procedure.
Jin, S., Gruber, D., & Harmuth, H. (2014). Engineering Fracture Mechanics, 116, 228-236.
Young’s modulus of magnesia-chrome refractories.
Wojsa, J., Podwórny, J., & Suwak, R. (2013). Materiały Ceramiczne/Ceramic Materials, 65(2), 151-155.
Thermal shock resistance of magnesia–chrome refractories—experimental and criterial evaluation.
Wojsa, J., Podwórny, J., & Suwak, R. (2013). Ceramics International, 39(1), 1-12.
Variation of Poisson's ratio of refractory materials with thermal shocks.
Podwórny, J., Wojsa, J., & Wala, T. (2011). Ceramics International, 37(7), 2221-2227.
Thermal shock damage in castables: Microstructural changes and evaluation by a damping method. In CFI.
Tonnesen, T., & Telle, R. (2007). Ceramic forum international (Vol. 84, No. 9). Göller.

Young’s modulus and various other elastic properties for refractory and castable materials

At room temperature, the elastic properties, among which the Young’s modulus, of refractories, refractory castables and materials for applications in the metallurgical and steel industry are continuously investigated in order to improve their performance against thermal shocks. In order to optimize the service life of refractory materials, the thermal shock resistance is currently determined based on the analysis of the Young’s modulus for refractory materials after different thermal shock cycles with the help of the non-destructive impulse excitation technique (IET).

Learn more about thermo-mechanical properties by the impulse excitation technique

At elevated temperatures, the Young’s modulus for refractory and castable materials is a mechanical property relatively sensitive to the evolution of the microstructure and can be characterized by the impulse excitation technique (IET) as function of the temperature.