Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables

JUNJIE YAN, WEN YAN*, STENFAN SCHAFFÖNER, YAJIE DAI, ZHE CHEN, QIANG WANG, GUANGQIANG LI, CANGJIAN JIA

— The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China.— e-mail: yanwen@wust.edu.cn

IntroductionPericlase-magnesium aluminate spinel refractory castables are widely used for tundish linings in steel making due to their excellent thermal shock and slag resistance as well as due to their superior mechanical properties. Usually periclase-magnesium aluminate spinel castables consist of dense sintered or fused magnesia aggregates as well as magnesia and alumina powders. The low porosity of magnesia aggregates in conventional periclase-magnesium aluminate spinel castables results in a high thermal conductivity and bulk density, which then cause excessive heat loss through the lining of the tundish. A major current research focus is therefore a lightweighting approach using microporous instead of dense aggregates. This lightweighting approach ensures stable service performance of refractories at high temperatures as well as reduced bulk density and thermal conductivity, which thus lowers heat losses. This concept was also recently applied to periclase-magnesium aluminate spinel castables. By replacing fused magnesia aggregates with a porosity of 6.9% by microporous periclase-spinel aggregates with a porosity of 23.3%, the bulk density and thermal conductivity were reduced by 9.2% and 18.8%, respectively. Generally, aggregates of refractories are produced by crushing and subsequent classifying of dense monolithic ceramics. As a rule, an increasing porosity reduces the strength of ceramic aggregates. However, unlike dense monolithic ceramics, refractories have a heterogeneous microstructure because they are composed of aggregates and a matrix. Therefore, the effect of the porosity on the strength of dense monolithic ceramics is considered to be different from the one on refractories. To elucidate this hypothesis, many studies investigated the effect of the aggregate porosity on mechanical and physical properties of refractories. The previous researches indicate that the increase of the porosity of aggregates does not necessarily decrease the strength of refractories. Even so, the water conte nt during the preparation of refractory castables is also a major factor modifying the resulting microstructure and mechanical properties. However, there remains a gap in understanding about the interaction of the level of microporosity in the aggregates and the water content during processing and how they affect the properties of refractory castables. Thus, in the present work, three microporous aggregates with different apparent porosities (12.8%, 30.8% and 39.3%) were used to prepare lightweight periclase-magnesium aluminate spinel castables. Subsequently, the effect of the apparent porosity of the microporous aggregates on the resulting microstructure, mechanical properties and fracture behavior of the castables was investigated in detail.

Junjie Yan, Wen Yan*, Stefan Schafföner, Yajie Dai, Zhe Chen, Qiang Wang, Guangqiang Li, Cangjian Jia, Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables, Ceramics International, 2021. https://doi.org/10.1016/j.ceramint.2020.10.240.The authors would like to thank the Key Project of the National Natural Science Foundation of China (Grant No. U1860205), and the Youth Program of National Natural Science Foundation of China (Grant No. 51902228) for financially supporting this work.

ConclusionThis study investigated the microstructure and mechanical properties of periclase-magnesium alumina spinel castables containing three aggregates with differing apparent porosity. The results support the following conclusions: (1) The apparent porosities of microporous magnesia aggregates affect especially the interface between the aggregates and the matrix of the castables by altering the necessary amount of water added in the forming process and the material transfer between the matrix and aggregate during the sintering process. In conjunction, these effects have a great influence on the mechanical properties of the castables. (2) When the apparent porosity of the aggregates was 12.8%, the cracks between the aggregates and the matrix caused by sintering shrinkage of the matrix reduced the strength to a large extent. When the apparent porosity of the aggregates increased from 12.8% to 30.8%, an excellent interlocking interface was formed between the aggregates and the matrix. This interlocking interface substantially increased the strength, but also reduced the fracture toughness. With a further increase of the apparent porosity of the aggregates to 39.3%, a small amount of micro-cracks formed between the aggregates and the matrix, which slightly lowered the strength, but barely affected the fracture toughness. (3) Compared to the castables C12.8 and C39.3, the castable C30.8 is considered to have the best set of properties, because it had a low bulk density (2.63 g/cm3), the highest compressive strength of 70.2 MPa as well as the highest flexural strength of 20.9 MPa. Even so, its fracture toughness needs to be improved.

Experimental procedure

The bulk densities and apparent porosities of the magnesia bricks and the fired samples were measured by Archimedes’ method using kerosene as the immersion medium. The microstructures were investigated by scanning electron microscopy (SEM, ISM-6610, JEOL, Japan). According to the SEM analysis, the pore size distributions of the aggregates and the matrices of the castables were statistically analyzed through an image analysis method. The flexural strengths and force-displacement curves were measured by three-point bending tests using an electronic digital control system (EDC 120, DOLI Company, Germany) according to the Chinese standard GB/T 3001-2007. The compressive strengths at room temperature were measured with a concrete compression testing machine (Hung Ta Instrument, Taiwan, China) with a loading rate of 1.0 MPa/s according to the Chinese standard GB/T 5072-2008. The specific fracture energy GF was determined according to the following equation by integrating the area under the load-displacement curve divided by the projected fracture area A during three-point bending tests: where δult is the ultimate displacement, δ is the horizontal displacement and F is the bending force.For each castable five specimens were used in total to determine all the above properties.

In order to prepare the castables, the raw materials were mixed with differing water contents according to the compositions given in Table 1. After homogeneously mixing the raw materials, they were cast into rectangle parallelepiped samples (140 mm×25 mm ×25 mm). All castables were cured for 24 h at room temperature, before they were dried at 110 ℃ for 24 h. Finally, the castables were fired at 1600 ℃ for 3 h in an electric furnace before they cooled down to room temperature.

Introduction

Results and discussion

Fig. 1. Microstructures of the aggregates.

The microstructure of the castables is shown in Fig. 1. The microstructure of the castables is shown in Fig. 2. It can be seen that the interface of the microporous aggregates and the matrix in the three castables was quite different. Some large cracks were observed between the aggregates and the matrix for castable C12.8. For castable C30.8, on the other hand, the cracks disappeared and a better interface bonding between the aggregates and matrix was formed. For castable C39.3 eventually some micro-cracks appeared between the aggregates and the matrix. The physical properties of the castables are listed in Table 2.

Fig. 2. Microstructures of the castables.

The force-displacement curves of the castables are shown in Fig. 3 . The average value of their f racture parameters are summarized in Table 3 with the respective variance in parentheses.

Fig. 3. Force-displacement curves of the castables.

The propagation of the the cracks during the three-point bending tests was investigated by SEM, which is illustrated in Fig. 4. To analyze the mechanisms of crack propagation of the castables, the crack propagation lengths within the aggregates, along the aggregate/matrix interface and within the matrix were measured, as is shown in Fig. 4. Then the percentages of these three mechanisms relative to the total length of the crack propagation path were calculated and named as PA, PAM and PM, respectively. The corresponding results are given in Fig. 5.

Fig. 4. Crack propagation paths of the castables during the three-point bending test.

Fig. 5. Percentages of the crack propagation paths in the castables during the three-point bending test.

Fig. 6. Schematic diagram influence of the apparent porosity of the aggregates on microstructure of the castables.

Overall their mechanical properties improved to a large extent when the apparent porosity of the aggregates increased from 12.8% to 30.8 and 39.3%. The effect of the apparent porosity of the aggregates will be analyzed by taking the forming and sintering processes into account, as is illustrated in Fig. 6.

Table 3. Characteristic values from the three-point bending test of the castables.