In addition to high temperature, oxidation and slag erosion, magnesia carbon brick also needs to withstand the impact and erosion of molten steel, which requires magnesia carbon brick to have high high temperature strength. The high temperature flexural strength has become the index to measure the high temperature strength of magnesia-carbon bricks, and it is also one of the important research directions in magnesia-carbon bricks.
There are many factors affecting the high temperature flexural strength, the most important of which are raw material purity, carbon content, binder, matrix composition and structure. The purity of the raw material is relatively simple, the purity of magnesia is high, the crystallization scale is large, the low melting point phase content distributed in the cubic magnesite grain boundary is low, the direct binding degree is high, and the high temperature bending strength is better: the purity and content of graphite are also affected. The research on matrix composition and organizational structure is relatively complex, and it is also the most concentrated research field to improve the high-temperature bending strength of magnesium-carbon bricks, which can be roughly divided into the following three directions.
1. Add metal powder
In terms of increasing the high temperature bending strength, the added metal powder mainly includes metal AI, S, etc. Its mechanism of action mainly includes: metal AI, Si, etc., reacts with graphite and resin carbon in magnesia carbon brick to form AI43, SiC, etc., which strengthens the combination between carbon and carbon and improves the strength; @ Metals A1, Si, etc. generate whiskers in magnesia carbon bricks. Fiber, etc., strengthen the material matrix :3) generate magnesium and aluminum spinel and other phases, improve ceramic bonding.
Wang Yulong et al. found that with the increase of metal Al, the high temperature flexural strength of low carbon MG-carbon brick increased, and more MgAlO and whiskers were found in the structure of MG-carbon brick with the addition of 6% metal A, as shown in Figure 1.
By introducing Zn powder and Al powder with different properties, it is found that when the added mass ratio of AI/Zn is 1 and the added mass fraction is 1%, the high temperature flexural strength of the sample treated at 1400C is greater. At this time, the expansion amount accompanied by metal carbonization reaction is moderate, and the stress of matrix aggregate binding tightly is small. In magnesia-carbon bricks, columnar or plate A4C3 interlace between aggregates or plug pores inside the sample, increasing the resistance of particle slip. Therefore, adding metal Al, AI4C3 is generated, and the high temperature bending strength of magnesia-carbon brick is enhanced. The addition of metal Si can also increase the high temperature strength of magnesium-carbon bricks, but the effect is not significant as metal AI.
2, in situ generation of carbides, ammoniates and other whiskers
The high temperature flexural strength of magnesia carbon brick is often improved by in situ generation of carbide and nitride whisker. Whiskers are generally one-dimensional junction materials of nanometer or submicron level, with few internal defects, and their strength and modulus are close to the theoretical values of crystalline materials. At the same time, the mesh distribution of whisker in the brick or the napping and locking function in the structure of magnesia carbon brick also give the material better strength. Yijingguang et al. found that with the increase of heat treatment temperature, the high temperature bending strength and the residual bending strength after thermal shock of magnesium-carbon bricks added with metal Si powder and A powder increase, and the bending strength of the sample after 1400° heat treatment is larger. Through the microstructure analysis, it is found that at 1400°C, not only needle-like AIN is formed in the brick, but also embedded on the surface of magnesia particles (FIG. 2), but also accompanied by a large number of SiC whiskers and needle-like BSi3N4 whiskers (FIG. 3). With such a microstructure, when the material is subjected to external forces, the stress can be transferred from the matrix to the whisker through the interface layer, and the whisker can disperse the stress on the matrix and reduce the damage effect. However, when the crack size of the sample is relatively small due to the action of thermal stress, the whisker plays a bridging role to inhibit the continued expansion of the crack: as the crack increases, the whisker at the crack is further destroyed, and the whisker is pulled out of the matrix and consumes energy. At this time, the pulling effect will give the magnesia-carbon brick higher mechanical properties at high temperature.
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