Generally, there are two types of microstructure of refractory products. One is the structure type of crystal particles cemented by silicate (silicate crystal mineral or glass) bond; The other is the crystal net synthesized by the direct cross junction of crystal particles. The difference in microstructure depends on the interface energy between phases and the wetting of liquid relative to solid phase.
Generally, the high-temperature performance (high-temperature mechanical strength, slag resistance and thermal shock stability) of products with direct bonding structure is much better than the structure of crystal particles cemented by silicate binder.
- Optimize the realization way of direct combination:
There are three ways to realize direct combination, namely, the selection of good raw materials, the optimization of process conditions and the introduction of the second solid phase.
(1) Selection and optimization of good raw materials:
In order to obtain products with good performance, from the perspective of composition, it is mainly to choose raw materials with high fire resistance and high purity. The content of impurities should be as small as possible and give full play to the performance of matrix raw materials, so as to ensure that the final products have high performance;
Considering the structure of the material, it is generally necessary to choose a compact structure with a strictly specified organizational structure in order to obtain the required characteristics. Because the final performance of products is determined by composition and structure.
(2) Optimization of process conditions:
1) Strictly control the particle size of raw materials. In order to obtain sufficient reactivity and sinterability of raw materials, micron powder is usually required, and the finer the particle size is, the more conducive to the reaction and sintering.
2) Selection of forming process. For a solid-phase reaction process without the participation of gas phase and liquid phase, the average distance between adjacent particles decreases and the contact area increases due to the increase of pressure, which is conducive to the reaction. High pressure forming is usually conducive to the improvement of sintered body density, and high pressure forming is particularly important.
However, generally, when the molding pressure increases to a certain extent, the influence on the solid-phase reaction process tends to be less obvious, and different molding pressures should be selected for the hardness of different raw materials, so reasonable molding pressures should be found out through experiments.
3) Determination of firing temperature and holding time. The specific firing conditions must be determined through tests according to the specific conditions such as the quality and performance of the raw materials used, proportioning ratio, particle size composition, etc.
(3) Reference optimization of the second solid phase:
According to the principle of the second solid phase and the experience in the research, production and use of magnesia refractories, it can be learned that in the production process of refractories, the introduction of the second solid phase in directly bonded refractories should meet the following conditions:
1) The second solid phase is also a high-temperature phase, and the eutectic temperature of the two phases is higher than the service temperature.
2) The second solid phase can be dissolved in the main crystalline phase.
3) The second solid phase generated by the reaction of the original can play a "bridging" role in the main crystal phase.
4) The crystal or particle of the second solid phase is easy to weave with the main crystal to form a network skeleton.
5) The second solid phase can be formed by in-situ reaction in the matrix or added as raw material.
- Selection and optimization of matrix materials:
The matrix material is the concentrated expression of the performance of refractories, and its properties determine the properties of refractories. From the selection of raw materials, it is wise to choose magnesium oxide as the matrix component for the development and utilization of magnesium composite sliding plate. Magnesia can be introduced through fused magnesia, sintered magnesia and light burned magnesia. Fused magnesia, also known as fused magnesia, is the main raw material for the formation of periclase in products.
Pure natural magnesite and light burned magnesia powder are mostly used for electric fused magnesia, which is heated and melted in a high-temperature electric arc furnace, and the melt is cooled naturally. The main crystalline phase periclase first crystallizes freely from the melt, grows and crystallizes, the grain is well developed, the crystal is coarse, the degree of direct combination is high, and the structure is dense, while a small amount of silicate and other combined mineral phases are isolated. This structural feature makes the electric fused magnesia in an oxidizing atmosphere, It can maintain stability below 2300 ℃, and has superior high-temperature structural strength and slag resistance. Fused magnesia can give full play to some superior properties of periclase. It is the best matrix material as magnesium composite sliding plate material.
- Selection and optimization of the second solid phase:
Magnesia composite refractories generally adopt silicate bonding and direct bonding. For magnesium composite refractories, the performance of silicate bonded products is worse than that of directly bonded products. Moreover, for sliding plate materials, silicate bonded magnesium composite refractories can not resist the chemical erosion of liquid steel components. This structure enables the direct bonding of grains between magnesite and between periclase and the second solid phase, and the refractories have higher high-temperature strength Mechanical stress resistance and high temperature resistance meet the standards quoted by the second solid phase. For example, the composite refractory of sliding plate generally adopts spinel composite material, such as aluminum magnesium (spinel) refractory castable, etc.