1 Introduction Refractory materials are gradually damaged, deteriorated, and eventually damaged due to high temperature, chemical attack, thermal shock, impact and wear of kiln materials, and other mechanical stress and other destructive factors.
Refractory materials can be regarded as macroscopically homogeneous materials with high melting point. On the mesoscale (resolution: 103~12um), the structural elements of many materials are island-like particles, matrix and the interface between the two; On the scale (resolution 102 ~ 10Vm), the microstructure elements are mainly crystalline, secondary crystalline, glassy and stomatal. The type, quantity, morphology, particle size, and distribution of each phase affect the properties or behavior of the material, and thus determine the quality of the material and the effect of use.
The change of process factors must be reflected in the microstructure of the product. The similar materials of different grades must have different microstructure characteristics. The effect of various damage factors on the refractories must be left in the residual lining of the material. Process information. Therefore, using microscopic analysis as a means and in combination with other research methods, it is of great significance to find problems, find solutions to them, and improve the quality and service life of refractory materials.
The development of modern testing technology has greatly enriched the human understanding of the microscopic world. It has become more convenient to directly observe the microstructure of materials and the information gained has become more and more abundant. However, the analysis of observations lags behind the development of testing techniques. The relationship between the microstructure of materials and the performance and damage mechanisms has always been a hot topic in materials science, but many experiences need to be upgraded to theory. Existing theories need to be improved and continuously enriched to form a complete knowledge system. This article will be based on the existing foundation. However, in addition to purity, other factors also greatly affect the performance of the material. Economic rationality requirements and resource conditions.
For two-phase materials, according to the distribution of the second phase can be divided into the following three categories: Dispersion uniform structure: the second phase particles in an island-like distribution in the matrix composed of the first phase material; aggregate particle structure: The second phase particles are agglomerated or discontinuously distributed in the matrix of the first phase; percolation-like structures: the two phases each have a three-dimensional structural network that is connected or substantially communicated.
The nature of the multiphase material is generally determined by the continuous phase therein, but is greatly influenced by the second phase particles that are encased therein. The increase of the content of the second phase increases the contact probability of each particle of the substance. When the content increases to a critical value, the appearance of the seepage structure causes significant changes in many properties of the material.
Although the structure of the high-quality electrofusion dolomite sand is dense, more MgO is solid-solubilized in CaO after high-temperature processing; however, it is semi-chained with periclase seeds as the second phase and calcite as the second phase. The aggregated particle structure. Once this material is hydrated, the destructive volume effect of Ca(OH) production will propagate along the continuous calcelous phase, resulting in a faster infectious erosion of the material.
There are many advantages for the cordierite kiln furniture materials, but its main weakness is poor mechanical properties at high temperature, and improving this weak link is a topic for many researchers. ~3 shows the microstructure of two such material matrices. In the material A, the cordierite (C), cordierite and mullite (M) form a connecting bridge; although the connecting bridge contains a higher Si2(S), it is better due to the formation of a more solid three-dimensional framework. High temperature performance. This bridge in material D is poorly developed and has poor high-temperature properties due to its relatively high impurities such as free Si2 and Na2. The flexural creep curves of cordierite-mullite materials A and D are shown in .
The flexural creep curve of cordierite-mullite material D 2.2 The crystal grain structure of the main crystal phase The ceramic is designed on the basis of the principle of minimum defects. The defect size is considered to be equal to the grain size, and the strength (f) of the material is related to d. (1). Dense fine-grained materials generally correspond to high strength and toughness, and thus are expected to have good thermal shock resistance and impact and erosion resistance.
The refractories pursue erosion resistance and the ability to resist deformation under high temperature. The flow stress required for deformation at high temperature and constant strain rate is given by formula (2). 131. This formula can be seen to increase the grain size when other conditions are the same (d ) Can effectively increase the material's resistance to deformation.
- Activation energy, T-absolute temperature, n, P-stress and grain index, A, P-constant.
The large grain size makes it difficult for the slag to intrude into the interior of the material along the grain boundaries, prevent the refractory main crystal phase from being eroded, and improve the erosion resistance of the material. Shows the effect of MgO grain size on the corrosion resistance of steelmaking slag with carbon-containing 10% Mg-C material. 21. Large-grain fused magnesia is used to improve the performance of magnesium-carbon materials, especially for the manufacture of steelmaking and Outer furnace refractories at high erosion areas.
However, there are still merits in the design concept of structural ceramics. For example, fine-grained corundum has higher strength and Embossing modulus 41 than electrofused corundum; and when Mg particles are treated with periclase, the thermal shock resistance of Mg particles is 400. Gradually descend with the increase of crystal grains (see).
Nitride bonded SiC kiln furniture contains a large number of pores, nitrides and SiC thermodynamic stability in the oxidizing atmosphere is limited, so the maintenance of product life depends to a large extent on the surface of the protective Si2 glaze layer. The life of the product depends on the quality and stability of the glaze layer, the ability of the pores to pass oxygen, and the oxidation resistance of the bound phase.
For example, material A used by Beijing Dongtao Company is high-purity SiC kiln furniture combined with granular Si2ON2, and SiC kiln furniture with fibrous Si3N4 is poor in purity. After use, the glaze layer of A is thin and dense, and the internal oxidation of the material is slow, so the service life is 3 to 4 years; the impurities migrate and accumulate on the surface glaze layer, glaze bubbling, cracking and loosening, and internal oxidation. Serious and only 1 year of life. The enrichment of impurities causes the softening of the shaft layer and the appearance of bubbles to loosen the oxide film, and the thick glaze layer is prone to cracking in the thermal shock, so that the above-mentioned glaze layer does not exert a good sealing effect, and the oxidation resistance of materials inside the material is weak. Significantly reduce its service life.
4 Blowholes Pores provide a channel for corrosive substances to penetrate into the material to enlarge the reaction area and severely impair the mechanical properties of the material. However, the characteristics of many refractory production processes determine that they cannot avoid the generation of air holes. A certain porosity is required for the sintered material to ensure its thermal shock resistance; it is also required for the insulating material to maintain a high voidage to ensure its low thermal conductivity and low thermal storage. Therefore, the key to the problem is to use the favorable side of the pore to limit its adverse effects. According to fracture mechanics, the following formula can be derived.
By studying the above-mentioned mechanism, Sonntag has manufactured a new type of recrystallized silicon carbide kiln furniture that significantly prolongs its service life by using chemical means to suppress the transition point of the aforementioned strength. It is speculated that the added material may enable the Si2 film to effectively seal the pore surface by effectively exploiting the potential of the aforementioned cristobalite material to effectively retard oxidation. From the micrographs provided by Somtag, it can also be seen that the pore spacing of the raw materials is small, and the connectivity between the pores is also high. Many small cracks generated during use are propagated through the pores, resulting in oxidation of SiC in the cracks. The volume effect is difficult to be dissipated and causes great destructive forces to propagate the cracks further, thereby creating new oxidation and destruction that causes the cracks to continuously generate and expand and connect into the network, thereby rapidly deteriorating the material properties and ultimately damaging it. The new material has been improved in the pore structure, which has played a role in reducing stress concentration and suppressing the generation of cracks.
5 Conclusions The effects of the refractories' microstructural elements such as the main crystalline phase, low melting phase, pores, etc., and their effects on material properties and damage mechanisms have been discussed. The progress of several refractory materials has been analyzed. By summarizing it can be concluded that the main crystal of the refractory material has a good thermodynamic stability, which means that the material should not have a harmful reaction in use and exhibit a certain "inertia" in the working environment. The stability of the organization is a prerequisite for stable performance, and it is also easy for the material to achieve a satisfactory service life.
The main crystalline phase characterizes the basic properties of the material, but the continuous relative performance in the material has a significant effect. When there is a high requirement for high-temperature mechanical properties, the refractory phase should be made into a solid three-dimensional framework and the low-melt phase should be torn down into islands.
Appropriately increasing the grain size is beneficial to improve the erosion resistance, erosion resistance, and high temperature mechanical properties of the material, which is particularly important when the grain boundary is weak.
Impairing the adverse effects of impurities is a topic of great significance. It is not only necessary to control the total amount of impurities, but also to control the proportion of impurity components and limit the content of the most harmful impurities.
When a protective film is formed on the surface of the refractory material, the quality and stability of the film are strongly influenced by the minor components, and some substances that are conducive to lowering the surface tension will migrate and accumulate in the film, thereby significantly affecting the erosion process and the use of materials. life.
The elastic modulus of the material, the thermal conductivity of the fracture work, etc. are all functions of the porosity, and the strength is the function of the porosity and the maximum pore size. Therefore, reducing the average pore size, especially the maximum defect size, is the solution to the conflict between porosity and strength.
When erosion is the main destructive factor and the erosion reaction is accompanied by a large expansive volume effect, cracks should be avoided or suppressed as much as possible. The reaction products generated between the crack surfaces will crack open, allowing it to expand and create a new round of erosion and expansion, thereby prompting material damage.
Sufficient space is one of the conditions for the self-growth of the fibers, but the reaction products are likely to have large voids or low melt phases. Therefore, appropriate measures such as hot pressing, impregnation, or crystallization should be used after the growth of the fiber to improve the organization and more effectively exert the effect of the fibrous substance.
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