Composition, function and creep fracture mechanism of high alumina brick

With the increase of the content of high-alumina brick A1203, the number of mullite and corundum components also increases, the glass also decreases accordingly, and the fire resistance and density of high-alumina refractory bricks also increase. When the content of high-alumina brick A1203 is less than 71.8% With the increase of A1203 content, the high-temperature stable crystal phase in high-alumina refractory bricks is mullite. The content of A1203 is more than 71.8% of high-alumina refractory bricks, and the high-temperature stable crystal phase is mullite and corundum. The content of 71.8% increases, the content of corundum increases, and the mullite decreases. The high-temperature performance of aluminum refractory bricks increases accordingly.

Refractory bricks with an A1203 content greater than 48% aluminum silicate are collectively referred to as high-alumina bricks. The content of A1203 is divided into three grades: I, etc. (A1203 > 75%); II, etc. (A1203 is 60%~ 75%); III, etc. (A1203 is 48%~ 60%). It can be divided into low mullite (including sillimanite) and mullite (A1203 is 48%~ 71.8%), mullite-corundum and corundum-mullite (A1203 is 71.8% to 95%), corundum (A1203) %~ 100%) and other refractory bricks.

The sintering temperature of high-alumina bricks depends on the sinterability of soil and mineral raw materials. Using bauxite clinker (bulk density) ≥ 2.80g/cm3), the raw material has a uniform structure and high impurity content, and it is easy to sinter the green body, but the sintering temperature range is narrow, which is easy to cause overburning or overburning. Using Ⅱ clear soil clinker cake (bulk density) ≥ 2.55g/cm3) Due to the swelling and loosening effects caused by secondary Molay petrochemical, the green body is not easy to sinter, and the sintering temperature is slightly higher. Using III alum clay clinker (bulk density) ≥ 2.45g/cm3), the structure is dense, the content of A1203 is low, and the firing temperature is low, generally slightly higher than the firing temperature of clinker clay bricks by 30-50 ° C. High-alumina refractory bricks are burned in an oxide flame.

Because the load softening temperature of high-alumina refractory bricks is an important property. The experimental results show that it changes with the change of A1203 content in high-alumina refractory bricks: when the A1203 content is lower than the mullite theoretical composition, the equilibrium phase of high-alumina refractory bricks is the mullite glass phase. With the increase of A1203 content, the mullite content increases, and the load softening temperature also increases accordingly.

The thermal shock resistance of high-alumina refractory bricks is worse than that of clay bricks, and the water cooling cycle at 850 ° C is 3 to 5 times. The main reason is that the thermal expansion of corundum is higher than that of mullite, and there is no crystalline transformation. The thermal vibration resistance of I and II high-alumina bricks is worse than that of III high-alumina refractory bricks.

In production, the method of adjusting the composition of mud particles is usually used to improve the particle structure characteristics of high-aluminum refractory bricks, thereby improving their heat resistance and impact properties. In recent years, a certain amount of synthetic cordierite has been added to the composition of high-aluminum refractory bricks to manufacture high-aluminum refractory bricks with high heat resistance and impact, and obvious results have been achieved.

With the increase of A1203 content, the slag resistance of high-alumina bricks also improves. Reducing the impurity content is beneficial to improve the corrosion resistance.

The difference between high-aluminum refractory bricks and clay bricks is that high-aluminum refractory bricks have good performance and longer service life than clay bricks, and have become one of the refractory bricks widely used in the building materials industry.

Creep fracture mechanism of high alumina brick

Another nonlinear fracture of high alumina bricks is the creep damage encountered when deformed at high temperatures. Under these conditions, the deformation of pure refractory oxide materials is mainly from the sliding of crystal boundaries. For small deformations, the sliding speed of crystal boundaries is proportional to shear stress; for larger deformations, due to the uneven crystal boundaries, their geometric non-integration can lead to occlusal between adjacent grains. When crystal boundaries migrate to adjust for this irregularity, the sliding rate of crystal boundaries decreases. Therefore, high tensile stress is formed in the crystal boundary region, resulting in the nucleation of cracks and pores. As the stretching continues, small pores expand. In polycrystalline materials, this process is a volume diffusion process. In materials with viscous crystal boundaries, the mechanism of pore growth may be viscous flow at crystal boundaries.

The consequence of the enlarged pores is that the solid area of the cross-section decreases, the stress per unit area increases, and then it breaks until it is damaged. The creep fracture form of the porous high-alumina brick with granular structure in high temperature use is taken from the actual example of the polished photo of the residual brick of the magnesia-alumina brick used on the top of the open hearth after being used under overheating conditions. The figure shows that under the action of its own weight, the pores gradually increase from the cold end, and when it continues to bear the action of its own weight, the cracks (fractures) that are almost parallel to the working surface.

Creep fracture of bricks containing high-liquid-phase aluminum is usually the result of diffusion. Therefore, the creep fracture phenomenon of high-aluminum bricks will vary with temperature, and the creep velocity will vary with temperature. When the inclination of the curve is greater than 1500 ° C, it indicates the fact that the temperature liquid phase begins to form sharply. Since creep fracture is the result of diffusion, the exact creep strength cannot be measured, but the time required for fracture will decrease as stress and temperature increase. This situation suggests that the method in which the experimental data is valid is the creep-fracture curve. If the logarithmic pair action stress is plotted before fracture, the data can be correctly represented in the range of high alumina brick creep-fracture processes. A practical example of a high temperature creep fracture path for a magnesia-alumina brick caused by overheating.

How to protect high alumina bricks in use?

For thermal equipment in metallurgical and other industrial sectors, only continuous automatic control of the state of the high-alumina brick lining and the system protection of the high-alumina brick can make the equipment operate reliably.

The initial protection measure for high alumina brick systems is to frequently measure the thickness of the lining during use.

We already know that there are several ways to find the damage rate of high-alumina bricks, such as naked eye, atoms, lining temperature, etc. Now laser interference analysis has been studied, and the residual thickness of high-alumina brick lining can be determined with an accuracy of less than 1mm.

The damage rate of high-alumina bricks in different areas of the lining is constantly measured, and the thickness of the masonry blank can be the same.

The protection of high alumina bricks has several aspects:

(1) Cool the high-alumina bricks of the masonry until the inner lining is completely replaced by a guard with water;

(2) Restore the high-alumina brick of the damaged layer by spraying, smearing, adhering, etc.;

(3) reduce erosion;

(4) High alumina brick lining, specifying the standard amount of operating temperature and gas system;

(5) The purpose of improving masonry components and masonry structures is to reduce thermomechanical stress.