1. Cause analysis
The characteristics of fused zirconium corundum bricks are narrow crystallization temperature range and low thermal conductivity. Therefore, its hardening process is actually continuous.
Under such continuous hardening conditions, the melt of large fused bricks will produce internal and external cracks due to the uneven temperature field along its cross section. During the melt hardening period, cracks first occur inside the fused bricks and finally appear on the outer surface.
According to research, the formation mechanism of this crack can be briefly expressed in the following way. In the first stage of melt cooling, the outer surface of the fused brick forms an elastic shell (. In the second stage of cooling, the entire melt hardens, and a temperature difference occurs on the cross section of the fused brick. At this time, there is no stress inside it, because in the previous period of time, the center of the fused brick is still in a plastic state, which can buffer the compressive stress caused by the shrinkage of the shell. When further cooled, the fused brick has completely hardened.
Due to the heat preservation inhibition effect of the insulation box, the base temperature of the fused brick drops faster than the surface temperature, and the temperature gradient of the fused brick begins to decrease. If the fused brick is completely hardened, there will be a temperature difference at the base. But in fact, this situation will not occur, because the entire size of the fused brick has been fixed due to the hardening of the surface. Because the change in the base temperature of the fused brick exceeds its change on the surface, the central part will be subjected to tensile stress when maintaining the length. When this tensile stress exceeds the temperature of the material itself at that temperature, the temperature gradient of the fused brick will decrease. When the maximum tensile strength under the temperature is reached, cracks will occur inside the product, and according to the size of the temperature difference, the cracks will either persist inside or extend to the surface and become visible cracks. This type of crack is generally called thermal crack.
In addition, the melt of fused zircon corundum bricks will also undergo a large volume change due to crystal transformation during the cooling and crystallization process, especially when the high-temperature cubic system is transformed into monoclinic zircon below 1008'℃, it will be accompanied by a large volume expansion.
Therefore, in addition to thermal cracks, we can sometimes see some reticular cracks on the cross section of fused bricks. The latter are mostly caused by the stress generated when the baddeleyite undergoes crystal transformation.
Therefore, when selecting annealing technology and formulating annealing criteria, we must consider the above two factors to prevent excessive stress from occurring during the cooling and crystallization process of fused bricks, which may cause cracks.
2. Solutions
During the cooling process of the cast fused zirconium corundum brick, as ZrO2 transforms from the high-temperature cubic system to the low-temperature monoclinic system, the volume of the fused brick expands rapidly. In order to prevent the product from cracking due to the large thermal stress caused by excessive cooling. Therefore, the insulation measures should be strengthened during its crystallization process. Usually, the insulation annealing of fused zirconium corundum brick is to put the sand mold into the insulation box and fill it with insulation medium. When casting, the molten liquid is injected into the sand mold and then placed in the annealing area for natural annealing. During the operation, the temperature difference between the casting and the annealing insulation medium should be minimized to slow down the cooling speed to avoid cracks and control the product out of the box temperature. In this way, a fused zirconium corundum brick product with uniform structure and improved high-temperature performance can be obtained. In view of the uneven stress distribution of special-shaped bricks and ultra-wide and ultra-large fused bricks, cracks are easily formed. In actual production, we adopt targeted auxiliary measures. Use a live basket to form the mold, inject the molten liquid, place it in a thousand insulation boxes, and finally bury it in a suitable insulation medium for insulation annealing. The annealing rate is maintained at about 60℃C/h. In this way, the crack tendency of the fused brick is significantly reduced.
With the acceleration of glass melting furnace technology, most melting furnaces are developing in the direction of heat preservation and energy saving. Therefore, the appearance quality requirements for fused zirconium corundum bricks are getting higher and higher, and the adoption of new casting processes is only a matter of time. There are several development directions in the future:
A. The use of integral resin sand molds
B. The promotion of vacuum casting. The integral resin sand mold has taken the appearance quality of the product to a new level. The casting has accurate dimensions and a smooth surface. The sand mold hardens quickly at room temperature and has good high-temperature self-collapse performance. The vacuum casting process also achieves accurate casting dimensions and a smooth surface. The mold sand can be reused without bonding to save money. Therefore, this process should be promoted and applied as much as possible and continuously improved.
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