What are the causes of cracks in fused zirconia corundum bricks

What are the causes of cracks in fused zirconia corundum bricks？

Cracks are the shortcomings of refractory bricks, which seriously affect the yield and economic benefits of the products. Today we have conducted some discussions on the types, distribution, causes and methods of eliminating cracks, and put forward some opinions for your reference.

 

1. Type of crack:

 

(1) According to the depth of the crack, it can be divided into surface cracks and deep cracks (i.e. internal cracks). Surface cracks include longitudinal cracks, transverse cracks, cracks and corner cracks. Deep cracks are mainly microcracks, which are caused by the phase change of baddeleyite.

(2) According to the thermal state of the melt, it can be divided into high temperature cracks and normal temperature cracks.

 

2. Spread of cracks:

Cracks generally start from the corners, with more cracks on the edges and fewer cracks in the center. The larger the brick, the more likely it is that cracks will develop, and the shape of the brick will be more complex, with more missing corners and edges. The darker the brick, the more cracks it will have. At the same time, the cracks of bricks are also related to the microstructure, which is fine and has few particle cracks. On the contrary, there will be many cracks. And there are many cracks near the pouring port.

 

3. Discussion of the causes of cracks:

 

After discussion, we believe that the causes of cracks are as follows:

(1) Thermal stress. During the cooling process of the product, the temperature of the surface layer drops faster than that of the inside, resulting in a large temperature difference between the inner and outer layers. This temperature difference causes the inner and outer layers of the brick to shrink inconsistently. The surface layer condenses first, which must be hindered by the non-condensed part of the inner layer, which is quite large. The inner layer puts a pressure on the outer layer, which is called thermal stress. Thermal stress damages the surface layer and causes cracks. The size of thermal stress is generally proportional to the size of the temperature difference.

 

(2) During the pouring process, due to the uneven surface of the casting mold, air stagnation, or impurities, the thickness of the condensation shell grows unevenly. In the thin areas of the condensation shell, thermal stress will concentrate and form the first crack. The cracked part is replenished by the residual liquid, but because the solute of this residual liquid has been concentrated, the condensation shell here is weaker than other parts. The stress caused by the condensation of the melt will be concentrated, resulting in 2nd crack.


 

(3) In fused zirconium corundum bricks, the volume change caused by the deformation of baddeleyite. Baddeleyite changes from a tetragonal strain to a monoclinic strain at about 1050-950°C, with a volume expansion of 9%. Under a high-resolution electron microscope, cracks can be seen in the eutectic of baddeleyite and corundum, and in the corundum. This is the basis for the phase transformation of baddeleyite. It should be noted that there is a difference in temperature between the inner and outer layers of the brick. The outer layer has cooled to 950°C and the phase change has been completed. The inner layer is undergoing phase transformation of baddeleyite, so the inner layer must give the outer layer a significant tensile stress. This stress is the main cause of internal cracks. If this internal crack develops further, the microcracks will be connected. , can also cause large cracks, causing the brick to be scrapped.

4. Ways to avoid cracks:

(1) The process is carefully operated. Cracks can be eliminated or reduced, such as the pouring temperature should not be too high and the pouring speed should not be too fast. Control the cooling rate of the melt to perform reasonable annealing.

(2) Pay attention to outgassing in the solution and use the oxidation method to produce products, which can reduce cracks. In addition, choosing the appropriate brick size can also reduce cracks.

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