High alumina bricks are a type of silicon-alumina series refractory bricks and are also neutral refractory materials. And the levels of high-alumina refractory bricks are also different. From high to low, they are: LZ-80, LZ-75, LZ-65, LZ-55, and LZ-48. The production process of high-alumina refractory bricks requires high-pressure molding and high-temperature sintering. The control of production details can often determine the quality of its performance.
So, what causes cracks in high-alumina refractory bricks? Today I will share with you the causes of cracks in high alumina refractory bricks.
1: Reasons for raw materials
The quality of raw materials is an important factor affecting the occurrence of cracks in high alumina bricks. If the raw materials contain too many impurities, especially impurities such as K2O and Na2O, they will cause uneven stress distribution within the brick during the sintering process, leading to cracks. In addition, the particle size of the raw materials and the amount of fine powder added are also key factors affecting the quality of the bricks. Uneven particle size distribution or improper addition of fine powder will cause inconsistent shrinkage of the bricks during the sintering process, thereby causing cracks.
Secondly, during the sintering process, if the temperature rises too fast or the drying process is improper, the moisture inside high alumina bricks will evaporate unevenly, resulting in a situation where the dehydration rate of the edges is greater than that of the middle. At this time, the water at the edges evaporates too quickly, while the water in the middle evaporates more slowly, causing the edges of the brick to shrink too quickly, resulting in cracks. In addition, the control of sintering temperature is also crucial. If the temperature is too high or too low, the internal structure of the brick will be unstable and cracks will occur.
In addition, factors such as the design of the forming mold, uniformity of cloth, operating skills, and forming pressure will all affect the quality of the bricks. If the mold design is unreasonable and there is a lack of vent grooves or vent holes, the internal gases in the brick body cannot be discharged during the molding process, resulting in cracks during the sintering process. At the same time, uneven distribution or improper operation will also lead to uneven stress distribution within the brick body, resulting in cracks.
2: Reasons for forming
First of all, the design of the forming mold has a decisive impact on the quality of high alumina brick. For complex molds with holes, tongues, grooves or corners, the structural design must be reasonable to avoid cracks caused by structural problems in the mold during the molding process. If there are many tongues and grooves and the multi-mold upper surface is difficult to form, it needs to be fully considered and optimized during mold design.
In addition, the exhaust system of the mold is also key. If the mold design lacks vent grooves and vent holes, the internal gases of the bricks will not be discharged smoothly during molding, resulting in cracks after the mold is released. Therefore, it is necessary to ensure a perfect exhaust system when designing the mold to avoid crack problems caused by gas retention.
Distribution uniformity is also an important factor affecting the quality of high alumina bricks. Uneven distribution will lead to inconsistent density within the brick body, which will cause stress concentration during the molding process and cause cracks. Therefore, it is necessary to strictly control the uniformity of raw materials during the distribution process to ensure that the density of each part of the brick body is consistent.
Operating skills and pressure control are also key aspects of the molding process. Improper operation or improper pressure will cause uneven force on the brick body, resulting in cracks. Therefore, operators need to have certain professional skills and experience to accurately control various parameters during the molding process to ensure that the quality of high alumina bricks is stable and reliable.
Finally, the force of each hammer during molding also needs to be precisely controlled. Excessive hammering will cause the gas inside the brick to be unable to be completely discharged, resulting in spalling and transverse cracks. Therefore, it is necessary to strictly control the hammering intensity and frequency during the molding process to ensure that the gas inside the brick can be smoothly discharged and avoid the occurrence of cracks.
Three: Reasons for burning
First of all, the heating rate is one of the key factors affecting the quality of high alumina bricks. If the temperature rises too fast, the temperature gradient between the inside and outside of the brick will increase, resulting in uneven stress distribution and cracks. Therefore, during the firing process, the heating rate needs to be strictly controlled to ensure that the bricks are heated evenly.
Secondly, atmospheric conditions also have a significant impact on the quality of bricks. During the firing process, the oxidizing or reducing nature of the atmosphere will directly affect the valence state and phase change process of the elements inside the brick, thus affecting its physical properties and chemical stability. If the atmosphere conditions are not appropriate, defects or incomplete phase transformation may occur inside the bricks, leading to cracks.
In addition, the shrinkage of the green body during the firing process is also one of the important reasons for cracks. Due to the complex composition of high alumina bricks, there are differences in thermal expansion coefficients and shrinkage rates between each component, so uneven shrinkage will occur during the firing process. If the shrinkage is too large or uneven, it will cause cracks on the surface of the brick.
Finally, secondary mullite-corundum recrystallization is an important stage in the firing process. During this stage, the mineral phases inside the brick undergo transformation and recrystallization, resulting in a volume change. If this process is not controlled properly, cracks can occur.
In order to reduce the risk of cracks in high alumina bricks during the firing process, optimization and improvement need to be made from multiple aspects such as raw material selection, formula design, and process control.
