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Two main factors cause damage to the cast material of the permanent lining of the ladle. First, repeated use produces excessive cracks. This is caused by the excessive shrinkage of the pouring material itself or the excessive linear shrinkage of the pouring material. The cracks expand after repeated use; when pouring in sections, the pouring material is not integrated at the joints, resulting in horizontal cracks. Cracks and cracks expand during use; the asbestos board near the steel shell has gaps after repeated use, releasing permanent lining stress and expanding cracks. Second, surface damage occurs due to insufficient strength of the casting material when disassembling the can.
A comparative test of different water addition amounts on low-cement high-aluminum permanent lining castables found that:
As the amount of water increases, the porosity of low-cement high-aluminum castables shows an upward trend, and the bulk density shows a downward trend, thereby reducing the compressive strength and flexural strength of the castables.
Through multiple comparative tests, it can be seen that as the amount of water added increases, the shrinkage rate of the castable material line increases significantly, which will expand the shrinkage cracks produced during the use of the castable material. When the water content of low cement castable increases by 2%, obvious shrinkage cracks will appear on the surface of the sample. In addition, as the amount of water added increases, the castable is prone to particle segregation, that is, particle sinking and powder floating. The shrinkage of the parts with more powder increases, which makes the internal stress of the castable uneven and prone to cracks.
Therefore, there are some small methods to improve the performance of the permanent layer castable of the ladle:
1. Control the amount of water added to the castable
The more water is added, the lower the strength and density of the cast material, with the consequent increase in porosity and post-burning shrinkage. Therefore, on the premise of ensuring its construction and operability, the water consumption of the pouring materials should be reduced as much as possible to ensure the best performance of the pouring materials.
2. Effectively ensure the connection of construction time
At present, the casting of permanent lining casting materials in steel plants uses bricks instead of molds and casts intermittently in sections. Generally speaking, the entire brick mold is divided into three parts for masonry. After the first section is built, casting begins, then the second section is built, then casting continues, and so on. High ambient temperature in summer or failure to cast in time due to special reasons can easily lead to the initial solidification of the first section material during the second section casting, resulting in poor connection between the two sections, resulting in horizontal delamination and horizontal cracks at the joints after use. Therefore, connection work should be done well during the construction process to reduce the interval between the two casting sections.
3. Develop a reasonable baking system
Low cement castables gain strength through hydration and cohesion bonding. Low-cement castables have strict requirements for baking, especially in the discharge stage of large amounts of physical water and crystallization water below 300°C. If baked improperly, it can easily peel and crack. A reasonable baking curve must be developed to allow the moisture in the molding body to be discharged smoothly. Depending on the thickness of the construction, the pouring will be cured naturally for 16 to 24 hours, and then the mold will be baked for about 3 days. Remove the brick form, complete the working lining, and bake for 16 more hours.
When there is no temperature control device, adjust the gas volume to control the temperature. It is generally baked over low, medium, and high heat. The maintenance and baking time is adjusted according to the size of the ladle and the thickness of the permanent layer, generally 60 to 80 hours. Usually, the three baking times are 24 hours, 24~32h, and 12~16h.
4. Optimize the formula of permanent lining ingredients
To solve the problem of excessive cracks, the castable formula was optimized and improved:
A low impurity expansion agent is added to the matrix to offset some of the shrinkage of the cast material at high temperatures. The pouring material with an expansion agent added improves the volume stability of the pouring material and improves the expansion of cracks after repeated use.
Increase the upper limit of the limit particle size of castable aggregate from the original 8mm to 12mm. After testing, after introducing large particles, the amount of water added can be reduced by 0.5%, and the flexural strength at 1100°C can be increased by 2mpa. This is because the presence of large particles can not only act as a skeleton but also change the direction of long cracks, helping to prevent crack expansion and improve the volume stability of the castable.
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