The quality of fused cast AZS block is related to many factors such as the chemical composition of raw materials, the melting process, the casting process and the annealing process. One of the main quality problems of the fused cast AZS block is cracking.
Fused cast AZS block is mainly composed of baddeleyite, corundum, and a small amount of mullite, glass phase and pores. When the Al2O3 content of the raw materials exceeds 30%, the raw materials is difficult to melt and the cracking rate will increase. So before feeding the raw materials into the furnaces, the Al2O3 content must be measured to ensure it does not exceed 30%.
The way of feeding will directly affect the melting. Once feeding all the raw materials may cause incomplete melting and the products will be easy to crack. Feeding small quantities repeatedly is better.
If the melting temperature is too high or low, the cracking rate will increase too. Melting the raw materials at the melting temperature and then increasing the temperature can reduce the cracking rate.
In order to reduce the residual thermal stress and avoid crack, there should be a certain amount of glass phase in the fused cast AZS to absorb the thermal stress. However, the glass phase is the weak point for the corrosion resistance of fused cast AZS. During the corrosion process, the glass phase is exuded first and replaced with the high temperature molten glass. Bubbles are released accompanying the exudation of the glass phase and pollute the glass. Fused cast AZS block (33#, 36# and 41#) contains 14-20% glass phase, thus the glass phase content has a big effect on its quality.
During the casting process, if the casting temperature is too high, the shrinkage rate and the temperature difference between the inside and outside increase and the products are easy to crack. Before casting, stop heating for 3-5 minutes to cool the molten glass to the casting temperature and then cast, which can greatly reduce the cracking rate.
During the annealing and cooling process, the casting products should be put in the insulating box. The initial temperature should not be too low. Then cool the products slowly. The crack rate can be reduced.
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