The Causes of Cracks in the Fused Cast AZS Block

Fused cast AZS block is characterized by a narrow range of the crystallization temperature and low thermal conductivity, so its hardening process is actually continuous.

The continuous hardening of the melt of AZS block will cause uneven temperature along its section. Inner and outer cracks will appear. During the hardening of the melt, the crack will be generated in the internal of the brick and then in the outer surface. 

 

In the first stage of cooling, fused cast AZS has flexible outer shell. During the second stage, the entire melt is hard but temperature difference is generated on the section of bricks. There is no stress in it, since the plastic core can buffer the stress caused by the shrinkage of the shell. After the bricks are further hardened, the bricks are entirely hard. 

 

Due to the insulation effect of the insulation box, the temperature in the core of bricks drops more quickly than that in the surface. The temperature difference becomes smaller and smaller. Since the temperature change in the core is bigger than that in the surface, the core should withstand stress when maintaining its length. When the stress is beyond its maximum strength at that temperature, cracks will be generated in the internal of the bricks or the outer surface of the bricks. Those cracks are called hot cracks. 

 

Besides, fused cast AZS block will have big volume change during the crystalline transformation, especially at 1008℃ when the cubic crystals are transformed into baddeleyite. 

 

Therefore, the two above factors must be taken into consideration when choosing annealing method to avoid cracks.