What is a reasonable cooling time for a tunnel kiln?
The final strength value and strength uniformity of a fired tile product depend on the phases formed in the product and the cooling rate. This issue often does not attract people’s attention. The influence of the thermal expansion value of each phase in the sintered product on the total thermal expansion of the product is proportional to their presence. Thermal expansion, or rather contraction, builds up stresses inside the product during the cooling phase, and this stress can cause cracks and lead to a loss of strength. The faster the cooling rate, the greater the temperature difference within the product, which will cause greater stress. At any specific temperature difference, the amount of stress formed depends on the thermal expansion of various phases. Sintered brick and tile products are far from homogeneous objects. They may have 4 to 8 independent basic phases, and these phases have an important impact on the final performance of the product.
Due to the diversity and regionality of the raw materials for bricks and tiles, their composition is very complex and no two factories can
It is evident that it can produce products with exactly the same performance, so it is necessary to take into account the differences in the roasted products.
The relationship between the various phases of matter. According to research results in the United States, in a fairly dense two-phase
When the linear thermal expansion coefficient difference between the two phases is 40×10-7cm/cm/℃ or greater, the weaker phase
The phase will crack due to cooling stress. Because the particle sizes of these phases are small,
Such cracks are called microcracks. It has been proven that the difference in thermal expansion coefficient between two compatible phases is 4×10-7cm/cm/℃. From this conclusion, people can understand the importance of whether cracks appear in the relative microstructure of objects that are together with each other. Microcracks are the beginning of complete destruction of the product. When microcracks appear, the strength of the product will be reduced, and the strength values between the products will also be unstable. When the irregular microcracks further expand, the microcracks become macrocracks, and the strength of the entire product is weakened. In the actual production site, if macro cracks (commonly known as cracks) occur due to cooling, the knocking sound will be dull, while those without cracks will have a crisp sound.
In terms of the thermal expansion coefficient of a single substance, the mutual compatibility of each phase in the sintered product can be judged.
For the common green body of mullite + quartz + glass body, it is very difficult to prevent microscopic cracks if it is cooled rapidly. In other aspects, combined green bodies of pseudowollastonite, diopside, corundum and iron mineral phases, such as those made from dolomite clay, can be cooled very quickly without macroscopic cracks. and damage the body strength. The average linear thermal expansion coefficient of quartz is the largest, which is 120×10-7cm/cm/℃. Therefore, the cooling performance of a green body with a high amount of free quartz is very sensitive, which is often referred to as the volume shrinkage during the transformation of quartz crystal form (a at 575℃ The transformation of one quartz into one quartz is a very dangerous stage). Therefore, when using river, river, and lake mud as raw materials, or some raw materials containing sandstone, special attention should be paid to the cooling during the roasting process.