What are the methods to improve the thermal shock resistance of refractory products?

August 31, 2018

What are the methods to improve the thermal shock resistance of refractory products?

In the course of use, refractory products are often subjected to abrupt changes in the ambient temperature, causing cracks in the product, eventually peeling off or even collapsing. This destructive effect not only limits the heating and cooling rate of products and kilns, but also limits the strengthening of the kiln operation, and is one of the main reasons for the rapid damage of products and kilns.

The property of a refractory product against sharp temperature changes without being destroyed is called thermal shock stability, which is also called thermal shock resistance or temperature sharpness.

Due to the intermittent operation characteristics of converter, strict requirements are imposed on the thermal shock resistance of magnesia-chrome refractory. In order to improve the thermal shock resistance of refractories, some measures can be taken to prevent crack propagation, consume crack propagation power, increase material fracture surface energy, increase plasticity, reduce linear expansion coefficient, and increase thermal conductivity.

(1) Appropriate porosity. Surface cracks do not immediately cause cracking, but are caused by internal thermal stress. When the porosity is properly increased, the crack length of the product becomes short and the numbers increase with the thermal shock. The cracks are interlaced and the degree of mesh formation is enhanced. Therefore, the fracture energy required for the product to break is increased, which can be effectively improved thermal shock resistance of product. The best porosity of refractory products is usually controlled at 13% to 20%.

(2) Control the particle grading of raw materials and select low expansion, high thermal conductivity materials. To obtain a magnesia-chrome refractory with good thermal shock resistance, it is required to increase the critical particle size and reduce the fine powder content in the chrome ore particles. Raw materials with small coefficient of linear expansion and raw materials having a high thermal conductivity such as Cu2O are used.

(3) Increase fine cracks and form a network structure. By utilizing the inconsistent characteristics of the refractory product particles and the linear expansion coefficient of the matrix and the volume effect of the phase change, fine cracks are generated in the product, which has a significant effect on resisting catastrophic damage (hot peeling or fracture) of the products. Tests have shown that increasing the A1203 content in the refractory products or adding the most suitable ZrO2 to the magnesia-chrome refractory can significantly improve thermal shock resistance. Compared with the sample incision, the sample with ZrO2 has a large number of fine cracks inside. It is because of the existence of these fine cracks that the energy of crack propagation is absorbed, which enhances the thermal shock resistance of the sample, but the addition amount should not exceed 5%.

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