Thermal shock resistance refers to the ability of refractory materials to resist damage caused by rapid changes in temperature. It has been called thermal shock stability, thermal shock resistance, resistance to rapid changes in temperature, resistance to rapid cold and heat.
Determination of thermal shock resistance according to different requirements and product types should be determined in accordance with the corresponding test methods, the main test methods are: ferrous metallurgy standard YB/T 376. 1-1995 refractory products thermal shock resistance test method (water rapid cooling method), ferrous metallurgy standard YB/T 376. 2-1995 refractory 2-1995 Test Method for Thermal Shock Resistance of Refractory Products (Air Rapid Cooling Method), Ferrous Metallurgy Standard YB/T 376. 3-2004 Test Method for Thermal Shock Resistance of Refractory Products Part 3: Water Rapid Cooling - Cracking Determination Method, Ferrous Metallurgy Standard YB/T 2206.1-1998 Test Method for
Thermal Shock Resistance of Refractory Castable Materials (Rapid Cooling Method of Compressed Air Flow), Ferrous Metallurgy Standard YB/T 2206.1-1998 Test Method for Thermal Shock Resistance of Refractory Castables (Compressed Air Flow Rapid Cooling Method), Ferrous Metallurgy Standard YB/T 376. Cold method), ferrous metallurgy standard YB/T 2206.2-1998 refractory castables thermal shock resistance test method (water rapid cooling method).
The mechanical and thermal properties of the material, such as strength, fracture energy, modulus of elasticity, coefficient of linear expansion, thermal conductivity and so on, are the main factors affecting its thermal shock resistance. Generally speaking, the smaller the coefficient of linear expansion of the refractory material, the better the thermal shock resistance; the higher the thermal conductivity (or thermal diffusion coefficient) of the material, the better the thermal shock resistance. In addition, the composition of the refractory particles, the density, the porosity of the microfine, the distribution of the pores, the shape of the product, etc. have an impact on its resistance to thermal shock. The presence of a certain number of micro-cracks and pores in the material is favourable to its thermal shock resistance; the large size and complex structure of the product will lead to serious uneven temperature distribution and stress concentration within the product, which will reduce the thermal shock resistance.
Some studies have shown that the thermal shock stability of refractory materials can be improved by preventing crack expansion, consuming the crack expansion power, increasing the fracture surface energy of the material, reducing the coefficient of linear expansion and increasing the plasticity. The specific technical measures are:
(1) Proper porosity
In addition to the existence of pores, there is a certain amount of fissures between the internal bone grains and bonding phase of the refractory material. Refractory materials in the fracture process, the internal pores and cracks can play a certain role in preventing and inhibiting the fracture extension cracks. Such as as high temperature thermal shock conditions used in refractory materials, in the service process, surface cracks do not cause catastrophic fracture of the material, the cause of its damage is mostly caused by the internal thermal stress caused by the structure of the spalling. When the internal porosity of the material is large, it will shorten the length of the cracks caused by thermal stress and increase the number of cracks. Short and many cracks cross each other to form a mesh structure, which increases the fracture energy needed when the material breaks, and can effectively improve the thermal shock stability of the material. It is generally accepted that when the porosity of the refractory material is controlled at 13%-20%, it has better thermal shock stability.
(2) Control the particle gradation of raw materials, particle critical size and shape
Relevant studies show that the surface energy caused by material fracture and the square of the particle size in the system are positively proportional. Therefore, through the introduction of large particles of aggregate in the material system, so that the cracks in the vicinity of the large aggregate steering, thereby improving the intergranular cracking properties, you can achieve the purpose of improving the thermal shock stability of refractory materials. Generally speaking, the modulus of elasticity of aggregates in refractory materials is significantly larger than that of the matrix, and this difference in modulus of elasticity enables large-grained aggregates to retard the expansion of the original cracks in the material. The larger the difference in modulus of elasticity, the more obvious the role of the aggregate in delaying the expansion of cracks. At the same time, the shape of the aggregate is also an important factor affecting the thermal shock stability of refractory materials. Such as in the material system to add the appropriate amount of rod or flake aggregate can improve the thermal shock stability of refractory products.
(3)Reasonable interface combination
Due to the refractories in the aggregate and matrix properties (such as density, coefficient of thermal expansion, etc.) is generally a large difference between the two combined interface on the expansion of thermal shock cracks, steering and other significant impact. Through the selection and pretreatment of aggregates and other technical measures, the formation of a suitable bonding interface between the aggregate and matrix, the formation of depolymerisation, particle pull-out, micro-cracking and other energy-consuming mechanisms, can inhibit the expansion of thermal shock cracks, so as to achieve the purpose of improving the toughness of refractory materials.
(4) Introducing or generating material phases with small coefficients of linear expansion
By introducing an appropriate amount of materials with low thermal expansion into the matrix, the thermal expansion mismatch within the material is caused, thus generating microcracks in the refractory firing process and hindering the expansion of thermal shock cracks. However, too much of the above microcracks will cause aggregation of microcracks and reduce the mechanical properties of the specimen. Therefore, the addition of low thermal expansion materials should be strictly controlled to obtain refractory products with more balanced thermal shock stability and mechanical properties.
(5) Introducing or generating a certain material phase (e.g., tetragonal ZrO2) so that it undergoes a phase transition at the crack tip to form an energy absorption mechanism.
Through the thermal mismatch of the phases in the material system, a non-catastrophic destructive system is generated within the refractory material and a complex non-linear fracture behaviour occurs, thus improving the thermal shock stability of the refractory products.
(6) Adding and uniformly dispersing fibres or fibrous materials
Through the introduction of fibers, whiskers or in situ formation of whiskers, etc., and to ensure that it is uniformly dispersed in the products, such as the addition of steel fibers in the casting material, etc., will increase the energy required for the fracture of refractory materials and present a significant nonlinear characteristics, thereby improving the toughness of the material.
(7) Add plasticity or viscous component
By adding plastic, viscous components in the refractory system or make products in the calcination process to form a high viscosity liquid phase, the use of their plastic deformation, absorbing the release of elastic strain energy, thereby improving the toughness of refractory products. For example, zircon - zirconia refractory materials in the calcination process, through the decomposition of zircon to form ZrO2 and high viscosity liquid phase SiO2, significantly improve the toughness of refractory materials.
From the above research progress of mullite-based materials and the research overview of thermal shock stability of refractory materials, it can be seen that, at present, the main technical way to improve the thermal shock stability of mullite-based refractory materials is to add SiC and ZrO2, etc., to improve the toughness of the materials through the microcracking and phase transformation, but this will also affect the mechanical strength of the materials.