Thermal shock resistance refers to the ability of refractories to resist damage caused by rapid changes in temperature. It has been called thermal shock stability, thermal shock resistance, temperature resistance, cold resistance and heat resistance. Magnesia carbon brick
The determination of thermal shock resistance according to the different requirements and product type should be in accordance with the corresponding test methods for determination of main testing methods are: black metallurgical standard YB/T 376. 1-1995 thermal shock resistance test method for refractory products (water quenching method), ferrous metallurgical standard YB/T 376. 2-1995 thermal shock resistance test method for refractory products (air quenching method), ferrous metallurgical standard YB/T 376. 3-2004 thermal shock resistance test method for refractory products part 3: Test method for thermal shock resistance of refractory castables in ferrous metallurgy standard YB/T 2206.1 -- 1998 (rapid cooling with compressed air), test method for thermal shock resistance of refractory castables in ferrous metallurgy standard YB/T 2206.2 -- 1998 (rapid cooling with water).
The mechanical and thermal properties of materials, such as strength, fracture energy, elastic modulus, coefficient of linear expansion and thermal conductivity, are the main factors affecting the thermal shock resistance of materials. Generally speaking, the linear expansion coefficient of refractory materials is small, 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 refractories, the density, whether the pores are fine, the distribution of pores, the shape of the products and so on have an effect on the thermal shock resistance of refractories. There are a certain number of microcracks and pores in the material, which are beneficial to its thermal shock resistance. The large size and complex structure of the products will lead to serious uneven temperature distribution and stress concentration in the internal, and reduce the thermal shock resistance.
Studies have shown that the thermal shock stability of refractories can be improved by preventing crack propagation, consuming crack propagation power, increasing fracture surface energy, reducing linear expansion coefficient and increasing plasticity. Specific technical measures are:
(1) Appropriate porosity
In addition to the existence of pores, there are some cracks between the bone grains and the bonding phase in the refractories. In the process of refractories fracture, the internal pores and cracks can prevent and inhibit the crack propagation. For example, when the refractory material is used under the condition of high temperature thermal shock, the surface crack will not cause the catastrophic fracture of the material in the service process, and the reason of its damage is mostly the structure spalling caused by the internal thermal stress. When the porosity in the material is large, the crack length caused by thermal stress will be shortened, and the number of cracks will be increased. Short and many cracks cross each other to form a network structure, which increases the fracture energy required by the material fracture, and can effectively improve the thermal shock stability of the material. It is generally believed that when the porosity of refractories is controlled at 13%-20%, they have better thermal shock stability.
(2) Control the particle grading, critical particle size and shape of raw materials
Related studies show that the surface energy caused by material fracture is positively proportional to the square of the particle size in the system. Therefore, the thermal shock stability of refractory materials can be improved by introducing large particles of aggregate into the material system to make the cracks turn around the large aggregate, thus improving the intergranular crack performance. Generally speaking, the elastic modulus of the aggregate in refractory materials is significantly greater than that of the substrate, and this difference in elastic modulus makes the larger aggregate slow the growth of the original cracks in the material. The larger the difference of the elastic modulus, the more obvious the effect of the aggregate on retarding the crack propagation. At the same time, the shape of aggregate is also an important factor affecting the thermal shock stability of refractories. The thermal shock stability of refractory products can be improved by adding a proper amount of rod or sheet aggregate into the material system.
(3) Reasonable interface combination
Since the properties (such as density, thermal expansion coefficient, etc.) of aggregate and matrix in refractories differ greatly, the interface between them has a significant effect on the thermal shock crack propagation and steering. Through the selection and pretreatment of aggregate and other technical measures, a suitable interface between aggregate and matrix is formed, and energy dissipation mechanisms such as depolymerization, particle pull-out and microscopic cracking are formed, which can restrain the crack propagation of thermal shock, thus achieving the purpose of improving the toughness of refractory materials.
(4) The phase with small linear expansion coefficient is introduced or generated
By introducing proper amount of materials with low thermal expansion into the matrix, the mismatch of thermal expansion in the material can be caused, so that the micro-cracks can be generated during the firing process of the refractory and the propagation of thermal shock cracks can be hindered. However, too many of the above microcracks will cause the aggregation of microcracks and reduce the mechanical properties of the sample. Therefore, the addition amount of low thermal expansion material should be strictly controlled in order to obtain the refractory products with more balanced thermal shock stability and mechanical properties.
(5) Some phase (such as tetragonal ZrO2) is introduced or generated to make it undergo phase transition at the crack tip to form an energy absorption mechanism.
Through the thermal mismatching of each phase in the material system, non-catastrophic failure system and complex nonlinear fracture behavior occur in the refractories, so as to improve the thermal shock stability of refractories.
(6) Add and evenly disperse fiber or fibrous material
By introducing fibrous whiskers, whiskers or in-situ whiskers, and ensuring their uniform dispersion in the product, such as adding steel fibers into castable, the fracture energy of refractories will be increased and show significant nonlinear characteristics, thus improving the toughness of the material.
(7) Add plastic or viscous components
The toughness of refractory products can be improved by adding plastic and viscous components or forming liquid phase with high viscosity in the process of calcination, using their plastic deformation to absorb the release of elastic strain energy. For example, zirconite - zirconia refractories in the calcination process, through the decomposition of zirconite to form ZrO2 and high viscosity liquid SiO2, significantly improve the toughness of refractories.
According to the research progress of mullite materials and the thermal shock stability of refractories mentioned above, the main technological approaches to improve the thermal shock stability of mullite refractories are to add SiC and ZrO2, etc., and to improve the toughness of the material through microcrack and phase transformation, which will also affect the mechanical strength of the material
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