thermal shock resistanceRefers to the ability of refractory materials to resist damage caused by rapid changes in temperature. It was once called thermal shock stability, thermal shock resistance, temperature resistance, and rapid heat resistance.
The determination of thermal shock resistance shall be determined according to the corresponding test methods according to different requirements and product types,Main test methodsThere are: ferrous metallurgy standard YB/T 376. 1-1995 refractory products thermal shock resistance test method (water quenching method), ferrous metallurgy standard YB/T 376-1995 refractory products thermal shock resistance test method (air quenching method), ferrous metallurgy standard YB/T 376-2004 refractory products thermal shock resistance test method part 3: water quenching-crack determination method, ferrous metallurgy standard YB/T 2206.1-1998 refractory castable thermal shock resistance test method (compressed air flow quenching method), ferrous metallurgy standard YB/T 2206. 2-1998 refractory castable thermal shock resistance test method (water quenching method).
The mechanical and thermal properties of materials, such as strength, fracture energy, elastic modulus, linear expansion coefficient and thermal conductivity, are the main factors affecting their thermal shock resistance. In general, the linear expansion coefficient of refractory materials is small, the better the thermal shock resistance; the thermal conductivity (or thermal diffusion coefficient) of the material is high, the better the thermal shock resistance. In addition, the particle composition, density, pore refinement, pore distribution, product shape, etc. of the refractory material have an impact on its thermal shock resistance. There are a certain number of micro cracks and pores in the material, which is conducive to its thermal shock resistance, and the large size and complex structure of the product will lead to serious uneven temperature distribution and stress concentration, reducing the thermal shock resistance.
Studies have shown that the thermal shock stability of refractory materials can be improved by preventing crack propagation, consuming the power of crack propagation, increasing the fracture surface energy of the material, reducing the linear expansion coefficient and increasing the plasticity. Specific technical measures are:
(1) Appropriate porosity
In addition to the presence of pores, there is a certain amount of cracks between the bone particles and the binding phase inside the refractory. In the fracture process of refractory materials, the internal pores and cracks can prevent and inhibit the fracture propagation crack. As a refractory material used under high temperature thermal shock conditions, in the service process, the surface crack does not cause catastrophic fracture of the material, and the cause of the damage is the structural spalling caused by the internal thermal stress. When the internal porosity of 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 when the material breaks, and can effectively improve the thermal shock stability of the material. It is generally believed that when the porosity of the refractory material is controlled at 13%-20%, it has better thermal shock stability.
(2) Control the particle gradation, critical particle size and shape of raw materials
Related studies have shown that the surface energy caused by material fracture is proportional to the square of the particle size in the system. Therefore, by introducing large particle aggregates into the material system, the cracks are diverted near the large aggregates, thereby improving the performance of intergranular cracks, and the purpose of improving the thermal shock stability of refractories can be achieved. Generally speaking, the elastic modulus of the aggregate in the refractory material is significantly greater than that of the matrix, and the difference in the elastic modulus makes the large particle aggregate delay the expansion of the original crack of the material. The greater the difference in elastic modulus, the more obvious the effect of aggregate on delaying crack propagation. At the same time, the shape of the aggregate is also an important factor affecting the thermal shock stability of the refractory. The thermal shock stability of refractory products can be improved by adding a suitable amount of rod or flake aggregate to the material system.
(3) interface with reasonable
Because the properties of the aggregate and the matrix in the refractory material (such as density, thermal expansion coefficient, etc.) are generally different, the bonding interface between the two has a significant effect on the expansion and steering of the thermal shock crack. Through technical measures such as selection and pretreatment of aggregates, a suitable bonding interface is formed between the aggregate and the matrix, and energy dissipation mechanisms such as depolymerization, particle pull-out, and micro-cracking are formed, which can inhibit the expansion of thermal shock cracks and improve the toughness of refractory materials.
(4) the introduction or generation of linear expansion coefficient of small phase
By introducing an appropriate amount of low thermal expansion material into the matrix, the thermal expansion mismatch within the material is caused, resulting in microcracks during the firing process of the refractory material, which hinders the propagation of thermal shock cracks. However, too many microcracks will cause the polymerization of microcracks and reduce the mechanical properties of the sample. Therefore, it is necessary to strictly control the addition of low thermal expansion materials to obtain refractory products with more balanced thermal shock stability and mechanical properties.
(5) the introduction or generation of a certain phase (such as tetragonal ZrO2), so that the crack tip phase change, the formation of energy absorption mechanism.
Through the thermal mismatch of each phase in the material system, a non-catastrophic failure system and complex nonlinear fracture behavior occur inside the refractory material, thereby improving the thermal shock stability of the refractory product.
(6) Add and evenly disperse fibers or fibrous materials
By introducing fibers, whiskers or whiskers formed in situ, and ensuring that they are evenly dispersed in the product, such as adding steel fibers to the castable, the energy required for fracture of the refractory material will increase and show significant nonlinear characteristics, thereby improving the toughness of the material.
(7) Add plastic or viscous components
By adding plastic and viscous components to the refractory system or making the products form a high-viscosity liquid phase during calcination, their plastic deformation is used to absorb the release of elastic strain energy, thereby improving the toughness of refractory products. Such as zircon-zirconia refractory material in the calcination process, through the decomposition of zircon to form ZrO2 and high viscosity of the liquid phase SiO2, significantly improve the toughness of the refractory material.
From the above research progress of mullite materials and the thermal shock stability of refractories, it can be seen that at present, the main technical way to improve the thermal shock stability of mullite refractories is to add SiC and ZrO2, etc., through microcracking and phase transformation to improve the toughness of the material, but this will also affect the mechanical strength of the material.