Thermal shock resistance refers to the ability of refractories to resist damage caused by rapid temperature changes. It was once called thermal shock stability, thermal shock resistance, resistance to temperature sudden change, and resistance to rapid cooling and heat. Magnesite manufacturer
The thermal shock resistance shall be measured according to the corresponding test methods according to different requirements and product types. The main test methods are: ferrous metallurgy standard YB/T 376 1-1995 Test method for thermal shock resistance of refractory products (water quenching method), ferrous metallurgy standard YB/T 376 2-1995 Test method for thermal shock resistance of refractory products (air quenching method), ferrous metallurgy standard YB/T 376 3-2004 Test method for thermal shock resistance of refractory products Part 3: Water quenching crack determination method, ferrous metallurgy standard YB/T 2206.1-1998 Test method for thermal shock resistance of refractory castables (compressed air flow quenching method), ferrous metallurgy standard YB/T 2206 2-1998 Test method for thermal shock resistance of refractory castables (water quenching method).
The mechanical properties and thermal properties of materials, such as strength, fracture energy, elastic modulus, linear expansion coefficient, thermal conductivity, are the main factors affecting their thermal shock resistance. Generally speaking, the smaller the linear expansion coefficient of refractory, 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 thermal shock resistance of refractory is affected by its particle composition, density, whether the pores are fine, the distribution of pores, and the shape of products. There are a number of microcracks and pores in the material, which is conducive to its thermal shock resistance; The large size and complex structure of the product will lead to serious uneven temperature distribution and stress concentration inside, reducing the thermal shock resistance.
Some studies have shown that the thermal shock stability of refractories can be improved by preventing crack growth, consuming crack growth power, increasing fracture surface energy, reducing linear expansion coefficient and increasing plasticity. Specific technical measures are:
(1) Appropriate porosity
In addition to pores, there are also some cracks between the bone particles and the bonding phase in the refractory. During the fracture process of refractory, the internal pores and cracks can prevent and restrain the fracture propagation cracks. For example, as a refractory used under the condition of high temperature thermal shock, the surface crack will not cause catastrophic fracture of the material during service, and its damage is mostly caused by structural spalling caused by 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 for material fracture, and can effectively improve the thermal shock stability of materials. 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
Relevant studies show that the surface energy caused by material fracture is in direct proportion to the square of particle size in the system. Therefore, the purpose of improving the thermal shock stability of refractory can be achieved by introducing large particle aggregate into the material system to make the crack turn around near the large aggregate, thus improving the intergranular crack performance. In general, the elastic modulus of the aggregate in the refractory is significantly greater than that of the matrix. This difference in elastic modulus enables the large particle aggregate to delay the expansion of the original crack of the material. The greater the difference of the above elastic modulus, the more obvious the effect of aggregate on delaying crack growth. At the same time, the shape of aggregate is also an important factor affecting the thermal shock stability of refractories. For example, the thermal shock stability of refractory products can be improved by adding an appropriate amount of rod or sheet aggregate in the material system.
(3) Reasonable interface combination
Because the properties (such as density, coefficient of thermal expansion, etc.) of aggregate and matrix in refractories are generally different, the bonding interface between the two has a significant impact on the propagation and turning of thermal shock cracks. By selecting and pretreating aggregates and other technical measures, an appropriate bonding interface is formed between aggregates and matrix, and energy consumption mechanisms such as depolymerization, particle pull-out, and micro cracking are formed, which can inhibit the expansion of thermal shock cracks, so as to improve the toughness of refractories.
(4) Introduce or generate phase with small linear expansion coefficient
By introducing a proper amount of materials with low thermal expansion into the matrix, the internal thermal expansion of the materials does not match, which results in microcracks during the firing process of refractories and hinders the expansion of thermal shock cracks. However, too many of these microcracks will cause the polymerization of microcracks and reduce the mechanical properties of the sample. Therefore, the addition of low thermal expansion materials should be strictly controlled to obtain refractory products with balanced thermal shock stability and mechanical properties.
(5) Introduce or generate a certain phase (such as tetragonal ZrO2) to make it phase change in the crack and form an energy absorption mechanism.
Through the thermal mismatch of each phase in the material system, a non catastrophic failure system will occur inside the refractory, and complex nonlinear fracture behavior will occur, thus improving the thermal shock stability of refractory products.
(6) Add and evenly disperse fibers or fibrous substances
By introducing fibrous substances, whiskers or whiskers formed in situ, and uniformly dispersing them in the products, such as adding steel fibers in castables, the energy required for fracture of refractories will increase and show significant nonlinear characteristics, thus improving the toughness of materials.
(7) Add plastic or viscous components
The toughness of refractory products can be improved by adding plastic and viscous components in the refractory system or making the products form high viscosity liquid phase in the process of calcination. For example, during the calcination of zircon zirconia refractories, ZrO2 and high viscosity liquid SiO2 are formed through the decomposition of zircon, which significantly improves the toughness of refractories.
It can be seen from the above research progress of mullite materials and the research overview of thermal shock stability of refractories that at present, the main technical way to improve the thermal shock stability of mullite refractories is to add SiC and ZrO2, and improve the toughness of materials through microcracks and phase transformation, but this will also affect the mechanical strength of materials.
