How many times can refractory bricks withstand rapid cooling and rapid heating, and how to strengthen thermal shock performance?

The thermal shock resistance of refractory bricks is the ability of refractory bricks to resist damage under the condition of rapid changes in temperature. Once known as thermal stability, thermal shock stability, thermal shock resistance, temperature change resistance, rapid cooling and heat resistance, etc., refractory bricks are brittle materials at low and medium temperatures and lack ductility. In the use of thermal equipment, refractory bricks are often subjected to rapid temperature changes, resulting in damage. Thermal shock resistance is one of the important properties of refractory bricks.

The thermal shock resistance of refractory bricks is the comprehensive performance of their mechanical properties and thermal properties under the condition of temperature changes. The rapid temperature change suffered by refractory bricks is called thermal shock. The new cracks generated by refractory bricks in the thermal shock center, as well as the cracking, spalling, and fracture caused by the new cracks and the original crack propagation, are called thermal shock damage of refractory bricks. Thermal shock damage is the result of thermal stress. When the temperature changes, the stress generated by the inhibition of the deformation of refractory bricks is thermal stress. If the change of the refiring line of the refractory brick is different when the temperature changes, the thermal expansion of the refractory brick will produce thermal stress if it is subjected to the uniform action of the temperature layer and the phase transition. The thermal stress is proportional to the elastic modulus and elastic strain of the refractory brick, and the elastic modulus strain of the refractory brick is equal to the product of the linear expansion coefficient and the temperature change.

The thermal shock resistance of refractory bricks refers to the resistance of the product to damage caused by rapid changes in temperature. Also known as thermal shock stability or resistance to quenching and quenching. During the use of refractory bricks, they are often subjected to strong quenching and quenching effects. For example, the temperature of the lining will change greatly in the kiln produced by gaps, kiln ignition, cease-fire and kiln maintenance. Due to the poor thermal conductivity of refractory bricks, stress will occur inside the product due to rapid changes in temperature. When this stress exceeds the structural strength of the product, cracking, peeling, and even masonry cracking will occur. The magnitude of this stress mainly depends on factors such as the organizational structure, thermal expansion, thermal conductivity and elastic modulus of the product.

1. The organizational structure of refractory bricks increases the number of critical particles and coarse particles of refractory bricks, which can significantly improve the thermal shock stability of most refractory bricks. Because there are small cracks and pores around the large particles, local bonds are formed in these parts. When thermal stress is generated in the product, the particles that are not tightly fixed will slide slightly with each other without cracking! Local elimination of some stress. On the contrary, dense refractory bricks composed of fine particles are not conducive to improving their thermal shock stability.

2. Thermal expansion When the temperature suddenly changes, there is a temperature difference between the surface and the interior of the refractory brick. If the thermal expansion coefficient of the product is large, the thermal stress caused by the temperature difference is also large, and its thermal shock stability is correspondingly poor.

3. Thermal conductivity, the stronger the thermal conductivity, that is, the greater the thermal conductivity, the smaller the temperature difference between inside and outside, the smaller the temperature difference stress, and the stronger the thermal shock stability of the refractory brick.

4. Elastic properties, the better the elasticity of refractory bricks, that is, the smaller the elastic modulus, the stronger the ability to buffer thermal stress. The thermal shock stability of the product is also better.

5. Structural strength The greater the structural strength of refractory bricks, the stronger the ability to resist thermal stress, and the better the thermal shock stability of the products.

Thermal shock test method for refractory bricks

(1) Heating-cooling method. After a certain size of the sample is directly placed in the furnace that has reached the specified temperature for a specified time, it is quickly removed from the furnace and quenched in a medium such as water or air. Repeat the above process until the specified number of thermal shock cycles is reached, observe the damage of the sample, or measure the retention rate of flexural strength before and after thermal shock to judge the thermal shock resistance of the material.

(2) Panel method. The panel method is to lay the refractory brick sample on the furnace wall or furnace door, and let it carry out heating and cooling cycles when one side is heated. In my country's current standard, the refractory brick sample is masonry on the test furnace door. After the furnace door is closed, one end of the sample is heated in the furnace. After the specified time is reached, the furnace door is turned over, and the hot refractory brick end is inserted into cold water for cooling. After repeated several times, the thermal shock resistance of the refractory brick is measured by the breakage rate of the heated end area. In the actual measurement process, the determination shall be carried out according to the procedures specified in the relevant standards. ( 3) The strip method. The strip-shaped sample is placed on the bracket, and there are gas burners and blowing nozzles under the heating surface of the sample. After heating the sample with gas for a specified time, use the blowing device to blow and cool for a period of time. After repeated several times according to the regulations, the retention rate of the flexural strength or elastic modulus of the sample before and after thermal shock is measured to measure its thermal shock resistance. Compared with the general heating-cooling method, both of them use elongated samples, the difference is that they are heated on one side in the strip method, and there is a certain temperature layer in the sample during the heating and cooling process.

In addition, the shape and size of the refractory brick, the structure of the furnace body and the masonry method all have a certain influence on the thermal shock stability of the product. The thermal shock resistance of previously sintered refractory bricks was determined according to the provisions of the Ministry of Metallurgy Standard YB376--75. It is measured by the number of cycles in which the sample is subjected to heating and water cooling. The thermal shock resistance of several types of refractory bricks is shown in the table. Since January 1, 1992, the determination of thermal shock resistance of fired dense shaped refractory bricks has been carried out according to the provisions of YB4018-9l. Name Thermal shock resistance test number Silica brick 1~ 2 Clay brick (coarse particles) 25~ 100 Clay brick (fine particles) 5~ 8 Ordinary clay brick 10~ 12 Ordinary magnesia brick 2~ 3 Magnesia aluminum brick 50~ 150 High aluminum brick 6~ 15.

How to improve the thermal shock resistance of refractory bricks?

The main factor affecting the thermal shock resistance of refractory products is the thermal stress caused by thermal expansion and contraction during heating or cooling. Generally speaking, the greater the thermal expansion rate, the worse the thermal shock resistance of materials, such as silicon bricks, magnesia bricks, etc.; the greater the thermal conductivity, the better the thermal shock resistance of materials, such as silicon carbide products.

Starting from the theory of thermal elasticity, the smaller the elastic modulus of the material, the greater the strength, the greater the thermal conductivity, and the better the thermal shock resistance of the product. The energy theory believes that when the product has a higher fracture surface energy, the thermal shock resistance of the product can be improved. That is to say, when the product has small pores, so that the product generates large internal stress when the temperature changes, and stores more internal energy, fine cracks can be generated through the product, and these may cause damage to the product. When the energy is released, the thermal shock resistance of the product can be greatly improved, that is, micro-cracks are intentionally introduced into the product, so that the degree of crack propagation is reduced to very small, which is one of the ways to improve the thermal shock resistance of the material.

For example, anti-spalling high-alumina bricks for cement kilns are used because a small amount of ZrO2 is added to the ingredients of high-alumina bricks, and the phase transformation of ZrO2 is used to form many tiny cracks in the products. When thermal stress is generated by temperature changes, these tiny cracks It may lead to the release of energy from the damage of the refractory material, thereby improving the thermal shock resistance of the refractory brick.