Thermal Shock Resistance

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 change resistance, rapid cold and heat resistance and so on. The thermal shock resistance of refractories is the comprehensive performance of their mechanical and thermal properties under the condition of temperature change.
Refractories are often affected by rapid changes in ambient temperature in the process of using them. Such as ladle in Shenggang, pouring and stopping process, converter, electric furnace and other steelmaking feeding, smelting, tapping or shutdown furnace temperature changes, other intermittent high temperature kilns or containers in the batch process, due to rapid changes in temperature, resulting in cracks in lining refractories, spalling or even collapsing. This kind of damage limits the heating and cooling rate of products and kilns, and limits the strengthening of kiln operation, which is one of the main reasons for the damage of kiln refractories.
The factors that affect the thermal shock resistance of refractories are very complex. According to the related theories of thermal shock fracture and thermal shock damage resistance of materials, the mechanical and thermal properties of materials, such as strength, fracture energy, elastic modulus, linear expansion coefficient, thermal conductivity and so on, are the main factors affecting their thermal shock resistance. Generally speaking, the smaller the linear expansion coefficient of refractories, the better the thermal shock resistance. However, the effects of strength, fracture energy and elastic modulus on thermal shock are related to the existence of microcracks and crack propagation.
In addition, the particle composition, density, whether the pores are fine, the distribution of pores and the shape of the products all affect the thermal shock resistance of refractories. There are a certain number of microcracks and pores in the material, which is beneficial to its thermal shock resistance, and the size of the product is large and the structure is complex, which will lead to serious uneven temperature distribution and stress concentration in the material, and reduce the thermal shock resistance.
Based on the analysis of the influencing factors of thermal shock resistance, the following technological measures can be taken to improve the thermal shock resistance of materials:
Selection of raw materials and admixtures: choose raw materials with low linear expansion coefficient and high thermal conductivity as far as possible, and add admixtures with low linear expansion coefficient and high thermal conductivity without affecting other properties of materials;
Microstructure optimization of materials: for example, the second phase or the second material (zirconia) is introduced into the material, and the microcracks are produced by phase transformation to toughen the material.
In the case of meeting the conditions of use, try to make small size, simple shape of the product.
For different kinds of refractories, the detection methods of thermal shock resistance are also different, including water quench method and air quench method.
The black metallurgical industry standard YB/T 376.1 / 1995 specifies a test method for the thermal shock resistance of some fired refractory products by water quench method. The principle is that under the conditions of specified test temperature (1100 ℃), water-cooled medium and a certain time, the thermal shock resistance of refractory products is determined by measuring the damage degree of the heated end face (damage rate of the heated end face) of the samples with a certain shape and size (200 × 230 mm in length, 100 × 150 mm in width and 50~100mm in thickness) under the conditions of specified test temperature (1100 ℃), water-cooled medium and a certain time. Damage rate of heated end face (P break) ) is calculated as follows:
P break = (A2/A1) × 100%
In the formula, P burst = the damage rate of the heated end surface of the test piece,%;
A1-the square number of the heated end surface of the sample in front of the sample, one;
A 2-the number of squares damaged on the heated end face of the sample after the test.
The Black Metallurgical Industry Standard YB/T 376.2 / 1995 sets out a test method for the thermal shock resistance of alkaline refractory products, siliceous products, molten casting products, etc., which cannot be determined by water cooling method. The experimental principle is as follows: when the specified test temperature is (950 ±10) ℃ and the compressed air flow quench medium (0.1MPa) is broken or ejected, the number of rapid cooling and rapid thermal cycles experienced, and the thermal shock resistance of refractory products are determined.
In addition, the metallurgical industry standard YB 4018 / 1991 also specifies the test method for the determination of thermal shock resistance of sintered dense shaped refractory products. The principle is that one side of 230mm × 114mm × 31mm or 230mm × 65mm × 31mm sample 230mm × 31mm is placed on the heating plate of the heating device, and one surface is heated to the test temperature at the specified rate. After holding it for a period of time, it is taken out of the heating device and cooled in the air. The damage degree of thermal shock was evaluated by the retention rate of bending strength before and after thermal shock.
Rr=Ra/Rb
In the formula, the retention rate of Rr= bending strength is%;
Bending strength of specimens after thermal shock of Ra=, MPa;
Bending strength of specimens before Rb= thermal shock, MPa.
The thermal shock resistance of refractory castable can be determined according to the black metallurgical industry standard YB/T 2206.1 (compressed air flow quench method) and YB/T 2206.2 (water quench method), respectively.