How to choose refractory castables? Load softening point, Al₂O₃ content, thermal shock resistance, o
2025-09-23 10:45:19
I. Load softening point temperature:
1. Definition: It refers to the temperature at which the refractory castable undergoes 4% compression deformation (or deformation of 4mm) as the temperature rises under a static load of 0.2MPa. It is a key parameter to measure the material's ability to resist load and maintain structural stability in a high-temperature environment.
2. Specific reference value:
(1) Match the temperature and load conditions of the use environment. The operating temperature and internal material load of different industrial furnaces vary significantly. For example, the hot blast furnace in the steel industry is at 1200-1400℃ for a long time and bears the weight of the furnace charge. When ordering, it is necessary to ensure that the material's load softening point is 10%-20% higher than the actual operating temperature to reserve a safety margin to avoid the collapse of the furnace structure due to plastic deformation at high temperature.
(2) Predict the high-temperature structural stability of the material. Materials with low load softening points are prone to shrinkage or deformation when approaching the critical temperature, resulting in cracking of brick joints, peeling of the lining, and shortening the life of the kiln. For example, if a castable with a load softening point of 1500℃ is used in the transition zone of a cement rotary kiln, and the actual operating temperature often reaches 1450℃, it is easy to cause bulging and deformation due to long-term proximity to the critical value, increasing the frequency of maintenance.
2. Al₂O₃ content:
1. Definition: Aluminum oxide (Al₂O₃) is a common high-refractory component in refractory castables. Its content is the core basis for distinguishing the material grade of castables (such as clay, high-aluminum, corundum, etc.), and directly reflects the matrix characteristics of the material.
2. Specific reference value:
(1) Relevant to the refractoriness and high-temperature strength of the material. Al₂O₃ has a melting point of up to 2050℃ and has strong chemical stability. Generally speaking, the higher the content, the higher the refractoriness of the material (e.g., Al₂O₃ content of corundum castables ≥90% can reach a refractoriness of over 2000℃; Al₂O₃ content of clay castables is about 30%-45%, and the refractoriness is mostly between 1600-1750℃). At the same time, the high-temperature compressive strength is also improved, making it suitable for areas that are severely eroded by materials (e.g., the tapping port of a steelmaking converter).
(2) Guide the judgment of corrosion resistance. In different atmospheres, the Al₂O₃ content affects the chemical stability of the material. For example, in the flue gas environment of a coal-fired boiler (containing acidic gases such as SO₂), high Al₂O₃ castables are more resistant to corrosion than siliceous materials; because the flue gas of a waste incinerator is complex, aluminate castables with 60%-80% Al₂O₃ are often used.
(3) Balance cost and performance. The higher the Al₂O₃ content, the higher the raw material cost (such as corundum sand, brown corundum, etc.). For example, the price of a corundum castable with 90% Al₂O₃ is 2-3 times that of a high-aluminum castable with 60% Al₂O₃. When ordering, avoid "blindly chasing high prices". For example, for low-temperature kilns (such as ceramic roller kilns, operating temperature ≤1200℃), clay castables with 40%-50% Al₂O₃ can meet the needs.
III. Thermal shock stability:
1. Definition: It refers to the ability of refractory castables to resist cracking and spalling when subjected to rapid temperature changes (such as sudden cooling and heating). It is usually expressed by the number of times the material can withstand repeated rapid cooling and heating cycles without damage (such as the number of water cooling cycles).
2. Specific reference value:
(1) Suitable for scenes with severe temperature fluctuations. Many industrial furnaces have frequent temperature increases and decreases. For example, in recent years, due to market reasons, many companies' steel rolling mill heating furnaces have been insulated during the day and produced at night; waste incinerators have temperature fluctuations of ±200°C due to unstable material calorific value. Casting materials with poor thermal shock stability will crack due to concentrated thermal stress, and then be corroded by melt or flue gas. For example, after a heating furnace replaced the material with thermal shock resistance for 30 cycles, its service life was extended from 3 months to more than 1 year. (2) Reflects the toughness of the material and the rationality of the structural design. Thermal shock resistance is related to the material (such as adding silicon carbide and zircon sand to improve toughness) and structure (such as density and pore distribution). For example, lightweight porous castables have better thermal shock resistance than high-density castables because their pores can buffer thermal stress, but their strength is lower. When ordering, it is necessary to select products that take into account both strength and toughness according to the working conditions. For example, fiber-reinforced thermal shock castables are preferred for the furnace door area. 4. Logic for Coordinating the Three Parameters:
1. If the operating environment is primarily subject to high-temperature static loads (e.g., blast furnace linings), ensuring the refractoriness under load meets the standard is a priority.
2. If the environment involves strong chemical attack or ultra-high temperatures (e.g., glass melting furnaces), focus on the Al₂O₃ content and the corresponding material.
3. If the environment is subject to frequent temperature fluctuations (e.g., forging furnaces), thermal shock resistance is the core screening criterion.
4. These three parameters do not exist in isolation. For example, while a corundum castable with a high Al₂O₃ content has high refractoriness, its thermal shock resistance may be poor if the structure is not optimized. Balancing these properties requires the addition of additives and other methods to achieve optimal cost-effectiveness and maximize service life.
1. Definition: It refers to the temperature at which the refractory castable undergoes 4% compression deformation (or deformation of 4mm) as the temperature rises under a static load of 0.2MPa. It is a key parameter to measure the material's ability to resist load and maintain structural stability in a high-temperature environment.
2. Specific reference value:
(1) Match the temperature and load conditions of the use environment. The operating temperature and internal material load of different industrial furnaces vary significantly. For example, the hot blast furnace in the steel industry is at 1200-1400℃ for a long time and bears the weight of the furnace charge. When ordering, it is necessary to ensure that the material's load softening point is 10%-20% higher than the actual operating temperature to reserve a safety margin to avoid the collapse of the furnace structure due to plastic deformation at high temperature.
(2) Predict the high-temperature structural stability of the material. Materials with low load softening points are prone to shrinkage or deformation when approaching the critical temperature, resulting in cracking of brick joints, peeling of the lining, and shortening the life of the kiln. For example, if a castable with a load softening point of 1500℃ is used in the transition zone of a cement rotary kiln, and the actual operating temperature often reaches 1450℃, it is easy to cause bulging and deformation due to long-term proximity to the critical value, increasing the frequency of maintenance.
2. Al₂O₃ content:
1. Definition: Aluminum oxide (Al₂O₃) is a common high-refractory component in refractory castables. Its content is the core basis for distinguishing the material grade of castables (such as clay, high-aluminum, corundum, etc.), and directly reflects the matrix characteristics of the material.
2. Specific reference value:
(1) Relevant to the refractoriness and high-temperature strength of the material. Al₂O₃ has a melting point of up to 2050℃ and has strong chemical stability. Generally speaking, the higher the content, the higher the refractoriness of the material (e.g., Al₂O₃ content of corundum castables ≥90% can reach a refractoriness of over 2000℃; Al₂O₃ content of clay castables is about 30%-45%, and the refractoriness is mostly between 1600-1750℃). At the same time, the high-temperature compressive strength is also improved, making it suitable for areas that are severely eroded by materials (e.g., the tapping port of a steelmaking converter).
(2) Guide the judgment of corrosion resistance. In different atmospheres, the Al₂O₃ content affects the chemical stability of the material. For example, in the flue gas environment of a coal-fired boiler (containing acidic gases such as SO₂), high Al₂O₃ castables are more resistant to corrosion than siliceous materials; because the flue gas of a waste incinerator is complex, aluminate castables with 60%-80% Al₂O₃ are often used.
(3) Balance cost and performance. The higher the Al₂O₃ content, the higher the raw material cost (such as corundum sand, brown corundum, etc.). For example, the price of a corundum castable with 90% Al₂O₃ is 2-3 times that of a high-aluminum castable with 60% Al₂O₃. When ordering, avoid "blindly chasing high prices". For example, for low-temperature kilns (such as ceramic roller kilns, operating temperature ≤1200℃), clay castables with 40%-50% Al₂O₃ can meet the needs.
III. Thermal shock stability:
1. Definition: It refers to the ability of refractory castables to resist cracking and spalling when subjected to rapid temperature changes (such as sudden cooling and heating). It is usually expressed by the number of times the material can withstand repeated rapid cooling and heating cycles without damage (such as the number of water cooling cycles).
2. Specific reference value:
(1) Suitable for scenes with severe temperature fluctuations. Many industrial furnaces have frequent temperature increases and decreases. For example, in recent years, due to market reasons, many companies' steel rolling mill heating furnaces have been insulated during the day and produced at night; waste incinerators have temperature fluctuations of ±200°C due to unstable material calorific value. Casting materials with poor thermal shock stability will crack due to concentrated thermal stress, and then be corroded by melt or flue gas. For example, after a heating furnace replaced the material with thermal shock resistance for 30 cycles, its service life was extended from 3 months to more than 1 year. (2) Reflects the toughness of the material and the rationality of the structural design. Thermal shock resistance is related to the material (such as adding silicon carbide and zircon sand to improve toughness) and structure (such as density and pore distribution). For example, lightweight porous castables have better thermal shock resistance than high-density castables because their pores can buffer thermal stress, but their strength is lower. When ordering, it is necessary to select products that take into account both strength and toughness according to the working conditions. For example, fiber-reinforced thermal shock castables are preferred for the furnace door area. 4. Logic for Coordinating the Three Parameters:
1. If the operating environment is primarily subject to high-temperature static loads (e.g., blast furnace linings), ensuring the refractoriness under load meets the standard is a priority.
2. If the environment involves strong chemical attack or ultra-high temperatures (e.g., glass melting furnaces), focus on the Al₂O₃ content and the corresponding material.
3. If the environment is subject to frequent temperature fluctuations (e.g., forging furnaces), thermal shock resistance is the core screening criterion.
4. These three parameters do not exist in isolation. For example, while a corundum castable with a high Al₂O₃ content has high refractoriness, its thermal shock resistance may be poor if the structure is not optimized. Balancing these properties requires the addition of additives and other methods to achieve optimal cost-effectiveness and maximize service life.