In the construction and operation of glass furnaces, the drying stage is one of the most critical and risky sections. The fused cast AZS blocks, as the core refractory material in the wall area, possess unparalleled excellent resistance to the erosion of molten glass. However, the cracking of fused cast AZS blocks during the drying process occurs frequently, causing either minor delays in production or even serious material leakage accidents, resulting in significant economic losses. This article will begin by examining the material characteristics of fused cast AZS blocks, thoroughly analyzing the causes of cracking, and proposing a comprehensive set of preventive strategies.
1. Understanding the characteristics of fused cast AZS blocks is the foundation for preventing cracking
2. Analysis of the main causes of fused cast AZS sidewall block cracking during the roasting process
3. Key strategies for systematic prevention of fused cast AZS block cracking in the furnace
4.Conclusion
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1. Understanding the characteristics of fused cast AZS blocks is the foundation for preventing cracking
Fused cast AZS blocks are not homogeneous materials. Their properties are closely related to the manufacturing process (casting method) and microstructure:
(1) Microstructure characteristics: Fused cast AZS blocks are mainly composed of corundum phase, rutile phase, and glass phase. Rutile crystals, wonderful crystals in co-crystals, are the key to resisting thermal stress and erosion by the glass melt. The higher the rutile content, the lower the glass phase content, and the brick's high-temperature volume stability, resistance to erosion, and thermal shock resistance are usually better.
(2) Density and porosity: The theoretical density of fused cast AZS blocks is high (>3.8g/cm³), but during the casting process, inevitable shrinkage cavities and a small amount of pores will be produced. Shrinkage cavities are huge structural defects that are stress concentration points and potential crack initiation points. Choosing high-quality fused cast AZS blocks (such as using tilt casting, reduced cavity cast technology) can significantly reduce the size and number of shrinkage cavities.
(3) Thermal conductivity and thermal expansion: The thermal conductivity of AZS is relatively good, which is conducive to heat transfer, but it also means that during a temperature shock, the temperature difference between the inside and outside of the brick generates large thermal stress. Its thermal expansion coefficient is in the middle to slightly higher range among refractory materials.
(4) Crystalline transformation and volume effect: During the heating process of the furnace (about 300°C to 900°C), the monoclinic phase of rutile within the fused cast AZS block will transform to the tetragonal phase, accompanied by a minor irreversible volume contraction (about 0.5%-0.9%). This phase transformation process is one of the main internal causes of the internal stress generated by the fused cast AZS block during the furnace heating period. The reverse phase transformation during cooling will be accompanied by volume expansion.
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(5) Anisotropy: Cast-formed fused cast AZS blocks have significant anisotropy. The thermal shock stability and resistance to glass liquid penetration are best perpendicular to the casting direction (usually the working surface direction), but the compressive strength parallel to the casting direction is the highest. The correct installation direction (the working surface perpendicular to the casting direction) is crucial.
2. Analysis of the main causes of fused cast AZS sidewall block cracking during the roasting process
Based on the characteristics of fused cast AZS blocks, the cracking mainly results from the combined effect of the following factors:
1. Excessive thermal stress: This is the most direct cause.
►Rapid heating/cooling: The heating/cooling curve of the roasting process is too steep, especially in the phase transition temperature range (300-900°C), the temperature rise is too fast, resulting in a steep temperature gradient inside the brick, generating huge thermal stress.
►Local overheating/overcooling: Improper flame adjustment, burner failure, malfunction of thermocouples, or unreasonable layout, leading to local sidewall blocks having temperatures much higher or lower than the overall area.
►Structural constraints: The sidewall blocks are tightly constrained by the steel structure and adjacent bricks, and the thermal expansion is restricted, resulting in huge compressive stress or tensile stress. When the stress exceeds the strength limit of the brick at this temperature, cracking occurs.
2. Phase transformation stress concentration: During the phase transformation temperature range, if the temperature rises too rapidly, the phase transformation process becomes intense and uneven, resulting in both contraction stress and heating stress. These combined stresses are likely to form stress concentrations at structural defects, leading to crack initiation and propagation.
3. Mechanical stress: Improper installation before the oven firing or deformation of the steel structure during the firing process applies additional stress.
►Insufficient expansion joint provision: There are insufficient and unreasonable expansion joints between adjacent AZS blocks and between the brick and the steel structure. The thermal expansion coefficients of different materials vary significantly.
►Excessive tightening of top bolts/secondary beams: The steel structure that tightens the sidewall is adjusted too tightly before the oven firing, without considering the thermal expansion of the bricks, causing excessive pressure on the bricks.
►Incorrect installation direction: The AZS working surface is not installed perpendicular to the casting direction, resulting in the inability of its optimal performance surface to effectively resist the erosion of the glass liquid and thermal shock.
4. Quality defects of the brick material: Excessive shrinkage and improper position, internal presence of latent cracks, uneven distribution of the glass phase, and large component fluctuations all significantly reduce the brick's thermal shock resistance and overall strength.
3. Key strategies for systematic prevention of fused cast AZS block cracking in the furnace
To avoid cracking, it is necessary to implement meticulous control throughout the entire process from material selection, design, installation, to furnace operation:
3.1 Select high-quality fused cast AZS blocks:
Material selection: For critical sidewall positions, prioritize the use of bricks with higher rutile content and lower glass phase content (such as AZS 41#). Their higher high-temperature strength, better volume stability, and thermal shock resistance are crucial for resisting the stress in the furnace.
Quality requirements: Choose reputable suppliers with stable production processes.Strict acceptance: Focus on checking the surface quality (no cracks,holes),the location and size of shrinkage cavities (should be away from the surface and small), and dimensional tolerances. Requirements for providing key performance indicators (such as chemical composition,volume density,apparent porosity,room temperature compressive strength,thermal expansion curve).
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Pre-processing and inspection:Carefully inspect each brick, removing defective ones. Perform precise cutting and grinding according to the design drawings to ensure precise dimensions and reduce installation stress.
3.2 Scientific design and standard installation:
(1) Accurate expansion joint design:
Between fused cast AZS blocks: Reserve sufficient horizontal and vertical expansion joints (usually 1.5-3mm/m, calculated based on the brick's thermal expansion coefficient and maximum operating temperature). Qualified refractory ceramic fiberboard (such as zirconium-containing types) or compressible felt must be used as filler. Do not use hard mud or broken bricks! Fiber materials allow the bricks to expand freely and absorb stress.
AZS and backing bricks/steel structures: Expansion space must also be reserved, taking into account the differences in expansion between different materials. Use appropriate sliding layer materials.
(2) Correct installation direction: Make sure that the casting working surface (dense, corrosion-resistant surface) of the fused cast AZS block is installed perpendicular to the casting direction and facing the glass liquid. This is the lifeline for its optimal performance. Mark the direction of the brick.
(3) Reasonable steel structure constraints: Sufficient adjustment margin must be left for restraint devices such as secondary beams and top screws during cold installation to ensure that the brick stack is stable but not crushed. During the heating process of the furnace, a dedicated person must be arranged to loosen the top screws in stages, symmetrically, and evenly according to the predetermined plan (combined with the heating curve) to release the expansion stress of the brick body. It is strictly forbidden to loosen or overtighten at one time.
(4) Fine masonry construction: Use special high-quality zirconium fire clay (with good fluidity, bonding strength, and thermal expansion matching). The mortar joints are full, uniform, and thin (≤1mm). Avoid strong hammering of the brick body into place. Thoroughly clean the brick joints after the masonry is completed.
3.3 Establish and implement a scientific furnace heating system: This is the core for preventing cracking!
(1) Customized temperature rise curve:
Critical temperature range slow heating: In the phase transition sensitive range from 300°C to 900°C, a very slow heating rate (usually ≤ 5°C/h, sometimes even lower) must be adopted. Ensure uniform temperature inside and outside the brick, and ensure a smooth phase transition process, fully releasing the phase transition stress.
Reasonable segmented insulation: Set sufficient long insulation time (several hours or even longer) at key temperature points (such as around 150°C for removing free water, 300-400°C for removing crystalline water, and opportune insulation during the phase transition range), allowing stress relaxation and temperature balance.
Overall smooth and controllable: The entire furnace heating curve should be smooth, avoiding any sudden rises or drops. The total furnace heating time should be sufficient, and it should not be compressed to meet the construction schedule.
(2) Strict temperature and pressure monitoring:
Multiple points dense temperature measurement: Install sufficient and qualified thermocouples at different heights and areas of the sidewall (especially in areas prone to overheating, such as near the burner and corners), and monitor and record the temperature at each point in real time, focusing on temperature differences. Any temperature difference between two points (especially for the same brick or adjacent bricks) should be controlled within the safe range.
Furnace pressure control: Maintain a slight positive pressure inside the furnace to prevent negative pressure from drawing in cold air, causing local rapid cooling. However, the positive pressure should not be too high to prevent excessive scrubbing of the sidewall by high-temperature gases.
(3) Uniform heating: Symmetrical burner points, regular rotation to ensure the temperature field inside the furnace is as uniform as possible. Use temporary insulation measures to protect weak parts of the furnace body.
3.4 Complete pre-furnace preparation and process management:
Sufficient pre-drying: If the construction environment is humid or after the masonry has been exposed to rain, before officially igniting the furnace for heating, a long-term (several days) low-intensity pre-drying (such as <150°C) must be carried out to eliminate physical water and some crystalline water in the brick and fire clay.
Detailed furnace heating plan and training: Develop a written, reviewed furnace heating temperature rise curve and operation manual (including top screw release plan). Provide adequate training to all operation, monitoring, and maintenance personnel, clearly defining responsibilities and emergency procedures.
Special personnel on duty and inspection: During the furnace heating period (especially during the critical temperature rise stage), there must be 24-hour dedicated personnel on duty. Regular manual inspections of the pool wall bricks should be conducted to check for cracks, abnormal sounds, the condition of the steel structure, and changes in expansion joints, etc.
Emergency plans: Prepare emergency plans to deal with situations such as thermocouple failure, burner failure, local overheating/undercooling, and the discovery of cracks. Once an abnormal temperature rise or suspected cracks are detected, the heating should be stopped or appropriately cooled and insulated, and the cause should be identified and dealt with before continuing.
Preventing the cracking of the fused cast AZS sidewall blocks during the baking process is a systematic project. A profound understanding of the unique properties of fused cast AZS sidewall blocks (especially the phase change behavior and structural defects) is a prerequisite. The key to success lies in:
►Select high-density and high-silica-zirconium content high-quality fused cast AZS blocks;
►Precisely design (especially scientifically reserved expansion joints) and install in accordance with specifications (ensuring correct direction and proper pre-tightening);
►Customize and strictly implement a scientific furnace heating-up system, especially in the phase transition sensitive zone (300-900°C), implementing ultra-low-speed heating and sufficient insulation;
►Strengthen the entire process monitoring (temperature uniformity, temperature difference, pressure, brick condition) and refined management (top screw release, emergency response).
Only by closely integrating materials science, engineering design, and rigorous operation management can the outstanding performance of fused cast AZS blocks be maximally unleashed, ensuring that the glass furnace safely and smoothly completes the high-risk stage of furnace firing, and laying a solid foundation for the subsequent long-term stable and efficient operation. Every successful furnace firing is a perfect demonstration of technical rigor and teamwork spirit.
Henan SNR Refractory Co., Ltd (SNR) focuses on producing high quality refractory materials.If you have any needs, please contact me.
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