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Silicon Nitride (Si₃N₄) Protection Tubes for the Molten Aluminum Industry
1. Why Protection Tube Material Matters in Molten Aluminum
In an aluminum foundry, the protection tube is the only barrier between a temperature sensor, an immersion heater, or a melt-contact component and a 660–900 °C bath of molten aluminum. It must survive direct contact with corrosive liquid metal, repeated thermal shock from raising and lowering, and the build-up of dross — while never contaminating the alloy it protects.
Traditional tubes fail these demands. Cast iron and alloy steel are attacked by the melt and leach iron into the aluminum, which destroys the ductility of the casting. Graphite oxidizes rapidly and introduces carbon contamination. Alumina offers reasonable corrosion resistance but is brittle and cracks under thermal cycling. Pure silicon nitride (Si₃N₄) solves all of these problems at once.
2. Core Advantages of Pure Silicon Nitride (Si₃N₄)
2.1 Non-Wetting to Molten Aluminum
Pure Si₃N₄ is thermodynamically stable in contact with molten aluminum and exhibits virtually zero wetting — the melt beads up and rolls off its surface instead of adhering. This eliminates dross build-up, keeps the melt free of impurities, and makes the tube essentially maintenance-free. It is a decisive advantage for high-purity structural castings where iron or carbon contamination is unacceptable, such as aluminum wheel hubs.
2.2 Exceptional Thermal Shock Resistance
Silicon nitride has one of the highest thermal shock resistance ratings among technical ceramics. A Si₃N₄ tube can be dipped directly into molten metal without preheating and withdrawn repeatedly without cracking — even across weekend shutdowns and intermittent production. This simplifies operation, removes the preheating step, and eliminates the cracking failures that plague alumina tubes.
2.3 Corrosion and Oxidation Resistance
Gas-pressure-sintered silicon nitride achieves near-zero porosity and resists chemical attack from molten aluminum and standard alloying elements such as magnesium and zinc. It is fully suitable for 5000 and 7000 series alloys without material degradation, and unlike graphite it does not oxidize in high-temperature service.
2.4 High Strength and Density
With a flexural strength of roughly 580 MPa or higher and a density of 3.1–3.3 g/cm³, silicon nitride withstands the high speeds and mechanical loads of degassing rotors and the repeated mechanical cleaning of thermocouple wells. It is denser and stronger than reaction-bonded and competitive ceramic grades.
2.5 Low Thermal Expansion and Long Service Life
A low coefficient of thermal expansion keeps dimensions stable over years of service, preserving measurement accuracy and component geometry. In real foundry service, pure Si₃N₄ protection tubes deliver 12 months to 2–3 years of continuous operation, far beyond cast iron (1–2 weeks), silicon carbide (1–3 months) or alumina.
3. Pure Si₃N₄ vs. Conventional Protection Tube Materials
Property
Cast Iron
Graphite
Alumina (Al₂O₃)
Pure Si₃N₄
Resistance to melt attack Poor
Poor (oxidizes)
Good
Excellent
Thermal shock resistance Fair
Poor
Fair
Excellent
Melt contamination Iron
Carbon
Low
None
Typical service life 1–2 weeks
Frequent replacement
3–12 months
12 months – 3 years
Maintenance Constant replacement
High
Moderate
Nearly zero
Table 1. Comparison of protection tube materials in molten aluminum service.
4. Real-World Application Cases
Case 1 — Die Casting: From Weekly Changes to Annual Reliability
A die-casting line suffering frequent cracking of its ceramic protection tubes replaced the standard (alumina) tubes with a single Si₃N₄ tube. The result: the one silicon nitride tube survived a full year of production, including numerous weekend shutdowns, eliminating thermal crack failures. The station saved the direct cost of 30+ alumina tubes and over 60 hours of maintenance labor, while consistent temperature readings improved shot consistency and reduced porosity-related scrap. (Source: SF-Foundry)
Case 2 — Molten Aluminum Casting: Scrap Rate Cut from 4% to 0.8%
A molten aluminum casting operation switched its thermocouple protection wells from cast iron to a silicon-nitride-based (Sialon) material. The high modulus of rupture withstood daily mechanical cleaning with zero breakage, and precise thermal control reduced the scrap rate from 4% to 0.8%. Although the initial unit cost was higher, eliminating weekly replacements and cutting scrap delivered a net saving of $42,000 per furnace per year. (Source: AdTech)
Case 3 — Immersion Heaters: 2–3 Years of Continuous Operation
Foundries protecting immersion heaters in in-line degassing and dosing systems (e.g., ALPUR, SNIF, Hertwich, Striko Westofen) report 2–3 years of continuous silicon nitride heater-tube service with minimal cleaning — far superior to cast iron or aluminium titanate riser tubes, which fail quickly or require frequent replacement. (Source: Sialon Ceramics)
Case 4 — Thermocouple Sheaths: 12 Months vs. Cast Iron’s 3–4 Weeks
In molten aluminum, zinc and magnesium service, a premium silicon nitride (Sialon) thermocouple sheath delivers a service life of up to 12 months. By comparison, cast iron sheaths erode in only 3–4 weeks while contaminating the melt with iron, demanding constant replacement and risking casting quality. (Source: Hongtai Heating Elements)
5. Service Life Comparison at a Glance
Material
Typical Service Life
Cast iron 1–2 weeks
Silicon carbide (SiC) 1–3 months
Alumina (Al₂O₃) 3–12 months
Pure Si₃N₄ / Sialon 12 months – 3 years
Table 2. Typical service life of protection tube materials in molten aluminum.
6. Application in the Field
Pure silicon nitride protection tubes are proven in aluminum melting, holding, degassing and die-casting around the world, as illustrated below.
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Silicon carbide pipe/nozzles in power plants
Silicon Carbide Products in power plant
The main function of the silicon carbide nozzle of the burner is to inject the pulverized coal air mixture (primary air) supplied by the pulverized coal preparation system and the secondary air required for combustion into the furnace at a certain concentration and speed, respectively, so as to achieve stable ignition and combustion in the suspended state.
- 1. The structure of silicon carbide nozzle must be simple, not easy to block, easy to repair and replace, long service life, and improve economic efficiency.
- 2. The silicon carbide nozzle adopts an integrated casting molding process, which is high temperature resistant, wear-resistant and reduces maintenance costs.
The burner nozzles of power plant generally adopts cast steel structure. The proper swing of the burner nozzle can change the position of the flame center to adjust the reheat steam temperature level; The thick and thin pulverized coal burner can prevent furnace coking and high-temperature corrosion, and has the characteristics of stable ignition, good combustion stability at low load, and low NOx emission. After the primary air cast steel nozzle of the burner is burnt and deformed, the deviation between the furnace flame center and the design value is large, and the deviation between the nozzle size and the original size is large. The initial momentum of the jet is reduced compared with that before the burning loss, the range is shorter, and the rigidity is poor. The pulverized coal ignites near the nozzle, and the temperature near the nozzle is high, which is easy to produce slagging. The primary air nozzle of the burner needs to be replaced after it is burnt. During the replacement, the powder pipes connected to the outside of the burner need to be disconnected, the primary air nozzle of the burner and the nozzle should be pulled out together, then replaced, and then loaded. The workload is large, the environment is poor, and it costs labor, time, and money, resulting in great economic losses.
The silicon carbide nozzle of the burner produced by our company is wear-resistant, high-temperature resistant, non-deformable, and its service life is 3-5 times that of the cast steel nozzle, which improves the boiler efficiency, reduces NOX emissions, saves energy and environmental protection, reduces maintenance costs, and improves economic benefits.
It adopts integrated molding process, which is resistant to high temperature, abrasion and deformation.
The defects of cast steel nozzle mainly include the following aspects.
1. The nozzle of cast steel is not wear-resistant, corrosion resistant, high temperature resistant and has short service life. The boiler needs to be shut down for replacement, resulting in great economic losses.
2. The cast steel nozzle is easy to be damaged during the operation of the unit, and the blunt body of the nozzle often falls off, threatening the operation of the burner and the safe operation of the slag extractor.
3. Because the nozzle material is G40Cr20Ni5, which works under high temperature for a long time, when the combustion of pulverized coal is unstable, it will cause uneven temperature distribution in the furnace or coking at the nozzle, resulting in thermal stress at the nozzle due to temperature difference, resulting in serious local deformation of the nozzle, short range of primary air, poor rigidity, pulverized coal ignition near the nozzle, high temperature near the nozzle, and local additional stress in the component. When the temperature fluctuation is large, The thermal stress changes greatly, causing thermal fatigue cracks on the nozzle surface, which will cause the burner nozzle and nozzle to burn over a long period of time.
4. Because the primary air flow is a high-speed gas-solid two-phase flow, the uneven scouring of pulverized coal particles leads to serious local wear of the blunt body and even wear off the blunt body. The nozzle and the blunt body are integrated. On the other hand, the thermal stress at high temperature is easy to cause serious deformation of the nozzle, even burning out, so the nozzle can not play its due role.
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NSiC nozzles – silicon carbide nozzles in power plants
The main function of the silicon carbide nozzle of the burner is to inject the pulverized coal air mixture (primary air) supplied by the pulverized coal preparation system and the secondary air required for combustion into the furnace at a certain concentration and speed, respectively, so as to achieve stable ignition and combustion in the suspended state.
- 1. The structure of silicon carbide nozzle must be simple, not easy to block, easy to repair and replace, long service life, and improve economic efficiency.
- 2. The silicon carbide nozzle adopts an integrated casting molding process, which is high temperature resistant, wear-resistant and reduces maintenance costs.
The burner nozzles of power plant generally adopts cast steel structure. The proper swing of the burner nozzle can change the position of the flame center to adjust the reheat steam temperature level; The thick and thin pulverized coal burner can prevent furnace coking and high-temperature corrosion, and has the characteristics of stable ignition, good combustion stability at low load, and low NOx emission. After the primary air cast steel nozzle of the burner is burnt and deformed, the deviation between the furnace flame center and the design value is large, and the deviation between the nozzle size and the original size is large. The initial momentum of the jet is reduced compared with that before the burning loss, the range is shorter, and the rigidity is poor. The pulverized coal ignites near the nozzle, and the temperature near the nozzle is high, which is easy to produce slagging. The primary air nozzle of the burner needs to be replaced after it is burnt. During the replacement, the powder pipes connected to the outside of the burner need to be disconnected, the primary air nozzle of the burner and the nozzle should be pulled out together, then replaced, and then loaded. The workload is large, the environment is poor, and it costs labor, time, and money, resulting in great economic losses.
The silicon carbide nozzle of the burner produced by our company is wear-resistant, high-temperature resistant, non-deformable, and its service life is 3-5 times that of the cast steel nozzle, which improves the boiler efficiency, reduces NOX emissions, saves energy and environmental protection, reduces maintenance costs, and improves economic benefits.
It adopts integrated molding process, which is resistant to high temperature, abrasion and deformation.
The defects of cast steel nozzle mainly include the following aspects.
1. The nozzle of cast steel is not wear-resistant, corrosion resistant, high temperature resistant and has short service life. The boiler needs to be shut down for replacement, resulting in great economic losses.
2. The cast steel nozzle is easy to be damaged during the operation of the unit, and the blunt body of the nozzle often falls off, threatening the operation of the burner and the safe operation of the slag extractor.
3. Because the nozzle material is G40Cr20Ni5, which works under high temperature for a long time, when the combustion of pulverized coal is unstable, it will cause uneven temperature distribution in the furnace or coking at the nozzle, resulting in thermal stress at the nozzle due to temperature difference, resulting in serious local deformation of the nozzle, short range of primary air, poor rigidity, pulverized coal ignition near the nozzle, high temperature near the nozzle, and local additional stress in the component. When the temperature fluctuation is large, The thermal stress changes greatly, causing thermal fatigue cracks on the nozzle surface, which will cause the burner nozzle and nozzle to burn over a long period of time.
4. Because the primary air flow is a high-speed gas-solid two-phase flow, the uneven scouring of pulverized coal particles leads to serious local wear of the blunt body and even wear off the blunt body. The nozzle and the blunt body are integrated. On the other hand, the thermal stress at high temperature is easy to cause serious deformation of the nozzle, even burning out, so the nozzle can not play its due role.
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Silicon carbide nozzle assembly – silicon carbide concentrator/wear plate/pipe fitting/gear ring/stop/tee in power plants
The main function of the silicon carbide nozzle of the burner is to inject the pulverized coal air mixture (primary air) supplied by the pulverized coal preparation system and the secondary air required for combustion into the furnace at a certain concentration and speed, respectively, so as to achieve stable ignition and combustion in the suspended state.
- 1. The structure of silicon carbide nozzle must be simple, not easy to block, easy to repair and replace, long service life, and improve economic efficiency.
- 2. The silicon carbide nozzle adopts an integrated casting molding process, which is high temperature resistant, wear-resistant and reduces maintenance costs.
The burner nozzles of power plant generally adopts cast steel structure. The proper swing of the burner nozzle can change the position of the flame center to adjust the reheat steam temperature level; The thick and thin pulverized coal burner can prevent furnace coking and high-temperature corrosion, and has the characteristics of stable ignition, good combustion stability at low load, and low NOx emission. After the primary air cast steel nozzle of the burner is burnt and deformed, the deviation between the furnace flame center and the design value is large, and the deviation between the nozzle size and the original size is large. The initial momentum of the jet is reduced compared with that before the burning loss, the range is shorter, and the rigidity is poor. The pulverized coal ignites near the nozzle, and the temperature near the nozzle is high, which is easy to produce slagging. The primary air nozzle of the burner needs to be replaced after it is burnt. During the replacement, the powder pipes connected to the outside of the burner need to be disconnected, the primary air nozzle of the burner and the nozzle should be pulled out together, then replaced, and then loaded. The workload is large, the environment is poor, and it costs labor, time, and money, resulting in great economic losses.
The silicon carbide nozzle of the burner produced by our company is wear-resistant, high-temperature resistant, non-deformable, and its service life is 3-5 times that of the cast steel nozzle, which improves the boiler efficiency, reduces NOX emissions, saves energy and environmental protection, reduces maintenance costs, and improves economic benefits.
It adopts integrated molding process, which is resistant to high temperature, abrasion and deformation.
The defects of cast steel nozzle mainly include the following aspects.
1. The nozzle of cast steel is not wear-resistant, corrosion resistant, high temperature resistant and has short service life. The boiler needs to be shut down for replacement, resulting in great economic losses.
2. The cast steel nozzle is easy to be damaged during the operation of the unit, and the blunt body of the nozzle often falls off, threatening the operation of the burner and the safe operation of the slag extractor.
3. Because the nozzle material is G40Cr20Ni5, which works under high temperature for a long time, when the combustion of pulverized coal is unstable, it will cause uneven temperature distribution in the furnace or coking at the nozzle, resulting in thermal stress at the nozzle due to temperature difference, resulting in serious local deformation of the nozzle, short range of primary air, poor rigidity, pulverized coal ignition near the nozzle, high temperature near the nozzle, and local additional stress in the component. When the temperature fluctuation is large, The thermal stress changes greatly, causing thermal fatigue cracks on the nozzle surface, which will cause the burner nozzle and nozzle to burn over a long period of time.
4. Because the primary air flow is a high-speed gas-solid two-phase flow, the uneven scouring of pulverized coal particles leads to serious local wear of the blunt body and even wear off the blunt body. The nozzle and the blunt body are integrated. On the other hand, the thermal stress at high temperature is easy to cause serious deformation of the nozzle, even burning out, so the nozzle can not play its due role.
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Silicon nitride bonded silicon carbide kiln furniture
SNBSC materials
The main products of KSINO include SNBSC protection tubes, SNBSC bricks, SNBSC plates, SiC nozzles, wear-resistant products, wear-resistant pipes, concentrator nozzle, wear-resistant nozzle of pulverized coal burner, central air duct of pulverized coal conveying, etc. The products are widely used in many industries, such as electric power industry, steel industry, non-ferrous metals industry, chemical and building materials industry and so on. They have advantages of energy saving, environmental protection, high temperature resistance, corrosion resistance and so on.
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NSiC blocks and bricks in steel,heat treatment and power industry
SNBSC materials
The main products of KSINO include SNBSC protection tubes, SNBSC bricks, SNBSC plates, SiC nozzles, wear-resistant products, wear-resistant pipes, concentrator nozzle, wear-resistant nozzle of pulverized coal burner, central air duct of pulverized coal conveying, etc. The products are widely used in many industries, such as electric power industry, steel industry, non-ferrous metals industry, chemical and building materials industry and so on. They have advantages of energy saving, environmental protection, high temperature resistance, corrosion resistance and so on.
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Wear resistant silicon carbide ceramic elbow and pipes
Wear resistant silicon carbide ceramic elbow and pipes Due to the poor corrosion resistance and wear resistance, easy penetration and short service life of the thickened wear-resistant cast steel elbow, the elbow penetration shall be repaired and replaced regularly, affecting the normal operation of the equipment. In order to solve the abrasion problem of the transmission pipeline, the anti-wear method of sectionalized cutting, pasting wear-resistant ceramic chips, and then butt welding resto... -
Burner silicon carbide concentrator
The main function of the silicon carbide nozzle of the burner is to inject the pulverized coal air mixture (primary air) supplied by the pulverized coal preparation system and the secondary air required for combustion into the furnace at a certain concentration and speed, respectively, so as to achieve stable ignition and combustion in the suspended state.
- 1. The structure of silicon carbide nozzle must be simple, not easy to block, easy to repair and replace, long service life, and improve economic efficiency.
- 2. The silicon carbide nozzle adopts an integrated casting molding process, which is high temperature resistant, wear-resistant and reduces maintenance costs.
