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In industrial furnaces and heat‑treatment equipment, furnace bottom plates, eye plates, and liner plates are three core components that are often confused with one another, yet each serves a distinctly different purpose. Although they all belong to the family of heat‑resistant steel castings for furnace use, they differ fundamentally in function, installation location, and material requirements. This article breaks down the key differences between the three to help you make precise selections and carry out scientific maintenance.
1. Furnace Bottom Plate – The Load‑Bearing Base for Workpieces
What Is a Furnace Bottom Plate?
A furnace bottom plate is a load‑bearing heat‑resistant steel plate installed at the bottom of an industrial furnace (such as box‑type furnaces, car‑bottom furnaces, pit furnaces, etc.). Its primary task is to support the workpieces being treated while isolating them from the heating elements and refractory materials beneath.
Core Functions
1. Supporting workpieces: The bottom plate bears the entire weight of the workpieces. In large car‑bottom furnaces, it must support several tons or even dozens of tons while maintaining flatness without deformation at high temperatures.
2. Protecting heating elements: Oxide scale that falls off during heating can cause short circuits or even burn‑out if it comes into contact with resistance wires or heating elements. A well‑designed bottom plate with tight jointing (such as interlocking lap joints) effectively prevents scale from dropping through.
3. Uniform heat transfer: A high‑quality bottom plate offers good thermal conductivity, ensuring even heat distribution from the furnace chamber to the workpieces and avoiding local overheating or underheating.
Common Materials
The material selection for bottom plates depends on the service temperature, load magnitude, and furnace type.
| Material Grade | Max. Service Temp. | Key Characteristics | Typical Applications |
|---|---|---|---|
| ZG40Cr25Ni20Si2 (2520/310S) | 1200°C | Excellent high‑temperature oxidation resistance, austenitic heat‑resistant steel | Standard choice for various high‑temperature industrial furnaces |
| ZG30Cr18Mn12Si2N (Cr‑Mn‑N) | 950°C | Good high‑temperature strength and thermal‑fatigue resistance | Furnace retorts, bottom plates, baskets, and other furnace components |
| ZG35Cr24Ni7SiN (Ni‑7‑N) | 950–1100°C | Nitrogen‑strengthened austenitic heat‑resistant cast steel, cost‑effective | Economical alternative to high‑nickel heat‑resistant steels |
| ZG40Cr9Si2 | 800°C | Relatively low high‑temperature strength | Low‑temperature furnace bottom plates, doors, etc. |
Structural Design and Selection Tips
- Thickness selection: Typical bottom plate thickness ranges from 10 to 22 mm; heavier loads require greater thickness.
- Innovative designs: In recent years, new designs such as quincunx‑shaped, corrugated‑edge, and hollow‑grid bottom plates have emerged. By optimising the geometry to disperse thermal stress, these designs significantly extend service life. Studies show that corrugated‑edge bottom plates generate lower maximum thermal stress during heat treatment than circular plates of the same specification.
- Modular assembly: Assembled bottom plates consist of multiple interlocking segments, facilitating maintenance and partial replacement.
- Regular cleaning: The bottom plate should be lifted weekly, and the oxide scale underneath blown out with compressed air to prevent blockage of the resistance‑wire grooves.
2. Eye Plate (Taphole Plate) – The “Throat Channel” for Furnace Charge
What Is an Eye Plate?
The term “eye plate” has two common interpretations in the industrial furnace field, depending on the specific context:
Interpretation 1: Taphole guard plate (for submerged‑arc furnaces / smelting furnaces)
In smelting equipment such as submerged‑arc furnaces and electric furnaces, “eye plate” generally refers to the guard plate or taphole brick around the taphole area – i.e., the protective components at the iron‑tapping or slag‑tapping outlet. The taphole is the passage through which molten metal or slag flows out of the furnace and is a critical part of the entire furnace lining.
Interpretation 2: Lifting eye plate (general industrial use)
In a broader industrial context, an “eye plate” may refer to a steel plate connector with mounting holes (eyes) used for lifting, rigging, or fixing. In furnace applications, this is often seen in furnace‑cover lifting or door suspension.
Core Functions of a Taphole Guard Plate
1. Protecting the taphole passage: The taphole comes into direct contact with incandescent molten metal at around 1600°C, withstanding both high‑temperature erosion and mechanical scouring. The role of the eye plate/taphole brick is to prevent this passage from being burned through or washed out.
2. Resisting scouring and oxidation: The material requirements for a taphole guard plate are far more demanding than for other parts of the furnace; it must have excellent resistance to scouring and oxidation.
3. Facilitating replacement and maintenance: A well‑designed taphole structure allows the eye plate or taphole brick to be replaced independently, avoiding scrapping of the entire furnace lining due to taphole damage.
Material and Selection
Material selection for taphole guard plates is extremely stringent:
- Silicon carbide bricks / silicon‑nitride‑bonded silicon carbide bricks: Resistant to corrosion, thermal shock, and high temperatures; widely used for tapholes in submerged‑arc furnaces.
- UCAR carbon bricks: Not easily oxidised and do not react with the liquid in the furnace.
- High‑refractoriness materials: Taphole bricks generally require a refractoriness of ≥1850°C.
Material for Lifting Eye Plates
If the term refers to lifting eye plates, the material is usually stainless steel (e.g., 304) or carbon steel, selected according to the load and corrosion‑resistance requirements.
3. Liner Plate – The “Armour Shield” for the Furnace Body
What Is a Liner Plate?
A liner plate is a protective plate installed on the inner walls, bottom, door, kiln inlet, and other areas of an industrial furnace. Its primary role is to protect the furnace structure from high temperatures, scouring, corrosion, and mechanical wear. While a bottom plate is the “pallet” for workpieces, the liner plate is the “armour” that protects the furnace itself.
Core Functions
1. Protecting the furnace lining structure: The liner plate isolates the furnace shell and refractory material from the high‑temperature flame, material scouring, and chemical attack inside the furnace chamber, thereby extending the overall service life of the furnace.
2. Resisting high‑temperature oxidation: In oxidising atmospheres, the liner plate surface forms a dense oxide film that prevents oxygen from penetrating into the material.
3. Withstanding material impact and abrasion: In equipment such as cement rotary kilns and sintering machines, liner plates are directly exposed to the continuous scouring and abrasion of hot clinker and feed materials.
Common Materials
The material system for liner plates overlaps with that for bottom plates, but with greater emphasis on wear resistance and scour resistance.
| Material Grade | Max. Service Temp. | Key Characteristics | Typical Applications |
|---|---|---|---|
| ZG45Cr28Ni48W5Si2 (2848W5) | 1300°C | Ultra‑high‑temperature resistance, top‑tier oxidation and creep resistance | Ultra‑high‑temperature kiln inlets and furnace chamber liners |
| ZG40Cr25Ni20Si2 (2520/310S) | 1200°C | Excellent overall performance | Standard choice for various high‑temperature furnace liners |
| ZG35Cr24Ni7SiN (Ni‑7‑N) | 950–1100°C | Nitrogen‑strengthened austenitic heat‑resistant cast steel; 3‑5 times the life of ordinary heat‑resistant steel | Cost‑effective guard plates replacing high‑nickel grades |
| ZG3Cr18Mn12Si2N | 950–1000°C | Superior wear resistance | Wear‑resistant liners for power‑station boilers, etc. |
| KMTBCr26 | — | High‑chromium wear‑resistant cast iron | Heavy‑wear applications such as incinerator liners |
Manufacturing Processes
Liner plates are predominantly produced by investment casting, lost‑foam casting, or sand casting as one‑piece components without welded joints. This avoids the risk of cracking in the heat‑affected zone at high temperatures. After casting, they undergo high‑temperature solution treatment to eliminate internal stresses and homogenise the microstructure.
4. Comparison of the Three and Selection Guidelines
At‑a‑Glance Comparison Table
| Aspect | Furnace Bottom Plate | Eye Plate (Taphole Plate) | Liner Plate |
|---|---|---|---|
| Core function | Supporting workpieces + protecting heating elements | Protecting the tapping/slagging passage | Protecting the furnace structure + resisting scouring |
| Installation location | Furnace bottom (load‑bearing surface) | Taphole / iron‑tapping or slag‑tapping outlet | Furnace walls, bottom, kiln inlet, doors |
| Main stresses | Vertical pressure + heat conduction | High‑temperature scouring + thermal shock | High‑temperature oxidation + material abrasion + thermal shock |
| Key required properties | High‑temperature strength + creep resistance | Scour resistance + thermal‑shock resistance | Oxidation resistance + wear resistance + thermal‑shock resistance |
| Typical materials | 2520, Cr‑Mn‑N, Ni‑7‑N | Silicon carbide bricks, UCAR carbon bricks | 2848W5, 2520, Ni‑7‑N |
Three‑Step Selection Guide
Step 1: Identify the purpose
- Supporting workpieces → choose a furnace bottom plate
- Protecting the taphole passage → choose a taphole guard plate (eye plate)
- Protecting furnace walls / kiln inlet → choose a liner plate
Step 2: Determine the temperature
- ≤950°C → Cr‑Mn‑N (ZG30Cr18Mn12Si2N) or Ni‑7‑N (ZG35Cr24Ni7SiN)
- 950–1100°C → Ni‑7‑N or 2520
- 1100–1200°C → 2520 (ZG40Cr25Ni20Si2)
- 1200–1300°C → 2848 series (ZG45Cr28Ni48W5Si2)
Step 3: Consider the specific operating conditions
- Severe scouring (e.g., submerged‑arc furnace taphole) → prioritise silicon‑carbide‑based taphole plates
- Severe abrasion (e.g., sintering‑machine liner) → prioritise high‑chromium wear‑resistant liner plates
- Frequent thermal shock (e.g., car‑bottom furnace bottom plate) → prioritise materials with excellent thermal‑fatigue resistance
5. Usage and Maintenance Highlights
Furnace Bottom Plate Maintenance
- Blow out the oxide scale underneath with compressed air on a weekly basis.
- Check joint gaps to ensure they are not too large, preventing scale from falling through.
- Replace promptly if deformation or cracking is found.
Eye Plate (Taphole Guard Plate) Maintenance
- Regularly inspect the taphole passage for the degree of scouring.
- Monitor the joint condition between the eye plate and the surrounding lining.
- If a taphole brick is damaged, replace it individually in time to avoid scrapping the entire lining.
Liner Plate Maintenance
- Regularly inspect the liner plate surface for cracks or spalling.
- Check the condition of fixing bolts/pins to prevent liner detachment.
- Replace immediately if severe wear or burn‑through is found.