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What Are Heating Furnace Slide Blocks?
Heating furnace slide blocks (also known as heat‑resistant blocks, skid blocks, or heat‑resistant slide rails) are critical high‑temperature wear‑resistant components mounted on the walking beams or fixed beams at the bottom of a heating furnace. They directly support and transport high‑temperature steel billets and play an indispensable role in both walking‑beam and pusher‑type reheating furnaces.
In hot‑rolling mills for plates, sections, and bars, slide blocks come into direct contact with steel billets at 1000–1350°C, withstanding extreme temperatures, sliding friction, and heavy loads. Their core functions are support and guidance: ensuring that billets move smoothly through the furnace (via the “lift‑advance‑lower” sequence of the walking beam), preventing direct contact between the billets and the furnace‑bottom refractory, avoiding local overheating, burning, or distortion at the billet bottom, and reducing wear on the furnace structure.
In essence, heating furnace slide blocks act as a “heat‑resistant and wear‑resistant track” for high‑temperature billets – they must not only endure temperatures above 1000°C but also remain flat and stable under repeated pushing and friction.
Core Functions of Heating Furnace Slide Blocks
1. Load‑Bearing and Support
Slide blocks directly carry the weight of high‑temperature steel billets and transfer the load to the water‑cooled beam system. In large furnaces, they must support several tons or even dozens of tons of billet weight while maintaining dimensional stability without deformation or sagging at high temperatures.
2. Guiding and Transport
In walking‑beam furnaces, slide blocks work with the walking beam’s stepping motion to achieve smooth billet movement. The surface flatness and wear resistance of the slide blocks directly determine whether billets can pass through the furnace without obstruction.
3. Reducing “Water‑Cooled Black Marks”
The water‑cooling system inside the furnace beams causes the billet bottom above the beams to be cooler, creating so‑called “water‑cooled black marks.” High‑quality slide blocks, through well‑designed structures (e.g., raised pads, perforations), increase thermal resistance and raise the top‑surface temperature of the blocks, thereby reducing the temperature difference between the billet surface and its centre. Studies show that optimised slide blocks can keep the black‑mark temperature difference within 30–40°C.
4. Protecting Billet Surface Quality
Slide blocks prevent direct contact between the billet and the furnace‑bottom refractory or water beams, avoiding scratches, indentations, and oxide‑scale adhesion on the billet bottom. Once a slide block wears out, cracks, or detaches, it will cause scratches (slide‑track black marks) on the billet surface, severely affecting the finished product yield.
Material Selection for Heating Furnace Slide Blocks
Material selection is the primary consideration in slide‑block design. Since temperature varies significantly along the furnace length, different zones require “zoned matching” of different materials to balance performance and cost.
Main Material Series
1. Cobalt‑Based Alloy Series (Co20 / Co40 / Co50)
Cobalt‑based alloys represent the premium material choice for heating furnace slide blocks. With excellent high‑temperature strength, thermal‑fatigue resistance, and oxidation resistance, they are widely used in the metallurgical and heat‑treatment industries.
| Grade | Cobalt Content | Continuous Service Temp. | Key Properties | Typical Applications |
|---|---|---|---|---|
| Co20 | ~20% | 800–1000°C | Good cost‑performance, basic oxidation and wear resistance | Preheating zone, light‑load pusher furnaces |
| Co40 | ~40% | 1000–1200°C | Outstanding thermal shock and creep resistance, strengthened with tungsten | Heating‑zone load‑bearing slide blocks |
| Co50 | ~50% | 1000–1300°C (peak 1350°C) | Superior high‑temperature wear and corrosion resistance, densest carbide hard phase | Soaking‑zone heavy‑duty slide blocks |
Co20 is positioned as an economical grade within the cobalt‑based series, suitable for moderate temperatures and light loads – for example, in preheating zones. It is a cost‑effective alternative to the more expensive Co40/Co50. Co40, with its excellent thermal‑fatigue resistance, can handle the thermal cycling stresses caused by frequent lifting and lowering of the walking beam. Co50, which directly contacts high‑temperature billets, significantly reduces surface scratching.
2. Chromium‑Nickel Heat‑Resistant Cast Steels
ZG40Cr25Ni20Si2 (also known as 2520 / 310S) is one of the most widely used heat‑resistant cast steels. Containing 25% chromium, 20% nickel, and 2% silicon, it achieves a good balance of high‑temperature strength, oxidation resistance, and thermal‑fatigue resistance, with a continuous service temperature of 1100–1150°C. For conventional high‑temperature environments around 1200°C, 2520 is a proven and reliable choice.
ZG40Cr28Ni48W5 (2848W5) is a high‑nickel‑chromium‑tungsten austenitic heat‑resistant cast steel designed for extreme high‑temperature conditions from 900°C to 1200°C. It offers excellent creep resistance and is suitable for heat‑treatment furnace pads in demanding applications.
3. New Composite Materials
In recent years, composite heat‑resistant slide blocks have become a technological trend. For example, composite slide blocks made of Sialon‑bonded SiC offer high strength, excellent thermal‑shock stability, and superior oxidation resistance. They undergo minimal temperature fluctuation during repeated contact with hot billets, significantly reducing temperature differences in the red‑hot billet and improving the quality of steel products. Another example is the composite slide block for walking beams, featuring a cermet layer combined with a nickel‑chromium alloy layer, offering extremely high high‑temperature strength, oxidation resistance, and wear resistance.
Zone‑Based Selection Recommendations
In practice, different temperature zones within a heating furnace should use different slide‑block materials:
- Charging end / Preheating zone (≤1000°C): Prioritise Co20 slide blocks/pads to effectively control costs while meeting support and wear requirements.
- Heating zone (1000–1150°C): Mostly use Co40 slide blocks to take advantage of their excellent thermal‑fatigue resistance under frequent beam lifting.
- Soaking zone / Discharging end (≥1200°C): Must use Co50 pads/blocks to prevent high‑temperature softening and collapse, and to minimise black marks or mechanical scratches on the billet bottom.
Structural Design of Heating Furnace Slide Blocks
Common Structural Types
- Rectangular slide blocks: Standard shape, strong versatility, suitable for conventional furnaces.
- Trapezoidal slide blocks: Wider top face and narrower bottom, reducing the contact area with the water beam to increase top‑face temperature.
- Slide blocks with mounting holes / dovetail grooves: Accurate positioning and installation, suitable for walking‑beam furnaces.
- Perforated slide blocks: Holes in the middle reduce the cooling effect of the water beam, increasing the surface temperature of the block and reducing “black marks” on the steel.
- Mushroom‑head slide blocks: Wide top and narrow bottom; used when the block width is ≥50 mm.
Key Design Considerations
- Height design: Increasing the pad height to 70–120 mm reduces thermal resistance, narrowing the temperature difference between the billet surface and centre. Large furnaces often use clip‑fixed heat‑resistant slide rails with heights below 120 mm.
- Bottom design: The bottom surface of the slide block is typically designed as an arc with the same radius as the longitudinal water pipe, achieving a gapless fit.
- Welding structure: The arc surface of the heat‑resistant block is welded to the outer wall of the water beam, often using a combination of TIG welding and manual arc welding.
- Thermal‑expansion relief: Features such as half‑slots effectively relieve internal longitudinal expansion forces in the heat‑resistant block, preventing fracture of the longitudinal water pipe.
Arrangement and Layout
Slide blocks are fixed on the bottom longitudinal water pipes of pusher‑type furnaces, typically spaced at 40–500 mm intervals, arranged in single‑line sequential order or double‑row staggered patterns.
Manufacturing Processes
The manufacturing process directly affects product quality and service life. Mainstream techniques include:
- Investment casting (lost‑wax casting): High dimensional accuracy, enables integrated forming of complex irregular shapes, ensuring uniform performance. Dimensional tolerances can be controlled to ±0.05 mm.
- Silica‑sol investment casting: Uses inorganic silica‑sol binders, suitable for high‑precision, complex‑shaped slide blocks.
- Sand casting: Suitable for large castings.
- Lost‑foam casting: Suitable for complex structural parts.
- Centrifugal casting: Dense microstructure with fewer defects.
Reputable manufacturers perform spectral material analysis on each batch to ensure composition compliance, and finished products undergo full stress‑relief treatment. Precision‑cast slide blocks can achieve tolerances within ±1 mm.
Usage and Maintenance
Pre‑Use Preparation
- Co20 and similar pads should be pre‑baked for 2–3 hours to remove internal moisture.
- Workpieces must be cleaned to avoid contaminating the slide‑block surface.
- Inspect blocks for cracks, deformation, or accumulated debris.
During Operation
- Monitor creep behaviour at high temperatures to prevent deformation.
- Pay attention to the weld condition between the block and the water beam – block detachment is primarily caused by weld failure at the connection.
- Poor heat resistance or insufficient strength of the block material will directly affect equipment life.
Regular Inspection
Slide blocks undergo creep, wear, and deformation during long‑term high‑temperature service. Inspection items include:
- Remaining height (directly affects the contact area between billet and block)
- Wear amount (calculated by comparison with original dimensions)
- Flatness (smoothness of the top surface)
- Relative elevation differences between blocks
Extending Service Life
Jinan Steel’s medium‑plate plant has successfully extended furnace campaign life through strict operating and thermal practices, with rational maintenance schedules and repair‑or‑replace strategies, saving RMB 3 million per furnace per year in maintenance costs. Using Co50 heat‑resistant slide blocks effectively reduces water‑cooled black marks and scratches on billets, improves rolling yield, and significantly lowers both block consumption and equipment maintenance costs.
Failure Analysis and Prevention
Common failure modes of heating furnace slide blocks include:
Wear: Sliding friction from high‑temperature billets gradually wears the block surface; when wear exceeds the threshold, replacement is required.
Cracking: In one steel plant, inspection revealed that 334 out of 448 heat‑resistant steel slide blocks had varying degrees of cracks, mostly at the block ends. Causes include thermal‑fatigue stress, casting defects, and weld quality issues.
Sagging deformation: In high‑temperature zones continuously above 1200°C, blocks may sag and develop plough‑groove wear on sliding surfaces within 30 days.
Weld fracture: Joining cobalt‑based superalloys to low‑carbon steel water beams involves dissimilar‑metal welding, which is technically challenging and requires careful selection of welding consumables and procedures.
Preventive measures include: zoned material selection (Co50 for high‑temperature zones), optimised welding procedures, regular inspection and timely replacement, and strict furnace‑temperature control.