Centrifugal casting has long been recognized as a highly effective method for producing high-performance mill rolls used in demanding rolling mill environments. Among the various centrifugal casting techniques, horizontal centrifugal casting followed by vertical secondary pouring stands out for its ability to create composite mill rolls with optimized material properties across different zones—delivering exceptional surface hardness, wear resistance, and structural integrity where it matters most.
Understanding the Dual-Pour Centrifugal Casting Process
The process begins with a horizontally mounted, rapidly rotating mold that simulates the outer contour of the final mill roll. Molten metal—typically a high-alloy white iron or high-chromium cast iron—is poured into this spinning mold. Centrifugal force pushes the liquid metal outward against the mold wall, ensuring dense, homogeneous solidification with minimal porosity or inclusions. This forms the working layer (or shell) of the mill roll, which directly contacts the hot steel during rolling operations.
Once the outer shell has cooled to a controlled temperature—usually between 900°C and 1050°C, depending on the alloy system—the mold is carefully rotated to a vertical orientation. At this stage, a second melt, often a ductile iron or low-alloy steel with superior toughness and tensile strength, is poured into the central cavity to form the core and neck regions. This dual-material approach leverages the best attributes of each alloy: extreme surface hardness from the outer layer and impact resistance from the inner core.
Material Selection and Performance Characteristics
The success of centrifugally cast mill rolls hinges on precise material pairing. The working layer must resist thermal fatigue, mechanical wear, and spalling under cyclic loading at elevated temperatures. Meanwhile, the core must absorb shock loads without cracking and maintain dimensional stability over thousands of rolling cycles.
Common combinations include:
- Outer Layer: High-chromium white iron (e.g., Cr15Mo3, Cr20Mo2), ledeburitic tool steel, or nickel-hard type alloys.
- Core Material: Spheroidal graphite (ductile) iron (e.g., EN-GJS-600-3), low-carbon steel, or medium-carbon alloy steel.
These pairings enable mill rolls to achieve surface hardness values exceeding 60 HRC while maintaining core elongation above 3%—a balance unattainable with monolithic castings.
Typical Specifications for Centrifugally Cast Mill Rolls
Below is a representative table of technical parameters for centrifugally cast composite mill rolls used in long product rolling mills (e.g., bar, rod, and section mills). These values reflect industry standards based on operational data from major steel producers and foundry practices.
| Parameter | Working Layer | Core / Neck | Notes |
|---|---|---|---|
| Material Type | High-Cr White Iron (Cr15–28%) | Ductile Iron (EN-GJS-600-3) | Alternative: Low-alloy steel for heavy-duty applications |
| Hardness (HRC) | 58–65 | 28–35 | Measured at 10 mm below surface |
| Tensile Strength (MPa) | ≥400 | ≥600 | Core value per EN 1563 |
| Elongation (%) | <1.0 | ≥3.0 | Critical for impact resistance |
| Roll Diameter Range (mm) | 300 – 1200 | Custom sizes available for specialty mills | |
| Working Layer Thickness (mm) | 60 – 150 | Adjustable based on roll life requirements | |
Advantages Over Conventional Casting Methods
Compared to static casting or continuous casting, centrifugal casting offers several distinct advantages for mill roll production:
- Uniform Hardness Profile: The rapid solidification under centrifugal force minimizes carbide segregation, resulting in consistent hardness from the surface to the interface—often within ±2 HRC across the entire working layer.
- Enhanced Cleanliness: Inclusions and gas pores are forced toward the inner diameter (later machined away), yielding a cleaner, denser surface layer critical for fatigue resistance.
- Optimized Material Usage: Expensive high-alloy materials are confined only to the wear-prone zone, reducing raw material costs by up to 30% compared to solid high-alloy rolls.
- Extended Roll Life: Field data from European and Asian steel mills show 20–40% longer service life versus forged or statically cast alternatives in hot bar rolling applications.
Critical Process Controls for Quality Assurance
Successful implementation of dual-pour centrifugal casting demands rigorous control over multiple variables:
- Mold Rotation Speed: Typically 300–800 rpm, calculated based on roll diameter to achieve a G-force of 60–100× gravity. Insufficient speed leads to poor compaction; excessive speed may cause mold erosion.
- Pouring Temperature: Outer layer: 1420–1480°C; Core: 1380–1430°C. Precise thermal management ensures proper bonding at the interface without remelting the shell.
- Shell Cooling Rate: Controlled air or mist cooling is applied during solidification to avoid thermal shock cracks. The shell must retain sufficient heat (≥900°C) to metallurgically bond with the core pour.
- Interface Integrity: Ultrasonic testing (UT) and macro-etching are standard NDT methods to verify absence of delamination or cold shuts at the bimetal junction.
Applications in Modern Rolling Mills
Centrifugally cast composite mill rolls are predominantly deployed in:
- Bar and Rod Mills: Finishing stands where surface finish and dimensional accuracy are paramount.
- Section Mills: Universal beam and rail mills requiring high load-bearing capacity and resistance to edge chipping.
- Wire Rod Mills: High-speed blocks operating at >100 m/s, demanding excellent thermal fatigue resistance.
Notably, these rolls are less common in cold rolling or plate mills, where through-hardened forged rolls or infinite chill cast iron rolls may be preferred due to different stress regimes.
Future Trends and Technological Refinements
Ongoing advancements focus on enhancing interfacial bonding and expanding material combinations. Recent developments include:
- Use of electromagnetic stirring during core pouring to refine grain structure and improve ductility.
- Integration of real-time thermal imaging to monitor shell temperature before secondary pour, minimizing human error.
- Exploration of functionally graded materials (FGMs) via controlled alloy addition during transition pouring, creating a gradual hardness gradient instead of a sharp interface.
Such innovations aim to further extend roll life in next-generation high-productivity mills operating under increasingly severe conditions.
For mill operators and maintenance engineers, selecting centrifugally cast composite mill rolls represents a strategic investment in uptime, product quality, and total cost of ownership. When sourced from foundries with proven process control and metallurgical expertise, these components deliver reliable performance across thousands of tons of rolled product—making them a cornerstone of efficient long-product steelmaking.