CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding of Continuous Casting Guide Rolls at Baosteel — Technical Analysis of Electroslag and Strip Cladding Methods
Literature Overview
This study examines the application of strip cladding technology to the guide rolls of a slab continuous casting machine at Baosteel (China Baowu Steel Group). Guide rolls in continuous casting machines are subjected to extreme conditions: high temperatures (up to 1,500°C at the roll surface), corrosive slag attack, and continuous mechanical loading from the moving slab. The base material is typically a high-chromium cast iron or a steel-ceramic composite, while the cladding layer provides enhanced wear and corrosion resistance. The study compares electroslag welding (ESW) strip cladding with conventional welding methods and evaluates the metallurgical quality, service life, and economic efficiency of the strip cladding approach.
Technical Background and Material Selection
Continuous casting guide rolls must withstand:
- Thermal cycling from 20°C (ambient) to 1,500°C (slab contact)
- Slag erosion from calcium-silicate-based casting slag
- Mechanical impact from slab weight (typically 2–5 tonnes per slab)
- Thermal shock from water cooling
The base roll material is typically HT300 gray cast iron or a high-chromium white cast iron (e.g., Cr20 equivalent). The cladding layer is selected from high-alloy materials such as:
- 1Cr13 martensitic stainless steel for moderate wear and corrosion resistance
- 4Cr13 or 9Cr18 for higher hardness requirements
- Ni-Cr alloy (e.g., Stellite equivalent) for severe slag attack environments
| Component | Material Specification | Key Properties |
|---|---|---|
| Roll base | HT300 gray cast iron | Tensile strength 300 MPa, castability |
| Cladding layer (standard) | 1Cr13 martensitic SS | Hardness 50–55 HRC after H&T |
| Cladding layer (severe service) | Ni-Cr alloy (Stellite 6) | Hardness 40–45 HRC, excellent corrosion resistance |
| Cladding thickness | 6–10 mm | Sufficient for 2–3 regrinding cycles |
Strip Cladding Process Analysis
Strip cladding using electroslag welding (ESW) offers several advantages over conventional arc welding for this application:
- Uniformity: The electroslag process produces a highly uniform deposit with consistent composition and microstructure across the entire cladding width.
- Low dilution: The slag pool acts as a thermal barrier, limiting dilution to 5–10%, which preserves the corrosion and wear resistance of the overlay alloy.
- High productivity: ESW strip cladding achieves deposition rates of 5–8 kg/h, significantly higher than GTAW or GMAW methods.
- Low distortion: The steady-state thermal profile of ESW produces minimal angular distortion, critical for maintaining roll geometry.
The typical ESW strip cladding parameters for guide roll application are:
| Parameter | Value | Notes |
|---|---|---|
| Welding current | 800–1,200 A | Depends on strip width |
| Voltage | 35–45 V | Maintains stable slag pool |
| Travel speed | 200–400 mm/min | Adjusted for deposit thickness |
| Strip width | 30–50 mm | Matches roll circumference |
| Preheat | 200–300°C | Prevents cracking in cast iron base |
| Post-weld cooling | Controlled furnace cool | Avoids thermal shock cracking |
Quality Assessment and Microstructure
The metallurgical quality of the ESW strip cladding is evaluated through:
- Macrostructure: The overlay should show uniform grain structure without segregation or banding. The interface between the base and overlay should be flat and free of cracks.
- Microstructure: The overlay should consist of martensite with retained austenite (for 1Cr13) or austenite with carbides (for Ni-Cr alloy). Carbide morphology is critical — fine, uniformly distributed carbides provide superior wear resistance.
- Hardness profile: A hardness gradient from the base (200 HB) through the transition zone to the overlay (500–600 HV) is expected. The transition zone should be at least 0.5 mm thick to accommodate thermal cycling without cracking.
- Bond strength: Tensile bond strength should exceed 400 MPa, verified by extraction testing on representative coupons.
Non-destructive testing includes ultrasonic testing (UT) for internal defects, magnetic particle inspection (MT) for surface cracks, and dimensional verification of the cladding thickness using ultrasonic thickness gauges.
Service Performance and Life Extension
The study reports that ESW strip cladded guide rolls achieve a service life of 15,000–25,000 slabs, compared to 5,000–8,000 slabs for uncladded rolls. This represents a 3–4 fold life extension, which translates to significant economic savings in a continuous casting operation. The cladding layer can be reground 2–3 times during its service life to restore the roll diameter, further extending the total number of slabs produced.
A key finding is that the failure mode of cladded rolls shifts from surface wear (dominant in uncladded rolls) to subsurface fatigue cracking. This is a beneficial failure mode because it provides a warning period — the crack initiates at the cladding-substrate interface and propagates slowly, allowing for scheduled replacement before catastrophic failure.
Key Engineering Considerations
The successful application of strip cladding to continuous casting guide rolls requires careful attention to several engineering factors:
- Thermal management: The cladding process must not introduce residual stresses that would promote thermal fatigue cracking during service. Post-weld stress relief at 600°C for 2 hours is recommended.
- Interface integrity: The cladding-substrate interface is the critical region for fatigue initiation. Any lack of fusion or porosity at this interface must be eliminated through rigorous NDT.
- Regrinding strategy: The cladding thickness must be designed to accommodate 2–3 regrinding cycles of 0.5–1.0 mm each, with a minimum remaining cladding thickness of 2 mm after the final grind.
- Roll balancing: After cladding, the roll must be dynamically balanced to ISO 1940 G2.5 grade to prevent vibration at operating speeds.
Summary and Reflections
The Baosteel guide roll cladding study demonstrates that ESW strip cladding is a mature and reliable technology for extending the service life of critical continuous casting components. The combination of low dilution, high productivity, and uniform microstructure makes it superior to alternative cladding methods for this application. The key to success lies in rigorous process control — from consumable qualification through to final NDT verification — and in designing the cladding thickness to accommodate the full service life including regrinding cycles. Engineers responsible for cladding specifications in continuous casting applications should prioritize interface integrity and thermal management over maximum overlay hardness, as the latter provides diminishing returns while the former directly determines service life.