CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Sintered Flux for Electroslag Welding Cladding of Continuous Casting Rolls
Literature Overview
Continuous casting rolls are critical components in steelmaking production lines, operating under extreme thermal cycling, mechanical loading, and corrosion from molten steel and flux. The overlay layer on roll surfaces must simultaneously exhibit high hardness, thermal fatigue resistance, and corrosion resistance. Electroslag welding (ESW) cladding is one of the most widely adopted methods for producing thick overlay layers on large-diameter rolls due to its high deposition rate and deep penetration characteristics. The sintered flux used in ESW cladding directly influences the dilution rate, microstructure evolution, and final performance of the overlay layer. This study note focuses on the development and optimization of sintered flux formulations specifically tailored for ESW cladding of continuous casting rolls.
Core Technical Approach
The fundamental challenge in developing sintered flux for ESW cladding lies in balancing several competing requirements: controlling dilution of the base metal into the overlay, ensuring stable slag composition during multi-pass welding, maintaining adequate fluidity of the slag pool, and producing a cladding layer with the desired microstructure and hardness profile. The research employs a systematic approach combining thermodynamic calculations, experimental trial welding, and microstructural characterization.
Flux Composition Design
The sintered flux composition typically includes the following major components:
| Component | Typical Range (wt%) | Function |
|---|---|---|
| SiO₂ | 12–20 | Slag former, viscosity control |
| CaO | 25–35 | Slag former, dilution control |
| Al₂O₃ | 8–15 | Refractoriness, slag stability |
| CaF₂ | 10–18 | Arc stability, slag fluidity |
| TiO₂ | 3–8 | Deoxidation, slag viscosity |
| MnO | 5–12 | Alloying, slag viscosity |
| Fe₃O₄ | 2–5 | Slag fluidity at low temperature |
The CaO/SiO₂ ratio is particularly critical as it directly governs the basicity of the slag, which in turn affects the selectivity of slag formation and the dilution rate of the base metal into the weld overlay. A higher basicity generally reduces dilution but may compromise slag fluidity at lower temperatures.
Dilution Control Strategy
Dilution is the primary factor determining whether the cladding layer achieves the desired alloy composition and microstructure. In ESW cladding, dilution rates typically range from 10% to 35%, which is significantly higher than in gas metal arc welding (GMAW) or plasma transferred arc (PTA) processes. The sintered flux controls dilution through several mechanisms:
- Selective slag formation: The flux composition is designed to preferentially absorb elements from the base metal (such as carbon and manganese from carbon steel substrates) into the slag phase.
- Slag-metal interface chemistry: The activity coefficients of alloying elements at the slag-metal interface determine the partitioning behavior.
- Slag viscosity: Higher viscosity reduces convective mixing between the molten pool and the base metal, thereby lowering dilution.
The study demonstrates that optimizing the CaF₂ content to 14–16 wt% combined with a CaO/SiO₂ ratio of 2.2–2.8 achieves dilution rates as low as 12–18% for stainless steel overlay on carbon steel substrates, which is a significant improvement over conventional fluxes that typically yield 25–30% dilution.
Process Parameters and Their Influence
ESW Cladding Process Parameters
| Parameter | Typical Range | Effect on Cladding Quality |
|---|---|---|
| Welding current | 350–600 A | Higher current increases dilution and penetration |
| Travel speed | 80–200 mm/min | Higher speed reduces dilution but may cause incomplete fusion |
| Electrode feed speed | 200–400 mm/min | Must be synchronized with travel speed |
| Arc voltage | 30–45 V | Affects slag pool stability and pool shape |
| Number of passes | 2–5 | First pass has highest dilution; subsequent passes lower |
| Preheating temperature | 150–250 °C | Reduces residual stress and cracking tendency |
Multi-Pass Strategy
In multi-pass ESW cladding, the first pass experiences the highest dilution because the molten pool is in direct contact with the bare base metal. Subsequent passes benefit from the alloying effect of the previous pass, progressively reducing dilution. The study recommends a minimum of three passes for achieving uniform composition throughout the cladding layer thickness, with the first pass serving as a transition layer.
Microstructural Evolution and Performance
Microstructure of the Cladding Layer
The overlay layer microstructure is governed by the cooling rate, solidification rate, and final composition after dilution. For a 316 stainless steel overlay on carbon steel substrate:
- Surface layers (passes 3–5): Dendritic austenite with 15–25% ferrite, hardness 220–260 HV
- Middle layers (pass 2): Mixed austenite-ferrite with some carbide precipitation, hardness 240–280 HV
- Transition layer (pass 1): Higher ferrite content (30–40%), possible formation of martensite due to higher carbon dilution, hardness 280–320 HV
Thermal Fatigue Performance
The cladding layer must withstand repeated thermal cycling between ambient temperature and 1200–1400 °C (molten steel contact temperature). The study reports that the optimized flux produces a cladding layer with:
- Thermal fatigue life exceeding 8000 cycles at 1200 °C peak temperature
- Crack initiation temperature of approximately 950 °C
- Linear thermal expansion coefficient matching within 5% of the base steel
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in cladding layer | High dilution causing martensitic transformation | Optimize flux basicity; increase ferrite content |
| Poor bond strength | Incomplete fusion at interface | Increase welding current; ensure proper preheating |
| Slag inclusions | Inadequate slag removal between passes | Improve slag removal procedure; adjust slag viscosity |
| Pore formation | Moisture in flux or base metal contamination | Control flux storage; clean base metal surface |
| Hardness variation | Non-uniform dilution across passes | Implement multi-pass strategy; control travel speed |
Integration with Engineering Practice
In industrial applications, continuous casting rolls typically require cladding thickness of 3–8 mm, with the overlay material selected based on the specific service conditions:
- Mold rolls: 310 stainless steel overlay for high-temperature oxidation resistance
- Secondary cooling zone rolls: 316 or 321 stainless steel for corrosion resistance in water environment
- Support rolls: High-alloy martensitic stainless steel (e.g., 410H) for wear resistance
The developed sintered flux has been validated on production rolls with diameters ranging from 400 mm to 800 mm, demonstrating reliable performance across different roll sizes and overlay materials. The economic benefit is substantial: the reduced dilution rate means less expensive overlay wire is consumed per unit area, while the improved thermal fatigue life extends roll service intervals from approximately 120 to 180 casting heats.
Key Technical Insights and Reflections
The most significant finding from this study is that the CaO/SiO₂ ratio in the sintered flux serves as the primary lever for dilution control, but it cannot be adjusted in isolation. The interplay between basicity, CaF₂ content, and slag viscosity creates a complex optimization landscape. A systematic approach using response surface methodology or similar optimization techniques is essential for identifying the true optimum rather than relying on trial-and-error experimentation.
Another important insight is that the transition layer (first pass) remains the weakest link in the cladding system regardless of flux optimization. The study suggests that even with optimized flux, the first pass will inevitably have higher dilution and potentially inferior properties. This reinforces the importance of the multi-pass strategy and the concept of designing the transition layer as a functional gradient rather than expecting uniform properties throughout the cladding thickness.
The practical implication for engineers is that flux selection should not be treated as a standalone decision but should be integrated with the overall cladding process design, including wire selection, parameter optimization, and post-weld treatment. The sintered flux is a powerful tool for dilution control, but it cannot compensate for fundamental process design errors such as inadequate multi-pass planning or improper preheating.
Conclusion
The development of sintered flux for ESW cladding of continuous casting rolls represents a mature but still evolving technology. The key to success lies in understanding the fundamental metallurgical principles governing slag-metal interactions and translating this knowledge into practical flux formulations that can be consistently produced and applied in industrial settings. The systematic approach combining thermodynamic analysis, experimental optimization, and field validation provides a robust framework for flux development that can be adapted to various cladding applications beyond continuous casting rolls.