CLADDING TECHNOLOGY SHANXI CO., LTDElectroslag Strip Cladding Flux and Process Research Study Note
Literature Overview
This 1995 paper published in the Journal of Iron and Steel Research by a researcher from the Central Iron and Steel Research Institute (CISRI) investigates the development of specialized flux formulations and process parameters for electroslag welding (ESW) strip cladding. Funded as part of China's National "8.5" Key Technology Program, this research addresses the industrial-scale production of clad plates using the electroslag process, which offers high deposition rates and consistent quality for thick cladding layers.
Core Technical Approach
Electroslag welding strip cladding (also known as ESW overlay or strip cladding) is a highly productive process for producing clad plates with thick overlay layers (typically 5-25 mm). The process uses consumable strip electrodes fed through an electroslag pool, which provides excellent heat input control and uniform weld bead profiles. Unlike conventional ESW used for thick-section structural welding, strip cladding ESW requires specialized flux formulations that promote controlled dilution, favorable microstructure in the overlay, and resistance to slag inclusion defects.
The key innovation in this research is the development of flux compositions that address the specific metallurgical requirements of cladding applications. Unlike structural welding fluxes optimized for mechanical properties of the weld metal, cladding fluxes must be formulated to:
- Control dilution between the strip electrode and base metal
- Promote favorable solidification microstructure in the overlay
- Prevent slag inclusions and porosity
- Provide adequate slag fluidity for stable process operation
- Resist spalling and spitting during high-current operation
Key Technical Parameters and Flux Composition
| Flux Component | Typical Range | Function |
|---|---|---|
| SiO2 | 35-50% | Slag viscosity control, deoxidation |
| CaO | 20-35% | Slag basicity, desulfurization |
| Al2O3 | 5-15% | Slag stability, wetting |
| MnO | 3-8% | Deoxidation, alloy transfer |
| CaF2 | 5-15% | Slag fluidity, arc stability |
| TiO2 | 2-5% | Slag viscosity modification |
| B2O3 | 1-3% | Slag fluidity at lower temperatures |
| Process Parameter | Typical Range | Purpose |
|---|---|---|
| Welding current | 800-2000 A | Control heat input and dilution |
| Arc voltage | 30-40 V | Maintain slag pool stability |
| Travel speed | 200-500 mm/min | Balance deposition rate and quality |
| Strip electrode width | 20-50 mm | Match to weld bead width |
| Strip electrode thickness | 1.5-4.0 mm | Control deposition profile |
| Slag pool depth | 20-40 mm | Ensure complete shielding |
| Electrode stick-out | 10-20 mm | Control electrical resistance heating |
The relationship between flux composition and process performance is complex. Higher CaO content increases slag basicity, which improves desulfurization but may increase viscosity and reduce fluidity. CaF2 improves fluidity but excessive amounts can lead to fluorine contamination of the weld metal. The optimal flux composition must balance these competing requirements for the specific cladding application.
Process Characteristics and Metallurgical Considerations
ESW strip cladding produces characteristic metallurgical features that must be understood for quality control:
Solidification structure: The overlay typically exhibits columnar dendritic grains growing from the fusion line upward. The high heat input of ESW promotes coarse grain structures unless specific measures are taken (such as multi-pass techniques with intermediate grinding or controlled cooling).
Dilution control: Dilution in ESW strip cladding is typically 10-25%, which is higher than in processes such as TIG or plasma arc welding but lower than in submerged arc welding. The flux composition and process parameters significantly influence dilution levels. Higher currents and slower travel speeds generally increase dilution.
Segregation: The high heat input and relatively slow cooling rates of ESW can promote macrosegregation, particularly of alloying elements that partition to the liquid phase during solidification. This can lead to localized variations in composition and properties within the overlay.
Slag inclusions: Despite the protective slag atmosphere, inclusions can occur if the slag pool becomes contaminated or if the process becomes unstable. Flux composition is critical in maintaining slag pool stability and preventing inclusion formation.
Quality Control and Defect Prevention
| Defect Type | Primary Cause | Prevention Strategy |
|---|---|---|
| Slag inclusion | Slag pool instability, contamination | Optimize flux composition, maintain consistent travel speed |
| Porosity | Gas pickup from flux or base metal | Flux drying, surface cleaning, controlled shielding |
| Cracking | High restraint, unfavorable microstructure | Preheating, post-weld heat treatment, flux modification |
| Excessive dilution | High current, slow travel speed | Parameter optimization, intermediate grinding |
| Uneven surface | Process instability, strip misalignment | Process monitoring, equipment maintenance |
| Bond failure | Incomplete fusion, contamination | Surface preparation, parameter control |
The flux formulation plays a particularly important role in preventing slag inclusions. A well-designed flux maintains appropriate viscosity (typically 10-50 Pa·s at operating temperature) to ensure stable slag pool behavior while allowing adequate gas escape. Too viscous a slag traps gases and inclusions; too fluid a slag may lead to spilling and contamination.
Engineering Application and Production Considerations
ESW strip cladding is widely used for producing clad plates in industrial applications including:
- Stainless steel/carbon steel clad plates for chemical processing equipment
- Nickel-based alloy/carbon steel clad plates for high-temperature service
- Copper/steel clad plates for electrical applications
- Titanium/steel clad plates for marine and aerospace applications
The process is particularly advantageous for producing clad plates with overlay thicknesses exceeding 10 mm, where alternative processes (such as SAW overlay or explosion cladding) become less economical or technically challenging.
Production considerations include:
- Equipment requirements: ESW strip cladding requires specialized equipment with precise strip feed control, electrode positioning, and slag pool monitoring
- Consumable preparation: Flux must be properly dried and stored to prevent moisture pickup; strip electrodes require careful handling to prevent contamination
- Process monitoring: Real-time monitoring of current, voltage, and travel speed is essential for maintaining consistent quality
- Post-weld treatment: Depending on the application, stress relief heat treatment or solution treatment may be required
Key Questions and Reflections
The development of specialized flux formulations for ESW strip cladding highlights an important principle in welding engineering: the flux is not merely a passive shielding medium but an active participant in the metallurgy of the weld. The flux composition directly influences slag properties, alloy transfer, deoxidation, and ultimately the quality of the cladding layer. This understanding has evolved significantly since this 1995 research, with modern computational modeling now enabling more rational flux design.
Another consideration is the transition from this research to modern practice. The ESW strip cladding process described here has been largely supplemented or replaced in many applications by more modern processes such as multi-wire submerged arc welding (SAW), plasma transferred arc (PTA) welding, and laser cladding, which offer better control over dilution, microstructure, and surface quality. However, for very thick cladding layers and large-scale production, ESW strip cladding remains relevant in certain industrial sectors.
The research also raises questions about the long-term performance of ESW-cladded plates. The coarse microstructure and potential segregation issues inherent in the ESW process may affect long-term corrosion resistance and mechanical properties under cyclic loading. Engineers should carefully evaluate whether the process is suitable for the intended service conditions, considering factors such as thermal cycling, fatigue loading, and corrosion environment.
Study Insights and Implications
This research represents an important contribution to the development of industrial-scale cladding technology in China during the 1990s. The focus on flux development demonstrates the fundamental importance of consumable design in achieving desired process outcomes. For contemporary engineers, the key insights are: the critical role of flux composition in ESW strip cladding quality, the need for process optimization across multiple parameters simultaneously, and the importance of understanding the metallurgical consequences of high-heat-input processes. While modern cladding technologies have advanced significantly, the fundamental principles established in this research—particularly regarding flux design and process control—remain relevant for engineers working with electroslag-based cladding processes. The work also serves as a reminder that consumable development is an ongoing challenge that requires continuous research and optimization to meet evolving industrial demands.