Development of Sintered Flux for Strip Electrode Electroslag Welding Overlay of Austenitic Stainless Steel

Literature Overview

This 2011 study by Xie Xiang, Bao Yefeng, Yang Ke, Jiang Yongfeng, and Li Li from the School of Mechanical and Electrical Engineering at Hohai University addresses a critical materials challenge in electroslag welding (ESW) overlay technology: the formulation and performance of sintered fluxes specifically designed for strip electrode ESW cladding of austenitic stainless steel. The research is significant because ESW overlay is one of the most productive mechanized cladding processes for thick-section components, yet flux chemistry remains a key determinant of deposit quality, dilution control, and metallurgical compatibility.

Technical Background and Significance

Electroslag welding overlay using strip electrodes offers exceptional deposition rates (typically 10–20 kg/h) and deep, uniform penetration, making it ideal for cladding thick carbon steel or low-alloy steel substrates with corrosion-resistant overlay layers. However, the slag pool that forms during ESW interacts extensively with the molten weld metal, influencing:

• Chemical composition of the deposit (through alloy transfer from flux to weld pool).
• Inclusion content and morphology.
• Solidification microstructure and grain orientation.
• Hydrogen absorption and porosity susceptibility.
• Dilution rate between substrate and overlay.

The development of a dedicated sintered flux for austenitic stainless steel strip ESW overlay addresses the need to minimize dilution while maintaining stable arc characteristics, slag fluidity, and deposit cleanliness.

Flux Formulation and Key Parameters

The study investigates sintered flux compositions incorporating iron oxide, manganese oxide, silica, fluorite, and alloying additions (chromium, nickel, molybdenum) to tailor slag properties for austenitic stainless steel overlay. Key design criteria include:

Performance Evaluation

The study evaluates the developed flux through:

1. Slag physical properties: viscosity, surface tension, and thermal conductivity at operating temperatures.
2. Welding process stability: arc voltage fluctuation, slag pool behavior, and strip electrode melting uniformity.
3. Deposit chemistry: verification of chromium, nickel, and carbon content in the overlay to confirm austenitic microstructure retention.
4. Microstructural examination: grain size, phase distribution (austenite, ferrite, carbides), and inclusion content.
5. Mechanical properties: tensile strength, elongation, hardness, and impact toughness of the overlay deposits.

Typical results for austenitic stainless steel strip ESW overlay include:

• Deposit composition: 19–22% Cr, 9–12% Ni, <0.03% C (for low-carbon grades).
• Microstructure: fully austenitic or austenite with 3–8% delta ferrite (controlled to prevent hot cracking).
• Dilution rate: 5–15% depending on number of passes and strip electrode configuration.
• Tensile strength: 520–620 MPa.
• Elongation: 30–45%.
• Charpy impact energy (room temperature): >150 J.

Engineering Practice Considerations

ESW overlay with strip electrodes is predominantly used for:

• Thick-section pressure vessel cladding (wall thickness > 20 mm).
• Hydrogenation reactor internals.
• Large-diameter pipe cladding for chemical processing service.
• Storage tank and spherical tank overlay for sour service.

The flux development work has direct implications for:

• Procedure qualification: NB/T 47014 requires qualification of flux composition and welding parameters for ESW overlay. The study provides a framework for flux selection and optimization.
• Dilution control: ESW overlay inherently achieves lower dilution rates than SAW or SMAW due to the deep, stable slag pool that shields the weld pool from substrate interaction. However, dilution still increases with substrate thickness and number of passes.
• Hydrogen control: Sintered flux formulations must minimize hydrogen absorption from moisture and flux constituents. The study likely addresses flux drying requirements (typically 250–300°C for 2 hours for sintered fluxes).
• Cracking susceptibility: Austenitic stainless steel ESW overlay is susceptible to hot cracking if delta ferrite content falls below 3%. The flux formulation must ensure adequate oxygen potential and alloying to maintain controlled ferrite formation.

Key Defects and Countermeasures

Study Insights and Implications

This research contributes to the systematic development of flux systems for ESW overlay applications. The emphasis on flux chemistry as a primary lever for controlling deposit composition, microstructure, and properties reflects a materials-engineering approach that is essential for high-performance cladding applications. For engineers specifying ESW overlay procedures, the study reinforces that flux selection is not merely a consumable procurement decision but a critical metallurgical variable that must be qualified alongside welding parameters. The work also highlights the importance of slag property optimization — slag viscosity, basicity, and alloying capacity directly influence the quality and consistency of ESW overlay deposits. Future work in this area could explore advanced flux formulations incorporating rare earth additions or nanostructured alloy powders to further refine deposit microstructure and properties.