Electroslag Weld Overlay Process and Applications in Boiler Technology

Literature Overview

This 1993 publication by Zhang Maolong and Sun Libing from Shanghai Boiler Works, published in the journal "Boiler Technology" (Guolu Jishu), provides a comprehensive treatment of the electroslag welding (ESW) overlay process and its applications in boiler manufacturing. Electroslag welding, originally developed for thick-section vertical welding of structural steel, was adapted for overlay applications to provide corrosion-resistant or wear-resistant linings on boiler components such as water walls, headers, and pressure parts.

Core Technical Principles of ESW Overlay

Electroslag weld overlay relies on the electrical resistance heating of a molten slag pool to melt the base metal and consumable electrode. Unlike conventional arc welding processes, the heat source is not an arc but rather the resistance of the molten slag itself, which produces a deep, narrow, and highly controlled weld pool.

Process Configuration

The ESW overlay setup consists of:

• A consumable electrode (wire or strip) fed into the slag pool
• A flux-covered backing plate to contain the molten slag
• A flux feeder to supply granular or powdered flux
• Power supply with appropriate current characteristics
• Mechanical or manual electrode guidance

Key Process Parameters

Metallurgical Characteristics

The ESW process produces welds with distinctive metallurgical features:

1. Low dilution rates: The deep slag pool provides excellent protection and limits base metal dilution to typically 5–15%, which is advantageous when overlaying dissimilar materials.
2. Uniform microstructure: The slow cooling rate from the deep slag pool promotes coarse-grained but uniform microstructure with reduced risk of hard phases.
3. Low hydrogen content: The slag atmosphere effectively excludes atmospheric hydrogen, reducing the risk of hydrogen-induced cracking.
4. High deposition efficiency: The process achieves deposition rates of 5–15 kg/h, significantly higher than conventional arc welding processes.

Applications in Boiler Technology

The Shanghai Boiler Works application of ESW overlay in boiler manufacturing addressed several specific engineering requirements:

Application Areas

Comparison with Alternative Overlay Processes

The ESW process's primary limitation is its restriction to near-vertical positions, which limits its application to specific boiler component geometries. However, for large-diameter cylindrical components such as headers and drums, the vertical orientation is natural and the process advantages are fully realized.

Process Control and Quality Assurance

Critical Process Control Points

1. Slag pool stability: Maintaining consistent slag pool depth and temperature is essential for uniform weld quality. Process monitoring systems track current, voltage, and travel speed to ensure slag pool stability.
2. Flux consistency: The flux composition and particle size distribution must be controlled to maintain proper slag viscosity and electrical resistance.
3. Surface preparation: The base metal surface must be clean and free of scale, rust, and contamination to ensure proper wetting and bonding of the overlay.
4. Travel alignment: The electrode must maintain precise alignment with the backing plate and the intended overlay path to avoid misruns and incomplete coverage.

Inspection Requirements

• Radiographic testing (RT): For detection of internal defects such as slag inclusion, porosity, and lack of fusion at the interface.
• Ultrasonic testing (UT): For bond strength verification and subsurface defect detection.
• Visual and dimensional inspection: Verification of overlay thickness, width, and surface finish.
• Hardness and tensile testing: Verification of mechanical properties of the overlay layer.
• Intergranular corrosion testing: For stainless steel overlays, per ASTM A263/A264 requirements.

Engineering Practice and Lessons Learned

The adoption of ESW overlay in boiler manufacturing during the early 1990s represented a significant technological advancement for the Chinese boiler industry. The process offered several compelling advantages for large-scale production:

• Productivity: The high deposition rate reduced welding time for large overlay areas, improving throughput in boiler fabrication.
• Quality consistency: The mechanized nature of the process reduced operator variability, leading to more consistent weld quality.
• Material efficiency: Low dilution rates meant less expensive overlay material was consumed per unit area of overlay.
• Reduced post-weld processing: The uniform microstructure and low residual stress of ESW welds often reduced or eliminated the need for post-weld machining.

However, the process also presented challenges:

• Capital investment: The specialized equipment required for ESW overlay represented a significant capital outlay.
• Position limitation: The restriction to vertical positions required careful fixture design and component orientation.
• Process sensitivity: The slag pool dynamics are sensitive to parameter variations, requiring skilled operators and well-controlled conditions.

Summary

This 1993 literature from Shanghai Boiler Works provides a valuable historical perspective on the application of electroslag welding for overlay purposes in boiler manufacturing. The process offered significant advantages in deposition rate, dilution control, and quality consistency for large cylindrical components. While modern processes such as plasma transferred arc (PTA) cladding and laser cladding have partially displaced ESW overlay in some applications, the fundamental principles of process control, slag pool management, and quality assurance described in this literature remain relevant for engineers working with thick-section overlay applications. The document serves as an important reference for understanding the evolution of overlay welding technology in heavy industry.