Development and Application of High-Speed Electroslag Overlay Welding

Literature Overview

The paper by Zhang Liangcheng and Ding Bixue from Shanghai Boiler Works Co., Ltd., published in Boiler Technology (2003, Vol. 34, Issue 5), reviews the development and industrial application of high-speed electroslag welding (ESW) for overlay applications. The study traces the evolution of ESW from its initial proposal at the 1982 IIW conference to its mature application in heavy industry, with particular emphasis on the breakthrough development of new flux formulations that enabled welding speeds up to 350 mm/min and single-pass deposition thicknesses as low as approximately 3 mm. The paper also discusses the incorporation of metal powders into the flux for wear-resistant overlay applications, representing a significant advancement in the versatility of the ESW process.

Core Technical Content and Process Principles

Electroslag welding for overlay applications operates on the principle of electrothermal heating, where the heat is generated by the resistance of the molten slag pool rather than by the arc itself. The consumable wire (or strip) is fed through the slag pool, and the electrical resistance of the slag provides the heat necessary to melt the wire and a small amount of the base metal. This fundamental difference from arc welding processes results in several distinctive characteristics that are particularly advantageous for overlay applications.

The development of advanced flux formulations was the critical enabling technology for high-speed ESW overlay welding. Traditional ESW fluxes were designed for structural steel welding and did not provide adequate wetting and stability for the high-speed operation required in overlay applications. The new flux formulations were specifically designed to work in conjunction with stainless steel and nickel-based welding strips, providing consistent slag pool stability, adequate heat generation, and proper gas shielding at welding speeds of up to 350 mm/min.

Engineering Applications and Process Advantages

The primary application areas for high-speed ESW overlay welding include:

• Boiler tube overlays for improved corrosion and erosion resistance in high-temperature service
• Large-diameter pipe overlay for chemical processing equipment
• Wear-resistant overlay on large rotating components such as grinding rolls and mill liners
• Corrosion-resistant overlay on pressure vessel heads and nozzles
• Thick-section overlay where multiple layers of conventional arc welding would be impractical

The single-pass capability of high-speed ESW is particularly significant for overlay applications because it reduces the number of passes required, thereby minimizing the total heat input and reducing the risk of distortion. The low dilution rate achievable with the advanced flux formulations is another critical advantage, as it ensures that the overlay composition meets the required specifications for corrosion or wear resistance.

The incorporation of metal powders into the flux represents a particularly innovative application of ESW technology for wear-resistant overlay. By adding carbide-forming elements such as chromium, molybdenum, tungsten, and cobalt directly into the flux, the effective composition of the deposited metal can be precisely controlled without requiring expensive alloy wire consumables. This approach is especially cost-effective for large-scale overlay applications where the volume of deposited metal is substantial.

Quality Control and Process Considerations

The quality of ESW overlay welds is governed by several critical factors that must be carefully controlled:

1. Flux composition and moisture content must be maintained within specified limits to ensure consistent slag pool stability and prevent porosity formation.
2. The wire feed speed and travel speed must be precisely synchronized to maintain the proper slag pool geometry and heat input.
3. The backing gas (typically Ar or Ar/CO2 mixture) must provide adequate shielding to prevent oxidation of the deposited metal.
4. Interpass temperature must be controlled to prevent excessive grain growth and maintain the mechanical properties of the overlay.
5. Non-destructive testing, including radiographic testing (RT) and ultrasonic testing (UT), must be performed to detect internal defects such as slag inclusions and porosity.

The microstructure of ESW overlay welds typically consists of a fine-grained structure due to the rapid solidification from the solid base metal, with the grain structure being columnar near the fusion line and equiaxed in the center of the weld. The hardness distribution across the overlay is generally uniform when proper process parameters are used, with slight variations near the fusion line due to dilution effects.

Summary and Engineering Implications

This study documents a significant advancement in ESW overlay welding technology, demonstrating that the combination of advanced flux formulations and optimized process parameters can achieve welding speeds and deposition thicknesses that were previously unattainable. The ability to perform single-pass overlay welding at speeds up to 350 mm/min represents a major productivity improvement that can significantly reduce manufacturing costs for large-scale overlay applications. The incorporation of metal powders into the flux further extends the versatility of the process, enabling the production of wear-resistant overlays with precisely controlled compositions. For engineers involved in the design and fabrication of heavy industrial equipment, high-speed ESW overlay welding offers a compelling solution that combines high productivity with excellent metallurgical quality, provided that the process parameters are carefully optimized and quality control protocols are rigorously implemented.