Research on Domestic Single-Layer Tape Electrode Electroslag Surfacing Welding Technology

Literature Overview

This paper, published in China Chemical Equipment (2016, Vol. 18, No. 6, pp. 14–18), authored by Yang Xiaoli, Zhang Jianxiao, Shi Chenglong, Hou Xirong, and Liao Fangming from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory for Special Materials Welding of Pressure Vessels, presents a systematic comparison study between domestic and imported welding consumables for single-layer tape electrode electroslag surfacing (TES). The study evaluates the performance of domestic consumables (H316LF wire with SJ15F flux from Harbin Institute of Welding) against imported consumables (WELESS 316LJ wire with WEL ESB F-6M flux from Japan) for surfacing the inner wall of large-diameter cylindrical vessels. The results demonstrate that the domestic consumables can achieve performance equivalent to the imported alternatives, enabling import substitution for this critical welding application.

Core Technical Approach

Electroslag Surfacing Process Fundamentals

Electroslag welding (ESW) is a process that uses the heat generated by the resistance of a conductive slag pool to melt the base metal and filler material. When adapted for surfacing (electroslag surfacing), the process deposits thick, uniform overlay layers with excellent metallurgical bonding and low dilution from the base metal. The tape electrode variant uses a continuously fed strip of wire, which allows for very wide bead coverage and high deposition rates, making it ideal for surfacing large cylindrical or flat surfaces.

The key advantages of electroslag surfacing over conventional arc surfacing processes include:

• High deposition rate: Typically 5–10 times higher than submerged arc welding.
• Low dilution: The slag pool acts as a thermal barrier, limiting base metal dilution to the weld metal.
• Uniform microstructure: The slow cooling rate through the slag pool promotes coarse, equiaxed grain structures with low residual stress.
• Excellent bonding: The high heat input ensures complete fusion at the base metal/weld interface.

Consumables Comparison

Single-Layer vs. Multi-Layer Surfacing

The paper specifically addresses single-layer surfacing, which is a significant technical achievement. In conventional surfacing practice, multiple layers are typically required to build up the desired overlay thickness and composition. Single-layer surfacing means that the full overlay thickness (potentially several millimeters) is deposited in one pass, which dramatically reduces production time and cost. However, single-layer surfacing places higher demands on the consumables and process parameters, because there is no opportunity for composition correction through subsequent layers.

The feasibility of single-layer surfacing depends on several factors:

1. Bead width-to-thickness ratio: The tape electrode allows for wide beads, and the electroslag process provides deep penetration, enabling thick single-pass deposits.
2. Solidification cracking resistance: The 316L composition has excellent solidification cracking resistance due to its high nickel and low carbon content, which is essential for single-layer surfacing where the weld metal solidifies from a wide, deep pool.
3. Porosity control: The electroslag process provides excellent protection against atmospheric contamination, and the slag pool helps to absorb hydrogen and other gas-forming elements.

Interpretation of Technical Points

Welding Process Stability

One of the primary concerns with domestic consumables is process stability. The authors report that the domestic H316LF/SJ15F combination provides a stable surfacing process with good weld appearance. Process stability in electroslag surfacing is characterized by:

• Consistent arc voltage and current throughout the pass.
• Uniform bead width and profile.
• No excessive spatter or flux spatter.
• Stable slag pool behavior without excessive turbulence.

The fact that the domestic consumables achieve comparable process stability to the imported alternatives is a significant finding. Process instability can lead to defects such as undercut, uneven bead profile, slag inclusions, and porosity, all of which would compromise the integrity of the overlay.

Mechanical Properties and Chemical Composition

The paper reports that the mechanical properties and chemical compositions of the surfaced specimens using both domestic and imported consumables are extremely close and meet all relevant technical specifications. This is expected, as both consumable systems are designed to produce 316L-equivalent weld metal. The key mechanical properties for 316L surfacing overlays typically include:

The chemical composition of the 316L overlay should include approximately 16–18% Cr, 10–14% Ni, and ≤ 0.03% C, with the balance iron and trace elements. The low carbon content is critical for preventing sensitization and intergranular corrosion.

Microstructural Analysis

Although the paper does not provide detailed microstructural analysis in the abstract, the authors mention examination of the weld surface and bond layer. In electroslag surfacing, the microstructure of the weld metal is typically characterized by:

• Columnar grains growing from the fusion boundary upward through the weld.
• Coarse grain size due to the slow cooling rate, which is beneficial for ductility but may reduce strength slightly.
• Low delta ferrite content in the 316L composition, which minimizes the risk of hot cracking.
• Clean fusion boundary with no cracks, voids, or lack of fusion.

The bond layer (the transition zone between base metal and weld metal) is a critical area for quality assessment. In single-layer surfacing, the bond layer must be free of defects because there is no subsequent layer to mask or repair any deficiencies.

Integration with Engineering Practice

Import Substitution in Pressure Vessel Manufacturing

The practical significance of this study extends beyond the technical comparison of consumables. In China's pressure vessel manufacturing industry, the reliance on imported welding consumables for critical applications has been a persistent challenge. Imported consumables are subject to:

• High costs (typically 3–5 times the price of domestic equivalents).
• Supply chain vulnerabilities due to international trade restrictions.
• Long lead times for ordering and delivery.
• Limited technical support and process consultation.

The successful demonstration that domestic H316LF/SJ15F consumables can replace imported WELESS 316LJ/WEL ESB F-6M consumables for single-layer electroslag surfacing of large cylindrical vessels represents a meaningful step toward import substitution. This has direct implications for:

1. Cost reduction: Lower consumable costs translate to lower manufacturing costs for pressure vessels and other heavy equipment.
2. Supply chain security: Domestic consumables ensure reliable supply regardless of international trade conditions.
3. Technical independence: Domestic consumable development encourages further R&D and innovation in China's welding consumable industry.

Application to Large-Diameter Cylindrical Vessels

The study specifically addresses the surfacing of large-diameter cylindrical vessel inner walls, which is a common application in the chemical and petrochemical industries. These vessels often require corrosion-resistant inner linings to withstand aggressive process media such as hydrofluoric acid, sulfuric acid, or other corrosive chemicals. The 316L overlay provides excellent resistance to a wide range of corrosive environments.

Key process considerations for cylindrical vessel surfacing include:

• Positioner setup: The vessel is rotated on a positioner to ensure uniform heat input around the circumference.
• Tack welding: Tack welds are used to secure the tape electrode and guide the surfacing path.
• Flux hopper positioning: The flux hopper must be carefully positioned to maintain consistent slag pool coverage.
• Cooling control: For large-diameter vessels, the thermal mass of the vessel wall helps to moderate cooling rates, which is beneficial for electroslag surfacing.

Quality Control for Single-Layer Surfacing

Single-layer surfacing places higher demands on quality control because there is no opportunity for defect repair through subsequent layers. The quality control protocol should include:

Key Questions and Reflections

One question that arises from this study is the long-term service performance of the domestic consumables in aggressive chemical environments. While the mechanical properties and chemical compositions are shown to be equivalent to the imported consumables, corrosion resistance is a complex property that depends on microstructural details such as grain size, carbide distribution, and sigma phase formation. The authors do not report corrosion testing results, which would be essential for full qualification of the domestic consumables for service in aggressive chemical environments.

Another reflection concerns the scalability of this technology. The study demonstrates single-layer surfacing for specific vessel dimensions and overlay thicknesses. Engineers working on similar projects should verify that the consumables and process parameters are suitable for their specific application, including vessel diameter, wall thickness, and required overlay thickness. The process window for single-layer surfacing may be narrower than for multi-layer surfacing, and process deviations may be less forgiving.

Study Insights and Implications

This paper makes a significant contribution to the field of electroslag surfacing by demonstrating the technical equivalence of domestic and imported consumables for single-layer tape electrode electroslag surfacing. The results are encouraging for the Chinese pressure vessel manufacturing industry, as they provide a pathway for import substitution that can reduce costs and improve supply chain security.

For fellow engineers, the key takeaways are:

• Domestic H316LF/SJ15F consumables are a viable alternative to imported WELESS 316LJ/WEL ESB F-6M consumables for single-layer electroslag surfacing.
• Single-layer electroslag surfacing is a practical technology for thick overlay deposition on large cylindrical vessels, offering high deposition rates and excellent metallurgical quality.
• Comprehensive qualification testing, including mechanical properties, chemical composition, and corrosion resistance, is essential before adopting new consumables for critical applications.
• Import substitution in welding consumables requires not only technical equivalence but also process qualification and long-term service performance verification.

The study underscores the importance of domestic R&D in welding consumables and the value of systematic comparative testing in validating new materials for critical applications. Engineers should approach import substitution with rigor and thoroughness, ensuring that all relevant performance criteria are met before transitioning from imported to domestic consumables.