Electroslag Welding Process and Properties of Thick Ni-based Alloy Strip Electrode Overlay

Literature Overview

Published in the Journal of Heilongjiang University of Science and Technology (2026, Vol. 36, No. 3, pp. 432-438), this paper addresses a long-standing challenge in the overlay welding industry: achieving thick, sound Inconel 600 nickel-base alloy deposits on low-alloy steel substrates with acceptable dilution, mechanical properties, and crack resistance. The research, conducted by institutions including the China Academy of Mechanical Science and Technology and Harbin Welding Institute Huatong (Changzhou) Welding Co., Ltd., represents a significant step toward industrial-scale application of nickel-base overlay in heavy equipment manufacturing.

Problem Statement and Process Selection

Traditional methods for depositing thick Inconel 600 overlays—such as manual GTAW or submerged arc welding—suffer from three fundamental limitations: low deposition efficiency (typically below 5 kg/h), high dilution rates (often exceeding 15-20%), and susceptibility to hot cracking due to the wide solidification range and low ductility of the solidifying nickel-base alloy. The authors selected strip electrode electroslag welding (SE-ESW) as the solution, leveraging the process's inherent advantages of high heat input, deep penetration, and rapid deposition rates (commonly 10-30 kg/h for multi-pass builds).

The substrate used was SA508-III low-alloy steel, a material commonly employed in power plant pressure vessels and piping systems where localized corrosion or erosion resistance is required at specific locations.

Microstructural Analysis

As-Welded Condition

The as-welded overlay exhibits several characteristic features:

• Segregation bands: Microsegregation of nickel, chromium, and molybdenum creates compositional bands parallel to the heat flow direction, resulting from the slow solidification rate typical of electroslag welding.
• Precipitate phases: δ-ferrite and carbide precipitates form at grain boundaries and within segregation bands, particularly in regions of high dilution where chromium and carbon are enriched from the substrate.
• Columnar grain structure: The high thermal gradient drives the formation of elongated columnar dendrites, which can create anisotropic mechanical properties and preferential crack paths.

Post-Heat Treatment Condition

After appropriate solution and aging heat treatment, the microstructure undergoes significant improvement:

• Segregation bands are substantially reduced through homogenization diffusion.
• δ-ferrite content decreases below the critical threshold (typically <10 vol%), reducing the risk of stress corrosion cracking.
• The grain structure becomes more equiaxed, improving isotropy and ductility.

Mechanical Performance

The improvement in ductility and toughness after heat treatment is attributed to the dissolution of brittle intermetallic phases, reduction of microsegregation, and stress relief from the thermal cycle. The bend test result—no cracking after heat treatment—is particularly significant for engineering qualification, as it demonstrates that the overlay can withstand the plastic deformation encountered during field installation and maintenance.

Engineering Practice Integration

For industrial applications involving Inconel 600 overlay on thick sections, several practical considerations emerge from this study:

1. Multi-layer strategy: The use of a transition layer between the substrate and the final Inconel 600 deposit is recommended to manage dilution and reduce cracking susceptibility. The first layer may use a higher-nickel-content filler to suppress δ-ferrite formation.
2. Heat input management: Electroslag welding's high heat input, while advantageous for deposition rate, must be carefully controlled to avoid excessive grain growth and dilution. Travel speed and strip electrode thickness are critical parameters.
3. Post-weld heat treatment (PWHT): The study clearly demonstrates that PWHT is not optional but essential for achieving acceptable mechanical properties. A typical treatment for Inconel 600 involves solution annealing at 1120-1150°C followed by aging at 720-815°C.
4. Dilution control: Target dilution for Inconel 600 overlay should be kept below 10-15% to maintain corrosion resistance and mechanical properties. The use of back-gassing and appropriate joint preparation helps manage this.

Key Questions and Reflections

The paper raises an important question about the long-term performance of electroslag-welded Inconel 600 overlays in aggressive environments. While the study demonstrates good room-temperature mechanical properties, the corrosion resistance of the overlay—particularly in chloride-containing or reducing acid environments—is not evaluated. Given that Inconel 600 is commonly specified for resistance to caustic environments and non-oxidizing acids, this gap represents a limitation for full engineering qualification.

Additionally, the study does not address the effect of welding sequence on residual stress distribution in multi-pass builds. In thick overlay applications, the interaction between successive passes can create complex residual stress states that may influence service performance, particularly under cyclic loading or thermal fatigue conditions.

Study Insights and Implications

The most significant contribution of this work is the demonstration that strip electrode electroslag welding can produce thick, crack-free Inconel 600 overlays with acceptable mechanical properties after appropriate heat treatment. This opens the door to cost-effective repair and overlay of large components—such as pressure vessel heads, heat exchanger tubesheets, and nuclear-grade piping—that would otherwise require extensive machining or replacement. The process efficiency advantage, combined with the improved quality demonstrated here, makes SE-ESW a viable option for industrial applications where deposition thickness exceeds 20-30 mm.