Strip Electrode Electroslag Surfacing of Nickel-Based Alloys on Thick Sections

Literature Overview

This study by Du Liping and co-authors from the China Machinery Science and Technology Group and related institutions addresses the challenge of depositing thick layers of Inconel 600 nickel-based alloy onto SA508-III low-alloy steel substrates using strip electrode electroslag surfacing (SESS). The work was published in the Journal of Heilongjiang University of Science and Technology and targets a critical engineering problem: traditional surfacing methods for nickel-based alloys suffer from low deposition efficiency, high dilution rates, and susceptibility to cracking when applied to thick sections.

Problem Statement and Methodology

Nickel-based alloys such as Inconel 600 are widely used in chemical processing, nuclear, and high-temperature applications due to their excellent corrosion resistance, oxidation resistance, and mechanical strength at elevated temperatures. However, depositing thick layers of these alloys onto ferrous substrates presents significant challenges. The large thermal gradient between the nickel-based overlay and the steel substrate, combined with the high dilution of base metal into the weld pool, often results in composition drift, segregation, and cracking.

The authors employed strip electrode electroslag surfacing, which uses a continuous strip of filler metal as the electrode rather than a consumable wire or powder. This technique offers several inherent advantages for thick deposition: the strip electrode acts as both a heat source and a filler metal reservoir, providing a stable, high-energy-density process with excellent metallurgical control. The electroslag process maintains a molten slag pool that shields the weld zone and provides a controlled cooling rate.

Microstructural Analysis

The as-welded surfacing layer exhibited segregation bands and precipitate phases, which are common features of electroslag deposited nickel-based alloys. These features arise from the relatively slow cooling rate inherent to the electroslag process and the large thermal mass of the deposited material. The segregation bands are attributed to dendritic solidification patterns that become locked in during the extended solidification period.

After appropriate heat treatment, the segregation bands were significantly reduced, indicating that the microstructure is responsive to post-deposition thermal processing. This is a crucial finding for engineering practice, as it means that the as-deposited microstructure can be optimized through controlled post-weld heat treatment (PWHT) rather than relying solely on process parameter adjustment during deposition.

Mechanical Performance

The improvement in ductility and impact toughness after heat treatment is particularly significant for thick-section applications where residual stresses are substantial and the risk of brittle fracture is elevated. The absence of cracks in bend tests after heat treatment confirms the structural integrity of the multi-layer, multi-pass surfacing deposit.

Engineering Practice Integration

For pipe fitting applications involving thick-wall components—such as large-diameter reducers, tees, or pressure vessel nozzles—strip electrode electroslag surfacing offers a practical solution for depositing nickel-based overlay layers of significant thickness. The high deposition rate of the SESS process (typically 5–10 kg/h compared to 0.5–2 kg/h for conventional arc surfacing) makes it economically viable for large-scale production.

Key process parameters for successful SESS deposition include:

• Strip thickness: typically 1.5–3.0 mm for Inconel 600
• Strip width: 20–40 mm depending on the desired bead width
• Slag composition: carefully controlled to maintain proper fluidity and protection
• Interpass temperature: maintained within specified limits to prevent excessive grain growth
• Post-weld heat treatment: solution treatment at 1050–1100°C followed by controlled cooling

The dilution control strategy employed in this study is particularly relevant for pipe fitting manufacturing, where the base metal composition directly affects the corrosion resistance of the final component. By using a multi-layer approach with progressively lower dilution rates in successive passes, the final surface layer achieves composition close to the intended Inconel 600 specification.

Study Insights and Reflections

This research underscores the importance of post-weld heat treatment in electroslag surfacing applications. The as-welded microstructure, while functional, contains features (segregation bands, coarse precipitates) that compromise mechanical performance. The ability to substantially improve toughness and ductility through heat treatment opens up design flexibility for engineers who must balance corrosion resistance with mechanical integrity.

For the piping industry, this work has direct implications for the selection of surfacing methods when thick nickel-based overlays are required. Conventional GTAW or GMAW surfacing would require an impractical number of passes to achieve the same thickness, with associated risks of distortion and cracking. The SESS method provides a practical alternative that delivers both efficiency and quality.

The findings also highlight the importance of considering the entire processing chain—from deposition through heat treatment—in the qualification of surfacing procedures for critical applications. A procedure that produces an acceptable as-welded deposit may not deliver optimal performance without appropriate PWHT, and vice versa.