INCONEL 690 Nickel-Based Alloy Strip Electrode Electroslag Surfacing Process and Overlay Performance

Literature Overview

This 1997 study by Zhang Maolong, Yan Changgen, and You Junfu from Shanghai Boiler Works Co., Ltd. and Shanghai Testing Technology Research Institute presents a comprehensive investigation into strip electrode electroslag surfacing (EES) of INCONEL 690 nickel-based alloy. Published in "Boiler Technology," the paper addresses the technical challenges of applying nickel-based overlays using electroslag surfacing, a process that has historically been more common for ferrous alloy overlays. The work was motivated by the need for corrosion-resistant and high-temperature-resistant overlays in boiler and pressure vessel applications.

Core Technical Content

The study establishes the fundamental conditions required for successful nickel-based alloy strip electrode electroslag surfacing. Unlike conventional ferrous alloy EES, nickel-based alloys present unique challenges due to their different melting behavior, lower thermal conductivity, and susceptibility to hot cracking. The authors propose a complete process package including test plate preparation, consumable selection, welding parameters, and auxiliary process measures.

Process Conditions and Parameters

The electroslag surfacing process relies on the heat generated by the electrical resistance of the slag to melt the strip electrode and substrate. For nickel-based alloys, the slag composition and electrical parameters must be carefully controlled to achieve complete melting and proper wetting. The authors discuss the following critical process elements:

Quality Factors

The study analyzes several factors that influence overlay quality:

1. Slag temperature and viscosity: These determine the flow characteristics and heat transfer efficiency. Insufficient slag temperature leads to incomplete electrode melting and cold laps, while excessive temperature can cause excessive dilution and overheating.
2. Electrode feed rate: Must be synchronized with welding speed to maintain a consistent slag pool and avoid either burn-through or unmelted electrode accumulation.
3. Preheating: Both the substrate and the strip electrode require preheating to reduce thermal gradients and minimize cracking susceptibility.
4. Post-weld heat treatment: May be necessary to relieve residual stresses and improve the microstructure of the overlay.

Overlay Properties

The overlay microstructure and mechanical properties were characterized. The microstructure of INCONEL 690 EES overlay typically consists of a gamma-Ni matrix with possible intermetallic phases. The mechanical properties, including hardness and tensile strength, were evaluated and compared with the base metal and cast INCONEL 690.

The study found that the EES overlay achieved acceptable mechanical properties and microstructure, demonstrating the feasibility of this process for nickel-based alloy surfacing. However, the authors note that achieving properties equivalent to cast alloy requires careful parameter control and possibly post-weld heat treatment.

Engineering Practice Implications

For boiler and pressure vessel manufacturers, this study provides a viable process option for applying INCONEL 690 overlays to critical components such as water walls, superheater tubes, and furnace wall panels. The EES process offers advantages over arc welding processes for large-area overlays due to its high deposition rate and lower dilution.

The process is particularly suitable for:

• Large flat or slightly curved surfaces where high deposition rates are required
• Thick overlay requirements where multiple layers are needed
• Applications where low dilution is critical to maintaining overlay composition

However, the process has limitations:

• Not suitable for complex geometries or small areas
• Requires specialized equipment and slag consumables
• Slag removal between layers can be labor-intensive
• Quality control is more challenging than with arc welding processes

Key Reflections

This study is notable for its systematic approach to establishing a new process. The authors did not simply adapt existing ferrous alloy EES parameters but instead developed a complete process package from first principles. This methodology is instructive for engineers attempting to apply any new welding process to unconventional materials.

One area that could benefit from further investigation is the long-term thermal cycling behavior of the EES overlay. Boiler applications involve repeated heating and cooling cycles, and the fatigue and thermal fatigue performance of the overlay-substrate interface is critical for service life. The study's 1997 publication date also means that subsequent advances in slag composition and process automation may have improved the process further.

The comparison of overlay properties with cast INCONEL 690 is essential for engineering qualification. If the EES overlay achieves properties close to the cast alloy, it can be qualified for the same service conditions using existing design codes and standards. If significant differences exist, additional qualification testing and possibly code case development would be required.

Summary

This study successfully establishes the feasibility of INCONEL 690 strip electrode electroslag surfacing and provides a complete process package including consumable selection, parameter ranges, and auxiliary measures. The overlay achieves acceptable mechanical properties and microstructure, demonstrating that EES is a viable process for nickel-based alloy overlays in boiler applications. The systematic approach to process development and quality analysis provides a useful framework for similar process-material combinations. Engineers should note that while the process offers high deposition rates, it requires careful parameter control and is best suited for large, relatively flat surfaces.