INCONEL 690 Strip Electrode Electroslag Cladding Process Development and Deposit Characterization

Literature Overview

This 1997 study by Zhang Maolong and colleagues from Shanghai Boiler Works and Shanghai Testing Technology Institute represents one of the earlier systematic investigations into nickel-base superalloy cladding using strip electrode electroslag welding (SEESW). The research focuses on developing a complete cladding process for INCONEL 690—a high-performance Ni-Cr-Mo superalloy developed for steam generator tube applications in nuclear power plants—and characterizing the resulting deposit properties. This work is historically significant as it addressed a critical technology gap in the cladding of nuclear-grade superalloys at that time.

Process Development and Parameters

The study establishes the fundamental conditions required for successful nickel-base alloy strip electrode electroslag cladding:

The development of this process required addressing several unique challenges associated with electroslag welding of nickel-base alloys:

1. Flux composition: Standard ESW fluxes are incompatible with nickel-base alloys due to potential chemical reactions that produce undesirable inclusion phases or promote cracking.
2. Preheat requirements: Nickel-base alloys have lower thermal conductivity than steel, requiring careful preheat management to prevent excessive temperature gradients and cracking.
3. Cooling rate control: The thick deposits achievable with ESW create unique solidification conditions that must be managed to avoid hot cracking in the susceptible solidification range of Ni-Cr-Mo alloys.
4. Dilution management: Strip electrode ESW produces significant dilution of base metal into the deposit, requiring careful control of the dilution ratio.

Deposit Microstructure and Properties

The electroslag cladding process produces thick, single-pass deposits with microstructural characteristics distinct from arc welding methods:

• Coarse grain structure: The slow cooling rates inherent to ESW produce significantly coarser grain sizes compared to TIG or MIG cladding
• Dendritic morphology: Equilibrium solidification conditions favor well-developed dendritic structures
• Precipitate distribution: Carbide and intermetallic precipitates have time to form and coarsen during the slow solidification
• Inclusion content: Flux-derived inclusions are present but controlled through flux composition optimization

The mechanical properties of the deposit, including tensile strength and elongation, were evaluated to confirm structural integrity. The microstructural analysis confirmed adequate metallurgical bonding between the deposit and substrate, with no evidence of cracking or delamination at the fusion interface.

Factors Affecting Cladding Quality

The authors systematically identify and analyze factors influencing deposit quality:

Engineering Practice Implications

The development of INCONEL 690 strip electrode ESW cladding technology has several practical applications:

• Steam generator tube repair: Enables cladding of damaged tube sections with corrosion-resistant overlay without requiring complete tube replacement
• Heat exchanger tube sheets: Provides corrosion protection for tube sheet materials exposed to aggressive primary coolant environments
• Large area cladding: The ESW process achieves deposition rates significantly higher than arc welding methods, making it economical for large surface areas
• Thick deposit capability: Single-pass deposits of considerable thickness are achievable, reducing the number of passes required for target thickness

The process is particularly suited for applications where thick corrosion-resistant overlays are required on large structural components, such as boiler pressure parts, reactor internals, and heat exchanger components in nuclear and fossil fuel power generation.

Key Questions and Reflections

The 1997 publication date places this work in an era before the widespread adoption of modern process monitoring and control systems. Several questions arise regarding the reproducibility and scalability of the developed process:

• Flux availability: The specialized flux composition required for nickel-base alloy ESW may have limited commercial availability, representing a supply chain risk.
• Process automation: The precision requirements for electrode positioning and travel speed control suggest that manual or semi-automated operation may limit productivity and consistency.
• Modern alternatives: Contemporary processes such as plasma arc cladding, laser cladding, and high-deposition-rate GTAW may now offer competitive alternatives with better process control and reduced dilution.
• Nuclear qualification: For nuclear service applications, the process would require extensive qualification testing including irradiation effects, thermal cycling resistance, and long-term creep behavior of the deposit.

Study Insights and Implications

This pioneering work established the technical feasibility of electroslag welding for INCONEL 690 cladding applications, providing a foundation for subsequent developments in nickel-base superalloy overlay technology. The systematic approach to process development—addressing flux selection, thermal management, dilution control, and quality factor analysis—represents a model methodology for developing new cladding processes. For modern engineering practice, the work demonstrates that ESW remains a viable option for thick, large-area cladding of nickel-base alloys where deposition rate and deposit thickness are prioritized over fine microstructural control. The identification of dilution ratio as a critical quality factor remains relevant regardless of the specific welding process employed, reinforcing the universal importance of composition control in superalloy cladding applications.

Summary and Cross-Reference Insights

Reviewing these five studies collectively reveals several overarching themes in overlay welding technology. The progression from conventional SAW (Topic 1) through laser cladding (Topic 2), hot wire pulsed TIG (Topics 3 and 4), and strip electrode electroslag welding (Topic 5) illustrates the evolution of process capabilities and their application to increasingly demanding service environments. A common thread across all studies is the critical importance of dilution control and interface quality in determining final deposit performance. Whether addressing wear resistance in Fe-Cr-B-C alloys, corrosion resistance in nickel-base superalloys, or crack resistance in ductile cast iron repairs, the fundamental metallurgical principles governing deposit-substrate interaction remain consistent. For engineers specifying overlay welding processes in piping systems, these studies collectively emphasize that process selection must be driven by the specific service requirements—wear, corrosion, or mechanical—while always accounting for the microstructural consequences of the chosen process on deposit integrity and long-term performance.