Electroslag Welding Overlay of INCONEL 690 Nickel Alloy Strip - Process Trials and Overlay Layer Performance Study

Literature Overview

This study focuses on the electroslag welding (ESW) overlay of INCONEL 690 nickel-based alloy strip on carbon steel substrates, a process of significant importance in nuclear power, petrochemical, and hydrogenation reactor applications where superior corrosion resistance is required. The literature examines the relationship between electroslag welding parameters and the resulting overlay layer quality, including microstructure, mechanical properties, and corrosion resistance. The INCONEL 690 alloy, with its high nickel and chromium content combined with molybdenum, offers exceptional resistance to chloride stress corrosion cracking and is widely specified in nuclear service for steam generator tubing and related components.

Core Technical Points

Electroslag Welding Process Parameters

The study systematically investigated the effects of key ESW parameters on overlay quality. The following table summarizes the typical process windows identified:

Parameter	Range Investigated	Optimal Range	Effect on Overlay Quality
Current (I)	600-900 A	700-800 A	Controls slag viscosity and heat input
Voltage (U)	32-42 V	34-38 V	Governs arc stability and penetration profile
Welding Speed (v)	120-200 mm/min	140-170 mm/min	Affects dilution and cooling rate
Flux Type	Rutile basic / Flux 10	Rutile basic	Influences slag composition and inclusion content
Strip Thickness	3-6 mm	4-5 mm	Determines single-pass deposition rate

The study emphasizes that the slag pool temperature, which directly controls the welding process stability, must be maintained between 1100 and 1400 degrees Celsius. Below this range, slag viscosity increases excessively leading to unstable arc and potential cold shuts. Above this range, excessive dilution occurs and the overlay layer composition deviates significantly from the nominal INCONEL 690 specification.

Microstructural Analysis

Metallographic examination of the overlay layer reveals a columnar dendritic structure growing from the fusion line toward the free surface. The dendrite arm spacing (DAS) is observed to range from 30 to 80 micrometers depending on the cooling rate, which is strongly influenced by the welding speed and preheat temperature. The study identifies three distinct zones:

The fusion zone (dilution zone) where substrate elements diffuse into the overlay, creating a gradient in composition and potentially forming brittle phases such as sigma phase or intermetallic compounds.
The fully deposited overlay zone where the microstructure is relatively homogeneous with a columnar-to-equiaxed transition occurring at approximately 60 to 70 percent of the overlay thickness.
The top surface zone where grain coarsening and potential porosity may develop if the slag coverage is insufficient.

Mechanical and Corrosion Properties

The overlay layer hardness is reported to range from 180 to 220 HV, consistent with the annealed condition of INCONEL 690. Tensile testing of overlay coupons demonstrates ultimate tensile strength of approximately 620 to 680 MPa with elongation exceeding 40 percent. The dilution rate, which is critical for maintaining corrosion resistance, is reported to be between 15 and 25 percent under optimal parameters. The study includes intergranular corrosion testing in accordance with ASTM G153 and confirms that overlay layers with dilution below 20 percent pass the critical strain test without evidence of sensitization.

Common Defects and Countermeasures

The literature documents several typical defects encountered during INCONEL 690 ESW overlay:

Defect Type	Root Cause	Countermeasure
Excessive dilution (>25%)	High current, low speed, insufficient preheat	Reduce current by 50-100 A, increase speed to 160-170 mm/min
Cracking at fusion line	High cooling rate, hydrogen pickup	Apply preheat of 150-200 degrees C, use low-hydrogen flux
Porosity in overlay	Inadequate slag coverage, flux contamination	Ensure proper flux pre-drying, maintain slag pool stability
Surface irregularity	Unstable arc, strip misalignment	Implement automatic wire feed control, monitor voltage fluctuations
Sigma phase formation	Excessive heat input, prolonged residence at 600-800 degrees C	Limit interpass temperature to below 150 degrees C

Integration with Engineering Practice

In engineering practice, the selection between ESW overlay and other overlay methods such as submerged arc welding (SAW) or gas metal arc welding (GMAW) depends heavily on the thickness of the required overlay layer and the geometry of the component. For thick overlay requirements (greater than 5 mm) on large flat plates or cylindrical shells, ESW offers the highest deposition rate, typically achieving 8 to 15 kg/h of metal deposition. However, the initial setup complexity and the need for proper backing arrangements must be considered.

A notable engineering case discussed involves the overlay of INCONEL 690 on a hydrogenation reactor shell where the base material is 16MnR (GB 150) with a wall thickness of 60 mm. The overlay specification requires a minimum 6 mm thickness of INCONEL 690 with dilution not exceeding 20 percent. The ESW process was selected for the first three passes to achieve bulk deposition rapidly, followed by a finishing pass using GMAW to ensure surface quality and dimensional accuracy. Post-weld heat treatment at 1050 degrees C for 2 hours followed by water quench was applied to eliminate sensitization and restore full corrosion resistance.

Key Questions and Reflections

The study raises several important questions for practitioners. First, the long-term stability of the overlay layer under cyclic thermal loading remains a concern, particularly for components subjected to frequent start-stop operations in chemical processing plants. The thermal mismatch between the nickel alloy overlay and the carbon steel substrate creates residual stresses that may promote delamination over time. Second, the effect of multiple welding passes on the cumulative dilution and microstructural evolution deserves further investigation, as the dilution from each subsequent pass interacts with the previous pass microstructure in a complex manner.

The study also highlights the importance of pre-weld preparation. Surface cleanliness of the base material is critical, as any oxide scale or contamination can lead to inclusion formation and reduced bond strength at the interface. The recommended surface preparation includes shot blasting to near-white metal finish followed by solvent cleaning within 4 hours of welding.

Study Insights and Implications

The literature provides valuable quantitative data on the process windows for ESW overlay of INCONEL 690, which can serve as a baseline for qualification procedures under NB/T 47014 or ASME IX. The emphasis on dilution control as the primary quality indicator is well-founded, as excessive dilution not only degrades corrosion resistance but also introduces brittle intermetallic phases that compromise mechanical integrity. Engineers should note that the optimal parameters reported are specific to the strip dimensions, flux composition, and substrate material used in the study, and qualification testing is essential for any new application.

The findings underscore the importance of integrating process monitoring with real-time quality assessment. The implementation of automated current and voltage monitoring, combined with periodic metallographic verification of dilution zones, provides a robust quality assurance approach. Overall, the study contributes significantly to the technical understanding of nickel alloy ESW overlay and provides actionable guidance for achieving reliable, high-quality overlay layers in demanding industrial applications.