CLADDING TECHNOLOGY SHANXI CO., LTDInconel 690 Strip Electroslag Welding Cladding Process Trial and Overlay Layer Property Study
Literature Overview and Research Background
Inconel 690 is a nickel-based superalloy widely used in nuclear power plant pressurizer nozzles, reactor coolant system piping, and other critical components where resistance to stress corrosion cracking (SCC) and intergranular corrosion (IGC) is paramount. The original specification for Inconel 690 was established to address the limitations of Inconel 600 in high-temperature aqueous environments, particularly in terms of carbide precipitation resistance and grain boundary strengthening. The study reviewed here investigates the application of strip electroslag welding (ESW) for overlaying Inconel 690 onto carbon and low-alloy steel substrates, with particular focus on process parameter optimization and resulting overlay layer properties.
The significance of this research cannot be overstated in the context of nuclear-grade pressure vessel fabrication. Electroslag welding offers several inherent advantages for thick-section cladding operations, including high deposition rates, deep penetration with narrow heat-affected zones relative to the deposited volume, and excellent metallurgical bonding between the overlay layer and the base metal. However, the process also introduces unique challenges related to slag chemistry, thermal input control, and solidification behavior that must be carefully managed to achieve acceptable overlay quality.
Core Technical Findings and Process Parameter Analysis
The study systematically examined the effects of key ESW process parameters on the quality and properties of the Inconel 690 overlay layer. The following table summarizes the typical process parameter windows identified during the investigation:
| Parameter | Typical Range | Influence on Overlay Quality |
|---|---|---|
| Stripping current | 400-700 A | Higher current increases deposition rate but may widen the heat-affected zone |
| Stripping voltage | 35-45 V | Affects slag fluidity and penetration profile |
| Welding speed | 150-350 mm/min | Inversely related to thermal input; slower speeds promote larger grain size |
| Flux composition | CaF2-SiO2-Al2O3 system | Controls slag viscosity, deoxidation, and inclusion removal |
| Preheat temperature | 150-250 °C | Reduces thermal gradient and minimizes cracking risk |
| Interpass temperature | 150-300 °C | Critical for controlling grain growth in multi-pass cladding |
One of the most important findings from the literature is the relationship between welding speed and the microstructure of the overlay layer. At lower welding speeds (below 200 mm/min), the extended heat input leads to coarse columnar dendrite growth along the solidification direction, which can compromise the mechanical properties and corrosion resistance of the overlay. Conversely, at higher speeds (above 300 mm/min), incomplete melting and poor bonding between the overlay and substrate become more likely. The optimal window identified in the study was approximately 220-280 mm/min, which balanced deposition efficiency with acceptable microstructural refinement.
The slag chemistry was found to play a decisive role in controlling the cleanliness and mechanical properties of the overlay. The CaF2-based flux system was selected for its ability to maintain appropriate slag viscosity while providing effective deoxidation through SiO2 and Al2O3 components. The study noted that excessive CaF2 content led to increased fluoride inclusion content in the weld metal, while insufficient SiO2 resulted in poor slag fluidity and inadequate protection of the molten pool.
Microstructural Characterization and Mechanical Properties
Metallographic examination of the overlay layer revealed a predominantly columnar dendritic microstructure with a characteristic cellular substructure within the dendrite arms. The grain size at the overlay-substrate interface was significantly refined compared to the interior of the overlay layer, a phenomenon attributed to the high thermal gradient at the fusion boundary during the initial passes. This refined zone, typically 0.5-1.5 mm in thickness, exhibited superior hardness and potentially enhanced crack resistance relative to the coarser-grained interior.
The hardness distribution across the overlay layer showed a gradient from approximately 210-240 HV at the overlay-substrate interface to 190-210 HV in the interior of the overlay. This gradient is consistent with the observed microstructural refinement near the fusion boundary. The base metal hardness remained unaffected beyond the immediate heat-affected zone, indicating that the thermal input was well controlled.
Mechanical property testing demonstrated that the overlay layer achieved tensile strengths of 620-680 MPa and elongations of 25-32%, which are in good agreement with the parent Inconel 690 alloy properties. The bond strength between the overlay and substrate, measured by the ring shear test method, consistently exceeded 280 MPa, well above the minimum requirements specified in relevant standards such as ASTM A265 and ASME Section II Part D.
Corrosion resistance testing, including potentiodynamic polarization in simulated reactor coolant conditions and intergranular corrosion testing per ASTM A262 Practice E, confirmed that the ESW overlay layer exhibited excellent resistance to both general and intergranular corrosion. The corrosion potential of the overlay was more noble than that of the base steel by approximately 250-350 mV, providing a robust electrochemical barrier.
Common Defects and Countermeasures
During the process trials, several defect types were encountered and analyzed:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracks at overlay-substrate interface | Excessive thermal gradient due to low preheat | Increase preheat to 200-250 °C; use higher interpass temperature |
| Porosity in overlay layer | Inadequate slag coverage or flux moisture | Dry flux to 250 °C for 2 hours; ensure adequate flux coverage |
| Incomplete fusion between passes | Welding speed too high or current too low | Reduce speed by 10-15%; increase current by 50-100 A |
| Excessive dilution from base metal | First pass thermal input too high | Use lower current for first pass; consider a transition layer |
| High fluoride inclusion content | Excessive CaF2 in flux | Reduce CaF2 content by 5-10% by weight |
The intergranular cracking issue was identified as the most critical defect mode, particularly in the first pass of the overlay where the dilution from the base metal can alter the solidification behavior of the weld metal. The study recommended the use of a transition layer of Inconel 625 or a similar alloy for the first pass when the base metal carbon and sulfur content exceeded certain thresholds, as this could significantly reduce the risk of cracking.
Integration with Engineering Practice
From an engineering practice perspective, this study provides valuable guidance for the fabrication of Inconel 690 clad components in nuclear applications. The process parameters identified are directly applicable to the manufacture of pressurizer nozzles, reactor vessel heads, and other critical pressure-containing components. The key engineering considerations include:
- Welding procedure qualification: The process parameters must be qualified per ASME Section IX or NB/T 47014 before production application, with particular attention to the multi-pass welding sequence and interpass temperature control.
- Heat treatment considerations: Post-weld heat treatment (PWHT) at 1050-1100 °C for solution treatment followed by controlled cooling is recommended to dissolve any carbides and restore full corrosion resistance. The cooling rate must be carefully controlled to avoid precipitation of brittle phases.
- Non-destructive examination: Ultrasonic testing (UT) per ASME V or JB/T 4730 is essential for detecting lack of fusion and cracking at the overlay-substrate interface. Radiographic testing (RT) should be applied to detect porosity and inclusions within the overlay layer.
- Dimensional control: ESW produces a relatively flat surface profile, but the overlay thickness may vary by 0.5-1.0 mm across the width. Machine finishing is typically required to achieve the final specified thickness.
Key Questions and Reflections
Several important questions arise from this study that warrant further investigation. First, the long-term stability of the overlay properties after prolonged exposure to reactor coolant conditions remains an area of concern. The microstructural evolution of the overlay layer during aging at 288-344 °C, which is typical of pressurizer operating conditions, has not been extensively characterized. Second, the effect of welding sequence on the residual stress distribution in multi-pass ESW overlays deserves further attention, as residual tensile stresses in the overlay layer can promote stress corrosion cracking in service. Third, the compatibility of ESW overlays with subsequent machining operations should be evaluated, as the high hardness of the overlay layer near the interface can affect machining performance and tool life.
A particularly noteworthy insight from this study is the recognition that the overlay-substrate interface represents the weakest link in the cladding system. The microstructural refinement observed at this interface, while beneficial for hardness and bond strength, may also create a region of increased susceptibility to stress corrosion cracking if the grain boundaries are sensitized. This observation underscores the importance of thorough non-destructive examination and, where feasible, destructive sampling from production welds to verify interface integrity.
Study Insights and Implications
The study of Inconel 690 strip ESW cladding provides a comprehensive framework for understanding the interplay between process parameters, microstructure, and performance in nickel-based alloy overlays. The key takeaway for practicing engineers is that successful ESW cladding of Inconel 690 requires not only precise control of electrical parameters but also careful management of slag chemistry, thermal input distribution, and post-weld heat treatment. The process is inherently well-suited to thick-section cladding operations where deposition efficiency is critical, but the challenge of maintaining consistent overlay quality across multiple passes demands rigorous procedural discipline and quality assurance.
The findings also highlight the importance of considering the entire fabrication sequence when planning Inconel 690 cladding operations. The interaction between welding sequence, residual stress development, and subsequent heat treatment must be integrated into a holistic fabrication strategy rather than treated as isolated process steps. Engineers involved in the design and fabrication of nuclear-grade clad components should view this study as a foundation upon which to build site-specific qualification procedures, rather than as a standalone reference for process parameter selection.