Next-Generation Welding Solutions for Nuclear Power Applications

Technical Summary: Next-Generation Welding Solutions for Nuclear Power Applications

Problem Statement
Inconel 690 (In-690) is a critical material in nuclear power systems, utilized for steam generator tubes and associated hardware. While the filler metal Inconel 52 (In-52) is widely used for welding In-690, it exhibits significant limitations in remote repair operations, specifically a "ductility dip" occurring between 1000–1200 K. This reduction in relative elongation coincides with the temperature range most susceptible to hot cracking, compromising the reliability of welds in radiation-intensive environments. Furthermore, the exact compositions and production principles of proprietary alloys like In-52 and the advanced dispersion-strengthened In-52MSS are not fully disclosed in open literature, necessitating the development of transparent, domestically produced analogues.

Methodology
The study employed a comparative approach to evaluate the structural, physical, and mechanical properties of commercial In-52 and In-52MSS against two locally developed Ukrainian analogues:

• Al-I: A Ni-Fe-Cr alloy (58Ni-11Fe-30Cr-0.5Nb-0.5Ti) serving as an analogue to In-52.
• Al-II: A dispersion-strengthened alloy (54.6Ni-11Fe-30Cr-3.5Nb-0.5Ti-0.4C) serving as an analogue to In-52MSS, reinforced with NbC precipitates.

Experimental procedures included:

• Sample Preparation: Multi-pass TIG overlay welding of In-52 and In-52MSS onto In-690 substrates. Al-I and Al-II were processed via argon-arc melting, cold rolling, and annealing.
• Microstructural Analysis: Utilization of SEM-EDS, EBSD, and TEM to examine grain boundary morphology, precipitate distribution (NbC, M23C6), and dislocation density.
• Mechanical Testing: Uniaxial compression and tensile testing conducted across a temperature range of 4.2 K to 400 K to assess yield strength, ultimate strength, and ductility.
• Acoustic Characterization: Measurement of dynamic Young's modulus and logarithmic decrement (internal friction) to detect relaxation processes and correlate them with mechanical behavior.

Key Contributions and Results

1. Microstructural Stabilization:

   • In-52 exhibited nearly straight grain boundaries with fine Cr-rich M23C6 carbides and a high dislocation density gradient at grain boundaries (~2·10¹¹ cm⁻²), leading to steep local stress gradients.
   • In-52MSS and Al-II displayed tortuous grain boundaries due to effective pinning by NbC precipitates formed prior to grain boundary development. This resulted in a uniform internal stress distribution and a significantly lower dislocation density within grain interiors (~5·10⁸ cm⁻²).

2. Elimination of Ductility Dip:

   • Mechanical testing confirmed that In-52 suffers from a pronounced ductility dip (reduced elongation) between 1000–1200 K.
   • In contrast, In-52MSS showed no such ductility dip. The dispersion strengthening provided by NbC precipitates acts as effective barriers to dislocation mobility, preventing the formation of sharp stress gradients that lead to hot cracking.

3. Acoustic-Property Correlation:

   • A wide diffuse acoustic absorption peak (P3) was observed in In-52 within the 1000–1200 K range, coinciding exactly with the ductility dip. This peak was absent in In-52MSS.
   • The study establishes a correlation between high-temperature acoustic absorption and plasticity failure, suggesting acoustic methods can serve as a non-destructive diagnostic tool for identifying hot-crack susceptibility.
   • At cryogenic temperatures, distinct relaxation peaks (P1 and P2) were identified. The P2 peak, associated with dislocation interaction with precipitates, was observed in In-52MSS and Al-II but not in In-52 or Al-I, confirming the role of carbide/nitride precipitates as effective pinning centers.

4. Performance of Domestic Analogues (Al-I and Al-II):

   • Al-II demonstrated superior mechanical properties compared to the carbon-free Al-I. The addition of carbon (0.4%) and the resulting fine dispersed carbide phase increased the yield strength at 300 K by more than twofold and the ultimate strength by 1.8 times.
   • Crucially, the addition of carbon in Al-II did not reduce plasticity; maximum plastic strain slightly improved (23.56% at 300 K for Al-II vs. 20.76% for Al-I).
   • Both analogues maintained stable structures and high strength/ductility across the 77–300 K range, exhibiting thermoactivated plastic deformation.

Significance and Claims
The paper asserts that the dispersion-strengthened In-52MSS alloy offers superior structural stability and resistance to ductility-dip cracking (DDC) compared to standard In-52, making it highly reliable for remote and manual welding in nuclear repair scenarios. The study validates that the locally developed analogue Al-II offers an economically viable replacement for proprietary foreign fillers, delivering comparable high-performance characteristics without the ductility dip.

The authors highlight that the correlation between acoustic absorption anomalies and mechanical ductility dips provides a novel basis for non-destructive testing (NDT) to detect states of reduced plasticity that may lead to hot cracking. The work concludes that these next-generation materials ensure reliable welding performance across a wide spectrum of operating conditions, from cryogenic to elevated temperatures, while offering a transparent, domestically produced alternative to proprietary nuclear-grade filler metals.