Dual-Tungsten Electrode Overlay Welding Technology in Nuclear Reactor Pressure Vessel Fabrication

Literature Overview

This study addresses the application of dual-tungsten electrode gas tungsten arc welding (GTAW) overlay technology in the fabrication of nuclear reactor pressure vessels (RVPVs), particularly for the cladding of inner surfaces of reactor pressure vessel heads and shells. The dual-electrode configuration represents a significant advancement over conventional single-electrode GTAW overlay, offering higher deposition rates while maintaining the fine grain structure and metallurgical quality essential for nuclear-grade applications governed by stringent codes such as ASME VIII Div.2, RBP, and GB/T 150.

Technical Background and Process Description

Conventional GTAW overlay welding achieves deposition rates of approximately 0.3–0.5 kg/h, which is prohibitively slow for large-scale nuclear pressure vessel fabrication where overlay thicknesses of 3–6 mm are required over surface areas exceeding 50 m². The dual-tungsten electrode configuration employs two independently controlled tungsten electrodes, each with its own arc, sharing a single shielding gas envelope. The two arcs are spatially offset by 6–10 mm and operate at individual currents of 80–120 A, resulting in a combined deposition rate of 1.2–2.0 kg/h — a 3 to 4 fold improvement over single-electrode GTAW.

Process Parameter	Single Electrode GTAW	Dual Electrode GTAW
Arc current (A)	160–200	80–120 (per electrode)
Arc voltage (V)	14–18	10–14 (per electrode)
Travel speed (mm/min)	60–100	150–250
Shielding gas flow (L/min)	12–15	20–25
Deposition rate (kg/h)	0.3–0.5	1.2–2.0
Electrode wire diameter (mm)	1.6	1.2 (per electrode)
Typical clad thickness per pass (mm)	0.4–0.6	0.8–1.2

Core Technical Findings

Heat Input Management

The dual-electrode configuration produces a wider but shallower weld pool compared to single-electrode GTAW. The peak temperature in the weld pool center is approximately 1800–2000 °C, comparable to single-electrode GTAW, but the thermal gradient is more distributed laterally. This wider thermal distribution is advantageous for nuclear applications because it reduces the peak thermal gradient at the weld boundary, thereby minimizing the risk of thermal cracking and reducing residual stress magnitudes. Post-weld residual stress measurements show peak longitudinal stresses of approximately 180–210 MPa for dual-electrode GTAW, compared to 220–260 MPa for single-electrode GTAW at equivalent total heat input.

Microstructural Characteristics

Metallographic analysis of the overlay layers reveals that the dual-electrode GTAW produces a finer columnar grain structure in the overlay layer compared to single-electrode GTAW. The grain size in the overlay layer is approximately 30–50 μm, compared to 50–80 μm for single-electrode GTAW. This refinement is attributed to the competing solidification front effect — the two arcs create a more complex thermal field that promotes nucleation at multiple sites. For nuclear-grade Inconel 625 overlay cladding, this finer grain structure translates to improved creep resistance and reduced susceptibility to intergranular corrosion, as confirmed by ASTM A263 intergranular corrosion testing.

Interfacial Metallurgy

The interface between the overlay layer and the base material (typically SA-508 Gr.3 Cl.1 low-alloy steel) shows a dilution ratio of 3–6 percent, which is within the acceptable range specified by ASME IX QW-440 for nuclear applications. The interface microstructure consists of a narrow martensitic transition zone approximately 50–80 μm wide on the base material side, with no evidence of carbide precipitation or intermetallic compound formation. This is particularly important for nuclear pressure vessels where the long-term irradiation behavior of the weld interface is a critical design consideration.

Standards and Code Compliance

The study evaluates compliance with multiple nuclear and pressure vessel codes:

Code/Standard	Requirement	Compliance Status
ASME IX QW-440	Dilution ratio ≤ 10%	Compliant (3–6%)
ASME VIII Div.2	Post-weld tensile strength ≥ base material	Compliant
GB/T 150	Overlay layer hardness ≤ 350 HV	Compliant (280–310 HV)
NB/T 47014	Qualification welding procedure	Qualified
RBP	RT inspection: no defects > 1 mm	Compliant
JB/T 4730.2	RT: no indications of crack or porosity	Compliant

Engineering Practice Implications

For nuclear pressure vessel fabrication, the dual-electrode GTAW overlay technology offers a compelling combination of improved productivity and enhanced metallurgical quality. The increased deposition rate reduces fabrication time by approximately 60 percent compared to single-electrode GTAW, which directly translates to cost savings on large-scale nuclear projects. However, the technology requires specialized equipment with independent current control for each electrode, precise torch positioning to maintain consistent arc spacing, and enhanced shielding gas coverage to prevent porosity formation in the wider weld pool.

Study Insights and Reflections

The most significant insight from this research is that the dual-electrode configuration does not merely increase productivity but also improves the metallurgical quality of the overlay layer through its unique thermal field characteristics. This challenges the common assumption that higher deposition rates necessarily compromise quality. The study demonstrates that with proper parameter optimization, dual-electrode GTAW can produce overlay layers with superior grain structure and lower residual stresses than conventional single-electrode GTAW. This finding has broad implications for the adoption of advanced multi-arc welding technologies in nuclear applications, and it should be considered in future revisions of welding qualification requirements in nuclear codes. The technology is particularly well-suited for Inconel 625 and 316L overlay cladding on SA-508 base materials, which are the most common material combinations in current nuclear pressure vessel designs.