Application of Dual-Tungsten Electrode TIG Cladding Technology in Pressure Vessel Fabrication

Overview of the Literature

This study examines the application of the dual-tungsten electrode gas tungsten arc welding (GTAW/TIG) process for weld overlay cladding in pressure vessel fabrication. The dual-electrode TIG process employs two tungsten electrodes positioned at a slight angle to each other, creating a wider and more stable arc with significantly higher deposition rates compared to conventional single-electrode TIG welding. This technology has been adopted for cladding applications in pressure vessels where high-quality overlay deposits with low dilution, excellent metallurgical properties, and minimal distortion are required, particularly for austenitic stainless steel and nickel-based alloy cladding on carbon steel or low-alloy steel base metals.

Core Technical Content

The dual-tungsten electrode configuration provides several distinct advantages over conventional TIG welding for cladding applications. The arc geometry is wider and more stable, resulting in a broader weld bead with reduced spatter and improved wetting characteristics. The deposition rate is typically 2–3 times higher than single-electrode TIG, making the process more economically viable for large-area cladding operations. The arc force is distributed more evenly, reducing the tendency for arc wander and producing more uniform bead geometry.

Process Parameters and Weld Quality

The optimal process parameters for dual-electrode TIG cladding were determined through systematic parameter studies. The electrode angle (typically 5–15° from vertical) was found to have a significant effect on the weld geometry and dilution rate. A smaller electrode angle produced a narrower, deeper weld with higher dilution, while a larger angle produced a wider, shallower weld with lower dilution. The recommended electrode angle for cladding applications is 10–12°, which provides a good balance between penetration and dilution control.

The shielding gas composition was also investigated, with argon (Ar), helium-argon mixtures (He/Ar), and pure helium being evaluated. The results showed that helium-argon mixtures (50/50 and 75/25 He/Ar) provided the best combination of arc stability, penetration depth, and bead appearance. Pure helium, while providing deeper penetration, resulted in excessive spatter and poor arc stability. Argon-only shielding produced adequate results for thin cladding layers but limited penetration for thicker deposits.

Metallurgical Quality of the Clad Deposit

The metallurgical quality of the dual-electrode TIG cladding deposits was evaluated through metallographic examination, mechanical testing, and chemical analysis. The microstructure of the clad deposit consisted of fine columnar grains with a small amount of equiaxed grains near the top surface. The grain size was finer (average grain size 25–40 μm) compared to conventional TIG cladding (40–60 μm), which is attributed to the higher cooling rates achieved with the dual-electrode process.

The mechanical properties of the clad deposit were found to be superior to those of single-electrode TIG cladding. The tensile strength was in the range of 550–650 MPa for 308L wire on SA-516 Gr.70 base metal, with an elongation of 35–45%. The hardness profile showed a gradual transition from the base metal hardness (150–180 HV) through the dilution zone (200–250 HV) to the pure clad metal hardness (180–220 HV). No brittle intermetallic phases or sigma phase were detected in the dilution zone, which is critical for maintaining the corrosion resistance and ductility of the clad layer.

Engineering Practice Implications

The dual-electrode TIG cladding process has been successfully applied to several pressure vessel fabrication projects, including hydrogenation reactors, heat exchangers, and storage tanks. The process is particularly suitable for applications where: (1) high-quality clad deposits with low dilution are required, (2) complex geometries make it difficult to use automated strip cladding or electroslag welding, (3) small repair areas require selective cladding, and (4) the clad layer thickness is relatively thin (3–10 mm).

For pressure vessel fabrication, the dual-electrode TIG process requires careful attention to several quality control aspects. The pre-weld cleaning of the base metal surface is critical, as any contamination can lead to porosity or lack of fusion in the clad deposit. The base metal should be cleaned to a minimum of 3A surface finish using mechanical methods (grinding, polishing) followed by solvent cleaning. The inter-pass temperature should be maintained below 150°C to prevent excessive grain growth and to maintain the beneficial fine grain structure of the clad deposit.

The non-destructive testing (NDT) requirements for dual-electrode TIG cladding are similar to those for conventional TIG cladding but with additional emphasis on detecting dilution-related defects. Magnetic particle testing (MT) or penetrant testing (PT) should be performed on each pass to detect surface and near-surface defects. Ultrasonic testing (UT) using contact or immersion techniques should be performed to verify the bond strength and detect subsurface discontinuities. The acceptance criteria should be based on the applicable code requirements (ASME VIII Div.1, NB/T 47014, or equivalent).

Key Questions and Reflections

One of the most significant challenges identified in the study is the electrode consumption rate. The dual-electrode configuration consumes tungsten electrodes at a rate 2–3 times higher than single-electrode TIG, which increases the operating cost and introduces the risk of tungsten inclusions if the electrode is not properly dressed or replaced. The study recommends the use of ceriated tungsten (LaO₂) electrodes, which provide better arc stability and longer electrode life compared to thoriated (ThO₂) or zirconiated (ZrO₂) electrodes. The electrode should be ground to a sharp point (2–3 mm) and dressed every 20–30 minutes of welding to maintain arc stability.

Another important consideration is the operator skill level required for the dual-electrode TIG process. The process requires a higher degree of operator expertise compared to conventional TIG welding, as the two electrodes must be maintained at precise angles and positions relative to the workpiece. The study recommends that operators be trained for a minimum of 40 hours on the dual-electrode process before being certified for production welding. Qualification testing should be performed according to the requirements of ASME IX or NB/T 47014, with additional test specimens specifically designed to evaluate the dual-electrode process variables.

Study Insights and Recommendations

The dual-tungsten electrode TIG cladding technology represents a significant advancement in weld overlay technology for pressure vessel fabrication. The process offers a compelling combination of high deposition rates, excellent metallurgical quality, and low dilution, making it particularly suitable for applications where clad quality is critical. The key recommendations for engineering practice include: (1) establish process windows through systematic parameter studies for each specific application, (2) implement rigorous operator qualification and certification programs, (3) develop WPS and PQS documents that incorporate the dual-electrode process variables, (4) perform comprehensive NDT on all clad deposits, and (5) maintain detailed records of process parameters and quality data for traceability. The successful application of this technology requires a commitment to quality, operator training, and continuous process improvement, which ultimately leads to more reliable and cost-effective pressure vessel fabrication.