Underwater Local Dry TIG Welding of Nuclear Positioning Pins Using Nitrogen-Controlled Austenitic Stainless Steel

Literature Overview

This paper published in Transactions of the China Welding Institution (Vol. 45, No. 2, 2024, pp. 7–12) by Sun Qingjie, Zhang Qinghua, Zhao Yongqing, and colleagues from Harbin Institute of Technology presents a systematic study on underwater local dry TIG welding of nuclear-grade positioning pins. The research is supported by the National Natural Science Foundation of China (Grants 52175299, U1960102) and the Shandong Provincial Natural Science Foundation Key Project (ZR2020KE010). The work addresses a critical engineering challenge in nuclear power plant maintenance and fabrication: achieving high-quality welds in submerged environments where conventional dry welding cannot be applied. The study focuses on the nuclear-grade nitrogen-controlled austenitic stainless steel Z2CN19-10, a material specified for nuclear-grade applications requiring resistance to stress corrosion cracking and irradiation effects.

Core Technical Approach

The central innovation of this research is the development of a dual-layer gas-protected local dry underwater TIG rotating welding torch. The motion trajectory is achieved by driving the tungsten electrode to rotate around a fixed diameter using a DC motor, which creates a consistent arc geometry and uniform heat distribution. This rotating mechanism is critical for maintaining a stable arc in the confined space around a cylindrical positioning pin, where axial welding approaches would be impractical.

The dual-layer gas protection system employs an inner layer and an outer layer of shielding gas, both supplied with argon. The inner layer provides direct arc stabilization and exclusion of water vapor from the immediate welding zone, while the outer layer acts as a secondary barrier against moisture ingress and ensures a controlled atmosphere around the molten pool. This configuration is essential because, in underwater environments, the surrounding water exerts significant pressure and introduces dissolved gases that can cause porosity, arc instability, and poor weld quality.

Weld Formation Analysis

The study systematically investigated the effects of welding current and welding speed on weld penetration depth and weld width. The key findings are summarized in the following table:

The results demonstrate that the dual-layer argon shielding produces excellent weld formation with a stable arc profile. Increasing the welding current enhances the heat input, leading to deeper penetration and wider weld beads. Conversely, reducing the welding speed increases the dwell time of the arc at any given point, similarly increasing both penetration and width. These findings are consistent with fundamental TIG welding principles, confirming that the local dry underwater environment does not fundamentally alter the thermal dynamics of arc welding, provided adequate gas protection is maintained.

Microstructure and Mechanical Property Analysis

A particularly insightful aspect of this study is the comparison between underwater and above-water weld joints. The thermal cycle curves and arc morphology were analyzed in conjunction with microstructural examination. The key microstructural findings are:

• Ferrite morphology transformation: Near the fusion line, the rapid cooling effect of water causes the ferrite phase to transform from a dendritic morphology (observed in above-water welds) to a lamellar morphology (observed in underwater welds). This is a significant metallurgical observation, as the cooling rate in underwater welding can be 2–5 times higher than in ambient conditions.
• Austenite content reduction: The faster cooling rate in underwater welding reduces the equilibrium austenite content in the weld metal, which has implications for the magnetic properties and corrosion resistance of the joint.
• Grain refinement: The center of the underwater weld exhibits finer grains compared to the above-water counterpart, attributable to the higher cooling rates promoting more nucleation sites.

Mechanically, the underwater welded joint exhibits slightly higher microhardness and mechanical properties compared to the above-water joint. This counterintuitive result—where faster cooling typically leads to harder but more brittle microstructures—can be attributed to the grain refinement effect and the transformation of ferrite morphology to a more favorable lamellar configuration that provides better load-bearing capacity.

Engineering Practice Implications

For nuclear power plant applications, this research has direct practical significance. Positioning pins are structural components used in reactor internals and support structures, and their integrity is paramount for safety. The ability to perform high-quality TIG welds in submerged conditions expands the range of feasible maintenance and repair operations without the need for complete dry-out of the reactor vessel, which is extremely costly and time-consuming.

From a quality assurance perspective, the following considerations must be addressed:

1. Gas supply logistics: Maintaining a continuous supply of high-purity argon to an underwater torch requires robust delivery systems capable of withstanding hydrostatic pressure.
2. Visual inspection limitations: Underwater visual inspection is inherently limited; supplementary non-destructive testing such as ultrasonic testing (UT) or phased array UT (PAUT) should be mandated for critical welds.
3. Post-weld heat treatment: The reduced austenite content and altered ferrite morphology may necessitate a post-weld heat treatment (PWHT) cycle to restore the desired microstructure and minimize residual stresses, particularly given the fast cooling rates.
4. Hydrogen embrittlement risk: Although the local dry method minimizes water contact with the arc, dissolved hydrogen in the surrounding water can still diffuse into the weld during the solidification period. Preheating the base metal to 100–150°C before welding and applying a controlled post-weld bake cycle are recommended mitigations.

Key Questions and Reflections

This research raises several important questions for further investigation. First, the long-term corrosion resistance of the underwater weld joint, particularly susceptibility to stress corrosion cracking (SCC) in chloride-containing environments, requires extended testing beyond the scope of this paper. Second, the effect of welding position and joint geometry on the dual-layer gas protection efficiency deserves systematic study, as the current work appears to focus on a specific pin geometry. Third, the rotational welding approach introduces mechanical vibration into the welding process; quantifying the effect of this vibration on arc stability and microstructural uniformity would be valuable for scaling the technology to larger components.

The transformation of dendritic to lamellar ferrite near the fusion line is a metallurgically significant finding. In austenitic stainless steels, the ferrite content and morphology directly influence the susceptibility to intergranular corrosion and SCC. Lamellar ferrite, while providing mechanical strength, may create preferential paths for intergranular attack if sensitization occurs. Future work should include electrochemical corrosion testing and long-term aging studies to validate the durability of these underwater welds in nuclear service conditions.

Study Insights and Implications

The most valuable insight from this paper is the demonstration that conventional TIG welding metallurgy can be successfully replicated in underwater conditions through engineering of the gas protection system, without requiring exotic welding consumables or fundamentally different welding processes. The dual-layer argon shielding with a rotating torch design is an elegant solution that leverages existing TIG equipment with minimal modification. For nuclear industry engineers, this means that the extensive qualification database and quality assurance infrastructure built around TIG welding can be partially transferred to underwater applications, reducing the time and cost of process qualification.

The finding that underwater welds exhibit slightly superior mechanical properties is encouraging but must be interpreted with caution. Mechanical property improvement does not automatically translate to improved long-term performance in service, particularly where fatigue, corrosion, and radiation embrittlement are governing failure modes. The grain refinement observed in underwater welds could be beneficial for fatigue resistance, but the reduced austenite content may compromise corrosion resistance. A balanced assessment considering all relevant degradation mechanisms is essential before deploying this technology in safety-critical nuclear applications.

In summary, this research represents a significant advancement in underwater welding technology for nuclear applications, providing a practical pathway for maintaining and repairing submerged structural components with conventional welding metallurgy. The dual-layer gas protection system and rotating torch design are innovative contributions that deserve further development and qualification for broader industrial deployment.