Zhuojin Pipe Fitting Co., LtdLocal Dry Underwater TIG Welding of Nitrogen-Controlled Austenitic Stainless Steel for Nuclear Power Plant Locating Pins
Literature Overview
Published in the journal "Welding Journal" in 2024 by Sun Qiejie and colleagues from Harbin Institute of Technology and Harbin Institute of Technology (Weihai), this paper presents a novel local dry underwater TIG welding process for locating pins in nuclear power plants. The research was supported by the National Natural Science Foundation of China and the Shandong Provincial Natural Science Foundation. The study develops a dual-layer gas protection local dry underwater TIG rotating welding torch, where the motion trajectory is achieved by a DC motor driving the tungsten electrode to rotate around a fixed diameter. The welding is performed on Z2CN19-10 nitrogen-controlled austenitic stainless steel, a nuclear-grade material used in critical reactor components. The research investigates weld formation, optimizes process parameters, and analyzes microstructure and mechanical properties of underwater and land-based welded joints.
Process Development and Torch Design
The local dry underwater welding process is a specialized technique that creates a dry welding zone beneath the water surface by using a multi-layer gas shielding system. The developed rotating welding torch incorporates dual-layer gas protection, where both the inner and outer layers are supplied with argon gas. The rotation mechanism, driven by a DC motor, ensures uniform heat input around the circumference of the locating pin, which is essential for achieving consistent weld quality in this axisymmetric geometry.
The dual-layer gas protection serves two critical functions. The inner layer directly shields the molten pool and tungsten electrode from water contamination, preventing hydrogen absorption and oxide inclusion formation. The outer layer creates a barrier that prevents water from penetrating the dry welding zone, maintaining a controlled atmosphere around the arc. The rotation of the torch ensures that the welding heat is distributed uniformly around the joint circumference, reducing the risk of directional distortion and achieving a consistent weld profile.
Weld Formation and Microstructural Analysis
| Parameter | Effect on Weld | Observation |
|---|---|---|
| Both layers argon | Good weld formation | Stable arc morphology |
| Increased welding current | Increased penetration and width | Larger weld pool |
| Decreased welding speed | Increased penetration and width | Longer heat input time |
| Underwater vs land weld | Ferrite morphology change | Dendritic to lamellar |
| Underwater vs land weld | Reduced austenite content | Quenched microstructure |
| Underwater vs land weld | Refined weld center grains | Faster cooling rate |
| Underwater vs land weld | Slightly higher hardness and strength | Quenching effect |
The most significant metallurgical finding is the effect of water's rapid cooling on the microstructure near the fusion line. In land-based welding, the ferrite near the fusion line exhibits a dendritic morphology typical of slow cooling conditions. In underwater welding, the rapid heat extraction by the surrounding water transforms the ferrite morphology from dendritic to lamellar, which is characteristic of faster cooling rates. The austenite content in the underwater weld is reduced compared to the land weld, and the weld center grain structure is refined. These microstructural changes result in slightly higher microhardness and mechanical properties in the underwater welded joints.
Nuclear Grade Material Considerations
Z2CN19-10 is a nitrogen-controlled austenitic stainless steel specifically designed for nuclear applications. The nitrogen addition provides solid solution strengthening and improves the material's resistance to stress corrosion cracking and irradiation-induced degradation. The base material composition typically includes approximately 19% chromium, 10% nickel, and controlled nitrogen content in the range of 0.1-0.2%. The nitrogen content is critical for maintaining the desired balance between austenite and ferrite phases, which influences both mechanical properties and corrosion resistance.
In underwater welding, the rapid cooling rate can significantly alter the phase balance in the weld metal and heat-affected zone. The reduction in austenite content and transformation of ferrite morphology must be evaluated against the material's performance requirements in the nuclear environment. The slightly improved mechanical properties of the underwater weld are a beneficial outcome, but the microstructural changes must be assessed for long-term radiation stability and corrosion resistance.
Engineering Practice Integration
The local dry underwater welding process offers significant advantages for nuclear power plant maintenance and repair operations. Underwater welding is necessary for repairing submerged components such as locating pins, which are critical for maintaining the position of fuel assemblies in the reactor core. Conventional underwater welding processes suffer from poor weld quality due to water contamination, hydrogen absorption, and uncontrolled heat input. The local dry underwater TIG process addresses these challenges by creating a controlled dry welding environment beneath the water surface.
The rotating torch design is particularly suited for axisymmetric joints such as locating pins, where uniform circumferential weld quality is essential. The DC motor-driven rotation mechanism provides precise control over the welding speed and trajectory, which is critical for achieving consistent weld geometry. The dual-layer gas protection system ensures reliable arc stability and molten pool protection, which are essential for producing high-quality welds in the underwater environment.
Key Questions and Reflections
Several important aspects of this study warrant further consideration. The study does not evaluate the long-term corrosion resistance and radiation stability of the underwater welded joints, which are critical performance requirements for nuclear applications. The hydrogen absorption behavior during underwater welding, which can lead to delayed hydrogen cracking, is not addressed. The study also does not compare the local dry underwater TIG process with alternative underwater welding methods such as submerged arc welding or electroslag welding.
The finding that underwater welding produces slightly higher mechanical properties than land-based welding is counterintuitive and merits further investigation. While the rapid cooling rate can refine the grain structure and increase hardness, excessive cooling rates can also lead to microcracking and reduced ductility. The study should evaluate the full range of mechanical properties including ductility, toughness, and fatigue strength to ensure that the underwater weld meets the performance requirements for nuclear applications.
Study Insights and Implications
This research presents a significant advancement in underwater welding technology for nuclear power plant applications. The development of a dual-layer gas protection local dry underwater TIG rotating welding torch provides a practical solution for welding critical submerged components such as locating pins. The process optimization and microstructural analysis demonstrate that the local dry underwater TIG process can produce welds with good formation and mechanical properties comparable to or slightly exceeding those of land-based welding. For nuclear industry engineers, this study provides a viable technology for underwater repair and maintenance operations that maintains the high quality standards required for nuclear-grade materials. The findings emphasize the importance of process development and material characterization for specialized welding applications in demanding environments. Future work should focus on long-term performance evaluation, hydrogen absorption control, and qualification testing for regulatory approval in nuclear power plant applications.