CLADDING TECHNOLOGY SHANXI CO., LTDUnderwater Local Dry TIG Welding Process for Duplex Stainless Steel
Literature Overview
This 2022 study by Ma Zhaoxuan, Liu Yibo, Wang Jianfeng, and Sun Qiejie from the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology and the Shandong Provincial Key Laboratory of Special Welding Technology investigates the underwater local dry TIG welding process for duplex stainless steel. Published in the Chinese Journal of Mechanical Engineering, the research was supported by the National Key Research and Development Program (2016YFB0300602), the National Natural Science Foundation of China (U1960102), and the Shandong Provincial Natural Science Foundation (ZR2020KE010). This work addresses the critical challenge of welding duplex stainless steel structures in submerged environments, which is essential for offshore engineering, submarine construction, and underwater repair operations.
Core Technical Content
Underwater welding of duplex stainless steel presents unique challenges due to the combined effects of hydrostatic pressure, limited access to shielding gas, and the sensitivity of duplex stainless steel to microstructural changes. The local dry TIG welding process creates a dry zone around the arc by using a combination of shielding gas, water displacement, and sometimes a welding cell or chamber, allowing conventional TIG welding parameters to be used while maintaining the dry conditions necessary for quality welds.
The duplex stainless steel used in this study typically contains approximately 22% chromium, 5-7% nickel, and 3-5% molybdenum, with a target ferrite content of 35-65% (in 304 equivalent) to balance corrosion resistance and mechanical properties. The key challenge in underwater welding is maintaining the austenite-ferrite balance during the welding thermal cycle, as excessive heat input or cooling rate can cause phase transformations that degrade the duplex structure.
The local dry welding process involves several key components:
- Welding cell or chamber: A pressurized enclosure that maintains a dry atmosphere around the weld area, typically using inert gas (argon or argon-helium mixture) to displace water and provide shielding.
- Arc stabilization: The TIG arc must be stabilized in the presence of water and hydrostatic pressure, which can cause arc wandering, increased arc voltage, and reduced arc stability.
- Heat input control: The cooling effect of the surrounding water must be compensated for by adjusting the welding parameters to achieve adequate penetration while avoiding excessive heat input that could cause phase transformation.
- Solidification control: The rapid cooling caused by the surrounding water must be managed to prevent the formation of harmful phases such as sigma phase or chromium carbide precipitation.
Process Parameters and Weld Quality
The study examined the effects of various process parameters on weld quality:
| Parameter | Underwater Range | In-air Equivalent | Effect on Weld Quality |
|---|---|---|---|
| Arc current | 100–250 A | 100–250 A | Higher current needed for penetration in water |
| Arc voltage | 15–25 V | 10–18 V | Increased due to water resistance and arc elongation |
| Travel speed | 3–15 cm/min | 5–25 cm/min | Reduced due to increased heat input |
| Shielding gas | Ar or Ar-He mix | Ar | He addition improves arc stability in water |
| Gas flow rate | 15–30 L/min | 10–20 L/min | Increased to maintain dry zone |
| Hydrostatic pressure | 0–50 MPa | 0.1 MPa | Affects arc stability and heat transfer |
The weld quality evaluation included metallographic examination, mechanical testing, and corrosion testing. The results showed that the local dry TIG welding process can produce welds with acceptable mechanical properties and corrosion resistance, provided that the process parameters are carefully optimized for the underwater conditions.
The microstructural analysis revealed that the weld metal maintains a duplex structure with a ferrite content in the target range (35-65% in 304 equivalent), while the heat-affected zone shows some changes in grain size and phase distribution. The cooling rate in the underwater environment is significantly higher than in air, which can lead to finer microstructures but also increases the risk of cracking if not properly managed.
Relevance to Pressure Vessel and Offshore Engineering
For engineers in the pressure vessel and offshore engineering industries, this research has direct practical applications. Duplex stainless steel is widely used in offshore platforms, subsea pipelines, and underwater storage tanks due to its excellent combination of strength, corrosion resistance, and cost-effectiveness. The ability to weld these structures underwater with acceptable quality is essential for construction, repair, and maintenance operations.
The local dry TIG welding process offers several advantages over other underwater welding methods:
- Quality: The dry conditions allow for better arc stability and reduced porosity compared to wet underwater welding.
- Flexibility: The process can be adapted to different underwater depths and conditions, making it suitable for a wide range of applications.
- Cost-effectiveness: Compared to hyperbaric welding in a full saturation diving chamber, the local dry process requires less equipment and can be deployed more quickly.
However, the process also has limitations that must be considered in engineering practice. The welding cell or chamber must be properly sealed to prevent water ingress, which can be challenging in rough underwater conditions. The process also requires careful management of the gas supply and pressure to maintain the dry zone throughout the welding operation.
Key Questions and Reflections
Several important questions arise from this research for practical engineering application. First, the scalability of the local dry welding process to large-scale offshore structures is a critical consideration. The study focuses on small-scale welding experiments, but the challenges of maintaining dry conditions over large weld areas in open ocean environments are significant.
Second, the long-term performance of underwater-welded duplex stainless steel joints under cyclic loading and corrosion conditions requires further investigation. The underwater welding process introduces additional variables such as hydrostatic pressure cycling and marine biological attack that can affect the long-term integrity of the weld.
Third, the standardization and qualification of underwater welding processes for pressure vessel applications remains an area of uncertainty. Current standards such as NB/T 47014 and ASME IX provide guidance for in-air welding qualification but do not specifically address underwater welding conditions. Engineering judgment and additional testing are required to qualify underwater welding procedures for critical pressure vessel applications.
Summary and Engineering Implications
The research by Ma et al. demonstrates that the local dry TIG welding process is a viable technique for welding duplex stainless steel in submerged environments, producing welds with acceptable mechanical properties and corrosion resistance. For engineers in the offshore and pressure vessel industries, this work provides a practical pathway for underwater construction and repair operations using duplex stainless steel. The key challenges lie in scaling the process to large structures, ensuring long-term performance under cyclic and corrosive conditions, and establishing appropriate qualification procedures. This research represents an important step toward enabling the use of high-performance duplex stainless steel in underwater applications and provides a foundation for further development of underwater welding technologies.