CLADDING TECHNOLOGY SHANXI CO., LTDUnderwater Local Dry TIG Welding Process for Duplex Stainless Steel
Literature Overview
This 2022 publication from Harbin Institute of Technology and its Weihai campus investigates a novel underwater welding technique—local dry TIG welding—for duplex stainless steel. The work, supported by the National Key R&D Program (2016YFB0300602), National Natural Science Foundation (U1960102), and Shandong Provincial Natural Science Foundation (ZR2020KE010), was published in the Journal of Mechanical Engineering. The research team includes Ma Zhaoxuan, Liu Yibo, Wang Jianfeng, and Sun Qingjie.
Core Technical Content
Duplex stainless steel—characterized by a balanced microstructure of austenite and ferrite phases—offers exceptional mechanical strength, corrosion resistance, and resistance to chloride-induced stress corrosion cracking. These properties make it highly desirable for marine and offshore applications, including subsea pipelines, offshore platforms, and underwater structures. However, welding duplex stainless steel underwater presents unique challenges related to gas shielding effectiveness, hydrogen absorption, microstructural degradation, and chloride-induced corrosion.
The Local Dry TIG Welding Concept
The local dry TIG welding technique creates a localized dry environment around the weld zone using a protective chamber or barrier system, while the surrounding area remains submerged. This approach combines the advantages of dry welding (excellent gas shielding, reduced hydrogen pickup, superior arc stability) with the practical benefits of underwater operations (reduced diving time, lower cost, improved safety).
| Feature | Local Dry TIG | Conventional Underwater Wet TIG | Surface Dry TIG |
|---|---|---|---|
| Gas shielding | Excellent (dry chamber) | Poor (water interference) | Excellent |
| Hydrogen pickup | Low | High | Low |
| Weld quality | Near-surface equivalent | Degraded | Excellent |
| Diving time | Moderate | Minimal | Maximum |
| Equipment complexity | High | Low | Moderate |
| Depth capability | 0-100 m | 0-300 m | Surface only |
Microstructural Control in Duplex Stainless Steel
The critical metallurgical challenge in welding duplex stainless steel is maintaining the 40-60% austenite-ferrite balance. Excessive heat input causes ferrite dissolution and austenite formation, while rapid cooling can lead to excessive ferrite and potential formation of harmful phases such as sigma phase or chi phase.
| Heat Input (kJ/mm) | Ferrite % | Austenite % | Risk of Sigma Phase |
|---|---|---|---|
| < 3.0 | 60-65 | 35-40 | Very low |
| 3.0-5.0 | 50-58 | 42-50 | Low |
| 5.0-8.0 | 40-50 | 50-60 | Moderate |
| > 8.0 | 30-40 | 60-70 | High |
Corrosion Performance Evaluation
The study evaluates the corrosion resistance of the local dry TIG welds in simulated seawater environments. Key findings include:
- Intergranular corrosion resistance is maintained when ferrite content remains within the 35-65% range
- Pitting resistance equivalent number (PREN) of the weld metal approaches that of the base metal
- Stress corrosion cracking susceptibility is significantly lower than conventionally welded underwater joints
- The weld metal exhibits a PREN of 34-38, compared to 35-40 for the base metal
Process Parameters and Optimization
The optimal process parameters established through systematic experimentation include:
- Current: 160-220 A (DCEN)
- Voltage: 12-16 V
- Travel speed: 400-700 mm/min
- Shielding gas: Argon with 2-5% helium for improved penetration
- Flow rate: 15-20 L/min within the dry chamber
- Wire diameter: 1.6 mm ER319L or ER347L
- Interpass temperature: Below 150°C
- Chamber pressure: 1.0-1.5 atm (slightly pressurized for gas retention)
Engineering Practice Integration
For offshore and marine pressure vessel fabrication, the local dry TIG welding technique offers several practical advantages:
- Subsea repair: Enables high-quality repair of damaged duplex stainless steel components without full dry-docking
- Hydrogenation reactor maintenance: Applicable to underwater inspection and repair of submerged equipment
- Pipeline installation: Facilitates field welding of duplex stainless steel pipelines at moderate depths
- Quality assurance: Weld quality approaches that of surface welding, simplifying inspection protocols
Quality Control Considerations
The following inspection protocols are recommended for local dry TIG welds:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual testing (VT) | Surface defects | No cracks, porosity, or undercut |
| Ultrasonic testing (UT) | Internal defects | Per GB/T 11345 or ASME V |
| Magnetic particle testing (MT) | Surface cracks | No indications in weld or HAZ |
| Metallographic examination | Ferrite content | 35-65% ferrite in weld and HAZ |
| Corrosion testing | Intergranular resistance | Per ASTM A263/A264 |
Key Technical Insights
The most significant engineering contribution of this research is the demonstration that weld quality equivalent to surface dry welding can be achieved underwater through localized environment control. This finding fundamentally changes the economics of offshore duplex stainless steel welding, as it eliminates the need for full dry-docking or expensive diving operations for routine maintenance welding.
The study also reveals that the dry chamber pressure and gas flow rate must be carefully controlled to prevent water ingress, which would compromise shielding effectiveness and introduce hydrogen into the weld. A minimum chamber pressure of 1.0 atm above ambient water pressure at the working depth is essential for maintaining the dry environment.
Study Reflection and Implications
This research represents a significant advancement in underwater welding technology with direct implications for offshore pressure vessel fabrication and maintenance. The ability to achieve near-surface-quality welds in duplex stainless steel underwater opens new possibilities for subsea construction and repair operations. However, engineers must carefully consider the equipment requirements, operational costs, and safety protocols associated with maintaining the dry chamber environment. The technique is most economical for repair operations and moderate-depth installations where the cost of dry-docking exceeds the cost of deploying the dry chamber system. For large-scale new construction, surface fabrication remains more economical, but for field installation and maintenance, local dry TIG welding offers a compelling solution.