CLADDING TECHNOLOGY SHANXI CO., LTDStainless Steel Arc-Assisted Active TIG Welding Process Development
Literature Overview
The study by Huang Yong, Fan Ding, Lin Tao, and Luo Huansheng from Lanzhou University of Technology, published in The International Journal of Welding & Joining in 2009, investigates the arc-assisted active TIG (A-TIG) welding process for stainless steel. This research addresses the challenge of achieving high-quality welds in austenitic stainless steels, which are widely used in cladding and bimetal pressure vessel applications due to their excellent corrosion resistance. The authors develop an A-TIG process that combines a main arc with an auxiliary arc to improve weld quality and productivity.
Stainless Steel Welding Challenges
Austenitic stainless steels such as 304, 316, 321, and 347 present unique welding challenges:
| Challenge | Description | Impact |
|---|---|---|
| High thermal expansion | ~17 μm/m·K | Distortion and residual stress |
| Low thermal conductivity | ~15 W/m·K | High heat concentration, wide HAZ |
| Sensitization | Chromium carbide precipitation at 500–800°C | Intergranular corrosion susceptibility |
| Solidification cracking | High sulfur and phosphorus | Transverse cracking in weld metal |
| Dilution | In dissimilar joints | Loss of corrosion resistance |
The A-TIG process offers potential advantages for stainless steel welding by providing higher heat input and better arc stability than conventional TIG, while maintaining the low dilution characteristic of TIG welding.
A-TIG Process Configuration and Parameters
The arc-assisted TIG process configuration includes:
| Component | Description |
|---|---|
| Main arc | Conventional TIG arc, provides primary heat input |
| Auxiliary arc | Secondary arc, positioned to assist the main arc |
| Arc configuration | Parallel, staggered, or perpendicular |
| Current ratio | Main:Auxiliary = 2:1 to 3:1 |
| Total current | 150–250 A |
| Travel speed | 200–500 mm/min |
| Shielding gas | 100% Ar or Ar + 2–5% CO2 |
| Filler wire | ER308L, ER316L, or ER347 |
The auxiliary arc provides additional heat input without significantly increasing the current density on the tungsten electrode. This allows for higher penetration and wider bead formation while maintaining arc stability and reducing tungsten erosion.
Weld Quality and Defect Analysis
The study compares the weld quality of A-TIG with conventional TIG:
| Defect Type | Conventional TIG | A-TIG |
|---|---|---|
| Porosity | Moderate, 5–10% area fraction | Reduced, 2–5% area fraction |
| Lack of fusion | Occasional, especially at root | Rare, improved fusion |
| Undercut | Common at high speeds | Reduced, wider bead |
| Cracking | Low susceptibility | Low susceptibility |
| Dilution | Low, 5–10% | Low, 8–12% |
The A-TIG process produces welds with reduced porosity and improved fusion, attributed to the higher heat input and better arc stability. The dilution ratio remains low, which is critical for maintaining the corrosion resistance of the weld in cladding applications.
Microstructural Characteristics
The microstructures of A-TIG welded stainless steel joints are as follows:
| Zone | Microstructure | Grain Size | Precipitates |
|---|---|---|---|
| Weld metal | Columnar dendrites + equiaxed grains | 50–100 μm | δ-ferrite 5–8% |
| Fusion line | Mixed columnar and equiaxed | 30–60 μm | δ-ferrite 8–12% |
| HAZ | Recrystallized grains | 100–200 μm | Carbides at grain boundaries |
| Base metal | Equiaxed grains | 100–150 μm | Solution carbides |
The δ-ferrite content in the weld metal is an important parameter for controlling solidification cracking and hot shortness. The A-TIG process produces δ-ferrite contents within the acceptable range of 3–10%, which provides good resistance to solidification cracking and hot shortness.
Engineering Practice Applications
For stainless steel cladding and bimetal pressure vessel applications, the A-TIG process offers several advantages:
- Improved productivity: The higher heat input allows for faster travel speeds while maintaining weld quality, reducing the number of passes required for overlay welding.
- Reduced dilution: The low dilution ratio is critical for maintaining the corrosion resistance of the cladding layer, especially when overlaying austenitic stainless steel on carbon steel substrates.
- Better fusion: The improved fusion reduces the risk of lack of fusion defects, which are critical in pressure-retaining welds.
- Consistent bead geometry: The stable arc and consistent heat input produce uniform bead geometry, which is essential for achieving uniform cladding thickness.
The A-TIG process can be applied to the fabrication of stainless steel clad plate pressure vessels, where the overlay layer provides corrosion resistance while the carbon steel base provides structural strength. The process can also be used for the repair of damaged cladding layers and the welding of dissimilar joints between stainless steel and carbon steel.
Key Reflections and Study Insights
This research demonstrates that the A-TIG process is a viable alternative to conventional TIG for stainless steel welding, offering improved weld quality and productivity. The systematic investigation of process parameters and their effects on weld quality provides a solid foundation for process optimization and WPS development.
However, the study raises important questions about the long-term performance of A-TIG welded joints under corrosive environments. The microstructural analysis shows the presence of carbides at grain boundaries in the HAZ, which may affect the intergranular corrosion resistance. The PWHT strategy for A-TIG welded joints should include a solution treatment to dissolve the carbides and restore the corrosion resistance.
The research also highlights the importance of process integration in achieving high-quality welds. The A-TIG process, when combined with appropriate filler metal selection, shielding gas composition, and post-weld treatment, can produce welds that meet the requirements of pressure vessel design codes. The systematic approach to process development demonstrated in this study is a model for the development of advanced welding technologies for demanding applications.
The findings of this research have direct implications for the cladding and bimetal pressure vessel industry, where the quality and integrity of the overlay layer are critical for long-term reliability. The A-TIG process offers a promising technology for the fabrication of high-quality stainless steel clad pressure vessels, and further research on the long-term performance of A-TIG welded joints is warranted.