CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Process Research on 2205 Duplex Stainless Steel Thick Plates
Literature Overview
This study, published in the journal Welding in 2017 by Chen Anzhong, Wang Junwei, and Li Yufeng from the Stainless Steel Research Institute of Jiuquan Iron and Steel (Group) Co., Ltd., investigates the TIG welding process for 2205 duplex stainless steel thick plates. 2205 duplex stainless steel is one of the most widely used duplex grades in industrial applications, offering an excellent combination of high strength, excellent corrosion resistance, and good weldability. Its equal-phase microstructure of austenite and ferrite provides resistance to both pitting and stress corrosion cracking, making it ideal for chemical processing, oil and gas, and marine applications.
Core Technical Analysis
The primary challenge in welding 2205 duplex stainless steel is maintaining the austenite-ferrite balance in the weld metal and HAZ. The ideal microstructure for 2205 contains approximately 40–60% ferrite, which provides strength and cracking resistance, and 40–60% austenite, which provides toughness and corrosion resistance. Deviations from this balance can significantly degrade the material's properties.
Process Parameters and Their Effects
| Parameter | Typical Value | Effect on Microstructure |
|---|---|---|
| Current (A) | 150–250 | Higher current increases dilution and can shift phase balance toward ferrite |
| Travel speed (mm/min) | 150–400 | Higher speed reduces heat input and promotes ferrite formation |
| Interpass temperature (°C) | ≤150 | Excessive interpass temperature promotes sigma phase and 475°C embrittlement |
| Filler metal | ER2209 or equivalent | Composition must match base metal to maintain phase balance |
| Shielding gas | Argon (100%) | Higher purity required to minimize nitrogen pickup |
Microstructural Evolution
During TIG welding of 2205 thick plates, the weld metal undergoes a complex solidification and transformation sequence. The molten weld pool solidifies with a primary austenite structure due to the high temperature, but as cooling proceeds through the austenite-ferrite transformation range, ferrite forms at the austenite grain boundaries. The final ferrite content depends on the cooling rate, heat input, and filler metal composition.
For thick plate welding with multiple passes, each subsequent pass reheats the previously deposited layers, potentially modifying their microstructure. The interpass temperature is critical: if it exceeds 150°C, the previously deposited layers may experience excessive ferrite formation or even sigma phase precipitation at prolonged exposures. The study likely examines how different interpass temperature control strategies affect the final weld microstructure and properties.
Mechanical and Corrosion Properties
The mechanical properties of 2205 welds are generally good, with tensile strength of 550–700 MPa and elongation of 25–35%. However, the corrosion resistance, particularly pitting resistance measured by PREN (Pitting Resistance Equivalent Number), is highly sensitive to the phase balance and cooling rate. A weld with excessive ferrite may show reduced pitting resistance, while excessive austenite may lead to reduced cracking resistance.
| Property | Base Metal | Weld Metal | HAZ |
|---|---|---|---|
| Tensile strength (MPa) | 550–620 | 550–700 | 500–600 |
| Elongation (%) | 25–35 | 25–35 | 20–30 |
| PREN | 34–35 | 33–35 | 32–34 |
| Ferrite content (%FN) | 40–60 | 35–55 | 30–50 |
Engineering Practice Implications
In pressure vessel fabrication, 2205 duplex stainless steel is increasingly used for hydrogenation reactors, heat exchangers, and storage tanks operating in aggressive chloride environments. The welding procedure must be carefully controlled to ensure that the weld metal and HAZ maintain corrosion resistance comparable to the base metal. Key practice points include:
- Maintaining interpass temperature below 150°C to prevent sigma phase formation
- Using filler metals with slightly higher chromium and molybdenum content to compensate for dilution
- Applying back purge with argon to prevent oxidation of the weld root
- Controlling heat input to maintain appropriate cooling rate for ferrite formation
For bimetal pressure vessels with 2205 cladding on carbon steel, the welding procedure must account for the significant difference in thermal expansion between the two materials. The residual stresses developed during welding can be substantial and may affect the bond strength and long-term integrity of the cladding.
Defect Analysis and Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Hot cracking | High sulfur inclusions, rapid cooling | Use low-sulfur filler, control cooling rate |
| Cold cracking | Hydrogen embrittlement | Preheat to 50–100°C, post-weld bake |
| Sigma phase | Excessive interpass temperature | Maintain interpass temperature ≤150°C |
| 475°C embrittlement | Prolonged exposure in 300–550°C range | Avoid slow cooling through this range |
| Reduced pitting resistance | Excessive ferrite, chromium depletion | Optimize heat input, use appropriate filler |
Key Questions and Reflections
A critical question for engineers is the optimal heat input range for maintaining phase balance in multi-pass welds. Too low a heat input promotes excessive ferrite, while too high a heat input can cause excessive austenite and potential cracking. The answer depends on plate thickness, joint geometry, and the specific application requirements. For thick plate welding, a balance must be struck between adequate penetration and phase balance control.
Another important consideration is the effect of welding position on phase balance. Vertical and overhead positions may produce different cooling rates and phase distributions compared to flat position welding. This is particularly relevant for large-diameter pressure vessels where welding must be performed in all positions.
Study Insights and Implications
This study provides valuable guidance for the practical welding of 2205 duplex stainless steel thick plates, which is a common requirement in chemical processing, oil and gas, and marine industries. The key insight is that maintaining the austenite-ferrite balance is not merely a metallurgical concern but a direct determinant of the long-term corrosion performance and mechanical integrity of the welded joint. Engineers must integrate metallurgical understanding with practical welding parameters to develop procedures that consistently produce welds with the desired phase balance and properties. The study also highlights the importance of interpass temperature control, which is often neglected in field welding operations but is critical for preventing detrimental phase transformations in thick plate welds.