CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Process Research on 2205 Duplex Stainless Steel Thick Plates
Literature Overview and Research Background
This 2017 study from Jiuquan Steel (Group) Co., Ltd. Stainless Steel Research Institute addresses the welding of 2205 duplex stainless steel thick plates using gas tungsten arc (GTAW/TIG) welding. The research is of considerable practical significance given the increasing adoption of 2205 duplex stainless steel in chemical processing, oil and gas, desalination, and marine applications where a combination of high strength, excellent corrosion resistance, and moderate cost is required.
2205 duplex stainless steel (UNS S31803/S32205) contains approximately 22% chromium and 5% nickel, with a balanced ferrite-austenite microstructure (typically 40-60% ferrite). This microstructure provides yield strength approximately twice that of 304/316 austenitic stainless steels, while maintaining excellent resistance to pitting, crevice corrosion, and chloride stress corrosion cracking. However, the welding of 2205 duplex stainless steel presents unique challenges related to microstructural evolution, phase balance control, and potential cracking susceptibility.
Core Technical Points and Welding Challenges
Microstructural Sensitivity of 2205 Duplex Stainless Steel
The ferrite-austenite balance in 2205 duplex stainless steel is highly sensitive to temperature. During welding, the following phase transformations occur:
| Temperature Range (°C) | Phase Transformation | Effect on Microstructure |
|---|---|---|
| >1400 | Complete austenitization | Loss of ferrite, all-austenite structure |
| 1150-1400 | Ferrite dissolution begins | Ferrite fraction decreases |
| 900-1150 | Sigma phase precipitation | Brittle intermetallic, reduced toughness |
| 700-900 | Sigma phase growth | Maximum brittleness risk |
| 400-700 | Sigma phase dissolution begins | Partial recovery of toughness |
| <400 | Stable duplex structure | Optimal ferrite-austenite balance |
The critical challenge is to maintain a ferrite content of 35-65% in the weld metal and HAZ to ensure:
- Adequate hot cracking resistance (ferrite acts as a crack arrestor)
- Sufficient toughness (excessive ferrite reduces impact energy)
- Resistance to intergranular corrosion (excessive ferrite promotes sigma phase formation)
Welding Process Parameters
| Parameter | Recommended Range | Effect on Weld Quality |
|---|---|---|
| Welding current | 150-220 A | Higher current increases heat input and ferrite dissolution |
| Travel speed | 4-8 mm/s | Lower speed increases heat input per unit length |
| Arc voltage | 12-18 V | Higher voltage widens the bead and increases dilution |
| Heat input | 0.8-1.5 kJ/mm | Must be controlled to prevent excessive ferrite dissolution |
| Shielding gas | Pure Ar or 98% Ar + 2% He | Helium admixture increases penetration and reduces arc voltage |
| Preheat | 50-100°C (for thick plates) | Reduces cooling rate, promotes balanced phase structure |
| Interpass temperature | <200°C | Prevents excessive grain growth and sigma phase formation |
| Fill material | ER2209 or ER2594 (higher Ni content) | Compensates for ferrite dissolution during welding |
Why Higher-Nickel Fill Material is Required
A key finding from this research is the necessity of using fill material with higher nickel content (such as ER2209 with ~9% Ni or ER2594 with ~12% Ni) rather than matching ER2205 wire. This is because:
- Ferrite dissolution during welding: The high temperatures in the weld pool (>1400°C) cause complete or partial dissolution of the ferrite phase.
- Rapid solidification: During solidification, the cooling rate determines the final ferrite content. Higher cooling rates generally promote higher ferrite content.
- Dilution effects: The base metal dilutes the weld metal, reducing the effective nickel content and potentially resulting in excessive ferrite.
- HAZ ferrite reduction: The HAZ experiences a thermal cycle that can reduce the ferrite content below the optimal range, particularly in the coarse grain HAZ.
By using higher-nickel fill material, the weld metal composition is shifted toward austenite, compensating for the ferrite dissolution that occurs during the welding thermal cycle. This ensures that the final weld metal retains an adequate ferrite content (35-65%) after solidification.
Process Optimization and Quality Control
Welding Sequence for Thick Plates
For thick plates (>12 mm), the welding sequence is critical to managing distortion and ensuring adequate penetration:
- Groove preparation: V-groove or X-groove preparation with included angle of 60-80° and root gap of 2-3 mm.
- Root pass: GTAW with low current (120-150 A) and high travel speed to achieve full penetration with minimal heat input.
- Fill passes: GTAW with progressively higher current, maintaining bead width of 8-12 mm and ensuring adequate fusion between passes.
- Cap pass: Final pass with controlled heat input to ensure surface quality and minimize cap porosity.
- Post-weld treatment: For critical applications, solution annealing at 1050-1100°C followed by rapid quenching may be required to restore the ferrite-austenite balance.
Non-Destructive Testing Requirements
| NDT Method | Application | Acceptance Criteria |
|---|---|---|
| RT (Radiographic Testing) | Internal defects (porosity, lack of fusion) | ASME Section V, Article 2 |
| UT (Ultrasonic Testing) | Volumetric and surface defects | ASME Section V, Article 4 |
| PT (Penetrant Testing) | Surface cracks, especially hot cracks | ASME Section V, Article 6 |
| MT (Magnetic Particle Testing) | Surface and near-surface defects | ASME Section V, Article 7 |
| TOFD/PAUT | Detailed volumetric characterization | EN ISO 17638 / ASME Section V |
Mechanical Property Verification
The welded joint of 2205 duplex stainless steel must meet the following mechanical property requirements:
| Property | Base Metal | Weld Metal | HAZ | Test Standard |
|---|---|---|---|---|
| Yield strength (MPa) | ≥450 | ≥450 | ≥400 | ASTM E8 |
| Tensile strength (MPa) | ≥550 | ≥550 | ≥500 | ASTM E8 |
| Elongation (%) | ≥25 | ≥20 | ≥15 | ASTM E8 |
| Charpy V-notch impact energy (J) | ≥70 | ≥50 | ≥40 | ASTM E23 |
| Ferrite content (%) | 40-60 | 35-65 | 35-65 | ASTM E1245 |
Integration with Engineering Practice
Application in Pressure Vessel Fabrication
2205 duplex stainless steel is increasingly used for:
- Hydrogenation reactors: Where high strength and resistance to hydrogen-induced cracking are required
- Chlor-alkali equipment: Where resistance to chloride stress corrosion cracking is critical
- Desalination systems: Where resistance to pitting and crevice corrosion in seawater is essential
- Oil and gas wellhead equipment: Where high yield strength and resistance to sour environment are needed
In pressure vessel fabrication, the welding procedure must be qualified in accordance with ASME IX or NB/T 47014, with additional requirements for:
- Ferrite content measurement of the weld metal and HAZ
- Intergranular corrosion testing (ASTM A263 Practice A or ASTM G150)
- Hydrogen-induced cracking (HIC) testing for sour service applications (ASTM G178)
- Sulfide stress corrosion (SSC) testing for sour service (ASTM G178)
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Excessive austenite in weld metal, high sulfur/phosphorus | Use higher-nickel fill material, control sulfur content |
| Cold cracking | Hydrogen embrittlement in HAZ | Preheat, low hydrogen electrode, post-weld bake |
| Sigma phase | Excessive heat input, slow cooling in 700-900°C range | Reduce heat input, increase travel speed, solution anneal |
| Excessive ferrite | Low nickel content, slow cooling | Use higher-nickel fill material, increase heat input |
| Insufficient ferrite | Excessive heat input, high nickel content | Reduce heat input, use matching fill material |
| Porosity | Gas shielding contamination, moisture in fill material | Ensure proper gas flow, dry fill material, clean base metal |
Study Insights and Reflections
This research provides valuable practical guidance for the welding of 2205 duplex stainless steel thick plates. Several key insights emerge:
- Fill material selection is the most critical process variable: The choice between ER2205, ER2209, and ER2594 fill material has a profound effect on the final ferrite content and mechanical properties of the weld. Engineers must carefully balance the need for adequate ferrite content against the risk of excessive ferrite leading to sigma phase formation.
- Heat input control is essential for thick plate welding: For thick plates, the cumulative heat input from multiple passes can lead to excessive ferrite dissolution and grain growth in previously deposited passes. The welding sequence and interpass temperature must be carefully controlled.
- Post-weld solution annealing may be required for critical applications: For pressure vessels operating in aggressive environments, post-weld solution annealing at 1050-1100°C followed by rapid quenching may be necessary to restore the ferrite-austenite balance and dissolve any sigma phase that may have formed during welding.
- The balance between strength and toughness must be maintained: 2205 duplex stainless steel offers approximately twice the yield strength of 304/316 austenitic stainless steels, but this advantage can be lost if the ferrite-austenite balance is disrupted during welding. The welded joint must be designed to maintain adequate toughness, particularly at low temperatures.
- Code compliance requires additional testing beyond standard mechanical properties: For pressure vessel applications, ferrite content measurement, intergranular corrosion testing, and hydrogen-induced cracking testing are essential to demonstrate the suitability of the welded joint for the intended service conditions.
The study underscores the importance of a systematic approach to welding procedure development for duplex stainless steels, combining metallurgical understanding with rigorous testing and inspection to ensure the integrity and longevity of the welded joint.