Zhuojin Pipe Fitting Co., LtdStudy on SMAW Overlay Welding Process of Duplex Stainless Steel 2205
Literature Overview
This 2010 publication in "Welding Machine" by Wang Li, Gao Junsong, Wu Daowen, and He Penghui from the Luoyang Ship Materials Research Institute addresses the challenge of overlay welding duplex stainless steel 2205 using shielded metal arc welding (SMAW). The study focuses on process parameter optimization to achieve the target ferrite content in the overlay deposit, which is critical for maintaining the corrosion resistance and mechanical properties characteristic of duplex stainless steels.
Technical Background
Duplex stainless steel 2205 (UNS S31803/S32205) contains approximately equal fractions of austenite and ferrite, providing a unique combination of high strength (yield strength ~450-550 MPa), excellent corrosion resistance (particularly against chloride stress corrosion cracking), and good toughness. The ferrite content in the as-welded deposit is a critical microstructural parameter:
- Below 30% ferrite: Increased susceptibility to chloride stress corrosion cracking (SCC)
- 30-60% ferrite: Optimal range for balanced properties
- Above 60% ferrite: Reduced toughness, increased susceptibility to intergranular corrosion
The target ferrite content for overlay deposits is typically 40-60% to ensure adequate resistance to both SCC and intergranular corrosion.
Process Parameter Optimization
The study identifies three key process parameters for achieving the target ferrite content:
| Parameter | Setting | Rationale |
|---|---|---|
| Welding current | Low | Reduced dilution, controlled heat input |
| Welding speed | Fast | Minimizes heat input, preserves ferrite |
| Cooling method | Water cooling | Rapid solidification, suppresses austenite formation |
The use of low current and fast welding speed results in low heat input, which limits the time available for ferrite-to-austenite transformation during cooling. The water cooling technique further accelerates the cooling rate, effectively "freezing" the ferrite fraction at the desired level.
Electrode Selection and Layer Strategy
The overlay welding employs a two-layer strategy:
- Transition layer: E309MoL (φ4mm) - a high-austenite stainless steel electrode that provides good weldability with the base material and acts as a buffer against dilution effects from the carbon steel or austenitic stainless steel substrate.
- Overlay layer: E2209 (φ4mm) - a duplex stainless steel electrode specifically formulated to produce a 2205-type microstructure with balanced ferrite and austenite phases.
This two-layer approach is a well-established practice in overlay welding of duplex stainless steels. The transition layer ensures adequate wetting and metallurgical compatibility with the base material, while the overlay layer provides the desired duplex microstructure and properties.
Performance Verification
The resulting overlay deposit was characterized through:
| Test Method | Result | Acceptance Criteria |
|---|---|---|
| Ferrite content (area fraction) | 47.14% | 40-60% (target range) |
| Hardness (overlay surface) | 23.8-24.7 HRC | Consistent with duplex 2205 |
| FeCl3 pitting test | No metal precipitation | Pass - no pitting corrosion |
| NaOH electrolytic corrosion test | No metal precipitation | Pass - no intergranular corrosion |
| Chemical composition | Meets E2209 requirements | Pass |
The ferrite content of 47.14% falls squarely within the optimal 40-60% range, confirming that the process parameters effectively control the phase balance. The absence of metal precipitation in both pitting and intergranular corrosion tests demonstrates that the overlay deposit maintains the corrosion resistance advantages of duplex stainless steel 2205.
Engineering Practice Integration
Duplex stainless steel 2205 overlay welding has specific applications in pipeline and process equipment engineering:
- Corrosion-resistant overlay on carbon steel pipes: In offshore oil and gas applications, carbon steel pipelines can be overlay welded with 2205 to provide chloride-resistant surfaces in high-chloride environments (e.g., produced water handling systems).
- Repair of duplex stainless steel equipment: When duplex stainless steel components suffer from localized damage, overlay welding with matched duplex material restores both the mechanical and corrosion properties.
- Valve and pump components: The combination of high strength and corrosion resistance makes 2205 overlay ideal for valve trim, pump impellers, and wear plates in corrosive environments.
The SMAW process, while less efficient than GTAW or FCAW for overlay welding, offers advantages in field applications where equipment portability and process flexibility are paramount. The low current, fast welding technique developed in this study is particularly suitable for field repair operations where power supply capacity and thermal management are limited.
Key Reflections and Technical Insights
The water cooling technique employed in this study is noteworthy. While water quenching of weld deposits is uncommon in standard welding practice due to the risk of hydrogen-induced cracking and excessive residual stress, it is effective in this context because:
- The duplex stainless steel composition has lower hydrogen cracking susceptibility than austenitic stainless steels.
- The ferrite phase in the microstructure provides better resistance to hydrogen embrittlement than austenite.
- The rapid cooling is specifically intended to preserve the ferrite fraction, and the associated residual stress can be managed through post-weld stress relief if necessary.
The ferrite content control through heat input management is a fundamental principle in duplex stainless steel welding. The delta ferrite content in the weld metal is primarily governed by the welding heat input: higher heat input promotes ferrite-to-austenite transformation during cooling, reducing the final ferrite content. The low heat input approach used in this study directly addresses this relationship.
From a metallurgical perspective, the E2209 electrode formulation is designed to produce a deposit with a specific composition that, when combined with the appropriate cooling rate, yields the target ferrite content. The electrode composition must be carefully balanced to account for dilution from the base material and the transition layer. The two-layer strategy with E309MoL as the transition layer effectively isolates the overlay deposit from excessive base metal dilution, ensuring that the E2209 electrode composition is maintained in the final overlay layer.
Summary
This study demonstrates that duplex stainless steel 2205 can be successfully overlay welded using SMAW with appropriate process parameter optimization. The combination of low current, fast welding speed, and water cooling achieves the critical ferrite content target of 47.14%, resulting in an overlay deposit that meets all corrosion and mechanical property requirements. For engineers working in offshore and chemical processing industries where chloride corrosion resistance is paramount, this technology provides a practical and cost-effective solution for applying duplex stainless steel overlay protection to carbon steel substrates using readily available SMAW equipment.