CLADDING TECHNOLOGY SHANXI CO., LTDSMAW Overlay Process Research for Duplex Stainless Steel 2205
Literature Overview
This technical paper investigates the application of shielded metal arc welding (SMAW) for overlaying duplex stainless steel 2205 (UNS S31803/S32205) on carbon steel and stainless steel base materials. Duplex 2205 offers exceptional combinations of high strength (yield strength approximately 450 MPa), excellent chloride stress corrosion cracking resistance, and good mechanical properties, making it ideal for overlay applications in chemical processing, offshore platforms, and desalination equipment. However, welding duplex stainless steels presents unique challenges related to phase balance control, microstructure evolution, and weldability. This paper addresses these challenges specifically in the context of SMAW overlay processes.
Core Technical Challenges
Phase Balance Control
The defining characteristic of duplex stainless steels is the approximately 50/50 balance between austenite and ferrite phases. This balance is critical for achieving optimal mechanical properties and corrosion resistance. During welding, the rapid solidification and cooling can shift the phase balance, leading to either ferrite-rich or austenite-rich microstructures. Ferrite-rich welds (above 65 percent ferrite) are susceptible to 475-degree embrittlement and intergranular corrosion, while austenite-rich welds (below 35 percent ferrite) are prone to hot cracking and lose the beneficial properties of the ferrite phase.
Dilution Effects
A major challenge in SMAW overlay of duplex 2205 is the dilution of the overlay material with the base metal. When overlaying on carbon steel, dilution can be as high as 30 to 40 percent in the first pass, significantly altering the effective composition of the overlay layer. This dilution introduces carbon and manganese from the base material, which can promote austenite formation and destabilize the duplex structure. The paper addresses this through multi-pass strategies and consumable selection.
| Overlay Configuration | First Pass Dilution | Final Dilution (3 passes) | Ferrite Content | Acceptable? |
|---|---|---|---|---|
| Carbon steel base | 35-40% | 15-20% | 55-65% | Borderline |
| 304 stainless base | 20-25% | 10-15% | 45-55% | Yes |
| 316L stainless base | 20-25% | 10-15% | 45-55% | Yes |
| Duplex 2205 base | 15-20% | 8-12% | 45-55% | Yes |
Process Parameter Optimization
Electrode Selection and Classification
The paper evaluates several electrode types suitable for duplex 2205 overlay welding. The recommended electrodes include E309L (for carbon steel base with multiple passes), E310 (for high-temperature applications), and specialized duplex electrodes such as E329L or equivalent compositions. The electrode composition is carefully designed to compensate for expected dilution and maintain the target phase balance in the final overlay.
Welding Parameters
The paper establishes optimal SMAW parameters for duplex 2205 overlay:
- Current range: 90 to 140 A for 3.2 mm electrodes, 120 to 180 A for 4.0 mm electrodes
- Arc length: maintained at 2 to 3 mm for consistent shielding and penetration
- Travel speed: 40 to 70 mm/min depending on joint geometry
- Interpass temperature: below 150 degrees Celsius to prevent excessive grain growth
- Weld leg length: limited to 100 to 150 mm to control heat input
- Preheating: generally not required for duplex 2205 overlay, but 50 to 100 degrees Celsius may be used for thick base materials
Multi-Pass Strategy
The paper strongly recommends a multi-pass approach for overlaying duplex 2205 on carbon steel bases. The first pass uses a highly alloyed electrode (such as E310 or high-nickel composition) to create a transition layer that reduces dilution effects on subsequent passes. Subsequent passes use duplex-specific electrodes to build up the final overlay with correct phase balance. This strategy typically requires three to four passes for adequate overlay thickness.
Microstructural Analysis and Performance Evaluation
Phase Balance Assessment
Ferrite content was measured using magnetic methods (Ferrite Scope) and confirmed by metallographic examination. The optimized process achieves ferrite content in the range of 45 to 55 percent in the final overlay layer, which is within the acceptable range for duplex stainless steels. The microstructure shows a well-distributed interlocking network of austenite and ferrite phases with minimal sigma phase formation.
Corrosion Resistance Testing
The overlay layers were evaluated for corrosion resistance through potentiodynamic polarization testing in 3.5 percent NaCl solution and intergranular corrosion testing per ASTM A262 Practice E. The optimized overlay demonstrates excellent corrosion resistance with pitting potential above 250 mV vs. SCE and no intergranular corrosion attack after 24-hour exposure. These results confirm that the phase balance control achieved through the optimized process is effective in maintaining the corrosion resistance properties of the duplex structure.
Mechanical Properties
The overlay layers exhibit yield strength of 400 to 480 MPa and ultimate tensile strength of 550 to 650 MPa, consistent with duplex 2205 specifications. The hardness is in the range of 240 to 280 HV, providing good wear resistance while maintaining adequate formability. The overlay-to-base bond strength exceeds 350 MPa in peel testing, indicating excellent metallurgical bonding.
Engineering Practice Considerations
Welder Qualification
The paper emphasizes the importance of welder qualification for duplex overlay welding. Due to the sensitivity of phase balance to welding parameters, welders must demonstrate consistent control of arc length, travel speed, and weaving technique. Qualification testing should include ferrite content measurement and corrosion testing in addition to standard mechanical testing. The recommended qualification procedure follows NB/T 47014 or ASME IX Section IX requirements with additional testing specific to duplex materials.
Inspection and Quality Assurance
Non-destructive testing requirements for duplex overlay welds include visual inspection, magnetic particle testing (MT) for surface defects, and ultrasonic testing (UT) for subsurface defects. The paper also recommends periodic metallographic examination to verify phase balance and absence of detrimental phases such as sigma or chi phases. Hydrogen testing should be performed to ensure hydrogen-induced cracking susceptibility is controlled.
Application Examples
The paper presents application examples in several industrial contexts: overlay of carbon steel pump casings in seawater service, corrosion-resistant lining of carbon steel storage tanks for acidic solutions, and repair of duplex 2205 piping systems. In each case, the SMAW overlay process was selected for its portability and flexibility, particularly for field repairs and in-service maintenance where other welding processes are impractical.
Study Insights and Conclusions
This paper provides comprehensive guidance on the practical application of SMAW for duplex 2205 overlay welding, addressing both the metallurgical challenges and the practical implementation considerations. The key insight is that successful duplex overlay requires a holistic approach that integrates consumable selection, process parameter control, multi-pass strategy, and rigorous quality verification. The phase balance control philosophy is central to the entire process design, and engineers must understand that even small deviations in welding technique can significantly affect the final microstructure and performance. The paper's emphasis on welder qualification and process discipline is particularly valuable, as it recognizes that the inherent sensitivity of duplex materials demands higher levels of procedural control than conventional stainless steel welding. For organizations implementing duplex overlay programs, this paper serves as both a technical reference and a practical implementation guide, bridging the gap between metallurgical theory and field application.