CLADDING TECHNOLOGY SHANXI CO., LTDFeasibility Study of 2507 Super Duplex Stainless Steel Strip Electroslag Weld Overlay
Material Background and Technical Challenges
UNS S32750 (2507) super duplex stainless steel (SDSS) offers exceptional combinations of strength (yield strength approximately 550 MPa), corrosion resistance (PREN > 40), and resistance to chloride stress corrosion cracking. Its application as an overlay material for pressure vessels and heat exchangers in the oil and gas industry is increasingly common. However, the strip electroslag welding (ESW) overlay process for 2507 presents unique challenges due to the material's high alloy content, susceptibility to cracking, and sensitivity to thermal input.
The feasibility study addresses whether strip ESW can be successfully applied to overlay 2507 on carbon and low-alloy steel substrates, considering the metallurgical compatibility, process stability, and mechanical performance requirements.
Metallurgical Considerations
The duplex microstructure of 2507 consists of approximately 50% austenite and 50% ferrite. Maintaining this balance during welding is critical, as deviation from the optimal ratio leads to either cracking (excessive austenite) or reduced corrosion resistance (excessive ferrite). The welding consumable for 2507 overlay must have a carefully balanced composition:
| Element | Base Metal (%) | Filler Metal (%) |
|---|---|---|
| Cr | 24-27 | 24-28 |
| Ni | 6-8 | 6-8 |
| Mo | 3-5 | 3-5 |
| N | 0.24-0.32 | 0.24-0.32 |
| C | ≤ 0.03 | ≤ 0.03 |
The nitrogen content is particularly important as it stabilizes the austenite phase and provides solid solution strengthening. During ESW, the high thermal input can cause nitrogen loss, shifting the balance toward ferrite. This necessitates careful control of slag composition and welding parameters to minimize nitrogen escape.
Process Parameters for Strip ESW Overlay
| Parameter | Recommended Range | Notes |
|---|---|---|
| Current | 300-500 A | Depends on strip thickness |
| Voltage | 25-35 V | Higher voltage for thicker strips |
| Travel speed | 200-400 mm/min | Balance with thermal input |
| Slag composition | CaO-SiO2-Al2O3 | Low basicity to control N loss |
| Preheat | 100-150 °C | Prevent cracking in HAZ |
| Interpass temp | < 200 °C | Maintain duplex balance |
| Strip thickness | 3-6 mm | Standard available range |
The thermal input for ESW is significantly higher than other welding processes, typically in the range of 25-45 kJ/mm. This high thermal input is advantageous for achieving complete melting and good bonding but challenging for maintaining the duplex microstructure. The key to success lies in optimizing the travel speed to achieve the correct cooling rate, which for 2507 should be in the range of 5-50 °C/s to maintain the duplex balance.
Dilution and Transition Layer Strategy
The dilution coefficient for strip ESW overlay is typically 20-35%, which is higher than processes like GTAW or PTA. For overlaying 2507 on carbon steel, a transition strategy is essential:
- First pass: Apply a 309L or 310L strip to create a high-nickel austenitic buffer layer. This layer absorbs the dilution from the base metal.
- Second pass: Apply the 2507 strip directly. The dilution from the transition layer is minimal at this point.
- Third pass (if required): For critical applications, a third pass of 2507 can be applied to ensure adequate thickness and metallurgical quality.
The resulting overlay structure, from base to surface, would be: carbon steel → 309L transition (with dilution) → 309L/2507 mix → 2507 (near-base composition).
Mechanical and Corrosion Performance Requirements
| Property | Requirement | Test Method |
|---|---|---|
| Yield strength | ≥ 550 MPa | ASTM A370 |
| Tensile strength | ≥ 620 MPa | ASTM A370 |
| Elongation | ≥ 15% | ASTM A370 |
| Hardness | HV 280-350 | ASTM E92 |
| PREN | ≥ 40 | Calculated |
| Intergranular corrosion | Pass | ASTM G48 |
| Pitting corrosion | Pass | ASTM G48 (0.5M NaCl, 60°C, 24h) |
| SCC resistance | Pass | ASTM G36 |
Feasibility Assessment and Key Findings
The feasibility study concluded that strip ESW overlay of 2507 is technically viable with the following conditions:
- A transition layer of 309L or 310L is mandatory for carbon steel substrates.
- The travel speed must be optimized to achieve a cooling rate of 5-50 °C/s, maintaining the duplex microstructure.
- The slag composition must be carefully formulated to minimize nitrogen loss while maintaining arc stability.
- Post-weld stress relief at 300-400 °C is recommended to reduce residual stresses without sensitizing the material.
- The overlay thickness should be at least 3 mm to provide adequate corrosion protection and allow for machining.
The main risk identified is the potential for hot cracking in the overlay, particularly at the grain boundaries of the ferrite phase. This can be mitigated by ensuring adequate nickel content in the filler metal and controlling the interpass temperature below 200 °C.
Engineering Implementation Considerations
For practical implementation, the following recommendations emerge from the study:
- Qualification testing per NB/T 47014 or ASME IX should include both mechanical and corrosion testing of the overlay.
- The weld procedure specification (WPS) should include detailed instructions for strip handling, slag management, and cooling rate monitoring.
- A qualified welding procedure specification (QWPS) should be developed with a range of travel speeds and currents to allow for flexibility in production.
- In-service monitoring should include periodic hardness surveys and, for critical applications, periodic PAUT inspection of the overlay-base interface.
The study demonstrates that while 2507 strip ESW overlay is more challenging than conventional austenitic overlay, it is achievable with proper process control and provides significant advantages in terms of productivity and cost-effectiveness for large-area applications.