CLADDING TECHNOLOGY SHANXI CO., LTDDuplex Stainless Steel 2507 Overlay Welding Technology
Literature Overview
This 2011 technical paper published in China Chemical Equipment (中国化工装备) by Guo Wenbin and Wang Qinghong from Jiangsu Yuanfang Diweier Container Co., Ltd. addresses the practical challenges of overlay welding duplex stainless steel grade 2507 (UNS S32750/S32750) onto pressure vessel substrates. Duplex 2507 is a high-strength, high-corrosion-resistance material with approximately 25% chromium, 7% nickel, and 3% molybdenum, along with 0.27% nitrogen, offering a PREN (Pitting Resistance Equivalent Number) of approximately 38–40.
Material Characteristics and Welding Challenges
Duplex stainless steel 2507 presents unique challenges in overlay welding due to its complex metallurgy:
| Property | Duplex 2507 | Comparison with 316L |
|---|---|---|
| Tensile strength (MPa) | 620–850 | 485–758 |
| Yield strength (MPa) | 450–600 | 170–310 |
| PREN | 38–40 | 24–26 |
| Pitting resistance (ASTM G48) | Excellent | Good |
| SCC resistance | Excellent | Fair |
| Weldability | Moderate | Excellent |
| Hot cracking susceptibility | Low-Moderate | Low |
The primary welding challenges for 2507 overlay include: (1) maintaining the austenite/ferrite balance (approximately 40–60% ferrite) in the weld metal; (2) preventing sigma phase formation in the heat-affected zone at elevated temperatures; (3) controlling dilution from the base metal to maintain alloy chemistry; and (4) managing residual stresses due to the high yield strength of the material.
Process Selection and Parameters
For pressure vessel applications, the process selection must comply with applicable codes such as ASME VIII Div.1, GB/T 150, or NB/T 47002. The following processes are commonly employed for 2507 overlay:
Recommended Welding Processes
| Process | Shielding Gas | Filler Metal | Typical Application |
|---|---|---|---|
| GTAW (TIG) | Ar (99.99%) | ER2209/ERNiCrMo-3 | First pass, thin sections |
| GMAW (MIG) | Ar + 2% O2 or Ar + 1% CO2 | ER2209 | Multi-pass overlay |
| SAW | Flux 5 (low dilution) | ER2209 + flux | Thick sections, high deposition |
| PTA | Ar (99.99%) | 2507 powder | Precision surface cladding |
| FCAW | Self-shielded flux core | 2507 wire | Field application |
Critical Process Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Heat input | 0.5–2.5 kJ/mm | Minimize sigma phase |
| Interpass temperature | <150°C | Prevent phase transformation |
| Travel speed | 200–500 mm/min | Control cooling rate |
| Arc voltage | 18–28 V (GMAW) | Maintain arc stability |
| Welding current | 150–300 A (GMAW) | Adequate penetration |
| Layer thickness | 1.5–3.0 mm per pass | Minimize dilution |
Quality Assurance and Inspection
For pressure vessel applications, the overlay weld must satisfy specific qualification requirements:
- Weld procedure qualification: Per NB/T 47014 or ASME IX, with mechanical testing of the weld overlay
- Non-destructive testing: 100% visual examination, magnetic particle testing (MT) or dye penetrant testing (PT) for surface defects, ultrasonic testing (UT) or radiographic testing (RT) for volumetric defects
- Bond strength verification: Shear bond strength test per ASTM A563 or equivalent, minimum 200 MPa for pressure vessel service
- Corrosion testing: Intergranular corrosion test (ASTM A263), ferrite number measurement (25–60 FN), and hardness survey
- Chemical analysis: Verification of weld metal composition within specified limits
Engineering Practice Cases
In chemical equipment manufacturing, 2507 overlay is commonly applied to carbon steel or low-alloy steel pressure vessels operating in chloride-containing environments at elevated temperatures. Typical applications include:
- Hydrogenation reactors: Where high-pressure hydrogen service requires excellent resistance to hydrogen-induced cracking and general corrosion
- Chlor-alkali equipment: Where chloride ion concentrations are high and pitting/SCC resistance is critical
- Offshore platform equipment: Where marine atmospheric and splash zone corrosion must be resisted
- Pulp and paper industry: Where acidic, chloride-containing process streams are handled
A critical engineering consideration is the number of overlay passes required. For thick sections (base metal >25 mm), 2–3 passes are typically required to achieve the full 2507 composition in the surface layer. The first pass will inevitably have significant base metal dilution, resulting in a composition between the base metal and 2507. Subsequent passes progressively dilute the previous overlay, approaching the 2507 composition. The final surface layer must be verified by chemical analysis to confirm adequate alloy content.
Key Insights and Reflections
The application of duplex 2507 overlay to pressure vessels represents a significant advancement over traditional 316L overlay in severe service conditions. The approximately 60% higher strength of 2507 allows for thinner overlay layers to achieve equivalent corrosion resistance, reducing material costs and maintaining vessel weight efficiency.
However, the engineering challenge extends beyond the overlay weld itself. The thermal cycling during multi-pass overlay welding affects the base metal heat-affected zone, potentially causing temper embrittlement in low-alloy steel substrates. Post-weld heat treatment (PWHT) may be required, but must be carefully controlled to avoid exceeding the sigma phase formation threshold temperature of approximately 450°C for extended durations.
The paper's practical orientation — written by engineers from a pressure vessel manufacturer — provides valuable field experience that complements academic research. The emphasis on process qualification and code compliance reflects the regulatory environment governing pressure equipment manufacturing in China.