Automatic Surfacing of Duplex Stainless Steel 2205 on Large Diameter Pipe Plates

Literature Overview

This paper by Wang Li et al. (2010), published in Welding Technology (Vol. 39, No. 2, pp. 61-63), documents the development and qualification of an automatic surfacing process for duplex stainless steel 2205 on large-diameter 16Mn forged pipe plates (tube sheets). The research is conducted at China Shipbuilding Industry Corporation's 725 Research Institute, reflecting the practical demands of shipbuilding and offshore engineering where duplex stainless steel overlay cladding is increasingly specified for corrosion resistance in harsh marine environments. The study provides a complete process qualification package including transition layer and overlay layer specifications, mechanical property verification, and corrosion testing results.

Core Technical Findings and Process Design

The surfacing process employs a two-layer approach—transition layer followed by overlay layer—which is the standard approach for duplex stainless steel surfacing on carbon or low alloy steel substrates:

The selection of D309MoL strip electrode for the transition layer is technically justified:

• The 309Mo (310Mo) chemistry provides high chromium and molybdenum content to resist dilution from the 16Mn base metal
• The strip electrode form enables high deposition rates suitable for large-area coverage on large-diameter pipe plates
• Submerged arc welding provides excellent weld quality with low gas inclusion and spatter

The selection of ER2209 wire for the overlay layer is appropriate:

• ER2209 is designed for welding and surfacing duplex stainless steels in the 2205 range
• The composition is designed to maintain the austenite-ferrite balance (approximately 50:50) even with some dilution from the transition layer
• MIG welding provides good bead profile control and productivity for large-area surfacing

Process Qualification and Performance Verification

The qualification testing included mechanical property tests and corrosion resistance tests, with results meeting the usage requirements of duplex stainless steel 2205:

Metallurgical Considerations and Process Challenges

The surfacing of duplex stainless steel 2205 presents several unique metallurgical challenges that this study addresses:

1. Ferrite content control: Duplex stainless steels derive their strength and corrosion resistance from the balanced austenite-ferrite microstructure. The ferrite content must be maintained in the 40–60% range. Excess ferrite (> 60%) leads to susceptibility to 475°C embrittlement and increased brittleness. Insufficient ferrite (< 40%) reduces strength and increases susceptibility to pitting corrosion.
2. Phase transformation sensitivity: The welding thermal cycle can cause phase transformations that alter the ferrite-austenite balance. The 1420°C phase (sigma phase) can precipitate during slow cooling through the 600–800°C range, severely degrading toughness and corrosion resistance.
3. Dilution management: The transition layer is critical for preventing excessive dilution of the overlay layer by the carbon steel base metal. Without an adequate transition layer, the carbon content from the base metal would promote sigma phase formation and reduce the ferrite stability of the overlay.
4. Residual stress management: Duplex stainless steels have higher thermal expansion coefficients than austenitic stainless steels, leading to higher residual stresses during surfacing. The automatic welding process provides consistent heat input, which helps minimize stress variation.

Engineering Practice Integration

For shipbuilding, offshore platform, and chemical processing engineers specifying duplex stainless steel surfacing on carbon steel components, the following practical considerations emerge:

• Applicability to piping systems: The process developed for large-diameter pipe plates is directly applicable to:
• Heat exchanger tube sheets
• Large pipe flanges requiring corrosion-resistant overlay
• Pump casings and valve bodies for aggressive service
• Distillation column internals and support structures
• Standards compliance: The process should be qualified in accordance with:
• ASME B31.3 Appendix Y for surface preparation and surfacing
• ASTM A790 for duplex stainless steel material requirements
• NACE MR0175/ISO 15156 for sour service requirements
• DNV-ST-F101 for offshore structural requirements
• Quality assurance measures:
• Ferrite number measurement (ferrite gauge) on each pass to verify phase balance
• Macrograph examination to verify layer thickness and interface quality
• Corrosion testing on production coupons for each heat lot
• UT inspection of the transition layer to detect lack of fusion

Key Questions and Reflections

The study provides a successful process qualification but leaves several questions for further consideration:

• Long-term corrosion performance: The corrosion testing demonstrates initial resistance, but long-term performance in specific service environments (e.g., seawater at elevated temperatures, sulfuric acid solutions) requires extended exposure testing.
• Thermal cycling resistance: For components experiencing temperature fluctuations during service, the stability of the ferrite-austenite balance under repeated thermal cycling is critical.
• Repair procedures: The study does not address repair procedures for damaged surfacing layers, which is a practical concern during component maintenance.
• Post-weld heat treatment: The study does not discuss whether post-weld heat treatment is required or beneficial. For critical applications, a solution heat treatment at 1050–1100°C followed by water quenching may be necessary to restore phase balance.

Study Insights and Implications

This paper provides a complete, qualified process for duplex stainless steel 2205 surfacing on 16Mn carbon steel pipe plates, demonstrating that the combination of SAW transition layer with MIG overlay layer achieves the required mechanical and corrosion performance. The practical significance of this work extends beyond the specific application studied—it establishes a process framework that can be adapted for various duplex stainless steel surfacing applications in the piping and pressure vessel industry. The two-layer approach with carefully selected consumables (D309MoL strip and ER2209 wire) represents a proven technology that balances corrosion resistance, mechanical strength, and processability. Engineers specifying duplex stainless steel overlay cladding should use this qualification as a reference baseline, adapting the specific parameters to their component geometry, service conditions, and applicable standards. The successful application to large-diameter pipe plates demonstrates that the technology is scalable and suitable for large-scale industrial applications, which is particularly important for offshore and shipbuilding sectors where component sizes are often substantial.