CLADDING TECHNOLOGY SHANXI CO., LTDGas Metal Arc Welding of Duplex Stainless Clad Steel Plates
Literature Overview and Research Background
This study addresses the gas metal arc welding (GMAW) of duplex stainless steel clad plates, which combine a duplex stainless overlay with a carbon steel or low-alloy steel backing plate. Duplex stainless steels, such as grades UNS S31803 (2205) and UNS S32750 (2507), offer exceptional combinations of yield strength, corrosion resistance, and resistance to chloride stress corrosion cracking. In pressure vessel fabrication, duplex clad plates are increasingly specified for applications involving high-pressure hydrogen service, sour gas environments, and aggressive chloride-containing media. The GMAW process, typically using solid wire or flux-cored wire, is the predominant method for field and shop welding of these components due to its high deposition rate and operational flexibility.
Core Technical Findings
The fundamental challenge in welding duplex clad plates is maintaining the ferrite-austenite balance in the weld metal and heat-affected zone (HAZ). Duplex stainless steels require a ferrite content of approximately 40-60% to achieve optimal mechanical and corrosion properties. During welding, excessive heat input can cause ferrite dissolution and precipitation of detrimental phases such as sigma phase (Cr23C6) and chi phase (Fe2Mo), which severely degrade corrosion resistance. The study emphasizes that GMAW parameters must be carefully controlled to limit heat input while achieving complete fusion and sound welds.
The research compares solid wire GMAW (ER3181 or ER32773) with flux-cored wire GMAW (E318FL or E327FL) for duplex clad plate welding. Solid wire GMAW provides superior weld metal cleanliness and tighter composition control, while flux-cored wire offers higher deposition rates and better arc stability in outdoor conditions. The study recommends solid wire for critical internal welds and flux-cored wire for structural welds where deposition rate is prioritized.
| Welding Parameter | Solid Wire GMAW (ER3181) | Flux-Cored GMAW (E318FL) | Recommendation |
|---|---|---|---|
| Wire diameter | 1.2 mm | 1.2 mm | Standard for most applications |
| Current range | 180-260 A | 200-320 A | Flux-cored allows higher current |
| Voltage range | 22-28 V | 24-32 V | Maintain short-circuit transfer |
| Travel speed | 150-250 mm/min | 180-300 mm/min | Faster with flux-cored |
| Heat input | 0.8-1.5 kJ/mm | 1.0-1.8 kJ/mm | Limit to 1.5 kJ/mm maximum |
| Shielding gas | Ar + 1-2% CO2 | Self-shielded or mixed | Ar+CO2 preferred for solid wire |
| Ferrite number (FN) target | 38-45 | 35-42 | Maintain within duplex range |
Heat Input Control and Phase Stability
The most critical technical consideration in duplex clad plate welding is heat input management. The study demonstrates that heat input exceeding 1.5 kJ/mm significantly increases the risk of sigma phase precipitation in the HAZ, particularly in the 2507 grade which has a higher Mo content. The recommended interpass temperature is limited to 150°C maximum, with mandatory cooling between passes. For multi-pass welds, the first pass (root pass) should be deposited with the lowest practical heat input, as this pass determines the dilution level and the initial ferrite content.
The dilution in clad plate welding is a particular concern because the carbon steel backing plate contributes significant carbon and manganese to the weld metal. Excessive dilution can lead to martensitic transformation in the weld metal, causing cracking susceptibility and loss of corrosion resistance. The study recommends using a higher-nickel filler metal (such as ER3181 with 6% Ni) to compensate for the dilution effect and maintain sufficient austenite stability.
Defect Analysis and Prevention
Cracking is the primary defect concern in duplex clad plate GMAW welding. Three types of cracking are most relevant: hot cracking in the weld metal due to delta-ferrite depletion, cold cracking in the HAZ due to hydrogen embrittlement, and reheat cracking in the HAZ of the carbon steel backing plate. The study provides a comprehensive FMEA analysis of these defects.
Hot cracking is mitigated by ensuring adequate delta-ferrite content (FN 38-45) in the weld metal, which acts as a crack arrestor during solidification. Cold cracking is controlled through preheating the carbon steel backing plate to 100-150°C, limiting hydrogen content in the shielding gas and wire, and applying post-weld heat treatment where specified. Reheat cracking in the carbon steel HAZ is addressed by controlling the carbon equivalent (CE) of the backing material and limiting heat input.
Integration with Engineering Practice
In pressure vessel fabrication, duplex clad plates are commonly used for hydrogenation reactor shells, sour gas separators, and offshore production equipment. The welding procedure must be qualified per ASME Section IX or NB/T 47014, with ferrite number measurement as a required acceptance criterion. The study recommends performing ferrite number measurements on representative weld samples using a portable ferrite scope, with acceptance limits of FN 35-50 for the weld metal and FN 30-50 for the HAZ.
For clad plate butt welds, the overlay side weld should be deposited with parameters that minimize dilution from the carbon steel side. A practical approach is to use a root pass with low heat input from the overlay side, followed by fill passes that progressively increase in heat input while maintaining ferrite control. The backing weld (carbon steel side) should be deposited using a carbon steel or low-alloy steel filler metal appropriate for the backing material grade.
Study Insights and Implications
The study reinforces the principle that duplex stainless welding is fundamentally about thermal management. The narrow window between adequate fusion and excessive heat input requires disciplined process control. Engineers should invest in real-time heat input monitoring equipment and enforce strict interpass temperature limits. The economic case for duplex clad plates over solid duplex construction is compelling, but only if the welding process is rigorously controlled to prevent degradation of the overlay layer. Future work should explore the application of hybrid processes such as GTAW root with GMAW fill for clad plate welds, which may provide superior dilution control in the critical root pass region.