CLADDING TECHNOLOGY SHANXI CO., LTDAutomatic Welding Overlay of Duplex Stainless Steel 2205 Process Development
Literature Overview
This study note examines the 2010 research by Wang Li, Gao Junsong, and Wu Daowen from the 725th Research Institute of China Shipbuilding Industry Corporation, published in the journal "Welding Technology." The work addresses the development of automatic welding processes for overlay cladding of duplex stainless steel 2205 (UNS S31803/S32205), a material of significant importance in marine engineering, chemical processing, and pressure vessel applications where both high strength and excellent corrosion resistance are required.
Technical Background
Duplex stainless steel 2205 contains approximately equal amounts of austenite and ferrite phases, providing a combination of high yield strength (450–550 MPa) and excellent resistance to chloride stress corrosion cracking. The material is widely used in marine applications, offshore platforms, chemical processing equipment, and high-pressure vessels. Overlay welding of 2205 onto carbon steel or low-alloy steel substrates is a common approach for creating corrosion-resistant linings without the expense of full-thickness duplex stainless steel construction.
2205 Duplex Stainless Steel Composition and Properties
| Parameter | Specification |
|---|---|
| Cr Content | 22–23% |
| Ni Content | 4.5–6.5% |
| Mo Content | 3.0–3.5% |
| N Content | 0.14–0.20% |
| Equivalent Pitting Resistance Number (PREN) | ≥ 34 |
| Yield Strength (as-welded) | 450–550 MPa |
| Ferrite Content (target) | 35–65% |
| Maximum Service Temperature | 250°C (to prevent 475°C embrittlement) |
Critical Process Challenges
The automatic welding overlay of 2205 presents several unique challenges that distinguish it from austenitic stainless steel overlay:
- Ferrite control: The ferrite content must be maintained in the 35–65% range. Excessive ferrite (>70%) leads to 475°C embrittlement and reduced toughness; excessive austenite (<30%) reduces stress corrosion cracking resistance. The ferrite content is sensitive to heat input, welding speed, and interpass temperature.
- Nitrogen loss: The nitrogen content in 2205 is critical for maintaining the duplex structure and corrosion resistance. During welding, nitrogen can be lost from the molten pool, particularly in processes with long arc exposure times. This requires effective shielding and potentially nitrogen addition to the shielding gas.
- Intermetallic precipitation: Sigma phase and other intermetallics can form during slow cooling or at elevated interpass temperatures, severely reducing toughness. Rapid cooling and limited interpass temperature are essential.
- Cracking susceptibility: Despite the duplex structure providing good crack resistance, 2205 overlay on dissimilar substrates can develop cracking due to thermal stresses, particularly at the bond interface.
Process Parameters for Automatic SAW Overlay
The submerged arc welding (SAW) process is the most commonly used method for thick overlay of 2205, offering high deposition rates and good slag protection. Key process parameters include:
| Parameter | Recommended Range | Effect |
|---|---|---|
| Welding Current | 400–700 A | Controls heat input and penetration |
| Arc Voltage | 28–35 V | Controls bead width and dilution |
| Travel Speed | 200–500 mm/min | Controls heat input and ferrite content |
| Wire Diameter | φ2.4–φ4.0 mm | Controls deposition rate |
| Flux Composition | Low-silica, basic flux | Controls slag properties and inclusion content |
| Interpass Temperature | ≤ 150°C | Prevents sigma phase formation |
| Preheat Temperature | 50–100°C | Minimizes cracking risk |
| Heat Input | 0.5–2.5 kJ/mm | Critical for ferrite content control |
Ferrite Control Strategy
The ferrite content in the weld metal is the single most important quality parameter for 2205 overlay. The following factors influence ferrite content:
- Heat input: Higher heat input promotes ferrite-to-austenite transformation during cooling, reducing ferrite content. Heat input above 3.0 kJ/mm typically results in ferrite content below 30%.
- Welding speed: Faster speeds reduce heat input, maintaining higher ferrite content. However, excessively fast speeds may cause incomplete fusion.
- Wire composition: The alloy composition of the filler wire (typically ER31803 or equivalent) must be slightly enriched in ferrite formers (Cr, Mo) to compensate for dilution from the base metal.
- Number of passes: Multi-pass welding with controlled interpass temperatures can achieve better ferrite balance than single-pass welding.
Ferrite Content vs. Heat Input Relationship
| Heat Input (kJ/mm) | Ferrite Content (%) | Acceptability |
|---|---|---|
| 0.5–1.0 | 55–65 | Acceptable (upper range) |
| 1.0–2.0 | 45–60 | Optimal |
| 2.0–3.0 | 35–50 | Acceptable (lower range) |
| >3.0 | <35 | Unacceptable—excessive austenite |
Consumable Selection and Flux Design
The selection of consumables for 2205 overlay requires careful consideration:
- Filler wire: ER31803 (AWS A5.9) or equivalent with slightly elevated Cr (23–24%) and Mo (3.5–4.0%) to compensate for base metal dilution. Wire diameter of φ3.2 mm is optimal for SAW overlay.
- Flux: Low-silica, low-sulfur, low-phosphorus flux is essential. The flux should have good fluidity at welding temperatures while maintaining sufficient viscosity to protect the molten pool. Flux with slight titanium oxide content can improve slag wetting and reduce spatter.
- Shielding gas (for GMAW alternative): 98% Ar + 2% N₂ is preferred to maintain nitrogen content in the weld metal. Pure argon leads to nitrogen loss and reduced pitting resistance.
Quality Control and Inspection
The quality assurance program for 2205 overlay must include:
- Visual inspection: 100% examination of all welds for surface defects, undercut, and porosity.
- Magnetic particle testing (MT): 100% coverage for surface and near-surface defects in the overlay and heat-affected zone.
- Ultrasonic testing (UT): 100% examination of the bond interface for lack of fusion. Phased array UT (PAUT) is preferred for its superior resolution at the interface.
- Ferrite content measurement: Magnetic ferrite gauge measurement on every weld bead, with acceptance criteria of 35–65% ferrite.
- Hardness testing: Vickers hardness measurement through the overlay thickness to verify hardness profile and detect excessive dilution.
- Intergranular corrosion testing: ASTM A262 Practice E or equivalent on the heat-affected zone to verify resistance to sensitization.
Engineering Practice and Application Cases
In marine engineering applications, 2205 overlay is commonly applied to carbon steel pressure vessels, heat exchangers, and structural components exposed to seawater or brine. A typical application involves overlaying 2–5 mm of 2205 on the interior surface of a carbon steel vessel using automatic SAW with flux-cored wire. The process requires:
- Surface preparation: Shot blasting to SA 2.5 standard, followed by solvent cleaning within 4 hours of welding.
- Preheating: 50–100°C to reduce thermal gradients and prevent cracking.
- Welding sequence: Back-plate welding followed by 2–3 overlay passes, with ferrite content verification after each pass.
- Post-weld treatment: No post-weld heat treatment is typically required, as the as-welded condition provides the optimal microstructure.
Study Insights and Practical Recommendations
This research underscores that 2205 overlay welding is fundamentally different from austenitic stainless steel overlay in its sensitivity to heat input and ferrite control. Engineers must recognize that the duplex structure is not merely a metallurgical detail—it is the source of the material's superior properties, and any process deviation that shifts the phase balance can result in catastrophic loss of performance. The key practical recommendations are:
- Always measure ferrite content during production welding, not just during qualification testing.
- Control heat input within the narrow window of 1.0–2.5 kJ/mm for optimal results.
- Maintain interpass temperatures below 150°C to prevent intermetallic precipitation.
- Use filler wire with slightly elevated alloy content to compensate for dilution.
- Implement real-time process monitoring (current, voltage, travel speed) to ensure parameter consistency throughout long welds.
The automatic welding approach described in this research provides the repeatability and consistency required for production-quality 2205 overlay, but it also demands a higher level of process understanding and control than manual welding. The investment in automated equipment and process monitoring is justified by the significant reduction in rework and the assurance of consistent overlay quality across large production volumes.