CLADDING TECHNOLOGY SHANXI CO., LTDNitrogen-Containing Shielding Gas for Duplex Steel Overlay and Sealing Welding
Technical Background and Metallurgical Rationale
Duplex stainless steels, characterized by a roughly equal mixture of austenite (γ) and ferrite (δ) phases, offer an exceptional combination of mechanical strength, corrosion resistance, and toughness. However, the maintenance of the optimal ferrite-austenite phase ratio during welding is a critical challenge, as the welding thermal cycle can shift the phase balance away from the desired 40–60% ferrite range. Excessive ferrite (>60%) leads to reduced ductility and increased susceptibility to stress corrosion cracking, while excessive austenite (<40%) reduces resistance to chloride stress corrosion cracking and pitting.
The use of nitrogen-containing shielding gases (Ar+N₂ or Ar+He+N₂) is a well-established technique for controlling the phase ratio in duplex steel welds. Nitrogen is a powerful austenite stabilizer that promotes the formation of austenite at the expense of ferrite. By adding controlled amounts of nitrogen (typically ≤3%) to the shielding gas, the nitrogen dissolves into the weld pool and shifts the phase balance toward austenite, compensating for the ferrite-forming tendency of the welding thermal cycle. This technique is particularly important for overlay welding and sealing welding applications where the weld metal composition must be carefully controlled to ensure compatibility with the base metal and adequate corrosion resistance.
Phase Control Mechanism and Gas Composition Optimization
Thermodynamic Basis of Nitrogen as Austenite Stabilizer
Nitrogen dissolves interstitially in austenite and has a strong austenite-stabilizing effect. The Schaeffler equivalent (SE) and the DeLong ferrite number (FN) can be used to predict the weld metal phase composition based on the chemical composition, including nitrogen content. The following relationship illustrates the effect of nitrogen on the ferrite number:
FN = f(Cr, Ni, Mo, N, Nb, Ti, ...)
Where nitrogen has a negative coefficient in the ferrite number equation, meaning that increasing nitrogen content decreases the ferrite number and increases the austenite fraction.
The following table shows the typical effect of nitrogen addition on the weld metal phase composition:
| N₂ in Shielding Gas (vol%) | Approximate N in Weld Metal (wt%) | Ferrite Number (FN) | Ferrite Fraction (%) |
|---|---|---|---|
| 0% (Pure Ar) | 0.01–0.02 | 45–55 | 50–60 |
| 1% | 0.05–0.08 | 35–45 | 40–50 |
| 2% | 0.08–0.12 | 25–35 | 30–40 |
| 3% | 0.10–0.15 | 20–30 | 25–35 |
The target ferrite fraction of 40–60% corresponds to a ferrite number of approximately 20–40, which provides the optimal balance of mechanical properties and corrosion resistance. The nitrogen addition should be carefully controlled to achieve this target without exceeding it, as excessive austenite can compromise chloride stress corrosion cracking resistance.
Gas Composition Selection
The selection of shielding gas composition depends on the specific welding process, base material, and desired weld metal properties:
| Application | Gas Composition | N₂ Content | Notes |
|---|---|---|---|
| Duplex 2205 overlay on steel | Ar + 2% N₂ | 2% | Standard for 2205 overlay |
| Duplex 2507 overlay on steel | Ar + 3% N₂ | 3% | Higher N for higher alloy content |
| Duplex 2205 sealing weld | Ar + 1% N₂ | 1% | Lower N for lower dilution |
| Super duplex 2507 TIG | Ar + 2.5% N₂ | 2.5% | Fine-tuned for 2507 |
| Duplex 2205 MIG | Ar + 2% N₂ + 5% He | 2% | He improves arc stability |
The nitrogen content should not exceed 3% for most duplex steel applications, as higher nitrogen levels can lead to excessive austenite and potential hot cracking due to the increased solidification range of the weld metal.
Welding Process Parameters and Technical Considerations
Process Parameters for Duplex Steel Overlay
| Parameter | TIG (GTAW) | MIG (GMAW) |
|---|---|---|
| Current | 100–250 A | 150–300 A |
| Voltage | 16–24 V | 20–28 V |
| Travel speed | 50–120 mm/min | 200–400 mm/min |
| Wire diameter | 1.6–2.4 mm | 1.2–1.6 mm |
| Gas flow rate | 15–25 L/min | 20–30 L/min |
| Heat input | 0.5–1.5 kJ/mm | 0.3–1.0 kJ/mm |
| Interpass temperature | <150°C | <150°C |
Critical Process Considerations
- Low heat input: Duplex steels are susceptible to phase transformation at elevated temperatures. Temperatures exceeding 300°C can promote the precipitation of intermetallic phases (σ, χ, R-phase) that reduce ductility and corrosion resistance. Therefore, the heat input should be kept low, and the interpass temperature should be maintained below 150°C.
- Rapid cooling: After welding, the weld zone should cool rapidly to avoid prolonged exposure to the temperature range where intermetallic phases can precipitate (300–600°C). This can be achieved by using low heat input, controlling the interpass temperature, and avoiding post-weld heat treatment.
- Shielding gas quality: The nitrogen content of the shielding gas must be precisely controlled, as variations of even 0.5% can significantly affect the weld metal phase composition. Gas mixing equipment with accurate flow control is essential.
- Surface preparation: The substrate surface must be thoroughly cleaned to remove any contamination that could affect the weld metal composition or introduce porosity. Any oxide scale, oil, or grease must be removed by mechanical grinding or chemical cleaning.
Defect Analysis and Quality Control
Common Defects in Duplex Steel Welds
| Defect | Cause | Consequence | Countermeasure |
|---|---|---|---|
| Excessive ferrite | Insufficient N₂, high dilution | Reduced ductility, SCC susceptibility | Increase N₂, reduce dilution |
| Excessive austenite | Excessive N₂, low dilution | Reduced pitting resistance | Decrease N₂, increase dilution |
| Intermetallic phase precipitation | High interpass temperature, slow cooling | Reduced toughness, corrosion resistance | Control interpass temperature, rapid cooling |
| Porosity | Gas contamination, inadequate shielding | Reduced mechanical properties | Improve shielding, clean surfaces |
| Cracking | High restraint, excessive austenite | Structural failure | Reduce restraint, control phase ratio |
Non-Destructive Testing Requirements
Duplex steel welds require comprehensive NDT to ensure quality:
- Visual inspection (VT): Check for surface defects, undercut, and profile irregularities.
- Magnetic particle testing (MT): Detect surface and near-surface cracks in the weld and heat-affected zone.
- Penetrant testing (PT): Detect surface-breaking defects that may not be visible to the naked eye.
- Ultrasonic testing (UT): Detect volumetric defects such as porosity, lack of fusion, and inclusions.
- Radiographic testing (RT): Detect volumetric defects and provide a permanent record of weld quality.
Metallographic Examination
Metallographic examination is essential for verifying the phase composition and microstructure of duplex steel welds. The ferrite fraction can be measured using the magnetic permeability method or optical microscopy with appropriate etchants (e.g., glycerol-nitric acid etchant). The target ferrite fraction of 40–60% should be verified at multiple locations in the weld, including the weld centerline, weld toes, and heat-affected zone.
Engineering Practice Cases
Case 1: Overlay Welding on Carbon Steel Reactor
A carbon steel reactor was overlaid with a 3 mm thick layer of duplex 2205 stainless steel to provide corrosion resistance in a sour gas environment. The overlay was performed using MIG welding with ER2209 wire and Ar+2%N₂ shielding gas. The welding procedure included:
- Preheating to 100°C to reduce cracking risk
- MIG welding with 200 A, 24 V, 300 mm/min travel speed
- Ar+2%N₂ shielding gas at 25 L/min
- Three layers of overlay, each approximately 1 mm thick
- Interpass temperature maintained below 150°C
- Post-weld inspection by UT and MT
The resulting overlay showed a ferrite fraction of 52%, within the target range of 40–60%. The overlay passed HIC and SSC testing, confirming its suitability for sour service.
Case 2: Sealing Weld for Duplex Clad Plate
A duplex 2205 clad plate was fabricated by welding the clad layer to the carbon steel backing plate using a sealing weld. The sealing weld was performed using TIG welding with ER2209 wire and Ar+1%N₂ shielding gas. The welding procedure included:
- Backing gas of pure Ar to prevent oxidation of the root side
- TIG welding with 150 A, 20 V, 80 mm/min travel speed
- Ar+1%N₂ shielding gas at 20 L/min
- Single pass welding to minimize heat input
- Post-weld inspection by UT and metallographic examination
The sealing weld showed a ferrite fraction of 48%, with excellent bond strength and no evidence of intermetallic phase precipitation.
Study Insights and Engineering Implications
The study of nitrogen-containing shielding gases for duplex steel welding highlights the importance of phase control in achieving the desired mechanical and corrosion properties. The addition of nitrogen to the shielding gas is a powerful tool for shifting the phase balance toward austenite, but it must be used with precision to avoid excessive austenite that can compromise chloride stress corrosion cracking resistance.
A key insight from this study is the interplay between nitrogen content, dilution, and the final weld metal phase composition. The nitrogen addition must be adjusted based on the expected dilution from the base metal, as higher dilution will introduce more ferrite-forming elements (Cr, Mo) and require a higher nitrogen content to compensate. Engineers should use the Schaeffler or DeLong diagrams to predict the weld metal composition and phase fraction, and adjust the nitrogen content accordingly.
Another important consideration is the control of interpass temperature and cooling rate. Duplex steels are susceptible to intermetallic phase precipitation at elevated temperatures, and the welding procedure must be designed to minimize the time spent in the critical temperature range (300–600°C). This can be achieved by using low heat input, controlling the interpass temperature, and avoiding post-weld heat treatment.
In conclusion, nitrogen-containing shielding gases are an essential tool for controlling the phase composition of duplex steel welds, and their effective use requires a thorough understanding of duplex steel metallurgy, welding process parameters, and quality control requirements. Engineers should always verify the weld metal phase composition through metallographic examination and perform comprehensive NDT to ensure the integrity and performance of duplex steel welds in demanding service environments.