CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Medium-Thickness High-Nitrogen Steel Double-Wire MIG Weld Joints
Literature Overview
This 2013 publication by Yang Wulin, Chen Donggao, and colleagues from the Ningbo Branch of the China Ordnance Science and Technology Institute investigates the microstructure and mechanical properties of double-wire MIG weld joints in medium-thickness high-nitrogen steel plates. High-nitrogen steels are advanced materials that achieve high strength through solid solution strengthening by nitrogen, offering a favorable combination of strength, toughness, and weight reduction potential for defense and industrial applications.
Core Technical Content
The study focuses on the welding of medium-thickness (typically 10-25 mm) high-nitrogen steel plates using double-wire metal inert gas (MIG) welding, a process that offers higher deposition rates than single-wire welding. The double-wire configuration allows for independent control of the two wire feeds, enabling optimization of the welding parameters for each wire to achieve improved weld quality. High-nitrogen steels present unique welding challenges due to their elevated nitrogen content, which can lead to porosity, nitride formation, and reduced weldability if not properly managed.
Welding Parameters for Double-Wire MIG
| Parameter | Wire 1 (Leading) | Wire 2 (Trailing) | Combined Effect |
|---|---|---|---|
| Current (A) | 200-280 | 200-280 | 400-560 total |
| Voltage (V) | 28-34 | 28-34 | Arc stability |
| Wire feed speed (m/min) | 5-8 | 5-8 | Deposition rate |
| Travel speed (mm/s) | 4-8 | 4-8 | Heat input control |
| Shielding gas | Ar + 2% CO2 | Ar + 2% CO2 | Arc stability, penetration |
| Preheat temperature | 100-150°C | - | Reduce cracking risk |
| Interpass temperature | <250°C | - | Control microstructure |
Interpretation of Technical Points
The double-wire MIG welding process provides several advantages for welding high-nitrogen steels:
- Higher deposition rate: The combined deposition rate of two wires is approximately 1.5-2 times that of single-wire welding, improving productivity for thick-section fabrication.
- Improved arc stability: The two-wire configuration creates a more stable arc with reduced spatter, which is critical for nitrogen-containing steels where spatter can introduce additional nitrogen into the weld.
- Optimized heat input: The ability to independently control the two wire feeds allows for fine-tuning of the heat input distribution, which is important for controlling the microstructure evolution in high-nitrogen steels.
- Reduced porosity risk: The improved arc stability and gas shielding reduce the risk of nitrogen porosity, which is a common defect in high-nitrogen steel welds.
Microstructure Characteristics
| Region | Microstructure | Nitrogen Content | Tensile Strength (MPa) | Impact Energy (J) |
|---|---|---|---|---|
| Base metal | Fine ferrite + martensite | 0.12-0.18% | 900-1100 | 60-80 |
| Weld metal | Fine acicular ferrite | 0.08-0.12% | 750-850 | 50-70 |
| HAZ (coarse) | Coarse martensite | 0.10-0.15% | 850-950 | 40-60 |
| HAZ (fine) | Fine martensite + bainite | 0.11-0.16% | 880-980 | 55-75 |
| Interpass region | Recrystallized ferrite + martensite | 0.11-0.16% | 870-970 | 50-70 |
Connection to Cladding and Overlay Applications
The welding of high-nitrogen steels has direct relevance to the fabrication of clad pressure vessels for high-pressure hydrogen service, where nitrogen-containing steels offer improved resistance to hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC). The double-wire MIG welding technique can be adapted for overlay welding of high-nitrogen steel cladding layers on conventional carbon steel substrates, providing a corrosion-resistant surface with reduced weight.
Key Challenges in High-Nitrogen Steel Welding
| Challenge | Mechanism | Mitigation Strategy |
|---|---|---|
| Nitrogen porosity | N2 gas evolution during solidification | High-purity shielding gas, preheat |
| Nitride formation | CrN, TiN precipitation | Wire composition control |
| Hot cracking | Low ductility of weld metal | Flux composition optimization |
| Cold cracking | Hydrogen embrittlement | Low-hydrogen electrodes, preheat |
| Softening in HAZ | Grain coarsening, tempering | Control interpass temperature |
Engineering Practice and Quality Control
For the fabrication of pressure vessels using high-nitrogen steel cladding, the welding procedure must be carefully qualified in accordance with applicable standards such as ASME Section IX and NB/T 47014. The qualification procedure should include:
- Welding procedure qualification (WPQ): Establishing the essential variables, including current range, voltage range, travel speed, preheat temperature, and interpass temperature limits.
- Welder performance qualification (WPQ): Ensuring that welders are competent in the double-wire MIG technique for high-nitrogen steels.
- Non-destructive testing (NDT): UT and MT inspection of the weld and HAZ for cracks, porosity, and incomplete fusion.
- Destructive testing: Tensile testing, impact testing, and hardness testing of the weld and HAZ to verify mechanical properties.
- Chemical analysis: Verification of nitrogen content in the weld metal and HAZ to ensure compliance with specifications.
- Corrosion testing: HIC and SSC testing of the weld and HAZ to verify resistance to hydrogen-induced damage.
Study Insights and Implications
The research demonstrates that double-wire MIG welding is a viable and productive technique for joining medium-thickness high-nitrogen steel plates, with the ability to achieve weld metal properties that approach those of the base metal. The key to successful welding lies in the careful control of shielding gas composition, preheat and interpass temperatures, and wire feed parameters to minimize nitrogen loss and prevent porosity. For cladding applications, the double-wire technique offers the potential to deposit high-nitrogen steel overlay layers with improved productivity and quality. Engineers should note that the nitrogen content in the weld metal is typically lower than in the base metal due to nitrogen loss during the welding process, which may affect the mechanical properties and corrosion resistance of the overlay layer. Future work should focus on developing specialized welding consumables with higher nitrogen retention for overlay applications, as well as optimizing the double-wire parameters for specific cladding geometries and service conditions.