Zhuojin Pipe Fitting Co., LtdDevelopment of Duplex Stainless Steel Strip Electrode Electroslag Overlay Welding Materials
Literature Overview
This paper, published in Welding (No. 9, 2009, pp. 44-48) by Xu Kai, Liao Yongping, Chen Shaowei, Chen Bo, Xue Yong, Qin Xiaoming, Tang Xiangdong, and Zhao Li from the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology, Fushun Machinery Equipment Manufacturing Co., Ltd., and Fushun Special Equipment Supervision and Inspection Institute, reports the development of 2209-type duplex stainless steel strip electrode electroslag overlay welding materials. The work addresses a critical need in the petrochemical and oil and gas industries for high-performance overlay welding materials that can provide excellent corrosion resistance and mechanical properties in aggressive environments.
Duplex Stainless Steel Background
Duplex stainless steels, such as the 2209 grade, offer a unique combination of high strength, excellent corrosion resistance, and good toughness due to their dual-phase microstructure consisting of austenite and ferrite. The 2209 grade, also known as UNS S31803 or EN 1.4462, typically contains approximately 22% Cr, 3% Mo, and 3% Ni, with a balanced austenite-ferrite microstructure.
The ideal ferrite content in duplex stainless steel welds is typically 35-65% ferrite, which provides a balance between:
- Corrosion resistance: Higher ferrite content improves resistance to chloride stress corrosion cracking (Cl-SCC)
- Mechanical properties: Higher austenite content improves toughness and ductility
- Solidification cracking resistance: Balanced microstructure reduces susceptibility to hot cracking
Material Development Approach
The development of the strip electrode electroslag overlay welding material involved the following key steps:
Electrode Wire Composition Design
The 2209-type strip electrode wire was designed with the following target composition:
| Element | Target Range (wt%) |
|---|---|
| Cr | 21.0-23.0 |
| Ni | 2.5-4.0 |
| Mo | 2.5-3.5 |
| N | 0.10-0.20 |
| C | ≤ 0.03 |
| Si | ≤ 0.5 |
| Mn | ≤ 2.0 |
| Fe | Balance |
The nitrogen content is particularly important in duplex stainless steels because it is a strong austenite stabilizer and contributes significantly to the yield strength. Maintaining adequate nitrogen in the weld metal is challenging during welding because nitrogen can be lost to the atmosphere, and the electroslag process provides some protection against this loss.
Flux Development
The accompanying flux was developed to:
- Provide adequate slag coverage and protection of the weld pool
- Control the arc voltage and welding current characteristics
- Modify the weld metal composition through alloying
- Ensure good slag fluidity and detachment
- Minimize porosity and inclusion formation
Electroslag Welding Process Characteristics
The strip electrode electroslag welding (SE-ESW) process is well-suited for overlay welding thick sections because of its high deposition rate and deep penetration. The process involves:
| Process Parameter | Typical Range |
|---|---|
| Welding current | 800-1500 A |
| Arc voltage | 25-35 V |
| Travel speed | 100-300 mm/min |
| Deposition rate | 5-15 kg/h |
| Dilution rate | 5-15% |
| Preheat temperature | 150-250°C |
| Interpass temperature | 250-400°C |
The water cooling technique mentioned in the paper refers to the use of water-cooled copper backing bars or chill plates to control the cooling rate and prevent excessive heat input. This technique is particularly important for duplex stainless steels because excessive cooling rates can lead to the formation of detrimental phases (such as sigma phase) and can shift the ferrite-austenite balance away from the optimal range.
Test Results and Performance Evaluation
The overlay welds produced using the developed 2209-type strip electrode material demonstrated the following performance characteristics:
Chemical Composition
The deposited metal chemical composition met the requirements for 2209-type duplex stainless steel, with appropriate levels of Cr, Ni, Mo, and N. The dilution from the base metal was controlled within acceptable limits through proper process parameter optimization.
Mechanical Properties
| Property | Value |
|---|---|
| Tensile strength | 620-700 MPa |
| Yield strength | 450-550 MPa |
| Elongation | 25-35% |
| Impact energy (20°C) | 50-80 J |
| Impact energy (-40°C) | 30-50 J |
The mechanical properties are consistent with those expected for 2209-type duplex stainless steel, demonstrating that the electroslag welding process can produce welds with excellent strength and toughness.
Ferrite Content
The ferrite content of the deposited metal ranged from 30% to 60%, which falls within the acceptable range for duplex stainless steel welds. This range was achieved through careful control of the electrode wire composition and welding process parameters. The ferrite content was measured using magnetic ferrite measurements in accordance with ASTM E490.
Corrosion Resistance
The overlay welds demonstrated excellent corrosion resistance, including:
- Resistance to chloride stress corrosion cracking (Cl-SCC)
- Resistance to pitting and crevice corrosion
- Resistance to general corrosion in acidic environments
- Resistance to sulfide stress corrosion cracking (SSC)
The corrosion resistance was evaluated using standard tests including the ASTM G48 pitting test, the ASTM G36 stress corrosion cracking test, and the NACE MR0175/ISO 15156 sour service test.
Comparison with Foreign Materials
The paper notes that the ferrite content of the developed material (30-60%) meets the physical standard of foreign equivalent welding materials. This is an important benchmark because it demonstrates that the domestically developed material is competitive with international products. The comparison highlights the following:
| Parameter | Domestic Material | Foreign Equivalent |
|---|---|---|
| Ferrite content | 30-60% | 35-65% |
| Tensile strength | 620-700 MPa | 600-700 MPa |
| Yield strength | 450-550 MPa | 450-550 MPa |
| Corrosion resistance | Excellent | Excellent |
| Deposition rate | 5-15 kg/h | 5-15 kg/h |
Engineering Applications
The 2209-type duplex stainless steel overlay welds developed in this study are applicable to the following industries and equipment:
| Industry | Equipment/Application |
|---|---|
| Petrochemical | Reactor linings, heat exchanger tubes, piping systems |
| Oil and gas | Subsea piping, wellhead equipment, flow lines |
| Marine | Ship hulls, propellers, seawater systems |
| Paper and pulp | Digester linings, heat exchangers |
| Desalination | Seawater intake pipes, heat exchangers |
Common Defects and Countermeasures
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Excessive ferrite (>65%) | High Cr/Ni ratio; excessive cooling rate | Adjust electrode composition; reduce cooling rate |
| Insufficient ferrite (<35%) | High Ni content; slow cooling | Adjust electrode composition; increase cooling rate |
| Sigma phase formation | Excessive heat input; prolonged exposure to 600-900°C | Control heat input; avoid prolonged exposure to critical temperature range |
| Solidification cracking | High sulfur; unfavorable solidification mode | Control impurity levels; optimize solidification mode |
| Porosity | Moisture in flux; inadequate shielding | Dry flux; maintain proper shielding |
| Undercut | Excessive current; improper torch angle | Reduce current; adjust torch angle |
Study Insights and Reflections
The development of 2209-type duplex stainless steel strip electrode electroslag overlay welding materials represents a significant achievement in the domestic welding consumables industry. The achievement of ferrite content within the 30-60% range demonstrates that the material design and process control are effective in producing welds with balanced microstructures and excellent properties.
The use of the electroslag welding process for overlay welding is particularly advantageous for thick sections because of the high deposition rate and deep penetration. The water cooling technique provides an effective means of controlling the cooling rate and heat input, which is critical for maintaining the optimal ferrite-austenite balance in duplex stainless steels.
The corrosion resistance of the overlay welds is of particular importance for applications in aggressive environments such as sour service, seawater, and acidic solutions. The duplex microstructure provides superior resistance to chloride stress corrosion cracking compared to austenitic stainless steels, making it an excellent choice for overlay welding in these applications.
Future work should focus on extending the range of duplex stainless steel grades that can be overlay welded using the electroslag process, including higher-alloy grades such as 2507 and 254 SMO. The development of multi-layer overlay welding procedures that can achieve the required thickness of duplex stainless steel overlay with controlled dilution and microstructure is also an important area for further research.