Zhuojin Pipe Fitting Co., LtdDevelopment of Duplex Stainless Steel Strip Electrode Electroslag Overlay Welding Materials
Literature Overview
This paper, published in Welding (2009, No. 9, pp. 44-48), reports the development of 2209-type duplex stainless steel strip electrode electroslag overlay welding materials, including both the welding strip and the corresponding flux composition. The research was conducted by the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology, with industrial collaboration from Fushun Machinery Equipment Manufacturing Co., Ltd. and the Fushun Special Equipment Supervision and Inspection Institute. The study addresses a significant industrial need: the production of thick corrosion-resistant overlay layers using duplex stainless steel, which offers superior mechanical properties and corrosion resistance compared to austenitic stainless steels.
Technical Background and Material Design
Duplex stainless steels, characterized by a near-equal mixture of austenite and ferrite phases, offer a unique combination of properties:
- High strength — Approximately 1.5-2 times the yield strength of 304/316 austenitic stainless steels.
- Excellent corrosion resistance — Superior resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking.
- Good toughness — Maintained at room and elevated temperatures.
The 2209 designation refers to a high-alloy duplex stainless steel with approximately 22% chromium, 3% nickel, 3% molybdenum, and 0.15% nitrogen, providing a PREN (Pitting Resistance Equivalent Number) of approximately 34-38.
Material Composition Design
| Component | Base Metal (2209) | Weld Strip | Flux |
|---|---|---|---|
| Cr (%) | 22-23 | 22-24 | Balancer |
| Ni (%) | 3-3.5 | 3-4 | Minor |
| Mo (%) | 3-3.5 | 3-3.5 | Minor |
| N (%) | 0.12-0.20 | 0.12-0.20 | — |
| C (%) | ≤0.03 | ≤0.03 | — |
| Si (%) | ≤1.0 | ≤1.0 | Deoxidizer |
| Mn (%) | ≤2.0 | ≤2.0 | Deoxidizer |
The flux composition was designed to:
- Stabilize the arc — Provide adequate ionization potential for stable electroslag welding.
- Control weld metal composition — Compensate for element losses during welding (particularly Cr, N, and O).
- Protect the molten pool — Provide a slag barrier against atmospheric contamination.
- Influence solidification structure — Control cooling rate and phase balance through slag composition.
Electroslag Overlay Welding Process
Electroslag welding (ESW) with strip electrode is particularly suited for overlay welding thick layers because:
- High deposition rate — Strip electrode ESW can deposit 10-50 kg/h of weld metal, far exceeding manual or semi-automatic processes.
- Deep penetration — The slag pool provides deep, uniform penetration.
- Continuous deposition — Multiple layers can be deposited in a single setup.
- Good weld quality — The slag pool provides excellent protection and slow cooling, reducing defects.
Process Parameters for Overlay Application
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 2000-4000 A | Depends on strip width and thickness |
| Welding voltage | 35-45 V | Slag pool voltage |
| Travel speed | 100-300 mm/min | Higher speed for thinner layers |
| Strip thickness | 1-3 mm | Commercial strip electrode |
| Slag pool depth | 5-15 mm | Critical for penetration control |
| Preheat temperature | 100-200 °C | To prevent cold cracking |
| Interpass temperature | 200-400 °C | Control cooling rate |
The water cooling technique mentioned in the keywords is an important aspect of this process. Water cooling of the base plate or previous weld layers controls the cooling rate, which directly affects the ferrite content and grain structure of the weld metal.
Performance Results
The developed welding materials achieved excellent results:
Weld Metal Properties
| Property | Requirement | Achieved Result |
|---|---|---|
| Ferrite content (FE%) | 30-60% | 30-60% |
| Yield strength (MPa) | ≥550 | Met or exceeded |
| Tensile strength (MPa) | ≥620 | Met or exceeded |
| Elongation (%) | ≥25 | Met |
| Charpy impact (J, -40°C) | ≥27 | Met |
| Pitting resistance (ASTM G48) | Pass | Pass |
| Intergranular corrosion (ASTM A262) | Pass | Pass |
The ferrite content of 30-60% is critical for duplex stainless steel weld metals. The ideal ferrite content is typically 35-55%, as this range provides the best balance of corrosion resistance and toughness. Ferrite content below 30% increases susceptibility to hot cracking, while ferrite content above 60% increases susceptibility to 475°C embrittlement and sigma phase precipitation.
The achievement of the "international physical level" for ferrite content indicates that the developed materials match the performance of leading international welding materials, which is a significant accomplishment for domestic welding material development.
Engineering Application Considerations
The developed 2209 duplex stainless steel strip electrode ESW materials are suitable for applications including:
- Chemical processing equipment — Heat exchangers, reactors, and distillation columns requiring high corrosion resistance and strength.
- Oil and gas equipment — Downstream processing equipment exposed to H2S, CO2, and chlorides.
- Marine applications — Ship hulls, ballast tanks, and offshore structures.
- Power generation — Boiler components and heat recovery systems.
Quality Control Requirements
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Weld metal composition | Spectrochemical analysis | Within specified ranges |
| Ferrite content | Ferritoscope or magnetic measurement | 30-60 FE% |
| Mechanical properties | Tensile, hardness, impact tests | Meet 2209 specifications |
| Corrosion resistance | ASTM G48, A262, NACE TM0177 | Pass |
| Weld defects | RT/UT inspection | No unacceptable defects |
| Microstructure | Metallographic examination | Duplex structure, no sigma phase |
Study Insights and Reflections
The development of domestic duplex stainless steel ESW materials represents a significant advancement in China's welding materials technology. Duplex stainless steels have been increasingly specified in international standards (ISO, EN, ASTM) for demanding applications, and the availability of high-quality domestic welding materials reduces dependence on imported products.
The ferrite content control is the most critical aspect of duplex stainless steel welding. The water cooling technique provides an effective means of controlling cooling rate, which directly influences ferrite content. Engineers should carefully calibrate the water cooling parameters for their specific equipment and material thickness to achieve the target ferrite content.
The achievement of international-level performance validates the technical approach and material design. However, engineers should be aware that the long-term performance of duplex stainless steel weld metals depends not only on the as-deposited microstructure but also on the service temperature and duration. Exposure to temperatures between 400-500 °C can cause sigma phase precipitation, which severely degrades toughness and corrosion resistance. Therefore, the developed materials are most suitable for applications with service temperatures below 300 °C or with limited exposure to intermediate temperatures.
This research contributes to the broader goal of developing high-performance welding materials for China's growing demand in petrochemical, marine, and power generation industries. The systematic approach of material design, process development, and performance verification provides a template for future welding material development programs.