CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Duplex Stainless Steel Strip Electrode Electroslag Cladding Materials
Literature Overview
This 2009 study, published in the journal Welding (Hànjie), was conducted by researchers from the Harbin Welding Research Institute, Fushun Machinery Equipment Manufacturing Co., Ltd., and Fushun Special Equipment Supervision and Inspection Institute. The work focuses on the development of duplex stainless steel strip electrodes for electroslag welding (ESW) cladding. This is a highly significant topic because ESW cladding is the dominant process for depositing thick corrosion-resistant linings on large pressure vessels, heat exchanger tubesheets, and nuclear reactor components, and duplex stainless steels offer superior mechanical and corrosion properties compared to austenitic grades.
Core Technical Content
Duplex stainless steels (e.g., 2205, 2507, and custom grades) contain approximately equal amounts of austenite and ferrite phases, providing a combination of high strength (yield strength approximately 450–550 MPa, nearly double that of 304/316L) and excellent resistance to chloride stress corrosion cracking, pitting, and crevice corrosion. However, ESW cladding of duplex stainless steels is challenging because the process involves high heat input and slow cooling rates, which can lead to phase imbalance (excessive ferrite or austenite), sigma phase formation, and reduced corrosion resistance.
Electroslag Welding Process Characteristics
| Parameter | Typical Value for Cladding |
|---|---|
| Welding current | 3000–6000 A |
| Arc voltage | 35–45 V |
| Travel speed | 200–500 mm/min |
| Slag basicity | 1.0–2.5 |
| Preheat temperature | 150–250 °C |
| Interpass temperature | 250–350 °C |
| Post-weld cooling rate | 5–50 °C/s |
| Cladding thickness per pass | 10–25 mm |
| Total cladding thickness | 20–100 mm |
The high heat input of ESW (typically 10–30 kJ/mm) results in slow cooling rates (5–50 °C/s), which is fundamentally different from GMAW, SAW, or TIG cladding (cooling rates of 100–1000 °C/s). This slow cooling promotes phase transformation and grain growth, which must be carefully controlled to maintain the duplex microstructure.
Duplex Stainless Steel Strip Electrode Design
The development of duplex stainless steel strip electrodes for ESW cladding requires careful attention to the following aspects:
- Chemical composition: The strip electrode composition must be designed to compensate for the dilution with the base metal and the slag. The base metal (typically carbon steel or low-alloy steel) dilutes the cladding layer with Fe, reducing the Cr and Ni content. The strip electrode must have higher Cr and Ni content than the target duplex composition to achieve the desired phase balance after dilution.
- Phase balance control: The ferrite content of the duplex stainless steel should be maintained in the range of 35–65% (by volume) to achieve optimal mechanical and corrosion properties. The ferrite content is influenced by the Cr, Ni, N, Mo, and Cu content of the alloy. The strip electrode composition must be designed to achieve the target ferrite content after ESW dilution.
- Nitrogen content: Nitrogen is a strong austenite stabilizer and is critical for achieving the duplex microstructure. However, nitrogen is lost during ESW due to the high temperature and long residence time in the slag pool. The strip electrode must contain sufficient nitrogen (typically 0.15–0.25 wt%) to compensate for losses and maintain the target ferrite content.
- Sigma phase prevention: Sigma phase (Cr₂₅Fe₇W₆) is a brittle intermetallic compound that forms at temperatures between 600–900 °C and severely reduces toughness and corrosion resistance. The cooling rate and interpass temperature must be controlled to avoid the sigma phase formation temperature range. The strip electrode composition should be designed to minimize sigma phase susceptibility by limiting Cr, Mo, and W content.
Typical Duplex Cladding Composition
| Element | Base Metal (Carbon Steel) | Strip Electrode | Target Cladding (After Dilution) |
|---|---|---|---|
| C | 0.15–0.25 | ≤0.03 | ≤0.03 |
| Cr | 0.30–0.60 | 24.0–26.0 | 21.0–23.0 |
| Ni | 0.30–0.60 | 6.0–8.0 | 5.5–7.0 |
| Mo | 0.20–0.50 | 3.0–3.5 | 3.0–3.5 |
| N | 0.01–0.02 | 0.20–0.25 | 0.14–0.18 |
| Cu | 0.10–0.20 | 1.0–1.5 | 0.8–1.2 |
| Fe | Balance | Balance | Balance |
Engineering Practice Integration
The application of duplex stainless steel ESW cladding in industry requires careful consideration of the following factors:
- Substrate compatibility: The base metal must be compatible with the duplex cladding layer. Carbon steel and low-alloy steel substrates are commonly used, but the dilution must be controlled to prevent excessive ferrite content in the cladding layer. For high-alloy substrates (e.g., 9% Cr steel), the dilution effect is less severe, and the cladding composition can be closer to the target duplex composition.
- Welding procedure qualification: The welding procedure must be qualified in accordance with applicable standards (e.g., ASME IX, NB/T 47014, ISO 15614-1). The qualification includes mechanical property tests (tensile, bend, impact), corrosion resistance tests (intergranular corrosion, pitting, crevice corrosion), and microstructural examination.
- Post-weld heat treatment: For thick ESW cladding layers, a post-weld solution treatment may be required to homogenize the microstructure and dissolve any sigma phase or other brittle phases that may have formed during welding. The solution treatment temperature is typically 1050–1100 °C with water quenching. However, this treatment must be performed with caution to avoid distortion and residual stress.
- Inspection requirements: For critical applications (e.g., nuclear reactors, pressure vessels), 100% non-destructive testing is required. The inspection includes:
- UT scanning of the bond line (100% coverage)
- PT of the surface (100% coverage)
- MT of the surface (if ferromagnetic)
- Macroetch examination of the cladding cross-section (representative samples)
- Hardness testing of the cladding layer and HAZ (representative samples)
- Corrosion resistance testing (representative samples)
- Slag management: The slag composition and basicity must be carefully controlled to achieve stable arc characteristics and proper metal-slag reactions. The slag must be basic enough to absorb sulfur and phosphorus but not so basic that it causes excessive oxidation of the cladding metal. The slag basicity is typically in the range of 1.0–2.5 for duplex stainless steel ESW cladding.
Key Questions and Reflections
A critical question is how to maintain the duplex microstructure during ESW cladding when the cooling rate is inherently slow. The solution lies in a combination of alloy design (high N content, controlled Mo and Cu), process control (interpass temperature, travel speed), and post-weld heat treatment. However, the post-weld heat treatment may not always be feasible for large in-situ cladding operations, and the alloy design must be robust enough to achieve the target microstructure without heat treatment.
Another important consideration is the effect of the number of passes on the microstructure. Each ESW pass is deposited on a partially solidified previous pass, creating a complex thermal history. The first pass experiences the highest cooling rate, while subsequent passes experience lower cooling rates due to the thermal mass of the previous passes. This can lead to a gradient in ferrite content and microstructure across the cladding thickness. The welding sequence and pass thickness must be designed to minimize this gradient.
The study also raises the question of long-term stability of the duplex microstructure under service conditions. Duplex stainless steels are susceptible to 475 °C embrittlement (ferrite decomposition) and sigma phase formation at elevated temperatures. For applications involving temperatures above 300 °C, the long-term stability of the duplex microstructure must be evaluated, and the service temperature must be limited to avoid these degradation mechanisms.
Summary
The development of duplex stainless steel strip electrodes for ESW cladding represents a significant advancement in the manufacturing of corrosion-resistant pressure vessels and heat exchangers. The key challenges lie in maintaining the duplex microstructure under the high heat input and slow cooling conditions of ESW, controlling dilution with the base metal, and preventing sigma phase formation. Engineers must integrate alloy design, process optimization, and quality assurance to achieve reliable and durable duplex cladding layers. The successful application of duplex ESW cladding extends the service life of pressure vessels and heat exchangers in aggressive chloride-containing environments, providing significant economic and safety benefits for the chemical, petrochemical, and nuclear industries.