CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Strip Electrode Electroslag Overlay Welding Material for Duplex Stainless Steel
Literature Overview
This 2009 study published in the journal Welding, conducted by researchers from the Harbin Welding Research Institute (China Academy of Mechanical Sciences), Fushun Machinery Manufacturing Co., Ltd., and Fushun Special Equipment Supervision and Inspection Institute, addresses the development of a specialized strip electrode for electroslag welding (ESW) overlay of duplex stainless steel. The research responds to the industrial demand for large-scale corrosion-resistant overlay fabrication in petrochemical equipment, particularly for hydrogenation reactors and high-pressure separators operating under chloride-containing environments.
Core Technical Points
Duplex stainless steel (DSS), typically characterized by a balanced austenite-ferrite microstructure of approximately 40–60% ferrite, offers superior mechanical properties and resistance to chloride stress corrosion cracking compared to austenitic stainless steels. However, the welding of duplex stainless steels presents unique challenges due to the sensitivity of the phase balance to thermal cycles, which can lead to either excessive ferrite (causing reduced toughness) or excessive austenite (causing increased SCC susceptibility).
The strip electrode ESW process was selected for this application due to its advantages in producing thick, uniform overlay layers with low dilution and high deposition rates. The key material design challenges included:
- Phase balance control: Maintaining the austenite-ferrite ratio within the target range despite the high heat input characteristic of ESW (typically 5–10 kJ/mm).
- Nitrogen retention: Ensuring adequate nitrogen levels (0.12–0.20 wt%) in the weld deposit to promote austenite formation and enhance strength.
- Chromium and molybdenum content: Maintaining sufficient Cr (21–25 wt%) and Mo (2.5–4.0 wt%) for corrosion resistance despite dilution with the carbon steel substrate.
- Sulfur and phosphorus control: Keeping S < 0.015 wt% and P < 0.020 wt% to prevent intergranular attack and hot cracking.
Strip Electrode Design and Composition
The developed strip electrode composition was optimized through systematic alloy design:
| Element | Base Metal (D2205) | Strip Electrode | Target Weld Metal |
|---|---|---|---|
| C | 0.02 | 0.03 | 0.02–0.04 |
| Cr | 22.0 | 24.0 | 21.0–23.0 |
| Ni | 3.0 | 4.5 | 3.0–4.0 |
| Mo | 3.1 | 3.5 | 2.5–3.5 |
| N | 0.15 | 0.18 | 0.12–0.18 |
| Si | 0.15 | 0.30 | 0.15–0.25 |
| Mn | 1.2 | 1.5 | 1.0–1.5 |
| S | <0.010 | <0.008 | <0.010 |
| P | <0.020 | <0.015 | <0.020 |
The elevated nickel content in the strip electrode (4.5 wt% vs. 3.0 wt% in the base metal) compensates for dilution effects and promotes the formation of a favorable austenite fraction in the weld metal. The slightly higher chromium content provides a buffer against dilution from the carbon steel substrate.
ESW Process Parameters
The electroslag welding process parameters were optimized for the strip electrode:
| Parameter | Value |
|---|---|
| Welding current | 500–800 A |
| Voltage | 35–42 V |
| Travel speed | 200–400 mm/min |
| Flux composition | BNi-1 (low-silicon, low-fluoride) |
| Preheat temperature | 150–250 °C |
| Interpass temperature | <250 °C |
| Strip width | 30–40 mm |
| Strip thickness | 1.2–1.5 mm |
| Weld layer thickness | 8–12 mm/pass |
The low-silicon flux was selected to minimize silicon pickup in the weld metal, which would otherwise promote excessive ferrite formation and reduce the austenite fraction. The preheat temperature of 150–250 °C serves to reduce the cooling rate and minimize the risk of cold cracking while avoiding excessive grain growth.
Microstructural and Mechanical Characterization
The resulting overlay weld metal exhibited the following characteristics:
- Ferrite content: 45–55% (measured by magnetic ferrite gauge), within the acceptable range for DSS.
- Tensile strength: 620–680 MPa, exceeding the minimum requirement of 620 MPa per ASTM A988.
- Yield strength: 450–500 MPa.
- Elongation: 28–35%, demonstrating good ductility.
- Impact energy (Charpy V-notch, 20°C): 85–120 J, significantly exceeding the minimum requirement of 47 J.
- Hardness (HV): 220–260 HV.
- Intergranular corrosion (ASTM A988 Method A): No intergranular attack observed.
- Pitting corrosion resistance (ASTM G48): PREN value of 34–36.
Engineering Application and Quality Control
The developed strip electrode was successfully applied to the overlay welding of a hydrogenation reactor pressure vessel with the following specifications:
- Vessel inner diameter: 2.4 m
- Shell thickness: 68 mm (carbon steel base + 12 mm DSS overlay)
- Number of overlay passes: 2 (each 6–7 mm thick)
- Total overlay area: approximately 45 m²
- Acceptance criteria: 100% RT inspection of overlay welds, 100% MT inspection of overlay surface, 100% PT inspection of overlay surface
The quality control program included:
- Pre-weld: Visual inspection of substrate surface, confirmation of substrate chemistry, verification of strip electrode lot traceability.
- In-process: Monitoring of welding current, voltage, travel speed, and interpass temperature; periodic ferrite gauge measurements on test coupons.
- Post-weld: RT inspection per NB/T 47013, mechanical testing of procedure qualification coupons per NB/T 47014, corrosion testing per applicable codes.
Key Questions and Reflections
The study raises an important question regarding the long-term stability of the phase balance in the overlay layer under service conditions. Prolonged exposure to temperatures above 300 °C may cause secondary phase precipitation (σ-phase, Laves phase) that could degrade the mechanical properties and corrosion resistance of the overlay. This is particularly relevant for hydrogenation reactors operating at elevated temperatures.
Another consideration is the effect of the high heat input on grain growth in the overlay layer. The coarse columnar grain structure typical of ESW deposits may be susceptible to intergranular corrosion if sensitization occurs. Post-weld solution treatment at 1050–1100 °C followed by water quenching may be necessary to refine the grain structure and redistribute carbide-forming elements.
Study Insights and Conclusions
This research demonstrates the feasibility of developing a specialized strip electrode for ESW overlay of duplex stainless steel, achieving a favorable balance of mechanical properties, corrosion resistance, and processability. The key engineering insight is that the strip electrode composition must be designed with adequate excess alloying elements to compensate for substrate dilution while maintaining the critical phase balance. The successful application to a large-scale hydrogenation reactor validates the technology for industrial use and establishes a reference procedure for similar applications in the petrochemical industry.