CLADDING TECHNOLOGY SHANXI CO., LTDFeasibility Study of Strip Electroslag Cladding for 2507 Super Duplex Stainless Steel
Literature Overview
This study note reviews the feasibility research conducted by Luo Yongjun, Wang Congyuan, Xu Shengdong, and Zhang Jianxiao, published in 2017 in the journal China Chemical Equipment. The study investigates the application of strip electroslag welding (ESW) cladding technology for depositing UNS S32750 (2507) super duplex stainless steel onto carbon steel substrates, targeting applications in oil and gas equipment where extreme corrosion resistance and high mechanical strength are simultaneously required.
Technical Background and Motivation
Super duplex stainless steel 2507 (UNS S32750) offers a unique combination of high yield strength (minimum 550 MPa), excellent pitting resistance (PREN > 40), and resistance to chloride-induced stress corrosion cracking. However, the high cost of solid 2507 plate limits its use to critical components only. Strip ESW cladding provides a cost-effective alternative by combining the economic strength of a carbon steel base with the corrosion resistance of a 2507 overlay layer, reducing material costs by 40–60% while maintaining equivalent corrosion performance.
| Property | 2507 Super Duplex SS | A105 Carbon Steel | Clad Assembly Target |
|---|---|---|---|
| Yield strength (MPa) | ≥ 550 | ≥ 180 | Base: ≥ 180; Overlay: ≥ 500 |
| Tensile strength (MPa) | ≥ 795 | ≥ 415 | Overlay: ≥ 750 |
| PREN | > 40 | < 5 | Overlay: > 38 |
| Pitting temperature (ASTM G48) | > 50 °C | < 10 °C | Overlay: > 45 °C |
| Cost index | 100 | 10 | 35–50 |
Electroslag Welding Cladding Process Parameters
Strip ESW cladding is a highly productive process that deposits thick overlay layers (typically 6–12 mm per pass) at rates 5–10 times higher than conventional arc welding processes. The process involves passing a strip electrode through a slag pool that acts as both a shielding medium and a heat source, providing deep, uniform penetration with minimal dilution.
Recommended ESW Cladding Parameters for 2507
| Parameter | Value | Notes |
|---|---|---|
| Strip material | UNS S32750 (2507) | 4–6 mm thick, 50–100 mm wide |
| Base material | A105 / SA-105 or P91 | Pre-machined to flat surface |
| Current | 3500–5000 A | DC, strip as positive |
| Voltage | 30–38 V | Including slag voltage |
| Travel speed | 150–350 mm/min | Depends on strip thickness |
| Slag composition | CaF₂ + CaO + SiO₂ + Al₂O₃ | Basic flux, low sulfur |
| Preheat | 100–150 °C | To prevent cracking in base |
| Interpass temperature | < 250 °C | Critical for duplex phase balance |
| Post-weld heat treatment | Solution treatment 1050–1100 °C / 1 h + water quench | To restore phase balance |
| Target overlay thickness | 6–12 mm | Minimum 3% of base wall thickness |
Metallurgical Challenges and Solutions
The primary metallurgical challenge in ESW cladding of 2507 is maintaining the duplex microstructure (approximately 40–60% ferrite / 40–60% austenite) throughout the overlay. Excessive heat input or high interpass temperatures can lead to austenite formation, loss of pitting resistance, and potential 475 °C embrittlement. Conversely, insufficient heat input can result in excessive ferrite and brittleness.
Phase Balance Control
| Condition | Ferrite % | Consequence | Solution |
|---|---|---|---|
| Normal ESW | 45–55% | Optimal properties | Maintain interpass < 250 °C |
| High interpass | 30–40% | Reduced toughness, increased SCC risk | Reduce interpass to < 200 °C |
| Low heat input | 65–75% | Brittle, reduced ductility | Increase current or reduce travel speed |
| Post-weld solution treated | 40–60% | Restored phase balance | Standard practice for 2507 |
Dilution Management
The dilution of base material into the overlay is the second critical challenge. In strip ESW cladding, dilution is typically 15–25% for the first pass and decreases to 5–10% for subsequent passes. High dilution introduces carbon and manganese from the carbon steel base, which can form sigma phase and reduce pitting resistance.
| Pass Number | Dilution (%) | Ferrite Number (FN) | Action Required |
|---|---|---|---|
| 1st (bonding) | 20–30% | 15–20 | Acceptable if followed by 2nd pass |
| 2nd | 5–15% | 25–35 | Monitor with FN probe |
| 3rd | 3–8% | 32–38 | Target range for final surface |
Quality Control and Inspection Requirements
The cladding of 2507 super duplex stainless steel requires rigorous quality control to ensure both mechanical integrity and corrosion performance. The inspection regime should follow NB/T 47002 and ASME IX requirements, supplemented by duplex-specific tests.
| Inspection Method | Acceptance Criteria | Standard Reference |
|---|---|---|
| Visual testing (VT) | No cracks, undercut > 0.5 mm, or porosity | NB/T 47013.1 |
| Magnetic particle testing (MT) | No linear indications > 3 mm | NB/T 47013.4 |
| Ultrasonic testing (UT) | No lack of fusion, no cracks at interface | NB/T 47013.2 |
| Ferrite number (FN) | 32–42 (Vickers FN) | ISO 8044 |
| Pitting resistance (ASTM G48) | No pitting at 45 °C in 3.5% NaCl | ASTM G48 Method B |
| Intergranular corrosion (ASTM A263) | No intergranular attack | ASTM A263 Practice E |
| Tensile test (overlay) | UTS ≥ 750 MPa, elongation ≥ 20% | ASME II Part A |
| Impact test (overlay) | ≥ 47 J at -40 °C | ASME II Part A |
Engineering Practice and Feasibility Assessment
The feasibility study concluded that strip ESW cladding of 2507 super duplex stainless steel is technically viable and economically advantageous for large-diameter pressure vessels, heat exchanger shells, and pipe spools in the oil and gas industry. The productivity advantage of ESW over conventional arc processes is substantial: a 2000 mm × 2000 mm clad panel can be produced in approximately 4–6 hours compared to 20–30 hours for SAW or FCAW overlay.
However, the study also identified several practical limitations:
- Equipment requirement: A dedicated ESW machine with high current capacity (5000 A+) and strip feeding mechanism is required, representing a significant capital investment.
- Geometry constraints: ESW is best suited for flat or gently curved surfaces; complex geometries such as nozzles, corners, and small-diameter pipes require supplementary GTAW or FCAW cladding.
- Post-weld heat treatment: The solution treatment requirement (1050–1100 °C) may be challenging for large components or those with embedded components, requiring careful thermal management.
- Surface quality: ESW deposits typically have a rough surface finish (Ra 25–50 μm) requiring machining to achieve the required surface quality for critical applications.
Summary and Study Insights
The feasibility study of strip ESW cladding for 2507 super duplex stainless steel demonstrates that this technology offers a compelling solution for large-scale corrosion-resistant cladding where productivity and cost-effectiveness are paramount. The key to successful implementation lies in rigorous control of interpass temperature, dilution management through multi-pass strategies, and mandatory post-weld solution heat treatment to restore the duplex phase balance. Engineers considering this technology should conduct thorough qualification testing on production-representative coupons and establish a comprehensive quality control plan that includes ferrite number monitoring, corrosion testing, and mechanical property verification. The technology is particularly well-suited for hydrogenation reactors, high-pressure separators, and chloride-containing process equipment in the petrochemical and oil and gas sectors.