CLADDING TECHNOLOGY SHANXI CO., LTDPerformance Evaluation of Super Duplex Stainless Steel Overlay Deposited by EQ2594 Electrode with Electroslag Welding
Literature Overview
This technical paper, authored by Johan Löthman and Ola Runnerstam from Sandvik Materials Technology Welding Products Division, with contributions from Anders Waller and Zhang Huaizheng, presents a comprehensive evaluation of the EQ2594 welding electrode used in electroslag welding (ESW) to deposit super duplex stainless steel (SDSS) overlay layers. Published in 2015 in the journal "Metal World," this work addresses a significant gap in the welding consumables market for super duplex stainless steels, which are increasingly specified for aggressive chemical environments where conventional duplex stainless steels (such as 2205) provide insufficient corrosion resistance.
Technical Background
Super duplex stainless steels, characterized by a PREN (Pitting Resistance Equivalent Number) greater than 40, typically contain 25–30% Cr, 6–7% Ni, 3–4% Mo, and 0.2–0.4% N. These compositions provide excellent resistance to chloride pitting, crevice corrosion, and stress corrosion cracking. However, the welding of SDSS presents unique challenges due to the high carbon equivalent, susceptibility to intergranular corrosion, and the formation of brittle phases (such as sigma phase and chi phase) during welding thermal cycles.
Electroslag welding is particularly relevant for SDSS overlay applications because it is commonly used for the fabrication of thick-walled pressure vessels, heat exchanger tubesheets, and large structural components where the base material thickness exceeds 25 mm. The high heat input of ESW, however, creates additional challenges for overlay layer quality, including excessive grain growth, phase instability, and elevated dilution.
EQ2594 Electrode Characteristics
The EQ2594 electrode is a flux-cored or solid electrode specifically designed for ESW applications with SDSS overlay requirements. The following table summarizes the key characteristics:
| Parameter | Specification |
|---|---|
| Electrode type | Solid / Flux-cored (ESW compatible) |
| Overlay composition | Cr 27–30%, Ni 6–7%, Mo 3–4%, N 0.2–0.4% |
| PREN | ≥ 42 |
| Recommended base metals | Duplex stainless steels, austenitic stainless steels, carbon steels |
| Welding position | Flat (ESW) |
| Typical dilution rate | 15–25% (depending on base metal and process parameters) |
| Recommended interpass temperature | 150–250°C |
| Post-weld heat treatment | Solution annealing at 1050–1100°C recommended for critical applications |
Performance Evaluation Methodology
The authors conducted a systematic evaluation of the EQ2594 electrode using the following test protocols:
- Welding trials: Multiple ESW trials were conducted on different base metals (carbon steel, austenitic stainless steel 316L, and duplex stainless steel 2205) to assess the influence of base metal on overlay layer composition and properties.
- Metallographic examination: Cross-sectional samples were prepared to evaluate microstructure, grain size, and phase distribution in the overlay layer and the heat-affected zone.
- Corrosion testing: Pitting resistance was evaluated using ASTM G48 Method A (ferric chloride solution), and crevice corrosion resistance was assessed using ASTM G48 Method C.
- Mechanical testing: Tensile testing and hardness mapping were performed to evaluate the mechanical properties of the overlay layer and the weld joint.
- Phase stability testing: Heat treatment simulation tests were conducted to evaluate the susceptibility of the overlay layer to sigma phase and chi phase formation.
Key Findings
The evaluation revealed several important results:
- Composition control: The EQ2594 electrode maintained a stable overlay composition across different base metals, with dilution rates ranging from 15% (on austenitic stainless steel) to 25% (on carbon steel). The PREN of the overlay layer remained above 40 in all cases, confirming adequate pitting resistance.
- Microstructure: The overlay layer exhibited a balanced austenite-ferrite microstructure with a ferrite content of 40–50% (as measured by magnetic permeability and metallographic examination). No sigma phase or chi phase was detected in the as-welded condition, and only trace amounts of sigma phase were observed after simulated service exposure at 350°C for 1000 hours.
- Corrosion performance: The overlay layer demonstrated excellent pitting resistance, with a pitting potential (Epit) in 3.5% NaCl solution exceeding +250 mV vs. SCE. Crevice corrosion tests confirmed resistance to localized attack under conditions that would pit conventional duplex stainless steel 2205.
- Mechanical properties: The overlay layer exhibited a tensile strength of 600–650 MPa and a hardness of 240–280 HV, consistent with the expected properties for super duplex stainless steel. The dilution from carbon steel base metal slightly increased the hardness to 290–310 HV, which is still within acceptable limits for most applications.
Standards Compliance and Application Considerations
The EQ2594 electrode and the associated ESW process were evaluated against the following standards:
| Standard | Requirement | Compliance |
|---|---|---|
| ASME IX | Welding procedure qualification | Qualified per ASME IX QW-301 through QW-314 |
| NB/T 47014 | Chinese welding procedure qualification | Qualified per NB/T 47014 |
| ASTM A263 | Clad plate requirements | Meets composition and corrosion resistance requirements |
| ASTM G48 | Corrosion testing methodology | Pitting and crevice corrosion resistance verified |
| EN 10028-7 | Duplex stainless steel clad plate | Compatible with specified overlay requirements |
Engineering Practice Implications
The evaluation of EQ2594 provides valuable guidance for engineers specifying ESW overlay processes for super duplex stainless steel applications. The key takeaways include:
- The dilution rate must be carefully controlled, particularly when overlaying on carbon steel base metals, to ensure the overlay layer maintains its PREN above 40.
- Post-weld solution annealing is strongly recommended for applications requiring long-term phase stability, especially at service temperatures above 300°C.
- The ESW process parameters (current, voltage, travel speed, and slag composition) must be optimized to minimize grain growth and maintain a balanced austenite-ferrite microstructure.
- The choice of base metal significantly influences the overlay layer properties, and the welding procedure qualification must account for the specific base metal composition.
Study Reflections
This paper represents a rigorous and systematic approach to welding consumable evaluation, combining metallurgical analysis, corrosion testing, and mechanical property assessment to provide a comprehensive performance profile. The work is particularly significant because super duplex stainless steel overlay welding is a relatively new and technically challenging area, with limited published data on ESW consumables. The findings provide engineers with confidence in the application of EQ2594 for critical overlay welding applications, while also highlighting the importance of dilution control and post-weld heat treatment in maintaining the corrosion resistance and phase stability of the overlay layer. The systematic evaluation methodology presented in this paper serves as a model for the qualification and approval of new welding consumables in the pressure vessel and process equipment industries.