Zhuojin Pipe Fitting Co., LtdFeasibility Study of Strip Electrode Electroslag Surfacing with 2507 Super Duplex Stainless Steel
Literature Overview
The paper authored by Luo Yongjun, Wang Congyuan, Xu Shengdong, and Zhang Jianxiao, published in China Chemical Equipment (Vol. 19, No. 5, 2017, pp. 3-5), presents a feasibility study on strip electrode electroslag surfacing (SES) using Sandvik EQ2594 strip electrode paired with Sandvik 47S flux to produce a 2507 super duplex stainless steel (SDSS) overlay layer. The study was conducted by Lanzhou Lanchi Heavy Equipment Co., Ltd. in collaboration with the Petroleum Tubular Engineering Research Institute of CNPC. The primary objective was to validate whether the strip electrode electroslag surfacing process could produce overlay metal with excellent mechanical properties and an appropriate ferrite-austenite phase ratio, thereby confirming its feasibility for equipment manufacturing applications.
This work is particularly significant in the context of chemical equipment manufacturing where 2507 SDSS is increasingly demanded for its outstanding resistance to chloride stress corrosion cracking (SCC), pitting, and crevice corrosion. Traditional surfacing methods such as GTAW or GMAW often struggle to achieve the required thickness and phase balance in a single pass, making SES a compelling alternative for thick overlay applications.
Core Technical Points and Process Parameters
The study focuses on the interplay between the strip electrode composition, flux chemistry, and welding parameters to achieve a stable duplex microstructure in the deposited metal. The Sandvik EQ2594 strip electrode is a 25Cr-7Ni-3Mo-N super duplex grade designed specifically for surfacing applications, while Sandvik 47S is a specialized flux formulated to promote proper slag fluidity, gas protection, and alloy retention.
| Parameter Category | Typical Range / Value | Rationale |
|---|---|---|
| Strip electrode grade | Sandvik EQ2594 (25Cr-7Ni-3Mo-N SDSS) | Ensures sufficient Cr and Mo for pitting and SCC resistance |
| Flux type | Sandvik 47S | Optimized for SES, promotes proper slag behavior and alloy recovery |
| Ferrite content (deposit) | 40-60% (target) | Maintains duplex balance for optimal mechanical and corrosion properties |
| Electrode feed speed | Process-optimized (moderate) | Controls dilution and heat input per pass |
| Travel speed | Low to moderate | Ensures full fusion and adequate deposition rate |
| Current type | DC, electrode negative | Standard for SES, ensures stable arc and slag pool |
| Number of passes | Multiple (typically 2-4) | Achieves required overlay thickness with controlled dilution |
The critical metallurgical challenge in SES of duplex stainless steel lies in maintaining the phase balance throughout the multi-pass build-up. The ferrite phase provides strength and SCC resistance, while the austenite phase contributes toughness and ductility. Excessive ferrite (above 60%) can lead to intermetallic phase precipitation (sigma phase) during cooling or subsequent service, while excessive austenite reduces the SCC resistance that makes 2507 valuable.
Phase Balance Control
The study demonstrates that the combination of EQ2594 and Sandvik 47S provides adequate chromium and nitrogen retention to maintain the duplex character. Nitrogen plays a particularly important role in stabilizing the austenite phase and improving pitting resistance equivalent number (PREN). The flux chemistry is designed to minimize chromium oxide formation in the slag, thereby preserving the alloying elements in the deposit. Metallographic examination and ferrite number (FN) measurement are essential quality control steps to verify that each pass maintains the target 40-60% ferrite range.
Dilution Management
A key advantage of SES is its relatively low dilution rate compared to other arc welding processes. The slag pool acts as a barrier between the base metal and the weld pool, limiting the amount of base metal alloying elements that dissolve into the deposit. This is particularly beneficial when surfacing a duplex stainless steel overlay onto a carbon steel or austenitic stainless steel substrate, where high dilution could compromise the corrosion resistance of the final overlay. The study confirms that the dilution rate remains within acceptable limits, preserving the chemical composition and phase balance of the deposited metal.
Integration with Engineering Practice
In chemical equipment manufacturing, components such as heat exchanger tubesheets, reactor internals, and pipeline spools frequently require thick corrosion-resistant overlays. Traditional GTAW or GMAW surfacing of 2507 SDSS is time-consuming and prone to phase imbalance in thick deposits. The SES process offers a significant productivity advantage with deposition rates that can be 3 to 5 times higher than conventional arc welding methods, while maintaining quality consistency.
The feasibility demonstrated in this study has direct implications for the manufacturing of pressure vessels and heat exchangers designed for aggressive service environments such as sour gas processing, seawater applications, and chemical storage systems. The 2507 SDSS overlay provides PREN values exceeding 40, offering resistance to chloride pitting and SCC at temperatures up to approximately 250°C.
Quality Assurance Considerations
From a quality control perspective, the following inspection protocols should be applied:
- Ferrite number measurement: Each pass should be tested using a Ferritescope to confirm 40-60% ferrite content.
- Chemical analysis: Spark OES or wet chemistry to verify Cr, Ni, Mo, N, and C content in the deposit.
- Mechanical testing: Hardness (HV), tensile strength, and impact toughness (Charpy V-notch) of the deposited metal.
- Corrosion testing: Pitting potential, crevice corrosion, and SCC tests to validate the corrosion resistance of the overlay.
- Bond strength testing: Peel or shear tests to ensure adequate metallurgical bonding between the overlay and substrate.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Excessive ferrite (>65%) | High dilution, insufficient Ni/N in flux | Reduce heat input, verify flux chemistry, increase Ni content |
| Sigma phase formation | Slow cooling in thick deposits | Control interpass temperature, optimize cooling rate |
| Cracking in deposit | High S/P content, excessive restraint | Preheat substrate, control S and P in consumables |
| Poor slag release | Inadequate flux coverage | Ensure proper flux distribution, check flux moisture content |
| Porosity | Moist flux, inadequate gas protection | Dry flux per standard, ensure proper shielding |
Study Insights and Implications
This study provides a valuable engineering validation of SES for super duplex stainless steel surfacing. The key insight is that the combination of a purpose-designed strip electrode (EQ2594) and a matching flux (Sandvik 47S) creates a self-consistent consumable system that simplifies process optimization and ensures repeatability. The study's emphasis on phase balance control highlights the fundamental metallurgical challenge of duplex stainless steel welding: the narrow processing window between ferrite and austenite stability must be maintained throughout the entire build-up process.
The practical implication for manufacturing engineers is clear: SES with 2507 SDSS consumables represents a viable, cost-effective alternative to conventional surfacing methods for thick overlay applications in chemical equipment. The productivity gains and quality consistency demonstrated in this study make this process particularly attractive for high-volume manufacturing of corrosion-resistant components. Future work should focus on extending the process parameters database to cover a wider range of substrate materials and overlay thicknesses, and on establishing comprehensive qualification procedures aligned with relevant standards such as ASME B31.3, ISO 15590, and NB/T standards for pressure vessel surfacing.