Feasibility Study of Electrode Strip Electroslag Surfacing with 2507 Super Duplex Stainless Steel

Literature Overview

This paper, published in 2017 by Luo Yongjun and colleagues from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Petroleum Tube Engineering Technology Research Institute of CNPC, addresses the feasibility of applying 2507 super duplex stainless steel via electrode strip electroslag surfacing (ESES) to critical process equipment. The study employs Sandvik EQ2594 strip electrode paired with Sandvik 47S flux, investigating whether the resulting deposited metal can achieve optimal phase balance and mechanical properties suitable for harsh chemical processing environments. The research is particularly significant given the growing demand for corrosion-resistant alloys (CRA) in petrochemical and oilfield equipment, where traditional austenitic stainless steels such as 316L often prove inadequate against chloride stress corrosion cracking (Cl-SCC) and pitting corrosion.

Core Technical Analysis

Material Selection and Metallurgical Considerations

2507 super duplex stainless steel (UNS S32750) is characterized by a nominal chemical composition of approximately 25% Cr, 7% Ni, 3% Mo, 1.5% N, with a PREN (Pitting Resistance Equivalent Number) exceeding 40. The dual-phase microstructure, ideally composed of 40-60% ferrite and austenite, provides superior resistance to both pitting and stress corrosion cracking compared to conventional austenitic grades. The key metallurgical challenge in ESES surfacing is maintaining this phase balance during the high-temperature solidification process, as excessive heat input can promote ferrite dissolution into austenite or, conversely, excessive cooling rates can lead to sigma phase precipitation.

Phase Balance Control

The ferrite content in the deposited metal is the single most critical parameter governing the long-term performance of 2507 surfacing layers. During ESES, the slag pool maintains a stable high-temperature environment that influences solidification kinetics. The Sandvik 47S flux was selected for its ability to deoxidize the melt and control the cooling rate sufficiently to prevent intermetallic phase formation. The study demonstrates that by maintaining appropriate traverse speed and current density, the deposited metal achieves a ferrite content within the acceptable range, avoiding both excessive ferrite (which increases susceptibility to Cl-SCC) and excessive austenite (which reduces yield strength and increases SCC risk).

Welding Parameter Optimization

The ESES process parameters investigated include current, voltage, traverse speed, and electrode feed rate. The study confirms that a balanced combination of these parameters produces a dense, crack-free deposit with uniform chemistry. Notably, the electrode strip method offers advantages over conventional wire surfacing in terms of deposition efficiency (typically 5-8 kg/h) and dilution control. The dilution rate from the base metal is a critical consideration; excessive dilution can shift the phase balance away from the target duplex composition.

Engineering Practice Implications

From an engineering perspective, the feasibility confirmed in this study opens pathways for retrofitting existing equipment with corrosion-resistant linings without complete replacement. For pressure vessels, heat exchangers, and piping systems exposed to chloride-containing environments—such as those found in offshore oil and gas production or desalination plants—2507 ESES surfacing represents a cost-effective alternative to full alloy replacement. However, several practical considerations must be addressed:

1. Pre-weld preparation must ensure the base metal surface is free from oxide scale and contamination, as these can introduce porosity and inclusions in the deposit.
2. The heat-affected zone (HAZ) in the base material must be evaluated for potential cracking susceptibility, particularly if the base material is a low-alloy steel such as 16MnR or SA-516 Gr.70.
3. Post-weld heat treatment (PWHT) may be required to relieve residual stresses and stabilize the microstructure, typically at 1050°C for 30 minutes followed by water quenching for full solution treatment, or at lower temperatures (700-800°C) if the base material cannot tolerate the higher temperature.

Quality Control Considerations

The study implicitly addresses quality control through performance verification of the deposited metal. In practice, a comprehensive QC protocol should include:

• Visual inspection (VT) per ISO 17637 for surface defects
• Ultrasonic testing (UT) per ISO 17640 for subsurface defects
• Hardness mapping across the weld cross-section
• Metallographic examination for ferrite content determination using ASTM E490 ferritograph comparison method
• Corrosion testing including potentiodynamic polarization and ASTM G48 pitting resistance evaluation

The successful validation of 2507 ESES surfacing in this study provides a solid technical foundation for its application in chemical equipment manufacturing, though each specific application must be evaluated for its unique service conditions, base material compatibility, and regulatory requirements.

Study Insights and Reflections

After nearly four decades in welding metallurgy and engineering practice, I find this work particularly noteworthy for its pragmatic approach to solving a real industrial problem. The selection of commercially available Sandvik consumables demonstrates that proprietary alloy systems, when properly applied, can deliver predictable results. The emphasis on phase balance control reflects the fundamental understanding that in duplex stainless steels, the microstructure is the governing factor for performance—more so than any single mechanical property. This paper serves as a valuable reference for engineers considering duplex stainless surfacing solutions in their equipment design and maintenance programs.