Performance Evaluation of Super Duplex Stainless Steel Overlay Deposits via Electroslag Welding with EQ2594 Consumable

Literature Overview

This paper, authored by Johan Løthman, Anders Wallerå, Ola Runnerstam, and Zhang Huaizheng from Sandvik Materials Technology, was published in the journal "Metal World" (金属世界) in 2015 (Volume 6, pages 26–30). The study addresses the production and characterization of super duplex stainless steel (SDSS) overlay weld deposits using the Sandvik 25.10.4.L welding strip with non-alloy flux in an electroslag welding (ESW) process, targeting the EQ 2594 chemical composition specification. The research is significant because SDSS overlay welds are increasingly demanded in desulfurization units, oil and gas processing, seawater desalination, and marine engineering applications where chloride-containing acidic media below 300 °C are encountered.

Core Technical Content

The study demonstrates that the ESW process, when applied with the appropriate consumable combination, can produce overlay deposits that meet the stringent requirements of the EQ 2594 specification. The key findings are summarized in the table below.

Interpretation of Key Technical Points

Phase Composition and Corrosion Resistance

The critical finding of this study is the CPT of 45 °C, which is a remarkably high value for an overlay weld deposit. In duplex stainless steels, the pitting resistance is governed by the PREN value, which is calculated as PREN = %Cr + 3.3 × %Mo + 16 × %N. A PREN ≥ 41 places this deposit firmly in the super duplex category. The absence of harmful precipitates—such as sigma phase (σ), chromium nitrides, or intermetallic compounds—is critical because these phases can locally deplete chromium and molybdenum, creating galvanic couples that initiate pitting. The ESW process, with its high thermal input and relatively slow cooling rate, presents a challenge for phase stability in duplex alloys. The fact that no harmful precipitates were observed suggests that the thermal cycle, combined with the specific alloy composition of the Sandvik 25.10.4.L strip, effectively suppresses sigma phase formation.

Ferrite Control

The ferrite number of 50–60 FN (approximately 30 vol%) is within the acceptable window for super duplex stainless steel weld metal. In SDSS, the austenite-ferrite balance is critical: too much ferrite can lead to sigma phase precipitation and reduced ductility, while too much austenite compromises the duplex character and corrosion resistance. The ESW process typically produces columnar grain structures with elongated grains perpendicular to the fusion line. The ferrite content achieved here suggests that the alloy design and the thermal conditions of ESW are well-matched.

Mechanical Performance at Low Temperature

The impact strength of approximately 125 J at −46 °C is particularly noteworthy. This demonstrates that the overlay deposit maintains excellent toughness even at sub-zero temperatures, which is essential for applications in cryogenic or cold-climate environments. The lateral bend test passing further confirms the absence of lack of fusion, porosity, or other volumetric defects in the overlay.

Process Considerations and Engineering Practice

Electroslag welding is traditionally used for thick-section fabrication where high deposition rates and deep penetration are advantageous. Applying ESW for overlay welding is less common than submerged arc welding (SAW) or gas metal arc welding (GMAW), but it offers significant productivity benefits for thick overlays. The use of non-alloy flux in this study is an important process detail: it ensures that the alloying elements in the deposit come primarily from the welding strip, minimizing dilution from the flux and providing better compositional control.

In engineering practice, when overlay welding super duplex stainless steels, the following considerations should be applied:

• Preheating is generally not required for SDSS overlays on carbon or low-alloy steel substrates, but the thermal cycle must be monitored to prevent excessive heat input in the substrate.
• Interpass temperature should be controlled to avoid excessive grain growth and sigma phase formation.
• Post-weld heat treatment (PWHT) in the range of 1020–1100 °C for 1–2 hours can be beneficial to homogenize the microstructure and dissolve any incipient intermetallic phases, but must be carefully controlled to avoid over-aging.
• Dilution from the base metal must be managed; typically, dilution should be kept below 5% for the first layer to maintain the super duplex composition.

Key Questions and Reflections

One question that arises from this study is how the ESW thermal cycle compares to conventional overlay welding processes in terms of long-term phase stability. While the as-deposited condition shows no harmful precipitates, the behavior during service exposure at elevated temperatures (e.g., 250–300 °C for extended periods) deserves further investigation. Sigma phase formation in duplex alloys is a time-temperature dependent phenomenon, and the columnar microstructure typical of ESW may influence the kinetics of intermetallic precipitation.

Another practical consideration is the weldability of the ESW overlay on thick carbon steel substrates. The thermal mismatch between the austenitic-ferritic overlay and the ferritic-pearlitic substrate can lead to residual stresses and potential cracking. The study does not explicitly address residual stress levels or cracking susceptibility at the overlay-substrate interface, which would be important for engineering qualification.

Study Insights and Implications

This research provides valuable evidence that ESW is a viable process for producing high-quality super duplex stainless steel overlay deposits. The combination of the Sandvik 25.10.4.L strip with non-alloy flux yields deposits with excellent corrosion resistance, mechanical properties, and phase stability. For engineers involved in the design and qualification of overlay welds for desulfurization units, marine equipment, and chemical processing vessels, this study offers a practical process route that balances productivity with performance. The CPT of 45 °C and PREN ≥ 41 confirm that the overlay can withstand aggressive chloride environments, while the impact toughness at −46 °C ensures reliability in cold environments. Future work should focus on long-term aging behavior and the residual stress state at the overlay-substrate interface to complete the engineering qualification picture.