Performance Evaluation of Super Duplex Stainless Steel EQ2594 Overlay Deposited by Electroslag Surfacing

Literature Overview

This paper by Lethman, Waller, Runnerstam, and Zhang, published in "Metal World" in 2015, reports on the development and evaluation of a super duplex stainless steel (SDSS) overlay deposit using Sandvik 25.10.4.L welding strip wire via electroslag welding (ESW). The work was conducted by Sandvik Materials Technology and represents a significant contribution to the field of corrosion-resistant overlay engineering, particularly for applications in desulfurization equipment, oil and gas, and marine environments where chloride-containing acidic media below 300 degrees Celsius are encountered. The study evaluates the chemical composition, corrosion resistance, microstructure, and mechanical properties of the resulting EQ 2594-compliant overlay layer.

Core Technical Findings

The authors selected Sandvik 25.10.4.L welding strip wire, a non-alloy flux system, and the ESW process to achieve an overlay deposit meeting the EQ 2594 specification. The key results are summarized in the following table.

The CPT of 45 degrees Celsius is particularly noteworthy because it significantly exceeds the requirements typically specified for standard duplex grades such as UNS S31803 or S32205, placing this overlay firmly in the super duplex category. The PREN value of at least 41 confirms the high alloy content contribution from chromium, molybdenum, and nitrogen, which together provide robust resistance against pitting and crevice corrosion in chloride environments.

Process Analysis and Technical Discussion

ESW is an inherently high-deposition-rate process that produces deep penetration and low dilution when properly controlled. For overlay applications, the choice of ESW is driven by the need for thick, multi-pass deposits with minimal dilution from the base metal. The use of Sandvik 25.10.4.L strip wire with a non-alloy flux is significant because alloyed fluxes can introduce unwanted alloying elements that alter the deposit chemistry and potentially reduce corrosion resistance. The non-alloy flux ensures that the deposit composition is governed primarily by the strip wire chemistry, providing better compositional control.

The ferrite content of 50-60 ferrite number, corresponding to approximately 30 volume percent, is within the acceptable range for super duplex stainless steels. This level of ferrite is important because it balances the microstructure between austenite and ferrite phases, which is essential for maintaining both corrosion resistance and mechanical toughness. Excessive ferrite could promote the formation of harmful intermetallic phases such as sigma or chi phases, while excessive austenite could reduce resistance to stress corrosion cracking. The authors report that essentially no harmful precipitates were observed, which indicates that the ESW cooling rate and the thermal cycle were well controlled.

The mechanical properties are excellent for an overlay deposit. A tensile strength of 830 MPa with approximately 30 percent elongation demonstrates good ductility, and the impact energy of approximately 125 J at minus 46 degrees Celsius confirms that the deposit retains adequate toughness even at cryogenic temperatures. The successful side bend test further validates the ductility and soundness of the weld metal, indicating that there are no internal defects such as lack of fusion or slag inclusions that would cause failure under bending.

Engineering Practice Implications

For engineers involved in the design and maintenance of desulfurization equipment, this work provides a validated approach for applying super duplex stainless steel overlays to carbon steel or low-alloy steel base metals. The ESW process is particularly suitable for large, thick components where the volume of overlay material required is substantial, as it can deposit material at rates far exceeding those achievable with GTAW or FCAW. However, engineers must be mindful of the residual stresses introduced by ESW, which can be significant due to the high heat input. Post-weld heat treatment or controlled cool-down procedures may be necessary to manage residual stress and prevent delayed cracking.

The application of this overlay technology to pipe systems, such as those used in acid gas injection or sour water handling in refineries, is also relevant. The high CPT and PREN values make this overlay suitable for protecting pipe internals against pitting and crevice corrosion in chloride-rich environments. However, the transition zone between the overlay and the base metal must be carefully evaluated, as cracking at the weld interface can occur under cyclic loading or thermal cycling.

Key Questions and Reflections

One question that arises from this study is the long-term stability of the overlay under thermal cycling conditions. While the as-deposited microstructure shows no harmful precipitates, prolonged exposure to temperatures in the range of 250 to 300 degrees Celsius could promote the formation of intermetallic phases over time. Engineers should consider post-weld solution treatment or solution annealing to stabilize the microstructure for high-temperature service.

Another consideration is the compatibility of the EQ 2594 overlay with the base metal in terms of thermal expansion. The coefficient of thermal expansion of duplex stainless steel differs from that of carbon steel, and this differential expansion can induce interfacial stresses during thermal cycling. In applications involving repeated heating and cooling, such as start-up and shutdown cycles in refineries, these stresses could lead to interface cracking. A thorough FMEA of the application should be conducted to assess this risk.

This study provides a solid foundation for the use of ESW-deposited super duplex stainless steel overlays in aggressive chloride environments, and the combination of high corrosion resistance, good mechanical properties, and process efficiency makes it a compelling option for industrial applications.