Performance Evaluation of Super Duplex Stainless Steel Cladding Layer by EQ2594 Electric Slag Welding Method

Literature Overview

This 2015 publication by Johan Lethman, Anders Wallen, Ola Runnerstam, and Zhang Huaizheng from Sandvik Materials Technology presents a comprehensive evaluation of the performance of a super duplex stainless steel (SDSS) cladding layer deposited using the EQ2594 electroslag welding (ESW) wire. The study is of significant interest to engineers involved in the fabrication of pressure vessels, heat exchangers, and other equipment requiring high-strength, corrosion-resistant overlays. Super duplex stainless steels, such as UNS S32750 and UNS S32760, offer a unique combination of high yield strength (approximately 550 MPa), excellent resistance to chloride stress corrosion cracking (SCC), and good resistance to pitting and crevice corrosion.

Core Technical Content

Super Duplex Stainless Steel: Microstructure and Properties

Super duplex stainless steels are characterized by a balanced microstructure of approximately 40–60% ferrite and 40–60% austenite, with minor amounts of intermetallic phases such as Cr2N (chromium nitride) and sigma phase when improperly heat-treated. The high alloy content, typically containing 25–27% chromium, 7–8% nickel, and 2.5–3% molybdenum, provides excellent corrosion resistance. The addition of nitrogen (0.15–0.25%) further enhances the pitting resistance and yield strength.

The key properties of the EQ2594 ESW overlay layer are summarized below:

Electroslag Welding Process for Cladding

The electroslag welding process is particularly well-suited for thick cladding layers on large components such as pressure vessels and heat exchanger shells. The process utilizes a conductive slag pool to generate the heat required for melting, resulting in a deep, narrow weld with high deposition rates. For cladding applications, the ESW process offers several advantages:

• High deposition rate: The deposition rate for ESW cladding can exceed 10 kg/h, significantly higher than SAW or GTAW overlay.
• Uniform microstructure: The slow, steady cooling rate of the slag pool promotes a uniform microstructure with consistent ferrite/austenite balance.
• Low dilution: The deep slag pool provides a shielding effect that reduces dilution from the base metal, typically achieving dilution rates below 20% for single-layer cladding.
• Low hydrogen content: The slag pool acts as a barrier to atmospheric hydrogen absorption, reducing the risk of hydrogen-induced cracking.

However, the ESW process also has limitations for cladding applications:

• Limited geometric flexibility: The process is primarily suitable for flat or slightly curved surfaces and is not easily adapted to complex geometries such as bosses or tubesheets.
• High heat input: The high heat input can lead to excessive grain growth and potential formation of intermetallic phases if the cooling rate is too slow.
• Positional constraints: The process is primarily limited to flat and horizontal positions, requiring careful setup for vertical or overhead applications.

Performance Evaluation Results

The study evaluates the EQ2594 ESW cladding layer through a comprehensive suite of tests:

Chemical composition: The overlay composition was verified to be within the specified range for super duplex stainless steel, with chromium, nickel, molybdenum, and nitrogen content meeting the requirements for high pitting and crevice corrosion resistance.

Microstructural examination: Metallographic analysis revealed a balanced ferrite/austenite microstructure with minimal intermetallic phase formation. The ferrite content was measured at approximately 45–55% by ASTM E490, within the target range. The grain size was fine and uniform, indicating good control of the cooling rate during the ESW process.

Mechanical properties: The overlay layer exhibited yield strength exceeding 550 MPa and tensile strength exceeding 750 MPa, consistent with the expected properties of super duplex stainless steel. The elongation was measured at approximately 25–30%, indicating good ductility. The hardness profile showed a uniform hardness of 250–350 HV across the overlay thickness, with a gradual transition to the base metal hardness.

Corrosion resistance: The pitting resistance equivalent number (PREN) was calculated to be greater than 40, indicating excellent resistance to chloride pitting. Crevice corrosion testing in 6% FeCl3 solution at 60°C showed no crevice corrosion initiation within the test duration. Intergranular corrosion testing per ASTM A923 Method A showed no intergranular attack, confirming the excellent resistance to sensitization.

Bond strength: The bond strength between the overlay layer and the base metal was evaluated through macrographic examination and hardness profiling. The hardness transition zone was narrow, indicating good metallurgical bonding without excessive dilution. No lack of bond or porosity was detected at the interface.

Process Analysis and Standards Context

The fabrication of the EQ2594 ESW cladding layer was performed in accordance with the following standards:

The procedure qualification required testing of the overlay layer for mechanical properties, corrosion resistance, and bond strength. The welder qualification required demonstration of the ability to deposit the overlay layer with acceptable quality on a test coupon of representative thickness.

Engineering Practice Integration

Application Considerations

The EQ2594 ESW cladding layer is particularly suitable for applications requiring high-strength, corrosion-resistant overlays on carbon steel or low-alloy steel substrates. Typical applications include:

• Hydrogenation reactors: Where the combination of high strength and resistance to hydrogen attack and corrosion is required.
• Heat exchangers: Where the overlay provides corrosion resistance to process fluids while the base metal provides structural strength.
• Columns and towers: Where the overlay protects against corrosion in the presence of chlorides and acidic media.
• Storage tanks: Where the overlay provides corrosion resistance to the stored product.

Quality Control Considerations

The quality control of ESW cladding requires attention to several critical parameters:

• Preheating: The base metal must be preheated to 150–250°C to reduce the cooling rate and minimize the risk of cracking. The interpass temperature should be maintained between 150–250°C.
• Slag composition: The slag composition must be carefully controlled to ensure proper fluidity, deoxidation, and shielding. The slag must be compatible with the super duplex stainless steel wire to avoid contamination.
• Travel speed and current: The travel speed and current must be optimized to achieve the desired dilution rate and microstructure. Excessive current or travel speed can lead to excessive dilution or poor penetration, respectively.
• Post-weld heat treatment: A stress relief treatment at 550–620°C may be required to relieve residual stresses and ensure the stability of the ferrite/austenite microstructure. However, the PWHT temperature must be carefully controlled to avoid excessive grain growth or intermetallic phase formation.

Key Questions and Reflections

A critical question addressed by this study is the long-term stability of the super duplex stainless steel microstructure under service conditions. Super duplex stainless steels are susceptible to the formation of intermetallic phases, particularly Cr2N and sigma phase, when exposed to temperatures in the range of 300–600°C for extended periods. The formation of these phases can significantly reduce the corrosion resistance and ductility of the overlay layer. The study implies that the slow cooling rate of the ESW process may promote some degree of intermetallic phase formation, and that careful control of the thermal cycle is essential to minimize this risk.

Another important reflection concerns the comparison between ESW cladding and other cladding processes such as SAW, GTAW, and laser cladding. While ESW offers high deposition rates and low dilution, it is limited in its geometric flexibility. For applications requiring cladding on complex geometries, such as tubesheets or small-diameter components, alternative processes may be more appropriate. The selection of the cladding process must therefore be based on a comprehensive evaluation of the application requirements, including geometry, required overlay thickness, corrosion resistance requirements, and production efficiency.

Study Insights and Implications

The study by Lethman and colleagues provides a valuable assessment of the EQ2594 ESW cladding layer for super duplex stainless steel applications. The key finding is that the ESW process can produce a high-quality super duplex stainless steel overlay with excellent mechanical properties, corrosion resistance, and bond strength when properly controlled. The study confirms that the ESW process is a viable option for thick cladding layers on large components, offering significant productivity advantages over other cladding processes.

The implications for the pressure vessel and heat exchanger industries are significant. As the demand for high-performance, corrosion-resistant equipment increases in the oil and gas, chemical, and power generation sectors, the ability to deposit high-quality super duplex stainless steel overlays becomes increasingly important. The study provides a practical framework for process development and quality control that can be adapted to other super duplex stainless steel cladding applications. The emphasis on microstructural stability, corrosion resistance evaluation, and bond strength verification is a model for best practices in the fabrication of high-performance cladding layers.