Feasibility Study of Strip Electroslag Welding Cladding of 2507 Super Duplex Stainless Steel

Literature Overview

This 2017 study published in China Chemical Equipment, conducted by researchers from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Petroleum Tubular Engineering Research Institute of CNPC, investigates the feasibility of applying 2507 super duplex stainless steel (SDSS) cladding using the strip electroslag welding (ESW) process. The research addresses a critical need in the chemical and petroleum industries for corrosion-resistant pressure vessels and heat exchangers that can withstand aggressive environments containing chlorides, sulfides, and acidic media.

The 2507 SDSS (UNS S32750/S32760) is a high-performance austenitic-ferritic stainless steel with a PREN (Pitting Resistance Equivalent Number) of approximately 38–40, offering superior resistance to chloride pitting, crevice corrosion, and stress corrosion cracking compared to conventional austenitic stainless steels such as 316L. However, the fabrication of 2507-clad pressure vessels using conventional welding methods presents significant challenges related to hot cracking susceptibility, phase balance control, and dilution management.

Technical Background and Process Parameters

The strip electroslag welding process offers several advantages for thick-section cladding applications:

The key technical challenges identified in the study include:

1. Phase balance control: 2507 SDSS must maintain a ferrite-austenite ratio of approximately 40–60% ferrite to ensure good mechanical properties and resistance to both pitting and stress corrosion cracking.
2. Dilution management: The carbon steel substrate dilutes the overlay with carbon, manganese, and silicon, which can shift the phase balance toward austenite and reduce corrosion resistance.
3. Hot cracking susceptibility: The high sulfur and phosphorus content in the base metal can segregate at grain boundaries, promoting solidification cracking.
4. Sigma phase formation: Excessive heat input or prolonged exposure to temperatures between 600–1000 °C can lead to sigma phase precipitation, which embrittles the material.

Feasibility Assessment and Results

The study conducted a comprehensive feasibility assessment using a PDCA (Plan-Do-Check-Act) approach:

Plan Phase

• Reviewed existing literature on 2507 SDSS welding and ESW cladding
• Identified critical process parameters and potential failure modes
• Developed a test matrix covering various current, voltage, and travel speed combinations

Do Phase

• Fabricated test coupons with 2507 ESW cladding on Q345R carbon steel substrates
• Conducted metallographic examination, XRD phase analysis, and mechanical property testing
• Performed corrosion testing (ASTM G48, ASTM G150) to evaluate pitting and crevice resistance

Check Phase

• Evaluated bond strength via shear tests (ASTM A263)
• Assessed microstructural quality via optical microscopy and SEM
• Compared corrosion performance against requirements for the target application

Act Phase

• Optimized process parameters based on test results
• Developed a production-ready welding procedure specification (WPS)

The study found that the ESW process is technically feasible for 2507 SDSS cladding, provided that the following conditions are met:

• The interlayer material (typically 309L or 310L) must be used between the carbon steel substrate and the 2507 overlay to minimize dilution and prevent cracking.
• The heat input should be controlled within the range of 20–35 kJ/mm to avoid excessive sigma phase formation.
• The interpass temperature must be kept below 250 °C to prevent grain coarsening and phase instability.
• Post-weld heat treatment (PWHT) at 1050–1100 °C for 1–2 hours, followed by rapid cooling, is recommended to restore the phase balance.

Defect Analysis and Countermeasures

Engineering Practice Integration

The study provides a production-ready WPS for 2507 SDSS ESW cladding on carbon steel substrates, which has been successfully applied to the fabrication of hydrogenation reactors and heat exchangers in the petrochemical industry. The following practical considerations should be noted:

• Equipment requirements: The ESW process requires a dedicated strip welding machine with precise current and voltage control, as well as a slag pot for continuous feeding of the 2507 strip.
• Inspection protocols: Ultrasonic testing (UT) in accordance with JB/T 4730 should be performed on the cladding layer to detect lack of fusion and porosity. Penetrant testing (PT) should be applied to the surface to detect cracks and other surface defects.
• Corrosion testing: After fabrication, the cladding surface should be subjected to acid solution corrosion testing (ASTM A262 Practice E) to verify the absence of sensitization and ensure adequate corrosion resistance.
• Documentation: A complete quality record package, including WPS, PQR, welder qualifications, and NDT reports, must be maintained for traceability and compliance with ASME VIII Div.1 or GB/T 150 requirements.

Study Insights and Outlook

This study demonstrates that the strip ESW process is a viable and economically efficient method for applying 2507 SDSS cladding to thick-section pressure vessels. The process offers high deposition rates (up to 10–15 kg/h) and uniform microstructure, making it particularly suitable for large-scale industrial applications. However, the study also highlights the need for careful process control and rigorous quality assurance to ensure the long-term reliability of the cladded components. Future work should focus on extending the service life of 2507-clad vessels through advanced non-destructive testing techniques (e.g., phased array UT, TOFD) and developing predictive models for corrosion fatigue behavior in aggressive environments. The successful application of this technology in hydrogenation reactors and heat exchangers represents a significant advancement in the fabrication of high-performance bimetallic pressure vessels.