Stainless Steel Strip Electrode Electroslag Overlay Welding for Hydrogenation Reactor Internals

Literature Overview

This paper by Du Bing and colleagues, published in Welding in 1996, presents a comprehensive study on the development and application of stainless steel strip electrode electroslag overlay welding for the internal lining of large-scale hydrogenation reactors. Hydrogenation reactors in petroleum refining operate under extremely harsh conditions of high temperature, high hydrogen pressure, and corrosive media, necessitating the use of corrosion-resistant stainless steel linings on carbon steel pressure vessels. The authors developed a proprietary stainless steel strip electrode material, conducted systematic process trials, and successfully completed the overlay welding of a large industrial reactor at Lanzhou Petrochemical Machinery Factory.

Technical Background and Challenges

Hydrogenation reactors typically operate at temperatures of 350–450°C and hydrogen partial pressures of 3–8 MPa. Under these conditions, carbon steel base materials suffer from hydrogen attack, which causes subsurface blistering, cracking, and material loss. The solution is to line the internal pressure surface with a corrosion-resistant stainless steel layer, typically 12–25 mm thick, to provide a barrier against hydrogen penetration and resist the corrosive environment.

The challenge lies in achieving a uniform, defect-free overlay layer on a large cylindrical surface with complex geometry. Conventional arc welding methods such as SMAW or GTAW are too slow for large-scale production, and they produce inconsistent layer thickness and quality. Electroslag welding offers the advantages of high deposition rate, deep penetration, and uniform microstructure due to the slag pool's insulating and purifying effects.

Strip Electrode Material Development

The authors developed a custom stainless steel strip electrode material specifically designed for electroslag overlay welding of hydrogenation reactors. The strip electrode eliminates the need for consumable wire feeding and provides a consistent composition and cross-section throughout the welding process.

The strip electrode composition was optimized to ensure adequate chromium and nickel content for corrosion resistance while maintaining sufficient ductility to accommodate thermal cycling during reactor operation. The carbon content was kept below 0.08% to minimize sensitization and intergranular corrosion susceptibility.

Process Development and Qualification

The electroslag overlay welding process involves a series of carefully controlled steps. The base surface is first prepared by machining to remove scale and provide a smooth, clean substrate. A starter block is placed at the beginning of each weld seam to establish the initial slag pool. The strip electrode is fed continuously as the welding head travels along the programmed path, which for a cylindrical reactor involves both longitudinal and circumferential passes.

The slag pool plays a critical role in the electroslag process. It provides thermal insulation, reducing cooling rates and promoting equiaxed grain growth in the weld metal. The slag also acts as a flux, removing impurities and deoxidizing the weld pool. The viscosity and temperature of the slag must be carefully controlled to ensure stable operation and uniform penetration.

Process qualification involved systematic variation of welding parameters to establish the optimal window for defect-free overlay welding. Key parameters included welding current, travel speed, strip electrode diameter, and slag composition. The qualification tests included visual inspection, ultrasonic testing, macrographic examination, and corrosion testing.

Microstructure and Performance of Overlay Layer

The microstructure of the electroslag overlay weld consists of equiaxed austenite grains with some delta ferrite, providing good ductility and resistance to cracking during thermal cycling. The grain size is typically 30–60 μm, which is coarser than in arc-welded deposits but acceptable for the application.

The overlay layer exhibits excellent resistance to hydrogen blistering and stress corrosion cracking in simulated reactor conditions. The chromium and nickel content provides a protective passive film, while the austenitic structure offers good resistance to intergranular corrosion. Mechanical properties are adequate, with tensile strength of 520–620 MPa and elongation of 35–45%.

Engineering Implementation

The successful application of this process at Lanzhou Petrochemical Machinery Factory demonstrated the industrial viability of electroslag overlay welding for large hydrogenation reactors. The production welding involved the overlay of multiple layers on a reactor with an inner diameter of approximately 4–5 meters and a length of 15–20 meters. The total overlay thickness achieved was 18–22 mm, with uniform thickness within ±1 mm across the entire surface.

Quality control included ultrasonic testing of each layer to detect lack of fusion, porosity, and other internal defects. The final overlay surface was inspected by magnetic particle testing to ensure freedom from surface cracks. Hydrogen blistering tests and stress corrosion cracking tests were conducted on coupon samples from the production weld.

Study Insights and Reflections

This study exemplifies the successful integration of materials development, process engineering, and industrial application in pressure vessel manufacturing. The development of a custom strip electrode material tailored for electroslag welding demonstrates the importance of material-process synergy. The electroslag process is particularly well-suited for thick overlay applications because of its high deposition rate and stable, repeatable weld quality. One area for further development would be the extension of this technology to more complex geometries, such as reactor heads and nozzles, where the welding path is not a simple helix. The economic benefits of electroslag overlay welding for large reactors are substantial, as the high deposition rate significantly reduces production time compared to conventional arc welding methods.