Stainless Steel Strip Electrode Electroslag Surfacing for Hydrogenation Reactors

Literature Overview

This 1996 paper by Du Bing and colleagues from the Harbin Welding Research Institute, in collaboration with Lanzhou Petrochemical Machinery Factory, reports on the development and industrial application of stainless steel strip electrode electroslag surfacing (ESS) technology for the internal lining of large-scale hydrogenation reactors. The work represents a significant milestone in pressure vessel engineering, where the internal stainless steel overlay provides corrosion resistance against hydrogen attack, amine corrosion, and high-temperature oxidation in hydroprocessing service.

Technical Background and Requirements

Hydrogenation reactors operating in petroleum refining and petrochemical processing are subjected to extremely harsh environments: operating temperatures of 350–450°C, hydrogen partial pressures of 5–15 MPa, and exposure to sulfurous and amine-containing process fluids. These conditions create multiple degradation mechanisms:

• High-temperature hydrogen attack (HTHA): Hydrogen diffuses into the steel matrix, reacts with carbides to form methane, causing internal cracking and loss of mechanical integrity.
• Sulfur attack: H₂S and organic sulfides attack chromium carbides, leading to intergranular corrosion and pitting.
• Amine corrosion: Ammonium chloride and ammonium sulfide solutions formed at cold spots cause localized attack on carbon and low-alloy steels.

The solution adopted in the industry is to apply a corrosion-resistant stainless steel overlay (typically 304L, 316L, or 321 grade) on the internal surface of the reactor shell, which is fabricated from low-alloy or carbon steel (e.g., 15CrMo, 12Cr1MoV). The overlay thickness typically ranges from 3 to 6 mm.

Electroslag Surfacing Process Characteristics

Electroslag surfacing (ESS) using strip electrodes offers distinct advantages over conventional arc surfacing methods for large-scale pressure vessel applications:

The strip electrode ESS process involves a continuous strip of stainless steel electrode (typically 15–25 mm wide, 1.5–3 mm thick) fed into an electrolytic slag pool formed between the electrode and the workpiece. The electrical resistance of the slag generates intense heat (approximately 1800–2000°C at the melting zone), producing a wide, shallow weld bead with excellent surface finish. The process is highly mechanized and suitable for large, curved surfaces such as reactor inner walls.

Materials Development

The authors developed proprietary strip electrode materials specifically for hydrogenation reactor applications. The material design addressed several critical considerations:

1. Low carbon content (≤0.03% C): To minimize intergranular corrosion susceptibility and reduce carbon dilution into the base metal.
2. Optimized chromium content (18–20% Cr): Provides adequate passive film stability in reducing environments.
3. Nickel addition (8–10% Ni): Stabilizes austenitic structure and improves ductility at elevated temperatures.
4. Molybdenum addition (2–3% Mo): Enhances resistance to chloride pitting and amine corrosion.
5. Titanium stabilization (5×C): Prevents chromium carbide precipitation at grain boundaries.

The developed strip electrode composition closely matched the 316L/321 grade specification, ensuring compliance with ASME Section VIII Division 2 requirements for overlay welds on pressure vessels.

Process Parameters and Performance

The electroslag surfacing process parameters optimized for the hydrogenation reactor application were as follows:

• Electrode feed speed: 0.3–0.5 m/min
• Travel speed: 0.15–0.3 m/min
• Voltage: 25–35 V
• Current: 600–1200 A
• Slag composition: CaF₂-based with Na₂SiF₆ fluxing agent
• Preheat temperature: 100–150°C
• Number of passes: 2–3 passes for 4–6 mm total thickness

The deposited overlay layer demonstrated the following performance characteristics:

• Hardness: 180–220 HV (austenitic structure)
• Tensile strength: ≥520 MPa
• Elongation: ≥35%
• Impact energy (25°C): ≥150 J
• Dilution rate: 12–18%
• Bond strength: Exceeds base metal strength in all test conditions

Industrial Application Results

The factory-level process qualification successfully completed the surfacing of a large hydrogenation reactor (inner diameter approximately 3.5 m, length approximately 12 m) with a 5 mm thick stainless steel overlay. Quality verification included:

• Radiographic testing (RT): No indications exceeding acceptance criteria per ASME Section V
• Magnetic particle testing (MT): No surface or near-surface defects detected
• Ultrasonic testing (UT): Confirmed proper bond between overlay and base metal
• Chemical analysis: Confirmed overlay composition within specification limits
• Hardness mapping: Uniform hardness distribution across the overlay surface

Study Insights

This paper exemplifies the successful integration of materials development, process engineering, and industrial scale-up. The strip electrode ESS technology addressed the economic challenge of applying corrosion-resistant overlays to large pressure vessels, where conventional methods would require excessive labor and time. The dilution rate of 12–18% is particularly noteworthy, as it represents a significant improvement over the 20–30% dilution commonly observed with submerged arc surfacing.

For engineers working on similar applications—such as hydrogen storage vessels, amine treating systems, or sulfur recovery units—this paper provides valuable guidance on material selection, process parameter optimization, and quality assurance methodology. The systematic approach of developing custom electrode materials, conducting laboratory-scale trials, and then proceeding to factory qualification demonstrates the appropriate technology development pathway for critical pressure equipment.