Single-Layer Strip Electroslag Welding Overlay on Hydrogenation Vessel Inner Walls

Overview of the Technology

Hydrogenation reactors operate under extreme conditions of high temperature, high hydrogen pressure, and highly corrosive media containing sulfur compounds and amines. The base material, typically Cr-Mo low-alloy steel such as 15CrMoR or 1.25Cr-0.5Mo, provides mechanical strength and hydrogen resistance but lacks sufficient corrosion resistance against the aggressive inner environment. Applying a single-layer stainless steel overlay on the inner wall using strip electroslag welding (ESW) offers an economical and technically sound solution. This study note examines the core principles, process parameters, quality assurance measures, and practical considerations involved in this technology.

Core Technical Principles

Strip electroslag welding overlay differs fundamentally from multi-pass ESW cladding in that only one layer of strip cladding material is deposited onto the prepared substrate surface. The key challenge lies in achieving a metallurgical bond of adequate strength while minimizing dilution from the base material into the overlay. In a single-layer configuration, the dilution rate can reach 30 to 50 percent depending on the strip geometry and process parameters, which must be carefully controlled to ensure the overlay retains sufficient chromium and nickel content for corrosion resistance.

The electroslag process involves a molten slag pool that shields the weld zone, provides thermal insulation, and acts as a heating medium. The strip electrode and consumable wire are fed into the slag pool, where the wire acts as a heat source and the strip serves as the filler material. The travel speed, wire current, and strip feed rate are the primary process parameters that govern dilution, penetration, and bond quality.

Key Process Parameters and Control Windows

The dilution rate is the most critical parameter for single-layer overlay. A dilution exceeding 40 percent can reduce the chromium equivalent of the overlay below the threshold required for resistance against hydrogen attack and sulfide stress corrosion. To manage dilution, the process should be optimized to minimize base metal penetration while maintaining a sound metallurgical bond. Increasing travel speed and reducing wire current are effective strategies, but they must be balanced against the risk of incomplete melting at the interface.

Interface Bond Quality and Defect Analysis

The metallurgical bond between the base metal and the overlay layer is the most critical quality attribute. Incomplete melting at the interface leads to lack of fusion defects that are extremely difficult to detect by conventional NDE methods and can result in catastrophic failure under pressure cycling. The bond strength should be verified through macrographic examination, where a fully melted transition zone with no unmelted base metal particles is required.

Common defects in single-layer strip ESW overlay include:

• Incomplete melting: Caused by insufficient heat input, excessive travel speed, or poor surface preparation. Countermeasure: increase wire current, reduce travel speed, and ensure thorough cleaning of the substrate surface.
• Cracking: Hydrogen-induced cracking in the heat-affected zone of the base material, especially in high-strength steels. Countermeasure: adequate preheating, low-hydrogen consumables, and post-weld heat treatment.
• Tungsten inclusion: Rare in ESW but possible if the wire tip contacts the slag pool. Countermeasure: proper gun alignment and wire feed control.
• Porosity: Gas porosity from contaminated surfaces or flux. Countermeasure: strict surface preparation and flux drying.

Quality Assurance and NDE Strategy

For hydrogenation vessels, the overlay quality must meet the requirements of applicable codes such as ASME VIII Division 1, NB/T 47014, and API 934. The NDE strategy should include:

1. Visual inspection (VT): 100 percent examination of the overlay surface for defects, undercut, and uniformity.
2. Magnetic particle testing (MT): 100 percent examination of the overlay and heat-affected zone for surface and near-surface cracks.
3. Ultrasonic testing (UT): 100 percent examination for lack of fusion at the interface. TOFD or PAUT methods are preferred over conventional contact UT for improved sensitivity at the bond line.
4. Macrographic examination: Required on test coupons and repair areas to verify the metallurgical bond and measure dilution.
5. Hardness testing: Across the overlay and HAZ to detect excessive hardness in the base metal HAZ, which could indicate potential cracking susceptibility.

Engineering Practice Considerations

In practice, the single-layer strip ESW overlay process for hydrogenation vessels must be qualified through welding procedure qualification per NB/T 47014 or ASME Section IX. The qualification coupon should be tested for dilution, bond strength, mechanical properties, and corrosion resistance. The single-layer approach is cost-effective compared to multi-layer cladding because it requires only one welding pass, reducing labor costs and production time significantly.

However, the single-layer approach has limitations. The overlay thickness is typically limited to 3-6 mm, which may be insufficient for applications requiring heavy corrosion allowances. Additionally, the single-layer overlay cannot accommodate significant geometric irregularities in the vessel inner surface, requiring high-quality machining of the base surface prior to welding.

The post-weld heat treatment (PWHT) is a critical step. For Cr-Mo steels, PWHT at 680-720 °C for a duration based on wall thickness is mandatory to relieve residual stresses and improve the microstructure of the HAZ. The PWHT cycle must be carefully designed to avoid sensitization of the stainless steel overlay, particularly for 304 or 316 stainless steel strips. The cooling rate through the sensitization range (450-850 °C) should be controlled to prevent chromium carbide precipitation at grain boundaries.

Study Insights and Reflections

The single-layer strip ESW overlay technology represents a pragmatic engineering solution that balances cost, quality, and performance for hydrogenation vessel applications. The key insight from studying this technology is that process parameter optimization must be driven by the dilution target, which in turn is determined by the corrosion resistance requirements of the specific service environment. Engineers should not rely solely on the welding procedure qualification but should also consider the long-term performance of the overlay under actual service conditions, including thermal cycling, pressure cycling, and exposure to hydrogen and sulfur compounds.

A thorough understanding of the metallurgical interaction between the Cr-Mo base steel and the stainless steel overlay is essential. The formation of a martensitic transition zone at the interface, if not properly managed, can lead to embrittlement and cracking during service. This underscores the importance of careful PWHT and the selection of appropriate strip compositions that promote a favorable interface microstructure.

In conclusion, the single-layer strip electroslag welding overlay technology for hydrogenation vessel inner walls is a mature and reliable method when properly designed, executed, and inspected. The engineer's responsibility extends beyond the welding procedure to encompass the entire lifecycle of the overlay, from material selection through fabrication, inspection, and in-service monitoring. A systematic approach incorporating FMEA principles to identify potential failure modes and implement preventive measures is strongly recommended for ensuring the long-term integrity of these critical pressure boundaries.