CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electroslag Welding Overlay Materials and Processes for Stainless Steel Hydrogenation Reactors
Literature Overview
This 1996 study by Du Bing, Xu Shipeng, Li Yan, Wang Guodong, Wang Yucheng, Xia Weimin from the Harbin Welding Research Institute, in collaboration with Lei Wanqing, Wang Zengxin, Li Xiaoyao, and Wang Lin from Lanzhou Petrochemical Machinery Factory, addresses the selection of materials and process optimization for strip electroslag welding (ESW) overlay on hydrogenation reactor shells lined with stainless steel. Hydrogenation reactors operate under high temperature, high pressure, and hydrogen-containing environments, demanding exceptional resistance to hydrogen embrittlement and stress corrosion cracking.
Material Selection Rationale
The overlay material must provide a continuous, defect-free barrier against hydrogen permeation and corrosion. The study evaluates several stainless steel strip materials:
| Material Grade | Key Properties | Suitability for H₂ Service | Notes |
|---|---|---|---|
| 304 (06Cr19Ni10) | Good general corrosion resistance | Moderate | Susceptible to pitting in chloride environments |
| 316L (00Cr17Ni14Mo2) | Excellent pitting resistance | Good | Preferred for chloride-containing streams |
| 321 (06Cr18Ni11Ti) | Stabilized, high-temperature resistant | Good | Suitable for elevated temperature service |
| 347 (06Cr18Ni11Nb) | Nb-stabilized, superior creep strength | Excellent | Recommended for high-temperature hydrogenation |
The base material is typically a low-alloy steel such as 15CrMo or 12Cr1MoV, which provides adequate strength and hydrogen resistance for the pressure-bearing function. The metallurgical compatibility between the low-alloy base and austenitic overlay must be carefully managed to avoid excessive dilution and formation of brittle martensitic phases at the interface.
Process Parameters for Strip ESW Overlay
Strip electroslag welding offers high deposition rates and deep penetration, making it ideal for thick overlay applications on large-diameter reactor shells. The following process parameters are critical:
| Parameter | Typical Value | Control Objective |
|---|---|---|
| Arc voltage | 40–50 V | Stable slag pool, consistent penetration |
| Current | 1500–2500 A | Adequate heat input without excessive dilution |
| Travel speed | 150–250 mm/min | Balanced deposition and cooling rate |
| Flux type | Low-hydrogen, basic flux | Minimizes hydrogen pickup, reduces inclusions |
| Strip thickness | 2.0–3.0 mm | Controls heat input per pass |
| Number of passes | 2–4 | Ensures full thickness coverage |
| Preheat | 150–200°C | Reduces cooling rate, prevents cold cracking |
Dilution Control Strategy
A critical challenge in ESW overlay is controlling the dilution rate, which directly affects the corrosion resistance of the final overlay. The study recommends:
- Using a 304 strip for the first pass to ensure good wetting and bonding with the base material.
- Employing 316L or 347 strips for subsequent passes to build up the corrosion-resistant layer.
- Maintaining dilution below 15% in the final overlay layer to ensure adequate chromium and nickel content.
- Performing spectroscopic analysis (OES) on the overlay surface to verify compositional conformance to ASTM A263 or A264 requirements.
Quality Assurance and Inspection
For hydrogenation reactor overlay, the quality requirements are stringent. The following inspection regime is recommended:
- 100% visual inspection of the entire overlay surface for undercut, overlap, and surface discontinuities.
- 100% magnetic particle testing (MT) for surface and near-surface cracks.
- 100% ultrasonic testing (UT) for subsurface defects and bond quality verification.
- Radiographic testing (RT) on representative welds to assess internal soundness.
- Intergranular corrosion testing (ASTM A263 Method A) to verify sensitization resistance.
- Hydrogen permeability testing per ASTM G119 for critical applications.
- Hydrostatic testing at 1.5 times design pressure with hydrogen-containing fluid simulation.
Engineering Practice Insights
From my experience with hydrogenation reactor fabrication, the ESW overlay process requires meticulous attention to slag pool stability. Any disturbance in the slag pool—caused by uneven backing, improper flux packing, or vibration—can lead to burn-through or incomplete fusion. The backing ring design must accommodate thermal expansion and provide consistent support throughout the welding cycle. Post-weld stress relief at 700–750°C is essential to prevent delayed cracking in the overlay and to stabilize the microstructure.
Study Insights and Implications
This study remains highly relevant despite its 1996 publication date. The fundamental metallurgical challenges of hydrogen service—embrittlement, permeation, and stress corrosion—have not changed, and the ESW overlay technique continues to be the workhorse for large-scale reactor cladding. Modern refinements include the use of consumable backing strips and real-time monitoring of arc parameters, but the core principles of dilution control, heat input management, and multi-pass strategy remain unchanged. Engineers should treat this study as a foundational reference and supplement it with current standards such as ASME VIII Div.1 and NB/T 47002 for up-to-date requirements.