CLADDING TECHNOLOGY SHANXI CO., LTDStainless Steel Strip Submerged Arc and Electroslag Overlay Welding for Hydrogenation Reactors
Literature Overview and Technical Significance
This 1990 publication by Zhou Weiyu and Fan Zheng from Sandvik Company addresses the application of stainless steel strip submerged arc welding (SSAW) and electroslag welding (ESW) overlay techniques for hydrogenation reactors. Hydrogenation reactors are among the most demanding pressure vessels in the petrochemical industry, operating at high temperatures (350-450°C) and pressures (10-20 MPa) in the presence of hydrogen and corrosive chemical species. The overlay welding of corrosion-resistant stainless steel or nickel-based alloy layers to carbon steel or low-alloy steel pressure vessels is a cost-effective alternative to fabricating the entire vessel from expensive alloy materials.
Hydrogenation Reactor Service Conditions and Material Requirements
Hydrogenation reactors face a unique combination of service challenges that require careful material selection and overlay welding process design.
| Service Parameter | Typical Range | Design Implication |
|---|---|---|
| Operating temperature | 350-450°C | High-temperature strength required |
| Operating pressure | 10-20 MPa | Thick vessel walls |
| Hydrogen partial pressure | 1-10 MPa | Hydrogen embrittlement risk |
| Sulfur content | 10-100 ppm | Sulfide stress cracking risk |
| Ammonia presence | Possible | Stress corrosion cracking risk |
| Design life | 20-30 years | Long-term integrity required |
The base material is typically a low-alloy steel such as 1.25Cr-0.5Mo (P91) or 2.25Cr-1Mo (P22), while the overlay material is typically a austenitic stainless steel such as 309, 310, or a nickel-based alloy such as Inconel 625 or Hastelloy C-276, depending on the specific corrosion environment.
Strip Submerged Arc Welding (SSAW) Process
The strip submerged arc welding process involves feeding a continuous strip of stainless steel or alloy material into the submerged arc weld pool, where it melts and bonds to the base metal. This process is particularly suited for thick overlay layers on large pressure vessel components.
| Parameter | Typical Value |
|---|---|
| Strip thickness | 2.0-4.0 mm |
| Strip width | 25-50 mm |
| Current | 800-1200 A |
| Voltage | 28-35 V |
| Travel speed | 100-300 mm/min |
| Flux coverage | Continuous, 10-20mm depth |
| Dilution rate | 10-25% |
| Overlay thickness per pass | 2-4 mm |
The SSAW process offers several advantages for hydrogenation reactor overlay welding:
- High deposition rate, enabling thick overlay layers to be built efficiently
- Deep penetration, providing excellent bond strength to the base metal
- Consistent overlay composition due to the controlled melting of the strip material
- Minimal spatter and gas inclusions due to the protective flux cover
Key Process Considerations for SSAW Overlay Welding
The success of SSAW overlay welding depends on several critical process parameters:
- Strip feed rate control: The strip must be fed at a rate that maintains a stable weld pool. Too fast a feed rate leads to incomplete melting of the strip, while too slow a rate leads to excessive dilution and potential burn-through of the base metal.
- Flux composition and coverage: The flux must provide adequate protection from atmospheric contamination and appropriate deoxidation. For stainless steel overlay, a low-chloride flux is essential to prevent chloride stress corrosion cracking in the overlay layer.
- Interpass temperature control: Maintaining interpass temperature below 250°C is critical to prevent excessive grain growth in previously deposited layers and to maintain the mechanical properties of the overlay.
- Welding direction and sequence: For large pressure vessel components, the welding sequence must be carefully planned to minimize residual stresses and distortion. Multi-directional welding sequences with balanced thermal input are preferred.
Electroslag Welding (ESW) Overlay Process
Electroslag welding overlay is an alternative process that uses the resistance heating of a conductive slag pool to melt the base metal and electrode material. The process is particularly effective for thick overlay layers and large surface areas.
| Parameter | Typical Value |
|---|---|
| Electrode diameter | 1.6-3.2 mm |
| Current | 600-1000 A |
| Voltage | 30-40 V |
| Travel speed | 50-150 mm/min |
| Slag depth | 50-100 mm |
| Dilution rate | 15-30% |
| Overlay thickness per pass | 3-6 mm |
The ESW overlay process provides even higher deposition rates than SSAW, making it suitable for very thick overlay layers. However, the higher heat input requires more careful control of interpass temperature and post-weld heat treatment to prevent adverse microstructural changes in the overlay layer.
Comparison of SSAW and ESW Overlay Processes
| Criterion | SSAW | ESW |
|---|---|---|
| Deposition rate | High | Very high |
| Heat input | Moderate | High |
| Dilution rate | Lower | Higher |
| Overlay thickness per pass | 2-4mm | 3-6mm |
| Equipment complexity | Moderate | Moderate |
| Surface quality | Good | Good |
| Flexibility | High | Moderate |
| Cost per kg deposited | Lower | Lower |
Quality Assurance for Hydrogenation Reactor Overlay Welds
The quality requirements for hydrogenation reactor overlay welds are among the most stringent in pressure vessel fabrication. The following quality assurance measures are essential:
- Welding procedure qualification (WPQ): The welding procedure must be qualified in accordance with ASME IX or NB/T 47014, including qualification of the specific overlay material, process parameters, and post-weld heat treatment.
- Non-destructive testing (NDT):
- Magnetic particle testing (MT) or liquid penetrant testing (PT) on all overlay surfaces to detect surface and near-surface cracks.
- Ultrasonic testing (UT) to detect lack of fusion at the bond line and volumetric defects within the overlay layer.
- Radiographic testing (RT) for critical areas where bond line integrity is paramount.
- Destructive testing:
- Bond strength testing by shearing off the overlay layer and measuring the fracture location and appearance.
- Hardness testing at multiple locations and depths to verify dilution gradient.
- Metallographic examination of the bond line and overlay microstructure.
- Intergranular corrosion testing of the overlay layer to verify resistance to sensitization.
- Post-weld heat treatment: The entire vessel, including the overlay layer, must undergo post-weld heat treatment to relieve residual stresses and stabilize the microstructure. The PWHT temperature must be carefully controlled to avoid sensitization of the stainless steel overlay layer. For austenitic stainless steel overlays, PWHT temperatures should be limited to below 425°C, or a separate solution heat treatment of the overlay layer may be required.
Study Insights and Engineering Implications
This research from Sandvik Company represents a significant contribution to the technology of overlay welding for critical pressure vessels. The systematic evaluation of both SSAW and ESW processes for hydrogenation reactor applications provides valuable guidance for engineers involved in the design and fabrication of these demanding equipment. The emphasis on quality assurance and the detailed discussion of process parameters and their effects on overlay quality reflects the high level of technical rigor required for nuclear-grade and petrochemical pressure vessel fabrication. The study reinforces the importance of process qualification, thorough non-destructive testing, and careful post-weld heat treatment in ensuring the long-term integrity of overlay welds in critical service. The lessons from this research remain directly applicable to modern hydrogenation reactor fabrication and provide a foundation for the continued development of overlay welding technology for demanding pressure vessel applications.