CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Domestic Single-Layer Band Electrode Electroslag Cladding Materials in Hydrogenation Reactors
Literature Overview
The technical report by Liu Baojian, Kong Fanhong, Wang Tianxian, and Fan Xibin from Qingdao Lanshi Heavy Machinery Co., Ltd., published in Pressure Vessel Technology in 2019, documents the successful application of domestically produced single-layer band electrode electroslag welding (ESW) cladding materials in hydrogenation reactor fabrication. This work is of significant practical importance in the context of China's push for domestic substitution of critical materials and welding consumables in the petrochemical and hydrogen energy industries. Hydrogenation reactors operate under severe conditions of high temperature, high hydrogen pressure, and aggressive chemical environments, making the cladding layer a critical barrier against corrosion and hydrogen damage.
Core Technical Content
The study addresses a specific challenge in hydrogenation reactor fabrication: the need for a corrosion-resistant overlay layer on carbon steel or low-alloy steel base materials that can withstand the combined effects of high-temperature hydrogen attack (HTHA), sulfide stress corrosion cracking (SSC), and general corrosion. Traditional approaches have relied on imported welding consumables, which are expensive, subject to supply chain disruptions, and often not optimized for the specific service conditions of Chinese hydrogenation reactors.
The domestically developed single-layer band electrode ESW cladding consumable is designed to produce an overlay layer with the following characteristics:
| Property | Requirement | Achieved Value |
|---|---|---|
| Overlay thickness | 3.0–5.0 mm | 3.5–4.5 mm (single pass) |
| Dilution rate | < 5% | 3–5% |
| Hardness (overlay) | 150–250 HV | 170–210 HV |
| Tensile strength (overlay) | > 500 MPa | 550–620 MPa |
| Elongation (overlay) | > 20% | 22–28% |
| Intercritical corrosion resistance | Pass | Pass (6% HCl, 60°C, 24h) |
| HIC resistance | < 10% | < 5% |
| SSC resistance | Pass | Pass (per NACE TM0177) |
The single-layer ESW process is chosen for its high deposition rate, low dilution, and excellent metallurgical bonding between the overlay and base material. The band electrode configuration allows for precise control of the overlay thickness and composition, which is critical for achieving the required corrosion resistance with minimal dilution.
Electroslag Welding Process Analysis
Process Parameters
The ESW cladding process involves several critical parameters that must be carefully controlled to achieve the desired overlay quality:
| Parameter | Typical Range | Control Objective |
|---|---|---|
| Welding current | 800–1200 A | Maintain stable slag pool and adequate heat input |
| Welding voltage | 35–45 V | Control arc length and slag pool stability |
| Travel speed | 0.2–0.5 m/min | Balance deposition rate with dilution control |
| Electrode diameter | 2.0–3.0 mm | Match current density and deposition rate |
| Slag composition | Flux-cored | Control cooling rate and dilution |
| Preheating temperature | 150–250°C | Prevent cold cracking in base material |
| Interpass temperature | 200–300°C | Control cooling rate and residual stress |
The single-layer approach is advantageous for hydrogenation reactor cladding because it minimizes the number of weld layers, reducing the risk of interlayer defects and residual stress accumulation. However, it requires precise control of the process parameters to ensure that the entire overlay thickness is achieved in a single pass with uniform composition and microstructure.
Metallurgical Considerations
The metallurgical quality of the ESW cladding overlay is critical for the long-term performance of the hydrogenation reactor. The key metallurgical concerns include:
- Dilution control: The dilution of base material into the overlay must be minimized to maintain the corrosion resistance of the overlay. For nickel-based alloy overlays, dilution above 5% can significantly reduce the corrosion resistance. The single-layer ESW process achieves dilution rates of 3–5%, which is within the acceptable range.
- Microstructure: The overlay microstructure should be fully austenitic (for stainless steel overlays) or fully austenitic with possible carbide precipitation (for nickel-based overlays). Any ferrite in a stainless steel overlay can reduce corrosion resistance, particularly in chloride-containing environments.
- Bond strength: The metallurgical bond between the overlay and base material must be strong enough to withstand the mechanical and thermal stresses during operation. The ESW process produces a strong metallurgical bond due to the high heat input and the molten slag pool that wets both surfaces.
- Residual stress: The ESW process produces lower residual stresses compared to arc welding processes due to the slower cooling rate and the higher heat input. However, residual stresses can still develop due to the mismatch in thermal expansion coefficients between the overlay and base material.
Quality Control and Inspection
The quality of the ESW cladding overlay is verified through a comprehensive inspection program that includes:
| Inspection Method | Standard | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | NB/T 47013.1 | No cracks, porosity, undercut |
| Magnetic particle testing (MT) | NB/T 47013.4 | No linear indications > 2 mm |
| Ultrasonic testing (UT) | NB/T 47013.2 | No lack of bond > 3 mm |
| Radiographic testing (RT) | NB/T 47013.2 | No porosity > 1 mm, no slag inclusions |
| Dye penetrant testing (PT) | NB/T 47013.15 | No surface cracks |
| Bond strength test | ASTM A263 | ≥ 100 MPa |
| Hardness mapping | ASTM E18 | Uniform within 50 HV |
| Chemical analysis | ASTM E4 | Composition within specification |
| Intergranular corrosion | ASTM A263 | No intergranular attack |
| HIC/SSC testing | NACE TM0284/TM0177 | < 10% HIC, pass SSC |
The study reports that all inspection methods were passed for the domestically produced consumable, demonstrating that the quality is equivalent to that achieved with imported consumables. This is a significant achievement that validates the domestic material development program.
Engineering Practice Implications
The successful application of domestically produced ESW cladding consumables in hydrogenation reactors has several important implications for the industry:
- Cost reduction: Domestic consumables are typically 30–50% less expensive than imported equivalents, which can significantly reduce the fabrication cost of hydrogenation reactors, particularly for large-scale projects.
- Supply security: Domestic production eliminates the risk of supply chain disruptions that can delay project schedules. This is particularly important for critical infrastructure projects where schedule delays can have significant economic consequences.
- Technical optimization: Domestic consumables can be tailored to the specific requirements of Chinese hydrogenation reactors, which may differ from those of Western designs. This includes optimization for specific base materials, welding procedures, and service conditions.
- Quality consistency: Domestic production allows for closer quality control and faster feedback loops, enabling continuous improvement of the consumable performance.
The study also highlights the importance of welding procedure qualification (WPQ) in ensuring the reliability of the cladding process. The WPQ must be performed in accordance with NB/T 47014 or ASME IX, with qualification parameters that cover the range of production welding parameters. The qualified welding procedure specification (WPS) must include detailed instructions for preheating, interpass temperature control, post-weld heat treatment (PWHT), and inspection requirements.
Key Questions and Reflections
A significant question that arises is the long-term performance of the domestically produced consumable in actual service conditions. While the laboratory testing and qualification testing demonstrate acceptable performance, the true test is in long-term operation under the severe conditions of hydrogenation service. The study does not provide long-term service data, which is a limitation that must be addressed through continued monitoring and periodic inspection of reactors fabricated with this consumable.
Another consideration is the effect of post-weld heat treatment on the overlay properties. Hydrogenation reactors typically require PWHT to relieve residual stresses and improve the mechanical properties of the base material. However, the PWHT can affect the overlay microstructure and properties, particularly if the temperature exceeds the solution treatment temperature of the overlay material. The study does not extensively address this interaction, but it is a critical consideration in the fabrication process.
Study Insights and Implications
This study represents a significant milestone in the domestic substitution of critical welding consumables for hydrogenation reactor fabrication. The successful qualification and application of domestically produced single-layer band electrode ESW cladding materials demonstrates that China has the technical capability to develop and manufacture high-quality welding consumables for the most demanding applications.
For engineering practice, the key takeaway is that domestic materials can achieve performance equivalent to imported materials when properly developed and qualified. This opens up opportunities for cost reduction, supply security, and technical optimization in the fabrication of hydrogenation reactors and other critical pressure vessels. The study also highlights the importance of a comprehensive quality control program that includes both destructive and non-destructive testing to ensure the reliability of the cladding overlay.
The broader implication is that the domestic substitution program should be extended to other critical welding consumables and materials used in pressure vessel fabrication, including nickel-based alloy consumables, titanium cladding materials, and specialty fluxes. This will further enhance the competitiveness of the domestic pressure vessel manufacturing industry and reduce the dependence on imported materials.