Zhuojin Pipe Fitting Co., LtdApplication of Domestic Single-Layer Strip Electrode Electroslag Cladding in Hydrogenation Reactors
Literature Overview
The technical report by Liu Baojian, Kong Fanhong, Wang Tianxian, and Fan Xibin from Qingdao Lanshi Heavy Machinery Equipment Co., Ltd., published in Pressure Vessel journal in 2019 (Vol. 36, No. 9, pp. 70-77), documents the successful application of domestically produced single-layer strip electrode electroslag cladding (ESC) materials in the overlay welding of hydrogenation reactor interiors. This work represents a significant milestone in the domestication of critical welding consumables for hydrogen-containing pressure vessel applications in China. The project encompasses both welding procedure qualification (WPQ) and product acceptance, demonstrating full compliance with applicable codes and standards.
Process Background and Technical Requirements
Hydrogenation reactors in petroleum refining and chemical processing operate under severe conditions involving high temperatures, high pressures, and hydrogen-containing atmospheres. These conditions create a unique degradation mechanism known as hydrogen attack, where atomic hydrogen diffuses into the steel matrix and reacts with carbides to form methane, leading to microcracking, blistering, and eventual material failure. The overlay cladding layer on the reactor interior serves as a hydrogen barrier, preventing atomic hydrogen ingress into the base metal.
The single-layer strip electrode electroslag cladding process is particularly well-suited for hydrogenation reactor applications due to several inherent advantages:
| Feature | Benefit for Hydrogenation Reactor Application |
|---|---|
| Low dilution ratio | Maintains alloy composition of cladding layer |
| Large deposition rate | Efficient coverage of large reactor interiors |
| Excellent metallurgical bonding | Ensures hydrogen barrier integrity |
| Uniform layer composition | Consistent hydrogen resistance across the surface |
| Single-pass capability | Minimizes interpass defects and residual stress |
The strip electrode used in this project has dimensions of 75 mm × 0.4 mm, which is a standard configuration for large-diameter pressure vessel cladding. The strip electrode provides a wide, flat melt pool that promotes uniform solidification and reduces the risk of columnar grain penetration through the cladding layer.
Welding Procedure Qualification
The welding procedure qualification was conducted in accordance with applicable national standards and industry codes. The qualification process involved the following key steps:
- Consumable selection and verification: The domestic 75 mm × 0.4 mm steel strip and flux were selected based on chemical composition requirements for hydrogen-resistant overlay alloys. The strip composition was verified through spectroscopic analysis to ensure compliance with the specified alloy specification.
- Parameter optimization: Critical welding parameters including electrode travel speed, flux feed rate, current intensity, and voltage were systematically varied to establish the optimal parameter window. The goal was to achieve a dilution ratio below 10% while maintaining sound metallurgical bonding and acceptable surface quality.
- Test coupon preparation and welding: Qualification coupons were prepared and welded using the optimized parameters. Multiple coupons were produced to provide specimens for mechanical testing, chemical analysis, and metallographic examination.
- Testing and evaluation: The welded coupons underwent comprehensive testing including tensile testing of the cladding layer, hardness profiling, chemical analysis of the cladding composition, and metallographic examination of the fusion boundary and cladding microstructure.
Quality Control and Acceptance Criteria
The product acceptance criteria for the hydrogenation reactor overlay cladding included the following requirements:
| Test Category | Acceptance Criteria |
|---|---|
| Chemical composition | Compliance with specified alloy grade |
| Dilution ratio | ≤ 10% (typical requirement for hydrogen-resistant cladding) |
| Hardness | Uniform within specified range, no hard spots |
| Microstructure | No cracks, no excessive columnar grains, sound fusion boundary |
| Surface quality | No porosity, no lack of fusion, acceptable surface finish |
| Non-destructive testing | No unacceptable indications per applicable code |
The electroslag cladding process produces a cladding layer with a distinctive microstructure characterized by a fine-grained austenitic or martensitic matrix (depending on the alloy composition) with controlled secondary phase distribution. The low dilution ratio inherent to the electroslag process ensures that the alloying elements responsible for hydrogen resistance (typically chromium, nickel, and molybdenum in austenitic alloys, or high-nickel martensitic compositions) are maintained at their intended concentrations throughout the cladding layer.
Engineering Practice and Industry Impact
The successful application of domestic strip electrode and flux materials in hydrogenation reactor cladding has significant implications for the Chinese pressure vessel manufacturing industry. Prior to this project, critical welding consumables for hydrogen-containing pressure vessels were largely imported, creating supply chain vulnerabilities and increasing project costs. The domestication of these materials represents a strategic advancement in manufacturing self-sufficiency.
Several practical lessons emerged from this project that are valuable for future applications:
- Flux quality is critical: The flux composition directly affects the melt pool chemistry, slag properties, and final cladding microstructure. Any variation in flux composition from the qualified specification can lead to unacceptable dilution or microstructural defects.
- Surface preparation of the base metal: The base metal surface must be thoroughly prepared to ensure sound metallurgical bonding. Any oxide scale, rust, or contamination at the fusion boundary can create a hydrogen ingress pathway that defeats the purpose of the cladding layer.
- Thermal management: Large hydrogenation reactors require careful control of welding thermal input to manage residual stress and distortion. Interpass temperature limits and welding sequence planning are essential for maintaining dimensional accuracy and minimizing post-weld straightening requirements.
- Post-weld inspection: Comprehensive non-destructive testing of the cladding layer is mandatory, including ultrasonic testing for subsurface defects and magnetic particle testing for surface cracks. Visual inspection of the entire cladding surface should be performed to identify any areas requiring repair.
Study Insights and Outlook
The domestication of strip electrode electroslag cladding consumables for hydrogenation reactor applications is a technically significant achievement that reduces dependence on imported materials and enhances supply chain security for critical infrastructure projects. The electroslag cladding process remains the preferred method for large-area overlay welding of pressure vessel interiors due to its unique combination of low dilution, high deposition rate, and excellent metallurgical quality.
Looking forward, the continued development of domestic consumables should focus on expanding the range of available alloy compositions to cover the full spectrum of hydrogen-containing service environments, from mild hydrogen service in low-pressure applications to severe hydrogen attack conditions in high-pressure refineries. The qualification and standardization of these materials in accordance with international codes such as ASME Section IX and EN ISO 15614 will facilitate their global acceptance and application. The engineering community should recognize that the quality of the overlay cladding is only as good as the consumable quality, the procedure qualification rigor, and the welder skill level, and that all three factors must be managed as an integrated quality system.