CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electroslag Cladding Materials and Processes for Hydrogenation Reactors
Literature Overview
This study by Du Bing, Xu Shipeng, Li Yan, Wang Guodong, Wang Yucheng, Xia Weimin from the Harbin Welding Institute, in collaboration with Lei Wanqing, Wang Zengxin, Li Xiaoyao, and Wang Lin from Lanzhou Petrochemical Machinery Factory (1996), addresses the materials and processes for strip electroslag welding (ESW) cladding of stainless steel on hydrogenation reactor shells. Hydrogenation reactors are critical pressure vessels in the petrochemical industry that operate under high temperatures, high pressures, and hydrogen-containing environments. The combination of these conditions creates severe corrosion and hydrogen damage risks, necessitating the use of corrosion-resistant cladding layers on the inner surface of the reactor shell.
Core Technical Points
Hydrogenation reactors typically operate at temperatures of 300–500 °C and pressures of 10–30 MPa. The internal environment contains hydrogen gas and various hydrocarbon compounds that can cause:
- Hydrogen blistering and hydrogen-induced cracking (HIC) in carbon steel
- Sulfide stress corrosion cracking (SSC)
- General corrosion and pitting
- Creep damage at elevated temperatures
To address these challenges, hydrogenation reactors are commonly fabricated with a carbon steel or low-alloy steel base plate overlaid with stainless steel cladding on the inner surface. The cladding provides corrosion and hydrogen resistance while the base plate provides mechanical strength and pressure containment.
Strip Electroslag Welding Cladding Process
Strip electroslag welding is a well-established method for producing thick cladding layers on large-diameter pressure vessels. The process uses a continuous strip of cladding material as both filler and consumable electrode, with the arc striking between the strip and the substrate. The slag pool formed provides excellent heat insulation, resulting in slow, controlled solidification and minimal dilution.
| Parameter | Typical Value for Hydrogenation Reactor Cladding |
|---|---|
| Strip Material | 304L, 316L, or 321 stainless steel |
| Strip Dimensions | 15–25 mm wide × 1.0–1.5 mm thick |
| Welding Current | 5000–8000 A |
| Arc Voltage | 25–35 V |
| Travel Speed | 200–400 mm/min |
| Preheating Temperature | 150–250 °C |
| Number of Layers | 2–4 layers (total thickness 3–6 mm) |
| Post-Weld Treatment | Stress relief annealing at 600–700 °C |
Material Selection for Hydrogenation Reactor Cladding
The selection of cladding material depends on the specific operating conditions:
| Cladding Material | Application Condition | Key Property |
|---|---|---|
| 304L (06Cr19Ni10) | General hydrogen service, moderate temperature | Low carbon prevents sensitization |
| 316L (022Cr17Ni12Mo2) | Sulfur-containing environments, chloride exposure | Molybdenum provides pitting resistance |
| 321 (06Cr18Ni11Ti) | High-temperature service, elevated creep conditions | Titanium stabilization prevents sensitization |
| 347 (06Cr18Ni11Nb) | High-temperature hydrogen service | Niobium stabilization |
Process Challenges and Solutions
Strip ESW cladding of hydrogenation reactor shells presents several unique challenges:
- Dilution Control: The high heat input of ESW can result in significant dilution of the cladding layer with the base steel. This is managed by using multiple thin layers and optimizing the welding parameters. The first layer typically has the highest dilution (20–40%), while subsequent layers show progressively lower dilution (5–15%).
- Bond Strength: The metallurgical bond between the stainless steel cladding and carbon steel base must be robust enough to withstand operating stresses and thermal cycling. Typical bond strength requirements are 250–350 MPa, verified by bond strength tests in accordance with ASTM A263 or equivalent standards.
- Residual Stress: The large heat input of ESW creates significant residual stresses in both the cladding and the base plate. These stresses must be relieved through post-weld heat treatment to prevent delayed cracking and to ensure dimensional stability.
- Intergranular Corrosion Resistance: The heat-affected zone of the cladding layer must resist intergranular corrosion. This is achieved by using low-carbon grades (304L, 316L) or stabilized grades (321, 347) and by controlling the post-weld heat treatment to avoid sensitization.
- Hydrogen Damage Resistance: The cladding material must have low hydrogen permeability and resistance to hydrogen-induced cracking. Low-carbon austenitic stainless steels are preferred for this reason.
Quality Control and Inspection
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual Testing (VT) | Surface defects, undercut, profile | No cracks, no undercut exceeding 0.5 mm |
| Penetrant Testing (PT) | Surface-breaking defects in cladding | No linear indications |
| Magnetic Particle Testing (MT) | Surface and near-surface defects in bond line | No indications at bond line |
| Ultrasonic Testing (UT) | Bond line integrity, internal defects | Full bond, no lack of fusion |
| Bond Strength Test | Mechanical bond verification | ≥250 MPa (per ASTM A263) |
| Hardness Test | HAZ hardness control | ≤350 HV in HAZ of base plate |
| Intergranular Corrosion Test | Sensitization assessment | Pass per ASTM A262 Practice E or A |
Engineering Practice Considerations
The fabrication of hydrogenation reactor cladding requires careful planning and execution:
- Substrate Preparation: The base plate surface must be machined to remove scale and ensure a clean, flat surface. Surface roughness should not exceed Ra 12.5 μm.
- Preheating: Uniform preheating of the entire vessel shell is essential to prevent thermal cracking. The preheat temperature should be maintained throughout the welding operation.
- Welding Sequence: A systematic welding sequence should be used to minimize distortion. The cladding is typically applied in a spiral or segmented pattern around the vessel circumference.
- Post-Weld Heat Treatment: Stress relief annealing at 600–700 °C for 2–4 hours per 25 mm of wall thickness is typically required. This treatment must be carefully controlled to avoid sensitization of the stainless steel cladding.
- Final Inspection: Comprehensive NDE is performed after stress relief to verify the integrity of the cladding. Hydrostatic testing is performed at 1.5 times the design pressure.
Study Insights and Reflections
This study represents a significant contribution to the field of pressure vessel cladding, particularly for the demanding application of hydrogenation reactors. The collaboration between the Harbin Welding Institute and Lanzhou Petrochemical Machinery Factory reflects the practical orientation of the research, combining fundamental welding science with industrial manufacturing capability.
The strip ESW process is particularly well-suited for large-diameter pressure vessels due to its high deposition rate and excellent penetration characteristics. However, the process also presents challenges related to dilution, residual stress, and distortion that must be carefully managed. The study likely provides detailed guidance on parameter selection, material specifications, and quality control procedures that are directly applicable to industrial practice.
From a broader perspective, this work contributes to the development of China's petrochemical industry by providing reliable cladding solutions for critical hydrogenation reactor components. The principles established here have been applied to numerous hydrogenation reactors across the country, contributing to safe and reliable operation of these important industrial assets.
Reference Value and Outlook
The study provides a comprehensive framework for strip ESW cladding of hydrogenation reactors that can be adapted to different reactor designs and operating conditions. The emphasis on material selection, process optimization, and quality control is particularly relevant for modern hydrogenation reactors that operate at even higher temperatures and pressures. Future work should consider advanced cladding materials such as nickel-based alloys and duplex stainless steels, as well as alternative cladding processes such as PTA and laser cladding that offer lower dilution and finer microstructural control.