CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electrode Electroslag Cladding Formation and Magnetic Control Device Application in Hydrogenation Reactor Overlay A Study Note
Literature Overview
This study addresses the application of strip electrode electroslag welding (ESW) for cladding hydrogenation reactor pressure vessels, with a particular focus on the development of a magnetic control device to improve weld formation quality. Hydrogenation reactors operating at high temperatures and pressures in hydrogen-containing atmospheres require corrosion-resistant overlay layers, typically nickel-based alloys such as Hastelloy C-276, Inconel 625, or Monel 400, to resist hydrogen embrittlement and sulfide stress corrosion cracking (SSC). The ESW process is widely used for this application due to its high deposition rate and deep penetration, but achieving consistent weld formation quality on thick clad plate pressure vessels remains a challenge.
ESW Cladding Process for Hydrogenation Reactors
The electroslag welding process for cladding involves the use of a strip electrode (typically 50-100 mm wide) fed through a molten slag pool to deposit the overlay material on the base plate. The slag pool provides excellent shielding, preheating, and heat control, resulting in low dilution rates and good metallurgical bonding. The process parameters are carefully controlled to maintain a stable slag pool and consistent weld bead formation.
| Parameter | Typical Range | Notes |
|---|---|---|
| Strip Width (mm) | 50-100 | Depends on plate thickness |
| Current (A) | 800-1500 | Higher for thicker deposits |
| Voltage (V) | 30-45 | Slag pool stability critical |
| Travel Speed (mm/min) | 100-300 | Slower for thicker sections |
| Slag Composition | CaF2-CaO-Al2O3 | Flux composition critical |
| Dilution Rate (%) | 5-15 | Lower is better for Ni alloys |
| Interpass Temp (°C) | 200-400 | Controlled for residual stress |
The cladding layer thickness for hydrogenation reactors typically ranges from 6-12 mm, built up in multiple passes. The first pass establishes the metallurgical bond with the base steel, while subsequent passes build up the required thickness. The top pass is critical for surface quality and corrosion resistance, and often uses a slightly different consumable composition to optimize surface properties.
Weld Formation Challenges and Defects
The primary challenges in ESW cladding for hydrogenation reactors include:
- Slag inclusion in the weld metal due to unstable slag pool
- Lack of fusion at the fusion boundary due to insufficient heat input
- Undercut at the strip edges due to excessive travel speed
- Porosity from hydrogen absorption in the slag pool
- Excessive dilution leading to reduced corrosion resistance of the overlay
The magnetic control device developed in this study addresses the slag inclusion and undercut problems by applying a controlled magnetic field to the slag pool. The magnetic field exerts a Lorentz force on the molten slag, which can be used to control the slag pool shape, improve slag pool stability, and reduce slag entrainment into the weld metal.
Magnetic Control Device Design and Operation
The magnetic control device consists of a set of electromagnetic coils positioned around the welding zone to generate a controlled magnetic field. The magnetic field is oriented to exert a downward force on the slag pool, promoting a more stable and uniform slag pool shape. The device parameters include magnetic field strength (0.5-2.0 T), coil geometry, and current control circuitry.
The magnetic field influences the welding process through several mechanisms:
- Lorentz force on the molten slag, which modifies the slag pool shape and stability
- Electromagnetic stirring of the weld pool, which promotes more uniform temperature distribution
- Magnetic pressure on the slag-metal interface, which reduces slag entrainment
- Control of the solidification front shape, which affects the final weld bead geometry
| Defect Type | Without Magnetic Control | With Magnetic Control |
|---|---|---|
| Slag Inclusion Rate (%) | 8-15 | 2-5 |
| Undercut Depth (mm) | 0.5-1.5 | 0.2-0.5 |
| Weld Width Variation (mm) | ±2.0 | ±0.8 |
| Surface Quality | Moderate | Good |
| Dilution Rate (%) | 10-15 | 8-12 |
The magnetic control device significantly improved the weld formation quality by reducing slag inclusions from 8-15% to 2-5%, reducing undercut depth by approximately 50-70%, and improving the consistency of the weld bead geometry. The reduction in dilution rate is attributed to the improved slag pool stability, which allows for better control of the heat input and slag pool depth.
Metallurgical Quality and Performance
The metallurgical quality of the ESW cladding layer with magnetic control was evaluated through metallographic examination, hardness testing, and corrosion testing. The overlay layer exhibited a uniform microstructure of austenite with some delta ferrite, consistent with the expected solidification behavior of nickel-based alloys. The hardness was in the range of 200-280 HV, appropriate for the intended service conditions.
Intergranular corrosion testing (ASTM A263) confirmed that the overlay layer met the required corrosion resistance for hydrogen service. The bond strength between the overlay and base steel was verified through mechanical testing and was found to be adequate for the design pressure and operating conditions of the reactor.
Engineering Practice and Standards Compliance
The ESW cladding process for hydrogenation reactors must comply with applicable codes and standards including ASME Section VIII Division 1 and 2, ASME Section IX, and API 934. The welding procedure qualification must demonstrate the ability to produce overlay layers with the required metallurgical properties, corrosion resistance, and bond strength. The magnetic control device does not change the fundamental welding process but improves the process stability and quality consistency, which is beneficial for meeting code requirements.
Quality assurance of the ESW cladding includes visual inspection, magnetic particle testing for surface cracks, ultrasonic testing for subsurface defects, and hardness profiling across the deposit thickness. The final overlay surface must be smooth and free of defects to ensure adequate corrosion resistance in hydrogen service.
Summary
This study demonstrates that the application of a magnetic control device to strip electrode electroslag welding significantly improves the weld formation quality for hydrogenation reactor cladding. The reduction in slag inclusions, undercut, and dilution rate leads to improved metallurgical quality and corrosion resistance of the overlay layer. The magnetic control device represents a practical engineering solution to a long-standing quality challenge in ESW cladding, and its application should be considered for critical hydrogen service pressure vessels where overlay quality is paramount for long-term reliability and safety.