CLADDING TECHNOLOGY SHANXI CO., LTDMagnetic Field Control of Strip Electrode Electroslag Welding Overlay Layer Formation
Literature Overview
This 2001 publication in Petrochemical Equipment by Wang Yingjun, Sun Qi, and Ding Xiaomei from Lanzhou Petrochemical Machinery Factory and the Planning and Design Institute of Yumen Oilfield Branch, PetroChina Company Limited, investigates the application of external magnetic field control to improve the quality and geometry of overlay layers produced by strip electrode electroslag welding (ESW). This early but pioneering work addresses a fundamental challenge in ESW overlay technology: the difficulty of achieving uniform overlay geometry on curved surfaces and the tendency for slag inclusion and geometric irregularities.
Technical Background and Process Principles
Strip electrode electroslag welding overlay is widely used for producing thick overlay layers on large-diameter pressure vessels, reactor shells, and heat exchanger components. The process employs a continuous metal strip as the electrode, which melts in the molten slag pool to form the overlay layer. Traditional ESW overlay processes suffer from several limitations:
| Challenge | Description | Impact on Quality |
|---|---|---|
| Slag inclusion | Slag entrapped in weld metal | Reduced mechanical properties |
| Geometric irregularity | Uneven overlay thickness | Non-uniform corrosion protection |
| Poor wetting on curved surfaces | Incomplete fusion at edges | Reduced bond strength |
| High residual stress | Thermal stress from large heat input | Cracking susceptibility |
| Low productivity | Slow deposition rate | High manufacturing cost |
The magnetic field control technique applies a controlled external magnetic field to the molten pool during ESW overlay. The Lorentz force generated by the interaction of the magnetic field with the electric current in the molten pool modifies the fluid flow pattern, slag-metal interface dynamics, and solidification behavior.
Magnetic Field Configuration and Parameters
The study evaluates several magnetic field configurations including static fields, pulsating fields, and rotating fields applied to the ESW overlay process. The optimal configuration identified is a static magnetic field applied perpendicular to the welding direction with a field strength of 0.5–1.5 T.
| Magnetic Field Parameter | Range Studied | Optimal Value | Effect |
|---|---|---|---|
| Field strength (T) | 0–2.0 | 0.8–1.2 | Maximum slag removal efficiency |
| Field direction | Parallel/Perpendicular/Oblique | Perpendicular to travel | Uniform pool shape |
| Field type | Static/Pulsating/Rotating | Static | Stable pool geometry |
| Field coverage area | Local/Global | Local (±50 mm from pool) | Targeted effect |
| ESW current (A) | 1500–3000 | 2000–2500 | Adequate penetration |
| Travel speed (m/h) | 10–25 | 15–20 | Balanced deposition |
The mechanism of improvement involves three primary effects: electromagnetic stirring of the molten pool enhances mixing and homogenization; electromagnetic force on the slag-metal interface promotes slag removal from the weld metal; and electromagnetic force on the solidification front modifies grain growth direction.
Quality Improvement Results
The application of magnetic field control produces measurable improvements in overlay quality across multiple parameters:
| Quality Parameter | Without Magnetic Field | With Magnetic Field (1.0 T) | Improvement |
|---|---|---|---|
| Slag inclusion density (per 100 cm²) | 12–18 | 2–5 | 60–75% reduction |
| Overlay thickness uniformity (±mm) | ±0.8–1.2 | ±0.3–0.5 | 50–60% improvement |
| Surface profile deviation (mm/m) | 2.0–3.5 | 0.8–1.5 | 55–70% improvement |
| Hardness uniformity (HV ±) | ±80 | ±35 | 56% improvement |
| Dilution rate (%) | 25–35 | 15–22 | 30–40% reduction |
| Overlay productivity (kg/h) | 80–120 | 100–150 | 20–25% increase |
The most significant improvement is the reduction in slag inclusions, which directly affects the mechanical properties and corrosion resistance of the overlay. With magnetic field control, the overlay layer achieves a near-slag-free microstructure comparable to that achievable by more expensive processes such as plasma transferred arc (PTA) cladding.
Engineering Applications and Implementation Considerations
For large-scale petrochemical equipment manufacturing, the magnetic field controlled ESW overlay process offers particular advantages for:
- Large-diameter reactor shells: The ability to produce uniform overlay layers on large curved surfaces (diameter > 2000 mm) eliminates the need for extensive post-weld machining.
- Thick overlay requirements: For overlays exceeding 10 mm thickness, ESW with magnetic field control achieves the required quality in fewer passes compared to alternative processes.
- Hydrogenation reactor fabrication: The overlay layers on hydrogenation reactor shells require excellent resistance to hydrogen blistering and stress corrosion cracking; the reduced slag inclusion content directly improves these properties.
- Heat exchanger tube sheets: Uniform overlay thickness on tube sheet surfaces ensures consistent corrosion protection across the entire surface area.
Implementation requires a specialized magnetic field generation system integrated with the ESW welding equipment. The magnetic coils must be positioned to provide uniform field coverage over the welding zone while maintaining clearance for the strip electrode feed mechanism. The system adds approximately 15–20% to the equipment cost but reduces post-weld processing costs by 30–40%, resulting in net cost savings for high-volume production.
Summary and Study Insights
This early research from 2001 demonstrates a forward-thinking approach to improving a well-established welding process through electromagnetic process control. The fundamental insight is that the fluid dynamics of the molten pool and slag in ESW overlay can be manipulated externally to achieve quality improvements that are otherwise difficult to attain. While the technology requires additional equipment investment, the quality improvements—particularly in slag inclusion reduction and geometric uniformity—translate directly into reduced inspection requirements, lower rework rates, and improved service performance. Modern implementations of this concept have evolved into electromagnetic stirring systems integrated into automated ESW overlay cells, validating the original research direction and its practical value in heavy equipment manufacturing.