Zhuojin Pipe Fitting Co., LtdEffect of Transverse AC Pulse Magnetic Field on Microstructure and Properties of Surfaced Metal
Literature Overview and Background
The study by Liu Zhengjun, Zhao Qian, Song Xingkui, and Yang Yang from Shenyang University of Technology, published in the Transactions of the China Welding Institute (2010, Vol. 31, No. 3, pp. 49-52), investigates the influence of an externally applied transverse alternating current pulse magnetic field on the microstructure and properties of plasma arc surfacing deposits. Plasma arc surfacing is widely used for depositing wear-resistant and corrosion-resistant overlay layers on base materials. However, achieving optimal microstructure and properties often requires careful control of welding parameters and post-weld treatments. This research explores an unconventional approach using electromagnetic fields to modify the solidification behavior of the overlay metal, potentially offering a non-contact method for improving overlay quality.
Core Technical Analysis
The experimental setup involves applying a transverse AC pulse magnetic field to the molten pool during plasma arc surfacing of a nickel-based self-fusing alloy onto low carbon steel substrate. The magnetic field is generated by a coil positioned perpendicular to the welding direction, creating an electromagnetic stirring effect within the molten pool.
Experimental Parameters
The study investigates the effects of varying magnetic field current and duty cycle on the overlay deposit properties:
| Parameter | Range Investigated | Optimal Value |
|---|---|---|
| Magnetic field current | 0-20 A | 10-15 A |
| Duty cycle | 20-80% | 50-60% |
| Plasma arc current | Constant | Standard process value |
| Travel speed | Constant | Standard process value |
The nickel-based self-fusing alloy contains carbide-forming elements such as chromium, molybdenum, and tungsten, which form hard phases responsible for wear resistance. The base material is low carbon steel, typical of industrial equipment components requiring surface hardening.
Microstructural Effects
The application of the transverse AC pulse magnetic field produces several beneficial effects on the overlay microstructure:
- Grain refinement: The electromagnetic stirring disrupts the directional solidification pattern, promoting equiaxed grain formation and reducing columnar grain size.
- Hard phase distribution: The magnetic field influences the nucleation and growth of carbide phases, increasing their number density and controlling their orientation.
- Reduced segregation: Enhanced fluid flow in the molten pool reduces macrosegregation, resulting in more uniform chemical composition throughout the deposit.
- Improved phase morphology: The hard phases exhibit more favorable shapes and distributions, enhancing their effectiveness in resisting wear.
Mechanical Property Improvements
The electromagnetic stirring effect leads to measurable improvements in overlay performance:
- Hardness increase: Up to 15-20% improvement in microhardness compared to deposits without magnetic field application
- Wear resistance enhancement: Significant improvement in wear resistance as measured by wet sand rubber wheel abrasion testing
- Improved toughness: More uniform microstructure reduces the likelihood of crack initiation and propagation
- Better adhesion: Reduced thermal stresses and improved metallurgical bonding between overlay and base metal
Mechanism of Electromagnetic Stirring
The transverse AC pulse magnetic field induces eddy currents in the conductive molten metal. These eddy currents interact with the magnetic field to produce Lorentz forces that drive fluid flow within the molten pool. The pulsing of the magnetic field creates time-varying forces that enhance mixing and disrupt the thermal gradients responsible for columnar grain growth.
The duty cycle of the pulse magnetic field is critical: too low a duty cycle provides insufficient stirring, while too high a duty cycle can destabilize the molten pool and cause surface irregularities. The optimal duty cycle represents a balance between effective stirring and pool stability.
Engineering Practice Implications
The concept of using external electromagnetic fields to modify weld metal properties offers several advantages for industrial applications:
- Non-contact process modification: Unlike conventional methods that require changes to welding parameters or post-weld treatments, electromagnetic stirring is applied externally without interfering with the welding process itself.
- Real-time control: The magnetic field parameters can be adjusted in real time to optimize the deposit properties for different sections of a large component.
- Reduced post-processing: Improved as-deposited properties may reduce or eliminate the need for post-weld heat treatment, saving time and cost.
- Applicability to existing equipment: The magnetic field coil can be added to existing plasma arc surfacing systems without major modifications.
Comparison of Overlay Properties
| Property | Without Magnetic Field | With Magnetic Field | Improvement |
|---|---|---|---|
| Microhardness (HV) | 700-800 | 820-950 | 15-20% |
| Wear resistance (mm³/g) | 20-25 | 28-35 | 30-40% |
| Grain size (μm) | 50-80 | 25-45 | 40-50% reduction |
| Hard phase density | Low | High | Significant increase |
Study Insights and Reflections
This research represents an innovative approach to improving overlay weld quality through electromagnetic process control. The concept of using external fields to influence solidification behavior is not new in metallurgy, but its application to plasma arc surfacing is relatively novel. For engineers involved in surface engineering and overlay welding, this study opens up new possibilities for process optimization.
The electromagnetic stirring effect addresses several fundamental challenges in overlay welding: achieving fine grain size, uniform hard phase distribution, and reduced segregation. These are traditionally addressed through careful selection of welding parameters, alloy composition, and post-weld treatments. The ability to influence these factors through external electromagnetic fields offers a complementary approach that can be combined with conventional optimization methods.
However, several practical considerations must be addressed for industrial implementation. The magnetic field coil adds complexity and cost to the welding system. The optimal magnetic field parameters may vary with material composition, deposit thickness, and welding parameters, requiring qualification for each specific application. Additionally, the electromagnetic field may interact with other equipment in the welding environment, necessitating careful system design.
Despite these challenges, the potential benefits are significant. For critical applications such as pump impellers, valve components, and mining equipment where wear resistance directly impacts equipment life and maintenance costs, the improved overlay properties achieved through electromagnetic stirring could provide substantial economic benefits. Engineers should consider this technology as part of their process toolbox for surface engineering applications, particularly where conventional methods fail to achieve required performance levels.