Zhuojin Pipe Fitting Co., LtdEffect of External Longitudinal Magnetic Field on Plasma Arc Overlay Microstructure and Properties
Literature Overview
This paper published in the Transactions of the China Welding Institute (2010, Vol. 31, No. 1, pp. 71-74) by Liu Zhengjun, Song Xingkui, Shao Dawei, and Zhao Qian from Shenyang University of Technology investigates the influence of an externally applied longitudinal magnetic field on the microstructure and mechanical properties of plasma arc overlay deposits. The study used a Fe5 iron-based alloy as the overlay material and systematically varied the magnetic field current to determine optimal conditions for enhancing overlay performance.
Core Technical Approach
Experimental Configuration
The researchers applied a longitudinal magnetic field during plasma arc overlay welding of Fe5 alloy powder. The magnetic field was generated by an external electromagnet, with the current varied to produce different field strengths. The overlay deposits were characterized through:
- Hardness testing
- Wear testing
- Microstructural examination (metallography)
- X-ray diffraction analysis
Key Findings
- Overlay deposits produced with longitudinal magnetic field application exhibited significantly higher hardness and wear resistance compared to deposits produced without magnetic field.
- The optimal magnetic field current was 3 A, at which the overlay deposit achieved the best combination of hardness and wear resistance.
- Under magnetic field influence, the α and γ solid solution phases in the overlay were substantially refined.
- Ideal hard phases—Cr₇C₃ and CrB—were obtained in the overlay microstructure under magnetic field conditions.
Interpretation of Technical Points
Magnetic Field Effects on Solidification
The application of a longitudinal magnetic field during solidification of the overlay weld pool influences several metallurgical processes:
- Dendrite growth modification: The magnetic field interacts with the thermoelectric currents generated at the dendrite tips during solidification (thermoelectric magnetic convection, TEMC). This interaction generates Lorentz forces that enhance convective mixing in the weld pool, promoting more uniform composition and finer dendrite spacing.
- Grain refinement: The enhanced convection disrupts the directional solidification pattern, breaking up dendrites and promoting equiaxed grain formation. Finer grains contribute to higher hardness through the Hall-Petch relationship and provide more nucleation sites for secondary phases.
- Hard phase morphology control: The magnetic field influences the morphology and distribution of Cr₇C₃ and CrB hard phases. Without the magnetic field, these phases may form as coarse, irregular particles or networks. With the magnetic field, they tend to form as finer, more uniformly distributed particles, which provide superior wear resistance.
Hardness and Wear Resistance Enhancement
The hardness improvement attributed to magnetic field application likely results from the combined effects of:
- Grain refinement (Hall-Petch strengthening)
- Finer hard phase distribution (more effective dispersion strengthening)
- Reduced porosity and segregation (improved density and uniformity)
The wear resistance improvement is directly related to the hard phase characteristics. Cr₇C₃ is a complex chromium carbide with high hardness (approximately 2000 HV) and excellent thermal stability. CrB is a chromium boride with hardness around 1500 HV. When these phases are finely distributed throughout the matrix, they provide effective resistance to abrasive wear through a combination of hard particle ploughing resistance and matrix support.
Optimal Magnetic Field Current of 3 A
The identification of 3 A as the optimal magnetic field current suggests a non-monotonic relationship between field strength and overlay performance. At low field strengths, the magnetic field effects may be insufficient to significantly modify solidification. At excessive field strengths, adverse effects may occur, such as:
- Arc instability due to Lorentz force deflection of the plasma arc
- Excessive convection leading to incomplete fusion or spatter
- Distortion of the weld pool geometry
The 3 A optimum represents a balance between beneficial solidification modification and avoidance of adverse process effects.
Engineering Practice Integration
| Parameter | No Magnetic Field | With Magnetic Field (3 A) | Improvement |
|---|---|---|---|
| Hardness | Lower baseline | Significantly higher | Substantial increase |
| Wear resistance | Lower baseline | Significantly higher | Substantial increase |
| α/γ grain size | Coarser | Substantially refined | Fine grain structure |
| Cr₇C₃ morphology | Coarse/irregular | Fine/uniform | Optimal dispersion |
| CrB morphology | Coarse/irregular | Fine/uniform | Optimal dispersion |
| Arc stability | Normal | Maintained at 3 A | No degradation |
The magnetic field plasma arc overlay technique has potential applications in:
- Repair of worn pump impellers and valve seats
- Hardfacing of mining equipment components
- Overlay of pipeline components subject to erosion-corrosion
- Surface engineering of turbine components
Key Questions and Reflections
The research raises several important considerations for practical implementation:
- Equipment requirements: Applying an external longitudinal magnetic field during plasma arc overlay requires additional equipment (electromagnet, power supply, field measurement). The cost-benefit analysis must justify this additional complexity for each application.
- Scalability: The laboratory-scale magnetic field application may not be easily scalable to large components or complex geometries. The magnetic field must be uniform across the weld pool, which becomes challenging for large weld areas.
- Process parameter interaction: The optimal magnetic field current may depend on other process parameters (plasma current, powder feed rate, travel speed, gas flow rate). A comprehensive parameter optimization study would be needed for production implementation.
- Microstructural uniformity: The magnetic field effects may vary with welding position and component geometry. A systematic study of field uniformity across different component configurations is necessary.
- Comparison with alternative grain refinement methods: The grain refinement achieved through magnetic field application should be compared with other methods (ultrasonic vibration, electromagnetic stirring, rapid solidification) to determine the relative advantages and disadvantages.
Study Insights and Implications
This research demonstrates that external magnetic field application is a viable non-consumable method for modifying overlay microstructure and enhancing performance. The technique offers several advantages over conventional approaches: it does not require changes to the overlay material composition, it does not introduce additional alloying elements that may have adverse effects, and it can be applied to existing overlay processes with minimal modification. For engineers involved in surface engineering and overlay welding, this approach represents an innovative tool for microstructural control that warrants further investigation for specific industrial applications where the cost of additional equipment can be justified by the performance improvement achieved.