Zhuojin Pipe Fitting Co., LtdEffect of External Longitudinal Magnetic Field Current on Microstructure and Wear Resistance of Plasma Arc Overlay Ceramic-Phase Reinforced Iron-Based Alloy
Literature Overview
This paper, published in Materials Protection (Vol. 46, No. 8, 2013, pp. 28–29) by Zong Lin and Liu Zhengjun from Shenyang University of Chemical Technology and Shenyang University of Technology, investigates the influence of an externally applied longitudinal magnetic field on the microstructure and wear resistance of in-situ synthesized ceramic-phase reinforced iron-based alloy overlay layers deposited by plasma arc surfacing on 20G carbon steel. The work is supported by the Liaoning Provincial Department of Education (L2012152) and explores an unconventional but promising approach to microstructure control in overlay welding.
Core Technical Findings
The study applied longitudinal magnetic field currents of varying magnitudes during plasma arc surfacing and evaluated the resulting overlay microstructure, hardness, and wear resistance. The key findings are summarized below:
| Magnetic Field Current | M₇C₃ Phase Content | Grain Size | Hardness | Wear Resistance |
|---|---|---|---|---|
| 0 A (no field) | Moderate | Coarse | Baseline | Baseline |
| 1.0 A | Increased | Refined | Improved | Improved |
| 2.0 A | Maximum (hexagonal, uniform) | Finest | Highest | Best |
| >2.0 A | Decreased | Coarsened | Decreased | Decreased |
The optimal magnetic field current of 2.0 A produced the highest volume fraction of hexagonal M₇C₃ ceramic hard phases, uniformly distributed throughout the overlay microstructure. Beyond this optimum, the performance declined due to electromagnetic damping effects becoming dominant.
Microstructural Mechanism
The application of a longitudinal magnetic field during plasma arc surfacing influences the solidification behavior of the overlay through several mechanisms:
- Magnetic force on moving charges: The Lorentz force acts on ionized particles in the molten pool, inducing forced convection that enhances heat and mass transfer.
- Grain refinement: The enhanced convection disrupts the natural columnar dendrite growth pattern, promoting equiaxed grain nucleation and growth.
- Carbide morphology control: The magnetic field influences the nucleation and growth of M₇C₃ carbides, promoting the formation of hexagonal morphology at the optimal current of 2.0 A.
Wear Mechanism Analysis
The superior wear resistance at 2.0 A magnetic field current is attributed to the combined effect of:
- Higher volume fraction of hard M₇C₃ ceramic phases, which resist abrasive ploughing and cutting.
- Uniform distribution of hard phases, preventing localized stress concentration and preferential wear.
- Finer grain size, which increases the number of grain boundaries and impedes crack propagation.
Process Analysis and Optimization
Plasma Arc Surfacing Parameters
Plasma arc surfacing is characterized by high energy density, narrow weld bead, and low dilution with the base metal. For 20G carbon steel substrate, typical parameters include:
| Parameter | Typical Range |
|---|---|
| Plasma current | 150–250 A |
| Arc voltage | 20–30 V |
| Travel speed | 100–300 mm/min |
| Shielding gas | Argon (10–20 L/min) |
| Wire feed rate | 100–200 mm/min |
The application of a longitudinal magnetic field requires additional equipment, including a DC power supply for the magnetic coil and a coil configuration that generates a uniform field along the welding direction. The magnetic field strength at the weld pool should be carefully controlled to achieve the optimal 2.0 A current without introducing electromagnetic interference with the plasma arc.
Electromagnetic Damping Effect
The decline in performance at magnetic field currents above 2.0 A is attributed to electromagnetic damping, which reduces the intensity of forced convection in the molten pool. At excessive field strengths, the Lorentz force opposes the natural fluid flow patterns, leading to reduced heat transfer and coarser solidification microstructures. This finding highlights the existence of an optimal window for magnetic field application, consistent with the general principle in welding that excessive process parameter values often lead to degraded performance.
Engineering Practice Integration
Application Scenarios
Ceramic-phase reinforced iron-based overlay alloys are widely used in applications requiring resistance to abrasive wear, including:
- Mining equipment components (shovels, bucket teeth, conveyor rollers)
- Cement mill internals (grinding rollers, mill liners)
- Coal handling equipment (chutes, hoppers, transfer points)
- Agricultural machinery (plowshares, harrow discs)
The application of magnetic field-assisted plasma arc surfacing could enhance the performance of these overlays by producing finer, more uniformly distributed ceramic phases.
Quality Control Considerations
When implementing magnetic field-assisted overlay welding, the following quality control measures should be considered:
- Magnetic field calibration: The magnetic field current should be precisely controlled and monitored during welding to ensure consistent microstructure.
- Microstructural verification: Metallographic examination of test coupons should be performed to confirm the presence and distribution of M₇C₃ phases.
- Hardness testing: Surface hardness should be measured at multiple locations on the overlay to verify uniformity and target values.
- Wear testing: Pin-on-disk or sand rubber abrasion tests should be conducted on representative samples to validate wear resistance improvements.
Study Insights and Reflections
This research demonstrates that external electromagnetic fields can be effectively used to control the solidification microstructure of overlay welds, offering a non-thermal means of microstructure refinement. The identification of an optimal magnetic field current of 2.0 A, beyond which performance degrades due to electromagnetic damping, provides a clear process window for practical implementation.
The finding that hexagonal M₇C₃ morphology is promoted at the optimal field strength is particularly interesting from a materials science perspective. The crystallographic orientation of carbide phases can significantly influence mechanical properties, and the magnetic field may preferentially stabilize certain crystallographic orientations during nucleation. Further research into the crystallographic texture of the M₇C₃ phases under magnetic field influence would provide deeper insight into the mechanism.
From an engineering economics perspective, the additional equipment and process complexity required for magnetic field application must be weighed against the performance improvement achieved. For high-value components where overlay performance directly impacts production efficiency and maintenance costs, the investment in magnetic field-assisted surfacing may be justified. For lower-value applications, conventional plasma arc surfacing without magnetic field assistance may be sufficient.
Reference Value and Outlook
This paper opens an interesting avenue for microstructure control in overlay welding through electromagnetic field application. The systematic investigation of magnetic field current effects provides a foundation for further optimization and scale-up. Future research could explore the combination of magnetic field application with other microstructure control techniques, such as ultrasonic assistance or electromagnetic stirring, to achieve synergistic effects. The principles demonstrated in this work may also be applicable to other welding processes and alloy systems, potentially offering a universal approach to solidification microstructure control.