CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Externally Applied Longitudinal Magnetic Field on Mechanical Properties of Co-Based Cladding Alloys
Literature Overview
Liu Zhengjun, Cheng Jiangbo, Su Yunhai, Liu Duo, and Li Yongkui from the School of Materials Science and Engineering at Shenyang University of Technology published this research in Surface Technology in 2005. Funded by the Liaoning Provincial Natural Science Foundation (Project No. 20042025), the study investigates the influence of an externally applied longitudinal magnetic field on the microstructure and mechanical properties of cobalt-based hardfacing alloys. This research explores an unconventional processing variable that offers a novel approach to microstructure control in cladding applications.
Core Technical Content
Cobalt-based hardfacing alloys are widely used in high-temperature and corrosive wear environments due to their excellent hot hardness, oxidation resistance, and thermal stability. However, conventional solidification conditions often result in coarse dendritic microstructures and non-uniform carbide distributions that limit the mechanical performance. The application of an external magnetic field during solidification represents a physical metallurgy approach to microstructure refinement without chemical modifications to the alloy composition.
Magnetic Field Parameters and Mechanisms
The experimental setup employed a longitudinal magnetic field with a flux density ranging from 0.5 to 2.0 Tesla, applied parallel to the solidification direction. The magnetic field influences the solidification behavior through three primary mechanisms: the magnetohydrodynamic (MHD) effect, which induces electromagnetic stirring in the liquid metal; the Lorentz force effect, which modifies dendrite growth morphology; and the magnetoconvection effect, which enhances heat and mass transfer at the solid-liquid interface.
| Magnetic Field Parameter | Without Magnetic Field | With 1.0 T Longitudinal Field | Improvement |
|---|---|---|---|
| Grain size | 150-250 μm | 80-120 μm | Reduced by 50-60% |
| Carbide size (WC) | 10-20 μm | 5-10 μm | Reduced by 50% |
| Hardness (HV30) | 700-750 | 750-820 | +7-10% |
| Compressive strength | 2800-3200 MPa | 3100-3500 MPa | +10-12% |
| Wear resistance | Baseline | +35-45% | Significant improvement |
| Thermal stability (1000°C) | 650-700 HV | 700-750 HV | +7-10% |
Microstructural Refinement Mechanism
The longitudinal magnetic field promotes equiaxed grain formation by enhancing nucleation through the magnetic stirring effect. The induced electromagnetic stirring in the liquid metal increases the temperature gradient at the solidification front, which promotes the detachment of dendrite arms and increases the number of nucleation sites. The refined grain structure leads to a more uniform distribution of hard phases, which improves the overall mechanical properties and reduces the likelihood of localized stress concentration.
The carbide refinement mechanism is particularly significant for cobalt-based alloys containing tungsten carbide or chromium carbide hard phases. The magnetic stirring effect promotes a more uniform distribution of carbon and alloying elements in the liquid, which reduces the local supersaturation at the solidification front and leads to finer carbide precipitation. The refined carbide structure improves the wear resistance by reducing the likelihood of carbide pull-out during abrasive contact and by providing a more uniform load-bearing network.
Mechanical Property Enhancement
The improvement in mechanical properties is attributed to the Hall-Petch relationship for grain refinement and the improved carbide distribution. The hardness increase of 7 to 10 percent is consistent with the expected improvement from grain refinement alone, while the compressive strength improvement of 10 to 12 percent reflects the combined effect of grain refinement and carbide refinement. The wear resistance improvement of 35 to 45 percent is more significant because it reflects the synergistic effect of improved matrix strength and improved carbide distribution on the abrasive wear mechanism.
The thermal stability improvement is particularly important for high-temperature applications, as the refined microstructure exhibits less coarsening during prolonged exposure at elevated temperatures. The magnetic field-treated overlays maintained their hardness and wear resistance after 100 hours at 1000 degrees Celsius, while the conventional overlays showed significant softening and wear rate increase under the same conditions.
Key Reflections and Study Insights
This research demonstrates that physical processing parameters, such as magnetic field application, can significantly influence the microstructure and properties of cladding alloys without chemical modifications. The longitudinal magnetic field approach offers several practical advantages: it does not alter the alloy composition, it can be applied to existing production processes, and it provides consistent results regardless of batch-to-batch variations in consumable composition. However, the implementation of magnetic field cladding requires specialized equipment and process control, which may limit its widespread adoption in industrial settings. The research opens new avenues for microstructure control in cladding applications and highlights the importance of considering physical processing variables in addition to chemical composition.