Effect of External Magnetic Field on Microstructure and Properties of Carbon Arc Surfacing Deposits

Literature Overview

This paper by Liu Zhengjun and colleagues from Shenyang University of Technology, published in the Transactions of the China Welding Institution (Vol. 28, No. 8, 2007), investigates the influence of a DC transverse magnetic field applied during carbon arc surfacing of a Cr-B-Ni-V iron-based alloy. The study addresses a practical engineering challenge: how to refine the microstructure of hardfacing deposits and control the morphology and distribution of hard phases without altering the base alloy composition or welding consumable. The authors systematically varied magnetic field intensity and surfacing process parameters, then evaluated hardness, wear resistance, and microstructural evolution of the resulting deposit layers.

Core Technical Findings

The central finding is that applying a DC transverse magnetic field during carbon arc surfacing yields deposits with higher hardness and superior abrasive wear resistance compared to deposits produced without any magnetic field. The optimal combination identified in the study is a magnetic field current of 3 A, a surfacing current of 180 A, and a travel speed of 12 cm/min. Under these matched conditions, the deposit hardness reaches its maximum, wear resistance is best, and the hard phases appear fine, uniformly distributed, and exhibit a distinctive hexagonal morphology with a consistent orientation.

Key Process Parameters and Outcomes

Interpretation of the Magnetic Field Mechanism

The application of an external magnetic field during solidification influences the growth of hard carbide and boride phases through magnetohydrodynamic (MHD) effects and magnetic force interactions with the dendritic solidification front. In a Cr-B-Ni-V system, the hard phases are predominantly Cr7C3, CrB, and VB-type carbides and borides. The transverse DC magnetic field generates Lorentz forces on the moving electrons and ions in the molten pool, which can suppress dendrite branching and promote equiaxed grain formation. More importantly, the magnetic field exerts an orientation force on anisotropic crystal phases, causing the hexagonal hard phases to align in a preferred direction. This alignment enhances the mechanical integrity of the deposit by creating a more coherent microstructure.

The requirement that the magnetic field parameter must be matched with the welding process parameters is a critical insight. An excessive magnetic field current relative to the welding current and travel speed can cause arc instability and spatter, while an insufficient field fails to exert meaningful influence on the solidification front. The optimal 3 A magnetic field current represents a balance where the magnetic force is sufficient to influence phase morphology without disrupting the arc.

Engineering Practice Implications

From a practical standpoint, this research has direct relevance to the repair and hardfacing of components in mining, power generation, and material handling industries where abrasive wear is a dominant failure mechanism. Carbon arc surfacing is widely used for field repairs because it is portable and does not require shielding gas. The addition of a simple DC magnetic field apparatus—essentially a pair of electromagnets positioned on either side of the weld zone—can significantly improve the performance of the hardfacing deposit without changing the consumable or the welding technique.

However, several practical considerations must be addressed before implementation. The magnetic field source must be designed to avoid interference with the carbon arc itself, which is inherently sensitive to electromagnetic disturbances. The positioning of the magnets must be carefully calibrated to ensure a uniform transverse field across the weld zone. Additionally, the magnetic field must be turned off before the deposit solidifies completely to prevent residual magnetic effects on the base material.

Key Questions and Reflections

One question that arises from this study is whether the magnetic field effect is limited to carbon arc surfacing or can be extended to other surfacing processes such as plasma arc surfacing or submerged arc surfacing. The fundamental mechanism—magnetic force interaction with the solidification front—should be process-independent, but the practical implementation may differ. Another important consideration is the scalability of this technique. The study was conducted on laboratory-scale specimens, and the transition to large-scale industrial hardfacing of components such as mill rolls, grinding rolls, and bucket teeth requires careful engineering of the magnetic field apparatus to ensure uniform field coverage over the entire weld length.

The hexagonal morphology of the hard phases under the magnetic field is particularly intriguing. In conventional hardfacing, the hard phases tend to be randomly oriented, which can lead to anisotropic wear behavior. The aligned hexagonal morphology observed in this study may provide a directional wear resistance advantage, which could be exploited in applications where the wear direction is predictable.

Study Insights and Outlook

This research demonstrates a novel and relatively simple approach to improving the performance of hardfacing deposits through magnetic field-assisted solidification. The key insight is that the magnetic field does not merely refine the grain structure but also controls the morphology and orientation of the hard phases, which is the primary determinant of wear resistance in iron-based hardfacing alloys. The matched-parameter concept—where the magnetic field strength must be optimized in conjunction with the welding parameters—is a valuable engineering principle that emphasizes the interdependence of physical phenomena in the welding process. Future work should explore the application of pulsed magnetic fields, higher field intensities, and multi-axis field configurations to further optimize the microstructure and properties of hardfacing deposits.