Effect of Transverse Magnetic Field on Iron-Based Wear-Resistant Cladding Layer in Reverse Polarity Plasma Arc Surfacing

Literature Overview

This paper by Ding Chunhui, Ai Xingyu, Liu Zhengjun, and Shao Hui, published in Hot Working Technology in 2019, investigates the influence of a transverse alternating magnetic field applied during reverse polarity plasma arc surfacing (RPAS) on the microstructure and wear resistance of iron-based wear-resistant cladding layers. The study is conducted at the Shenyang Special Equipment Inspection Research Institute and Shenyang University of Technology, combining practical inspection expertise with academic materials science research.

Experimental Design and Parameters

The experimental approach involves applying a transverse alternating magnetic field to the plasma arc during RPAS surfacing. The magnetic field is generated by a coil arrangement positioned to create a field perpendicular to the welding direction. The key experimental parameters and their optimized values are as follows:

The choice of reverse polarity plasma arc surfacing is significant. In RPAS, the workpiece is the cathode and the electrode is the anode. This configuration provides higher energy density at the workpiece surface, deeper penetration, and better dilution control compared to forward polarity. For wear-resistant surfacing applications, RPAS is often preferred because it allows for more controlled dilution of the base metal into the cladding layer, which is critical for maintaining the desired composition of the wear-resistant alloy.

Microstructural Analysis and Mechanism

The key findings of the microstructural analysis reveal that the transverse magnetic field produces several beneficial effects on the cladding layer:

1. Arc stirring effect: The magnetic field induces Lorentz forces on the electrically conductive molten pool, creating electromagnetic stirring that promotes homogeneous mixing of alloying elements and reduces compositional segregation.
2. Grain refinement: The electromagnetic stirring disrupts the directional solidification pattern, promoting equiaxed grain formation and reducing grain size. Finer grains contribute to higher hardness through the Hall-Petch relationship.
3. Increased Cr7C3 precipitate density: The magnetic field promotes the formation of Cr7C3 carbide particles, which increase in both quantity and uniformity. These hexagonal carbide particles are distributed uniformly throughout the cladding layer and provide a "pinning" effect that impedes dislocation motion and crack propagation.

The Cr7C3 carbide is particularly important for wear resistance. As a hard, refractory carbide with a melting point exceeding 2000°C, Cr7C3 provides excellent resistance to abrasive wear. The uniform distribution of these particles, facilitated by the magnetic field, ensures that the wear-resistant properties are consistent throughout the cladding layer rather than being concentrated in localized regions.

Wear Performance Evaluation

The wear test results demonstrate a significant improvement in abrasive wear resistance when the magnetic field is applied at the optimized parameters. The wear weight loss of 0.0524 g represents a substantial reduction compared to cladding layers deposited without the magnetic field. The hardness increase to 61.5 HRC, while not extreme for iron-based wear-resistant alloys, is notable given that the improvement is achieved through microstructural refinement rather than through increasing carbon or chromium content, which could compromise toughness.

This approach is particularly attractive from a practical standpoint because it achieves improved properties through process modification rather than through material composition changes. The same welding consumable can be used, but the process parameters are enhanced by the magnetic field application. This means that existing production setups can be upgraded with magnetic field equipment without requiring changes to material specifications or supply chains.

Engineering Practice Implications

For engineers considering the practical implementation of magnetic field-assisted surfacing, several factors deserve attention. The magnetic field equipment adds cost and complexity to the welding setup, and the optimal parameters are highly dependent on the specific welding process, consumable, and application. The paper's optimized parameters (Im = 2.5 A, f = 40 Hz, I = 160 A) should be regarded as starting points for process development rather than universal values.

The concept of electromagnetic stirring in welding is not new, but its application to surfacing for wear-resistant applications represents a valuable extension. In my experience with welding process development, the most effective improvements often come from understanding the fundamental physics of the process and finding ways to manipulate the process variables to achieve desired metallurgical outcomes. The magnetic field approach is a prime example of this philosophy.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the long-term stability of the Cr7C3 precipitates under thermal cycling or in-service conditions is not addressed. If the carbides undergo coarsening or dissolution during service, the wear resistance improvement may be temporary. Second, the effect of the magnetic field on the weld dilution ratio is not explicitly discussed, which is critical for understanding the actual composition of the cladding layer. Third, the scalability of this technology to larger components or production environments warrants further study.

The use of a transverse rather than axial magnetic field is an interesting choice. A transverse field creates a more complex flow pattern in the molten pool, which may be more effective at disrupting the directional solidification pattern. However, the implementation of a transverse field is mechanically more challenging than an axial field, which may limit its practical application.

Summary

This paper presents a compelling demonstration that the application of a transverse alternating magnetic field during reverse polarity plasma arc surfacing can significantly enhance the microstructure and wear resistance of iron-based wear-resistant cladding layers. The optimized parameters yield a hardness of 61.5 HRC and a wear weight loss of 0.0524 g, with the improvement attributed to grain refinement and increased uniform Cr7C3 carbide precipitation. The approach offers a process-based pathway to improved wear resistance without requiring changes to material composition, making it potentially attractive for industrial implementation where existing consumable supply chains and specifications must be maintained.