Microstructure and Wear Resistance of Ni-Based Plasma Arc Surfacing Layer under DC Transverse Magnetic Field

Literature Overview

This paper by Liu Zhengjun, Li Lecheng, Zong Lin, Su Yunhai, and Su Ming, published in the Welding Journal (Volume 33, Issue 2, 2012, pages 53-56), investigates the effect of a direct current (DC) transverse magnetic field on the microstructure and wear resistance of nickel-based alloy plasma arc surfacing layers deposited on low carbon steel. The research was supported by the Liaoning Provincial Natural Science Foundation (20042025). The study systematically examines the influence of the magnetic field on the hardness, wear resistance, and morphology and distribution of hard phases in the overlay layer. This work explores an innovative approach to enhancing the properties of overlay coatings through the application of external magnetic fields during welding, which has potential applications in various industrial settings where magnetic field equipment is available.

Magnetic Field Effects on Weld Pool Behavior

The application of a DC transverse magnetic field during plasma arc surfacing introduces several physical phenomena that influence the weld pool behavior and, consequently, the microstructure and properties of the overlay layer. The magnetic field exerts a Lorentz force on the moving electrons and ions in the weld pool, which can alter the fluid flow patterns, the heat distribution, and the solidification behavior of the molten metal.

The Lorentz force acts perpendicular to both the direction of current flow and the magnetic field direction. In the case of a DC transverse magnetic field applied to a plasma arc surfacing process, the Lorentz force can induce additional convection currents in the weld pool, which can affect the mixing of the molten metal, the transport of heat, and the distribution of alloying elements. These effects can lead to changes in the microstructure of the overlay layer, including the grain size, the morphology and distribution of hard phases, and the segregation of alloying elements.

The paper investigates the effect of different combinations of welding current and magnetic field current on the properties of the overlay layer. The authors found that the optimal performance was achieved when the welding current was 140 A and the magnetic field current was 2 A. At this combination, the overlay layer exhibited a hardness of 66.3 HRC and a wear rate of 0.0767 g in the pin-on-disk wear test. The microstructural analysis revealed that the hard phases in the overlay layer were most numerous and uniformly distributed under these conditions.

Hard Phase Morphology and Distribution

The microstructural analysis of the overlay layer under different magnetic field conditions revealed significant variations in the morphology and distribution of the hard phases. The hard phases in the Ni-based alloy overlay layer typically include carbides such as Ni₃C, Ni₇C₃, and various chromium and molybdenum carbides, as well as intermetallic compounds such as Ni₃Mo, Ni₄Mo, and other compounds depending on the specific composition of the Ni-based alloy.

Under the optimal magnetic field conditions (welding current 140 A, magnetic field current 2 A), the hard phases were observed to be most numerous and uniformly distributed throughout the overlay layer. The morphology of the hard phases was also refined, with smaller and more regular shapes compared to the overlay layer deposited without a magnetic field. This refinement and uniform distribution of hard phases contribute to the enhanced hardness and wear resistance of the overlay layer.

The mechanism by which the magnetic field improves the hard phase distribution is related to the enhanced fluid flow in the weld pool. The Lorentz force induced by the magnetic field creates additional convection currents that promote the mixing of the molten metal and the transport of alloying elements. This enhanced mixing reduces the segregation of alloying elements and promotes a more uniform nucleation and growth of hard phases throughout the weld pool. Additionally, the magnetic field may influence the solidification behavior of the molten metal, promoting a finer grain structure and a more uniform distribution of the hard phases.

Wear Testing and Mechanical Property Analysis

The wear testing of the overlay layer was conducted using a pin-on-disk tribometer, which is a standard method for evaluating the wear resistance of coatings. The wear rate was measured as the mass loss of the overlay layer under a specified load and sliding distance. The results showed that the overlay layer deposited under the optimal magnetic field conditions exhibited the lowest wear rate, indicating the highest wear resistance.

The hardness testing was conducted using a Rockwell hardness tester, which is a common method for measuring the hardness of coatings. The results showed that the overlay layer deposited under the optimal magnetic field conditions exhibited the highest hardness, which is consistent with the improved wear resistance. The correlation between hardness and wear resistance is well-established in tribology, as harder materials generally exhibit lower wear rates under abrasive wear conditions.

The mechanical property analysis also included tensile testing and impact testing to evaluate the ductility and toughness of the overlay layer. The results showed that the overlay layer deposited under the optimal magnetic field conditions maintained good ductility and toughness, indicating that the enhanced hardness and wear resistance were achieved without a significant loss of mechanical integrity. This is an important finding, as many hard alloy overlays suffer from reduced ductility and toughness, which can lead to cracking and spalling under impact or cyclic loading conditions.

Engineering Practice Implications

For engineers considering the use of magnetic field-assisted plasma arc surfacing, several practical considerations must be addressed. First, the availability and cost of magnetic field equipment must be evaluated. The application of a DC transverse magnetic field requires the installation of electromagnetic coils or permanent magnets around the welding area, which may require modifications to the existing welding setup. The cost of the magnetic field equipment and the associated modifications must be weighed against the benefits of the improved overlay properties.

Second, the magnetic field must be carefully controlled to avoid adverse effects on the welding process. Excessive magnetic field strength can cause arc deflection, which can lead to poor weld quality and inconsistent overlay properties. The paper demonstrates that the optimal magnetic field current is 2 A, which is relatively low and may be easily achieved with commercially available electromagnetic equipment. Engineers should perform a thorough process evaluation to determine the optimal magnetic field parameters for their specific application.

Third, the magnetic field-assisted welding process must be integrated into the existing quality control and inspection procedures. The application of a magnetic field during welding may affect the magnetic properties of the overlay layer, which can influence the results of magnetic particle inspection (MT). Engineers should be aware of this potential interference and develop appropriate inspection procedures to ensure the quality of the overlay welds.

Study Insights and Reflections

This paper demonstrates the potential of magnetic field-assisted plasma arc surfacing to enhance the properties of Ni-based alloy overlay coatings. The application of a DC transverse magnetic field during welding improves the distribution and morphology of hard phases, resulting in enhanced hardness and wear resistance without a significant loss of ductility and toughness. The systematic investigation of the welding current and magnetic field current combinations provides valuable guidance for engineers seeking to optimize the magnetic field-assisted welding process.

One reflection from this study is the importance of process optimization in achieving the desired properties in overlay coatings. The optimal performance was achieved only at a specific combination of welding current and magnetic field current, highlighting the need for careful process development and control. Engineers should invest in the necessary process development and optimization efforts to achieve the best results from magnetic field-assisted welding.

Another reflection is the potential for further research in this area. The study opens up several avenues for future investigation, including the use of different magnetic field configurations (e.g., longitudinal, axial, or rotating magnetic fields), the application of magnetic fields to other welding processes (e.g., laser cladding, electron beam welding), and the investigation of the long-term performance of magnetic field-assisted overlay coatings under cyclic loading and thermal cycling conditions.

Summary

This paper investigates the effect of a DC transverse magnetic field on the microstructure and wear resistance of Ni-based alloy plasma arc surfacing layers deposited on low carbon steel. The authors demonstrate that the application of a magnetic field during welding improves the distribution and morphology of hard phases, resulting in enhanced hardness (66.3 HRC) and wear resistance (wear rate 0.0767 g) at optimal conditions (welding current 140 A, magnetic field current 2 A). The study provides valuable insights for engineers considering the use of magnetic field-assisted welding to enhance overlay coating properties, while also highlighting the need for careful process optimization and integration into existing quality control procedures.