Effect of Hardfacing Speed on Microstructure and Properties Under Transverse Magnetic Field

Literature Overview

The 2010 paper by Feng Lifeng, Liu Ke, Su Yunhai, and Liu Zhengjun, published in Welding Technology (Vol. 39, No. 5, pp. 24-26), investigates the influence of hardfacing speed on the microstructure and mechanical properties of iron-based alloy carbon arc hardfacing deposits when a DC transverse magnetic field is applied. Supported by the Liaoning Provincial Natural Science Foundation (Project 20042025), this research explores the interaction between external magnetic fields and the welding arc/melt pool to achieve microstructural refinement and improved wear resistance.

Fundamental Mechanisms of Magnetic Field Effect

The application of a transverse DC magnetic field during welding introduces Lorentz force effects on the molten metal and the plasma arc. The key physical mechanisms include:

• Arc deflection and concentration: The magnetic field causes the arc to deflect and concentrate, increasing the energy density and altering the heat input distribution
• Melt pool convection enhancement: Lorentz forces drive additional convection currents in the molten pool, promoting more uniform temperature distribution and finer grain structure
• Solidification microstructure refinement: Enhanced cooling rates and modified nucleation conditions result in finer grain structures
• Hard phase morphology control: The magnetic field influences the shape, size, and distribution of carbide and other hard phases within the deposit

Experimental Parameters and Results

The study systematically varied hardfacing speed and magnetic field current to determine the optimal combination for maximizing deposit properties. The key findings are summarized below:

The optimal conditions of 12 cm/min hardfacing speed with 3A magnetic field current produced the highest hardness (54.4 HRC) and lowest wear loss (0.0335 g), representing a significant improvement over the baseline condition without magnetic field application.

Process-Property Relationships

The study reveals a critical interaction between hardfacing speed and magnetic field intensity:

1. Low speed with strong field: Excessive cooling rate can lead to microcracking and poor wetting
2. High speed with weak field: Insufficient heat input results in poor fusion and incomplete hard phase formation
3. Optimal speed-field combination: Balanced cooling rate promotes fine grain structure and uniform hard phase distribution

The magnetic field's influence on hard phase morphology is particularly significant. In iron-based hardfacing alloys, the primary hard phases are typically M₇C₃ carbides and other transition metal carbides. The magnetic field application was found to:

• Reduce the size of primary carbides
• Promote more uniform spatial distribution of hard phases
• Suppress the formation of continuous carbide networks at grain boundaries
• Enhance the matrix-hard phase bonding interface quality

Engineering Application Considerations

For steel pipe manufacturing and pipe fitting production, the magnetic field-assisted hardfacing technique offers several potential applications:

• Roll surface hardfacing: In pipe rolling mills, the roll surfaces experience extreme thermal and mechanical loading. Magnetic field-assisted hardfacing could produce finer, more uniform hardfacing layers with improved wear resistance
• Pipe joint repair: Field repair of worn pipe components where conventional hardfacing may not achieve adequate performance
• Tooling surface treatment: For precision forming dies and mandrels used in pipe fitting production, the microstructural refinement achieved through magnetic field application could improve dimensional stability and surface finish quality

The technique requires additional equipment for magnetic field generation, which adds complexity and cost. However, the demonstrated improvements in hardness (approximately 9% improvement over baseline) and wear resistance (approximately 39% reduction in wear loss) may justify the additional investment for critical applications.

Study Insights and Reflections

This research demonstrates that external physical field manipulation during welding can serve as a powerful tool for microstructural control without requiring changes to the base material composition or welding consumables. The finding that optimal results require careful matching of hardfacing speed and magnetic field intensity highlights the importance of process parameter optimization in advanced welding techniques.

From a practical standpoint, the technique's applicability in industrial settings depends on the feasibility of maintaining stable magnetic field conditions during production welding. The relatively modest magnetic field currents used (3A) suggest that the equipment requirements may be manageable for production applications. The wear testing methodology and the systematic parameter variation approach provide a rigorous framework that can be adapted for evaluating other process modification techniques in steel pipe manufacturing environments.

The work also underscores a broader principle in welding metallurgy: that solidification conditions—cooling rate, thermal gradient, and convection patterns—exert profound influence on deposit microstructure and properties. The magnetic field serves as one means of controlling these conditions, and understanding the underlying mechanisms enables engineers to predict and optimize outcomes for specific applications.