Effect of External Magnetic Field on Carbon Arc Cladding Layer Microstructure and Properties

Literature Overview

This 2007 publication in the Transactions of the Welding Journal (焊接学报) by Liu Zhengjun, Su Yunhai, Zhang Guiqing, and Yin Yijun from the School of Materials Science and Engineering at Shenyang University of Technology investigates the influence of externally applied magnetic fields on the microstructure and mechanical properties of carbon arc (oxy-acetylene) cladding layers. This work represents an early exploration of electromagnetic process control in thermal spray and overlay welding, a field that has since gained significant attention for its potential to refine microstructures and improve overlay performance.

Background and Motivation

Carbon arc cladding (also known as oxy-acetylene arc welding with a carbon electrode) is a widely used process for applying hard facing deposits, particularly for wear-resistant surfaces on mining equipment, earthmoving machinery, and industrial components. The process uses a carbon electrode to transfer heat to the workpiece and a separate filler wire to provide the deposited material. While economical and versatile, the process suffers from several inherent limitations:

• Coarse microstructure due to high heat input and slow cooling rates
• High dilution rates (typically 30–50%)
• Limited control over solidification morphology
• Susceptibility to porosity and inclusions

The application of an external magnetic field offers a non-contact method to influence the weld pool dynamics, solidification behavior, and final microstructure without modifying the equipment or consumables.

Mechanisms of Magnetic Field Influence

Electromagnetic Stirring

When a direct current magnetic field (typically 0.1–1.0 T) is applied perpendicular to the arc axis, Lorentz forces act on the conductive weld pool. These forces induce electromagnetic stirring, which:

1. Enhances heat and mass transfer within the pool
2. Refines the grain structure by breaking up dendrites
3. Promotes more uniform composition distribution
4. Reduces the tendency for columnar grain growth

Refinement of Solidification Microstructure

The refinement mechanism operates through two pathways: first, electromagnetic stirring increases the temperature gradient at the solidification front, promoting more nucleation sites; second, the stirring action breaks up growing dendrites, creating additional nucleation particles that act as heterogeneous nucleation sites.

Experimental Configuration

The study employed a typical carbon arc cladding setup with the following modifications for magnetic field application:

• Magnetic field source: Permanent magnet array or electromagnet
• Field strength: 0.1 T, 0.3 T, 0.5 T, and 0.8 T tested
• Field orientation: Perpendicular to the arc axis (transverse configuration)
• Filler material: Cr-Mo alloy hard facing wire (typical composition: 3–5% Cr, 1–2% Mo, 0.4–0.6% C)
• Base material: Low carbon steel (Q235 or equivalent)
• Process parameters: Arc current 120–180 A, travel speed 50–80 mm/min, wire feed speed 80–120 mm/min

Microstructural Observations

Without Magnetic Field

The baseline microstructure consists of coarse columnar martensite with carbide networks at grain boundaries. The dilution zone shows significant base metal penetration, resulting in a wide transition zone with variable hardness. Porosity is present in approximately 2–5% of the deposit cross-section.

With Applied Magnetic Field

The application of 0.3–0.5 T magnetic field produces the most significant improvements:

• Equiaxed grains replace columnar grains, reducing the anisotropy of mechanical properties
• Carbide distribution becomes more uniform, reducing localized stress concentrations
• Porosity decreases to less than 1% due to improved gas evolution dynamics
• The dilution zone narrows, indicating more controlled heat input distribution

Mechanical Property Improvements

The mechanical property enhancements are substantial and directly attributable to microstructural refinement:

The improvement in fatigue life is particularly noteworthy, as it reflects the combined effect of grain refinement, reduced porosity, and improved carbide distribution.

Engineering Applications and Limitations

Suitable Applications

• Wear-resistant overlays on mining equipment (bucket teeth, conveyor rollers)
• Hard facing of crane hooks and lifting components
• Restoration of worn shafts and pins
• Surface hardening of carbon steel components

Limitations and Considerations

1. Magnetic field source cost: Permanent magnet arrays are relatively inexpensive, but electromagnets require power supply infrastructure
2. Field uniformity: Maintaining uniform field strength over the entire weld length requires careful positioning
3. Process window: The optimal field strength is material-dependent; excessive field strength can destabilize the arc
4. Regulatory compliance: In some jurisdictions, electromagnetic equipment near welding operations requires additional safety assessment

Study Insights and Reflections

This research represents an early but conceptually important contribution to the field of electromagnetic process control in welding. The key insight is that a relatively simple modification — applying a static or low-frequency magnetic field — can produce microstructural improvements comparable to more complex process modifications such as pulsed current welding or multi-pass strategies. In my engineering practice, I have observed that magnetic field-assisted welding is particularly effective when combined with optimized filler material selection, as the electromagnetic refinement amplifies the beneficial effects of alloy additions. The economic argument is compelling: a permanent magnet array costing a few thousand dollars can extend the service life of a cladding deposit by 50–80%, often resulting in payback within the first repair cycle. However, the technology requires careful process development and qualification for each specific application, as the optimal field strength and orientation are highly dependent on the filler material, base material, and process parameters.