Effect of Magnetic Field Control on Microstructure and Properties of Plasma Arc Overlay Weld Metal

Overview and Research Significance

The study by Liu Zhengjun and Sun Jinggang (2009), published in the Journal of Shenyang University of Technology, investigates the influence of longitudinal intermittent alternating magnetic field application on the microstructure and mechanical properties of plasma arc overlay weld metal deposited on low carbon steel using Fe5 alloy powder. Funded by the Liaoning Provincial Natural Science Foundation, this work represents a sophisticated application of electromagnetic process control to improve overlay weld quality. The findings are directly applicable to engineers seeking to enhance the wear resistance and mechanical properties of overlay deposits in piping, pressure equipment, and heavy machinery applications.

Plasma arc surfacing (PAS) is widely used for depositing hardfacing alloys on base materials that require enhanced wear resistance. However, the microstructure and properties of plasma arc overlay deposits are sensitive to cooling rate, solidification morphology, and phase distribution, all of which are influenced by the thermal history of the deposit. Magnetic field application offers a non-thermal means of influencing solidification behavior and microstructure development.

Theoretical Basis of Magnetic Influence on Solidification

Lorentz Force and Molten Pool Dynamics

The application of an alternating magnetic field to the plasma arc welding zone generates Lorentz forces within the electrically conducting molten pool. These forces influence:

• Convection patterns within the molten pool
• Temperature distribution and uniformity
• Solidification front stability
• Grain growth direction and morphology

Crystal Growth Under Magnetic Field

During solidification, the magnetic field interacts with the moving solidification front in several ways:

1. The field suppresses constitutional supercooling by enhancing convective mixing, promoting planar solidification over cellular or dendritic growth
2. The field exerts directional forces on growing crystal faces, potentially aligning grain growth in preferred orientations
3. The field influences the nucleation rate and distribution of secondary phases through its effect on solute partitioning

The combination of these effects can produce refined grain structures, controlled phase distribution, and improved mechanical properties without altering the alloy composition or thermal parameters of the welding process.

Experimental Configuration

Magnetic Field Parameters

The study employed a longitudinal intermittent alternating magnetic field, with the following characteristics:

Welding Configuration

• Base material: Low carbon steel
• Consumable: Fe5 alloy powder (iron-based hardfacing alloy)
• Process: Plasma arc surfacing (PAW)
• Powder feeding: Central or lateral (typical of PAS)

Characterization Methods

• Optical metallography for microstructure observation
• X-ray diffraction (XRD) for phase identification
• Microhardness testing for hardness mapping
• Wet sand rubber wheel wear testing for wear resistance evaluation

Key Results and Analysis

Microstructural Refinement

The application of appropriate magnetic field parameters produced significant grain refinement in the overlay weld metal. The refined microstructure is attributed to:

1. Enhanced convection within the molten pool, which increases the effective cooling rate at the solidification front
2. Suppression of dendrite arm coarsening through continuous thermal disturbance
3. Promotion of equiaxed grain nucleation through improved thermal uniformity

Hard Phase Distribution

A particularly significant finding was the ability of the magnetic field to increase the quantity of hard phases within the overlay deposit and to control their growth direction. The hard phases in Fe5-type alloys typically include carbides (Fe3C, M7C3) and intermetallic compounds. The magnetic field influence on these phases manifests as:

• Increased volume fraction of carbide precipitates
• More uniform spatial distribution of hard phases
• Controlled orientation of elongated carbide particles
• Reduced coarsening of carbide particles during solidification

Hardness Enhancement

Microhardness testing confirmed that magnetic field application increased the hardness of the overlay deposit. The hardness improvement is attributed to the combined effects of grain refinement (Hall-Petch strengthening), increased hard phase volume fraction, and more uniform phase distribution.

Wear Resistance Improvement

Wet sand rubber wheel wear testing demonstrated enhanced wear resistance under magnetic field-assisted conditions. The wear resistance improvement correlates with the microstructural changes: refined grains provide more slip system resistance, increased hard phase content provides abrasion resistance, and controlled phase orientation prevents preferential wear along weak interfaces.

Process Optimization

The study identified optimal magnetic field parameters that maximize the beneficial effects on microstructure and properties. The optimization involves balancing:

1. Field strength: Sufficient to generate meaningful Lorentz forces without causing arc instability
2. Field frequency: Appropriate for the solidification timescale of the specific alloy system
3. Field duration pattern: Intermittent application that provides periodic influence without excessive energy input
4. Field orientation: Longitudinal alignment that influences grain growth along the critical direction

The optimization is process-specific and must be conducted for each combination of base material, consumable alloy, and welding parameters.

Engineering Application Considerations

For engineers applying magnetic field-assisted plasma arc surfacing in industrial settings, the following considerations are important:

Equipment Requirements

1. Magnetic field generation system capable of producing controlled longitudinal alternating fields
2. Integration with existing plasma arc welding equipment
3. Field monitoring and control system for consistent application
4. Shielding considerations to prevent interference with nearby equipment

Process Integration

The magnetic field application must be synchronized with the welding parameters to ensure consistent influence throughout the deposit build-up. For multi-pass deposits, the field parameters may need adjustment for each pass to account for the changing thermal conditions as the deposit thickness increases.

Quality Verification

Post-weld verification should include:

• Metallographic examination to confirm grain refinement and phase distribution
• Hardness mapping to verify property improvement
• Wear testing (if applicable) to confirm functional performance
• Comparison with non-magnetic-field baseline deposits for quantification of improvement

Code and Standards Compliance

For critical applications, the magnetic field parameters must be incorporated into the qualified welding procedure specification. The field parameters should be classified as essential or supplemental variables depending on their demonstrated influence on mechanical properties. ASME Section IX and ISO 15614 provide frameworks for this classification.

Critical Reflection

This study demonstrates that electromagnetic process control can produce meaningful improvements in overlay weld quality without altering the fundamental welding parameters or consumable composition. The approach is particularly attractive because it offers a non-invasive means of microstructure control that does not require changes to equipment design, consumable procurement, or operator training beyond the magnetic field system integration.

The intermittent alternating field configuration is cleverly chosen to avoid magnetic saturation while providing dynamic influence on the solidification process. This is a significant practical advantage over DC magnetic fields, which can cause saturation in ferromagnetic materials and produce unidirectional forces that may destabilize the arc.

However, the study's scope is limited to a single alloy system (Fe5 on low carbon steel) under specific welding conditions. The generalizability of the findings to other alloy systems, base materials, and welding configurations requires further investigation. Additionally, the long-term stability of the microstructure under service conditions (thermal cycling, mechanical loading) has not been evaluated.

Conclusion

The application of longitudinal intermittent alternating magnetic field to plasma arc overlay welding represents a sophisticated and effective approach to microstructure control and property enhancement. By influencing convection patterns, solidification morphology, and phase distribution without altering thermal parameters or alloy composition, magnetic field control offers a pathway to improved overlay performance that is complementary to traditional optimization approaches. For engineers in the piping and pressure equipment industry, this technology provides an additional tool for achieving demanding performance requirements in overlay applications, particularly where wear resistance and mechanical property uniformity are critical. The work reinforces the principle that process control extends beyond thermal and geometric parameters to encompass electromagnetic influences on the fundamental metallurgical processes governing deposit quality.