CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Iron-Based Wear-Resistant Weld Overlay Under Reversed-Polarity Plasma Arc with Magnetic Field
Literature Overview
This 2019 publication by Ding Chunhui, Ai Xingyu, Liu Zhengjun, and Shao Hui from the Shenyang Special Equipment Inspection Research Institute and Shenyang University of Technology investigates the microstructural evolution and mechanical properties of iron-based wear-resistant weld overlay deposits produced using reversed-polarity plasma transferred arc (PTA) welding under the influence of an applied magnetic field. The study addresses a significant gap in the understanding of how external electromagnetic fields interact with the welding arc and molten pool to influence the solidification behavior and final properties of weld overlay deposits. Wear-resistant overlays are extensively used in mining, cement, power generation, and material handling industries, where components are subjected to severe abrasive and erosive conditions.
Fundamental Principles of Magnetic Field Influence on Welding
The application of a magnetic field during arc welding affects the weld pool through several physical mechanisms:
- Electromagnetic stirring: The interaction between the arc current and the applied magnetic field generates Lorentz forces that induce fluid flow within the molten weld pool. This stirring effect promotes homogeneous mixing of alloying elements and reduces macrosegregation.
- Arc constriction and deflection: The magnetic field modifies the arc shape and energy distribution, potentially increasing arc stability and energy density.
- Solidification modification: Enhanced convection in the weld pool alters the thermal gradient and growth rate, which in turn affects grain morphology, dendrite spacing, and phase distribution.
The following table summarizes the experimental parameters used in this study:
| Parameter | Specification |
|---|---|
| Welding process | PTA (plasma transferred arc) |
| Polarity | Reversed polarity (DCRP) |
| Plasma gas | Argon |
| Arc current | 80-150 A |
| Arc voltage | 22-30 V |
| Travel speed | 200-500 mm/min |
| Powder feed rate | 100-300 g/min |
| Magnetic field strength | 0-500 mT |
| Magnetic field orientation | Parallel and perpendicular to weld direction |
| Powder composition | Fe-based with Cr, Mo, W, C additions |
| Base material | Q235 carbon steel |
Microstructural Analysis
The reversed-polarity PTA process inherently produces a different solidification pattern compared to straight polarity. Under reversed polarity, the workpiece acts as the cathode, resulting in a narrower, deeper weld bead with higher energy density. The application of a magnetic field further modifies the weld pool dynamics.
At a magnetic field strength of 0 mT (no field), the overlay deposit exhibits a typical columnar dendritic microstructure with Widmanstätten ferrite and acicular ferrite in the heat-affected zone. The dendrite arm spacing (DAS) is relatively large, approximately 40-60 μm, indicating a moderate cooling rate.
When a magnetic field of 200-500 mT is applied perpendicular to the weld direction, the Lorentz force induces a rotational flow pattern in the weld pool. This flow breaks up the columnar dendrites and promotes equiaxed grain formation. The DAS decreases to 20-30 μm, and the volume fraction of equiaxed grains increases from less than 10% to over 60%. The refinement of the microstructure is attributed to the enhanced nucleation rate caused by the increased thermal gradient fluctuations and the mechanical detachment of dendrite fragments.
The presence of carbide phases in the iron-based overlay is directly related to the carbon and alloying element content. Under magnetic field influence, the distribution of cementite (Fe3C) and alloy carbides (Cr7C3, Mo2C, W2C) becomes more uniform due to improved mixing. The carbide size decreases from 2-5 μm to 0.5-2 μm, and the volume fraction increases from approximately 15% to 25-30%.
Mechanical Properties and Wear Resistance
The microstructural changes induced by the magnetic field translate into measurable improvements in mechanical properties and wear resistance:
| Property | 0 mT | 200 mT | 500 mT |
|---|---|---|---|
| Hardness (HV0.3) | 480-520 | 540-580 | 560-600 |
| Compressive strength (MPa) | 2200-2400 | 2500-2700 | 2600-2800 |
| Abrasive wear volume loss (mg) | 120-140 | 70-90 | 60-80 |
| Wear rate reduction (%) | 0 (baseline) | 33-43 | 38-50 |
The hardness improvement is attributed to the combined effect of grain refinement, increased carbide volume fraction, and finer carbide distribution. The Hall-Petch relationship predicts that grain refinement alone can account for approximately 30-40 HV of the total hardness increase, with the remaining contribution coming from carbide precipitation strengthening and solid solution strengthening.
The wear resistance improvement of 38-50% under optimal magnetic field conditions represents a significant practical benefit. In industrial applications where wear life directly impacts equipment availability and maintenance costs, such improvements can translate into extended service intervals and reduced total cost of ownership.
Effect of Magnetic Field Orientation
The orientation of the applied magnetic field relative to the weld travel direction has a notable influence on the weld pool dynamics and resulting microstructure. When the magnetic field is applied parallel to the weld direction, the Lorentz force induces a flow pattern that elongates the weld pool in the travel direction, resulting in a flatter bead profile and reduced dilution. When the field is applied perpendicular to the weld direction, the flow pattern is more turbulent, promoting better mixing but potentially increasing spatter and porosity.
The optimal magnetic field orientation for wear-resistant overlay applications is perpendicular to the weld direction at a field strength of 400-500 mT. This configuration maximizes the grain refinement effect while maintaining acceptable weld pool stability and bead geometry.
Engineering Implications and Process Optimization
The findings of this study have direct implications for the design and operation of PTA welding systems used in industrial wear-resistant overlay applications. The integration of a permanent magnet or electromagnetic coil into the welding head is technically feasible and can be achieved with minimal modification to existing equipment. The additional cost of the magnetic field generation system is estimated at 5-10% of the total equipment cost, while the resulting improvement in wear life can exceed 40%.
Process optimization should consider the following factors:
- Magnetic field strength should be maintained within the range of 400-500 mT for optimal microstructural refinement.
- The field orientation should be perpendicular to the weld travel direction to maximize turbulent mixing.
- The powder feed rate should be adjusted upward by 10-15% to compensate for the increased weld pool fluidity under magnetic field influence.
- The travel speed should be reduced by 5-10% to ensure adequate heat input for complete melting and bonding.
Study Insights and Outlook
This research represents a pioneering investigation into the use of external electromagnetic fields to control weld overlay microstructure and properties. The results demonstrate that magnetic field-assisted welding is not merely a theoretical concept but a practically viable technology that can deliver significant improvements in wear resistance. The underlying physics of electromagnetic stirring and its interaction with solidification dynamics provides a powerful tool for tailoring the microstructure of weld overlays to specific service requirements.
Future research should explore the combined effect of magnetic fields with other process variables such as pulsed current, variable polarity, and hybrid welding configurations. Additionally, the long-term service performance of magnetic field-treated overlays under actual industrial conditions should be validated through field trials in mining, cement, and power generation applications. The potential for extending this technology to other cladding processes, such as laser cladding and cold spray, warrants further investigation.