Effect of Low-Frequency Longitudinal Magnetic Field on Hardness and Wear Resistance of Weld Overlay

Literature Overview

This study investigates how applying a low-frequency longitudinal magnetic field (LF-LMF) during the weld overlay process influences the microstructure, hardness distribution, and tribological performance of the deposited layer. The research is significant because magnetic field-assisted welding has emerged as a promising non-contact technique to refine grain structure and homogenize the heat-affected zone without introducing external materials or mechanical interference. The work was conducted on a typical medium-carbon steel substrate with a high-chromium cast iron overlay, using submerged arc welding (SAW) as the base process. The magnetic field was applied at frequencies ranging from 1 Hz to 100 Hz, with field intensities between 0.5 mT and 3 mT, aligned longitudinally along the weld axis.

Core Technical Points

The primary mechanism identified is the Lorentz force acting on molten metal in the weld pool when an external magnetic field is superimposed. This force induces micro-convection within the liquid pool, enhancing heat transfer and promoting more uniform cooling rates. The resulting effect is a refinement of the columnar dendrite structure in the overlay layer, a reduction in microsegregation of alloying elements such as Cr and C, and a decrease in the width of the heat-affected zone (HAZ).

Key findings from the study include:

• At an optimal frequency of approximately 10 Hz and a field strength of 1.5 mT, the overlay layer hardness increased by approximately 12% compared to the baseline condition without a magnetic field.
• The Vickers hardness distribution across the overlay layer became more uniform, with reduced variation between the top surface and the fusion boundary.
• Pin-on-disc wear tests demonstrated a 15-18% improvement in wear resistance under dry sliding conditions at a normal load of 10 N.
• Metallographic examination revealed that the dendrite arm spacing decreased by approximately 30%, contributing to finer carbide precipitation and a more homogeneous microstructure.

Process Parameters and Mechanism Analysis

The study employed a systematic approach to determine the optimal magnetic field parameters. The frequency was varied from 1 Hz to 100 Hz while maintaining a constant field intensity, and vice versa. The results showed a non-monotonic relationship between magnetic field parameters and overlay performance, with an optimum existing at moderate frequencies.

At very low frequencies (below 2 Hz), the magnetic field changes too slowly to induce significant Lorentz forces in the rapidly solidifying weld pool. At very high frequencies (above 50 Hz), the skin effect becomes pronounced, and the magnetic field penetration depth into the molten pool is reduced, limiting its effectiveness. The optimal window of 5-20 Hz represents a balance between sufficient electromagnetic stirring and adequate field penetration.

The longitudinal orientation of the magnetic field was found to be superior to transverse or rotating configurations for this specific application. A longitudinal field promotes preferential grain growth in the direction of heat extraction, which is beneficial for reducing transverse cracking susceptibility in the overlay layer. This observation is particularly relevant for thick-section overlay applications where residual stress management is critical.

Engineering Practice Implications

For practical implementation, the study suggests that LF-LMF can be integrated into existing welding production lines with relatively modest capital investment. The magnetic field generator can be designed as a solenoid coil surrounding the welding head, with the welding process parameters (current, voltage, travel speed) remaining unchanged. This makes the technology retrofittable to existing SAW and flux-cored arc welding (FCAW) systems used in overlay applications.

However, several practical considerations must be addressed before widespread adoption. The magnetic field must be carefully shielded to avoid interference with nearby electronic controls and instrumentation. The field strength and frequency must be monitored in real time to ensure consistent results across different production runs. Additionally, the effect of LF-LMF on the mechanical properties of the HAZ and the substrate material requires further investigation, particularly for applications involving dissimilar metal joints.

From a quality assurance perspective, the introduction of a magnetic field parameter adds an additional variable to the welding procedure qualification process. According to NB/T 47014 and ASME IX requirements, the magnetic field parameters must be documented and included in the qualified welding procedure specification (WPS). Non-destructive testing (NDT) methods such as ultrasonic testing (UT) and magnetic particle inspection (MT) must be validated to ensure they are not adversely affected by residual magnetism in the workpiece.

Study Insights and Reflections

This research demonstrates that electromagnetic field manipulation offers a viable pathway to improve overlay layer quality without altering the consumable composition or welding parameters. The concept of "process-aided" microstructure control is particularly attractive for industries where consumable supply chains are constrained or where frequent changes to overlay alloy specifications are impractical. The findings also raise questions about the potential of magnetic field assistance in other welding processes, such as electroslag welding (ESW) overlay and plasma transferred arc (PTA) cladding, where the weld pool dynamics are fundamentally different.

The study's limitation lies in its focus on a single substrate-overlay combination under laboratory conditions. Industrial overlay applications often involve multi-layer deposition, varying substrate geometries, and complex residual stress states that may interact with the magnetic field effects in unpredictable ways. Future research should address multi-pass overlay scenarios and the cumulative effect of LF-LMF on residual stress distribution and distortion.

In conclusion, the application of low-frequency longitudinal magnetic fields to weld overlay processes represents a promising approach to enhancing overlay layer hardness and wear resistance through microstructure refinement. The technology is sufficiently mature for pilot-scale implementation in high-value overlay applications, such as mining equipment components and power plant wear parts, where even modest improvements in service life can yield significant economic returns.