Electromagnetic Force Distribution in Electroslag Strip Electrode Hardfacing of Flat Workpieces

Literature Overview

Published in Welding (2011, Issue 6), this paper by authors from Shanghai Jiao Tong University investigates the electromagnetic force distribution during electroslag strip electrode hardfacing (ESSW) of flat workpieces. The study was supported by the National Natural Science Foundation of China (50975176, 50475021). The authors developed a numerical simulation model to analyze the electromagnetic field and electromagnetic force distribution in the workpiece, with particular attention to the edge effects that cause undercut defects at the weld toes.

Physical Mechanism of Undercut in ESSW

Electroslag strip electrode hardfacing (ESSW) is a high-deposition-rate welding process that uses a wide strip electrode and a consumable or permanent flux to create a molten slag pool through which the welding current passes. The electromagnetic forces generated by the welding current interact with the magnetic field to produce Lorentz forces that act on the molten metal.

In the center of a wide workpiece, the electromagnetic force distribution is relatively symmetric, and the molten metal is constrained by the surrounding solid metal. However, at the edges of the workpiece, the symmetry is broken, and the electromagnetic force distribution becomes asymmetric. This asymmetry results in a net electromagnetic force directed toward the weld toe, creating a strong contraction force that pulls molten metal away from the weld toe region. The result is a characteristic undercut defect at the weld toes.

The paper's key quantitative finding is that the ratio of electromagnetic contraction force on the left and right sides of the weld toe reaches 2.1 times when the weld is positioned at the edge of the workpiece. This significant force imbalance is the root cause of the undercut defect.

Numerical Simulation Results

The authors developed a 2D electromagnetic field model for the flat workpiece and performed numerical simulations under various conditions. The simulation results reveal several important phenomena:

Simulation Condition	Key Finding	Impact on Weld Quality
Weld at workpiece center	Symmetric electromagnetic force distribution	Uniform weld profile
Weld at workpiece edge	Asymmetric electromagnetic force distribution	Undercut at weld toes
Non-uniform electrode voltage	Distorted current and magnetic field distribution	Difficulty in achieving uniform weld profile
Double-sided grounding	Altered current path and electromagnetic force distribution	Weld profile control problems

The electromagnetic force distribution in the workpiece is directly related to the position of the welding electrode relative to the workpiece geometry. When the electrode is positioned at the center of the workpiece, the current flows symmetrically through the workpiece, generating a symmetric magnetic field and electromagnetic force distribution. When the electrode is positioned at the edge, the current path becomes asymmetric, leading to an asymmetric magnetic field and electromagnetic force distribution.

Process Optimization Strategies

Based on the simulation results, several strategies can be employed to mitigate the undercut defect in ESSW:

Electrode positioning: Positioning the electrode slightly inward from the workpiece edge can reduce the asymmetry of the electromagnetic force distribution. However, this approach requires additional welding passes to cover the edge region.
Current distribution control: Using multi-electrode configurations or current shunting can help distribute the current more uniformly across the workpiece, reducing the electromagnetic force asymmetry.
Flux management: Optimizing the flux composition and distribution can help compensate for the electromagnetic force-induced molten metal movement at the weld toes.
Welding parameter optimization: Adjusting the welding current, travel speed, and electrode voltage can help balance the electromagnetic forces and improve the weld profile.
Workpiece geometry modification: For critical applications, modifying the workpiece geometry (e.g., adding backing plates) can help control the current path and electromagnetic force distribution.

Engineering Practice Implications

The findings of this paper have direct implications for the practical application of ESSW in industrial hardfacing operations:

Edge welding: When hardfacing the edges of flat plates or components, the electromagnetic force asymmetry must be accounted for in the process design. Standard welding parameters developed for center-of-plate welding may not be suitable for edge welding.
Multi-pass welding: For wide hardfacing applications, the welding sequence should be planned to minimize the number of edge welds and to position subsequent passes to compensate for any profile irregularities from edge passes.
Quality control: Visual inspection of the weld toes is essential for detecting undercut defects. The characteristic appearance of electromagnetic-force-induced undercut (typically a smooth, gradual depression rather than a sharp notch) can be distinguished from other types of undercut.

The concept of electromagnetic force distribution analysis is also applicable to other welding processes, particularly those involving high current densities and wide weld pools, such as submerged arc welding and electroslag welding for thick plate fabrication.

Key Reflections

This paper provides a valuable theoretical understanding of a common practical problem in ESSW. The numerical simulation approach allows for the systematic investigation of the electromagnetic force distribution under various conditions, which would be difficult to achieve through experimental measurement alone. The quantitative finding of the 2.1× force ratio at the weld toe provides a clear target for process optimization.

The study also highlights the importance of considering the interaction between the welding process and the workpiece geometry. In many industrial hardfacing applications, the workpiece geometry is fixed and cannot be modified. Therefore, the welding process must be adapted to the geometry, rather than the other way around. This requires a deep understanding of the physical mechanisms governing the welding process, as demonstrated in this paper.

One limitation of the study is the use of a 2D model, which may not fully capture the three-dimensional effects of the electromagnetic force distribution. Future work could benefit from 3D modeling to provide more accurate predictions of the weld profile and defect formation.

Conclusion

This paper provides a rigorous numerical analysis of the electromagnetic force distribution in electroslag strip electrode hardfacing, offering valuable insights into the physical mechanisms responsible for undercut defects at weld toes. The quantitative findings and the proposed process optimization strategies provide a solid foundation for improving the quality and reliability of ESSW hardfacing operations. For practitioners in the field, this work underscores the importance of understanding the electromagnetic aspects of welding processes, particularly for edge welding applications where force asymmetries are most pronounced.