Effect of Low-Frequency Magnetic Field on Submerged Arc Surfacing Microstructure and Performance of Rolling Mill Rolls

Literature Overview

This study by Chang Yunlong and colleagues from Shenyang University of Technology and Dalian Heavy Industry Group investigates the influence of externally applied low-frequency pulsed longitudinal magnetic fields on the microstructure and mechanical properties of submerged arc surfacing (SAS) deposits on rolling mill rolls. Published in China Surface Engineering (2011, Vol. 24, No. 6, pp. 98-103), the work addresses a practical metallurgical challenge: how to improve the hardness and grain refinement of surfacing layers without altering the base alloy composition or significantly changing the welding consumable specification. The research is funded by Shenyang Talent Program (2008030103055) and Liaoning Provincial Department of Education Key Laboratory Project (2009S072), reflecting the industrial relevance of hardfacing technology in heavy equipment manufacturing.

Core Technical Approach and Experimental Design

The experimental methodology involves applying a low-frequency pulsed longitudinal magnetic field during the submerged arc surfacing process on rolling mill roll surfaces. The magnetic field parameters are systematically varied, with the optimal condition identified at pulse current Ip = 300 A, bias current Ib = 200 A, and frequency f = 5 Hz. The surfacing layer is analyzed through hardness testing (Rockwell C scale) and metallographic examination of surface specimens.

The underlying physical mechanism proposed is electromagnetic stirring of the molten pool. When the longitudinal magnetic field interacts with the electric current flowing through the arc and molten weld pool, a Lorentz force is generated that induces fluid motion within the melt. This electromagnetic stirring effect alters the nucleation and growth kinetics of crystallization, thereby refining the grain structure of the solidified surfacing layer.

Key Technical Findings and Quantitative Results

The most significant finding is that at the optimal magnetic field parameters, the grain size is reduced to 30.1 μm, representing a substantial refinement compared to the baseline condition. The corresponding hardness improvement of 4.5 HRC, while modest in absolute terms, is practically meaningful for rolling mill roll applications where even small increments in surface hardness translate to extended service life under severe abrasive and adhesive wear conditions.

Mechanism Analysis: Electromagnetic Stirring and Grain Refinement

The mechanism can be understood through the Hall effect and Lorentz force interactions. The longitudinal magnetic field, combined with the arc current, generates a rotational component in the arc column itself. This arc rotation induces electromagnetic stirring in the molten pool, which has several metallurgical consequences:

1. Enhanced nucleation: The fluid motion disrupts the thermal gradient at the solidification front, promoting heterogeneous nucleation events and increasing the number of active nucleation sites.
2. Suppressed grain growth: The convective stirring breaks up growing dendrites and columnar grains, limiting their longitudinal development and promoting a more equiaxed microstructure.
3. Modified heat transfer: The electromagnetic stirring alters the heat dissipation pattern within the weld pool, creating conditions favorable for finer grain formation.

This mechanism is consistent with established theories of electromagnetic solidification, where the interaction between magnetic fields and electrically conductive molten metals produces controlled fluid flow that influences solidification morphology.

Engineering Practice Implications

For rolling mill roll manufacturers, this technology offers a non-consumable modification to the surfacing process. The magnetic field apparatus does not require changes to the surfacing alloy, flux composition, or welding parameters beyond the addition of the external field generator. This makes the technology retrofittable to existing production lines with relatively modest capital investment.

However, several practical considerations must be addressed before industrial implementation:

• The magnetic field apparatus must be compatible with the existing roll surfacing equipment geometry and access constraints.
• The pulsed nature of the field requires careful synchronization with the welding travel speed and wire feed rate to ensure consistent application across the entire surfacing track.
• The 5 Hz frequency represents a low-frequency regime, which may impose specific requirements on the power supply design and magnetic coil configuration.
• Field testing under actual rolling mill operating conditions is essential to validate the laboratory results, as the thermal cycling and mechanical loading during production may affect the long-term stability of the refined microstructure.

Critical Reflections and Study Insights

From a metallurgical perspective, this work demonstrates that process-side modifications can achieve meaningful microstructural improvements without altering the fundamental alloy chemistry. This philosophy is particularly attractive in industrial hardfacing applications where consumable cost is a major economic driver and alloy changes may require extensive requalification.

The 4.5 HRC improvement, while not dramatic, should be contextualized within the typical hardness ranges of rolling mill roll surfacing layers. For high-alloy carbide-containing surfacing alloys operating in the 45-55 HRC range, a 4.5 HRC increase represents approximately a 10% relative improvement, which could translate to 15-20% extension in roll service life depending on the wear mechanism dominance.

One area that warrants further investigation is the interaction between the magnetic field parameters and multi-pass surfacing. The study appears to focus on single-pass or limited-pass conditions, but industrial rolling mill rolls typically require multiple surfacing layers. The cumulative effect of electromagnetic stirring across successive passes, and its interaction with the interpass temperature, remains an open question that could significantly impact the practical adoption of this technology.

The research also raises interesting questions about the optimal magnetic field waveform. The use of pulsed rather than DC magnetic fields suggests that the time-varying nature of the field may provide additional benefits through dynamic arc manipulation. Further parametric studies exploring waveform shapes, duty cycles, and phase relationships between the magnetic field and the arc current could reveal additional optimization opportunities.

Reference Value and Outlook

This study contributes a valuable data point to the growing body of knowledge on electromagnetic processing of welds. The systematic identification of optimal magnetic field parameters provides a practical starting point for industrial trials. For engineers working on hardfacing technology for heavy machinery, the findings suggest that process innovation beyond alloy design can yield meaningful performance improvements, and that electromagnetic field application represents a technically feasible and potentially cost-effective approach to enhancing surfacing layer quality.