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

Literature Overview

This study by Chang Yunlong et al. (2011), published in China Surface Engineering (Vol. 24, No. 6, pp. 98–103), investigates the influence of externally applied low-frequency pulsed longitudinal magnetic fields on the microstructure and mechanical properties of submerged arc surfacing deposits on rolling mill rolls. The research was conducted jointly by Shenyang University of Technology and Dalian Heavy Industry Group, and was supported by Shenyang Talent Program and Liaoning Provincial Key Laboratory grants. The work addresses a critical industrial challenge: improving the hardness and durability of hardfacing overlays on heavy-duty rolling mill components subjected to severe abrasion and thermal fatigue.

Core Technical Findings

The authors applied a low-frequency pulsed longitudinal magnetic field during the submerged arc surfacing process and systematically varied the magnetic field parameters, including pulse current (Ip), bias current (Ib), and frequency (f). The key finding is that the optimal magnetic field parameters—Ip = 300 A, Ib = 200 A, f = 5 Hz—produced the most significant grain refinement, reducing the grain size to 30.1 μm and increasing the hardness by 4.5 HRC compared to the baseline condition without magnetic field application.

Technical Mechanism Interpretation

The electromagnetic stirring effect of the low-frequency longitudinal magnetic field fundamentally alters the solidification behavior of the molten pool. The rotating arc caused by the magnetic field interaction creates a dynamic stirring action within the molten pool, which has several important metallurgical consequences:

1. Enhanced nucleation: The electromagnetic stirring disrupts the thermal gradient at the solidification front, promoting heterogeneous nucleation and increasing the number of nucleation sites.
2. Suppressed grain growth: The continuous stirring prevents the preferential growth of columnar grains, favoring equiaxed grain formation throughout the deposit.
3. Refined dendrite arm spacing: The stirring action breaks up dendrite arms, further contributing to microstructural refinement.

The frequency of 5 Hz is particularly significant because it corresponds to the characteristic time scale of the submerged arc welding process, ensuring that the electromagnetic stirring is synchronized with the natural oscillation frequency of the molten pool. This resonance-like effect maximizes the grain refinement capability.

Engineering Practice Integration

From a practical standpoint, this research has direct implications for the maintenance and refurbishment of rolling mill rolls in steel mills. Rolling mill rolls typically undergo periodic hardfacing to restore wear surfaces, and the quality of the hardfacing overlay directly affects roll life and product surface quality. The application of low-frequency magnetic fields during the surfacing process offers several advantages:

• Extended roll service life: The 4.5 HRC hardness improvement translates to enhanced wear resistance, potentially extending roll life by 20–30%.
• Reduced downtime: Improved overlay quality means fewer roll changes, reducing production interruptions.
• Cost-effectiveness: The magnetic field equipment is relatively inexpensive compared to the cost of roll replacement or frequent surfacing.

However, several practical considerations must be addressed when implementing this technology:

• Equipment integration: The magnetic field generation system must be compatible with existing submerged arc welding equipment.
• Parameter control: Precise control of Ip, Ib, and f is essential to achieve consistent results.
• Safety considerations: The high currents involved (300 A pulse) require appropriate safety measures.

Key Questions and Reflections

A critical question that arises from this study is whether the observed grain refinement is solely due to electromagnetic stirring or if there are additional contributions from magnetic pressure effects on the solidification front. The distinction is important because magnetic pressure effects would be dependent on the spatial distribution of the magnetic field, whereas electromagnetic stirring is primarily a function of the induced currents in the molten pool.

Another important consideration is the scalability of this technology. The study was conducted on laboratory-scale specimens, and the magnetic field parameters that are optimal for a small sample may not be directly transferable to large-diameter rolling mill rolls where the molten pool volume and geometry are significantly different.

Study Insights and Implications

The most valuable insight from this research is the demonstration that external electromagnetic fields can be effectively used to control the microstructure of surfacing deposits without altering the base material or the welding consumables. This represents a paradigm shift in hardfacing technology, moving from a purely consumable-driven approach to a process-parameter-driven approach.

The practical significance extends beyond rolling mill rolls to any application where submerged arc surfacing is used for wear or corrosion protection. In the steel pipe manufacturing industry, for example, submerged arc welding of corrosion-resistant alloy (CRA) overlays on carbon steel pipes could potentially benefit from similar magnetic field treatments to improve the microstructure and performance of the overlay.

This research also highlights the importance of fundamental understanding of solidification mechanisms in welding processes. The ability to predict and control grain size through external field manipulation represents a significant advance in welding metallurgy and opens new avenues for process optimization in industrial hardfacing applications.