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

Literature Overview

This paper 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. The research was supported by the Shenyang Talent Special Project (208030103055) and the Liaoning Provincial Department of Education Key Laboratory Project (2009S072), published in China Surface Engineering (Vol. 24, No. 6, 2011, pp. 98-103). The core objective is to improve the mechanical performance of the deposited metal layer by manipulating the solidification process through electromagnetic stirring, which is directly relevant to extending the service life of heavily loaded rolling mill rolls in steelmaking and metal forming applications.

Core Technical Approach

The experimental design involves applying a low-frequency pulsed longitudinal magnetic field during the submerged arc surfacing process on rolling mill rolls. The magnetic field parameters studied include the excitation current (Ip), the magnetic flux density current (Ib), and the frequency (f). The key parameters identified in the study are summarized below:

The electromagnetic stirring effect generated by the low-frequency pulsed longitudinal magnetic field influences both the nucleation and growth processes of grains during solidification. The rotating arc induced by the magnetic field creates a stirring action within the molten pool, which disrupts the normal columnar grain growth pattern and promotes equiaxed grain formation. This is a physically significant mechanism because it directly addresses one of the most persistent challenges in surfacing technology: the tendency for coarse columnar grains to form under the high heat input conditions typical of submerged arc welding.

Microstructure Analysis and Grain Refinement Mechanism

The study demonstrates that the magnetic field interacts with the electric arc during the welding process, causing the arc to rotate and thereby stirring the molten pool. This electromagnetic stirring action has several metallurgical consequences. First, it increases the temperature gradient within the molten pool, which promotes the formation of more nucleation sites. Second, it disrupts the directional heat flow that normally leads to columnar grain growth, encouraging the formation of finer equiaxed grains. The optimal parameters of Ip = 300 A, Ib = 200 A, and f = 5 Hz resulted in the most pronounced grain refinement, reducing grain size to 30.1 μm.

The grain refinement mechanism can be understood through the lens of solidification theory. In conventional submerged arc surfacing, the high heat input and large molten pool volume tend to produce coarse columnar grains due to the steep thermal gradient and relatively low cooling rate. The introduction of electromagnetic stirring increases the nucleation rate by creating thermal fluctuations and mechanical agitation that promote heterogeneous nucleation on floating particles and at the solid-liquid interface. The stirring also breaks up existing dendrites, increasing the number of potential nucleation sites. This is consistent with the well-established principle that grain refinement in welding is governed by the interplay between the thermal gradient (G) and the cooling rate (R), where the grain size is approximately proportional to 1/(G/R).

Hardness Improvement and Engineering Significance

The hardness of the surfacing layer increased by 4.5 HRC compared to the baseline condition without magnetic field application. This improvement is attributed to the Hall-Petch relationship, which establishes that yield strength (and by extension, hardness) increases with decreasing grain size according to the equation σ_y = σ_0 + k_y · d^(-1/2). The reduction in grain size to 30.1 μm translates directly into higher resistance to plastic deformation and, consequently, improved wear resistance and fatigue performance of the surfacing layer.

For rolling mill rolls, which are subjected to extreme cyclic loading, abrasive wear, and thermal fatigue during hot rolling operations, even modest improvements in hardness and grain refinement can significantly extend service life. The ability to achieve grain refinement through external magnetic field application offers a non-invasive process modification that does not require changes to the consumables or base material composition. This is particularly valuable in industrial settings where process flexibility and minimal disruption are prioritized.

Key Questions and Reflections

Several technical questions arise from this study that warrant further consideration. First, the long-term stability of the grain refinement effect under repeated surfacing passes is not explicitly addressed. In industrial surfacing of large-diameter rolls, multiple overlapping passes are typical, and the thermal cycling from subsequent passes could partially reverse the grain refinement achieved in earlier passes. Second, the study focuses on surface hardness and microstructure but does not report on toughness or fatigue properties, which are equally critical for roll performance under impact loading. Third, the scalability of the magnetic field application to large-scale industrial surfacing operations, where roll diameters can exceed 1.5 meters, needs practical evaluation.

The study also raises an important process parameter interaction question: how does the magnetic field interact with the flux composition and shielding gas composition used in submerged arc surfacing? The flux in SAS serves both as a shielding medium and as a source of alloying elements, and the electromagnetic stirring could potentially affect the composition of the molten pool through enhanced mixing of flux-derived elements. Understanding this interaction is essential for optimizing the combined effect of magnetic field and flux composition on deposit properties.

Study Insights and Implications

This research demonstrates a promising approach to improving surfacing layer properties through process physics manipulation rather than consumable modification. The electromagnetic stirring concept has broader applicability beyond rolling mill roll surfacing and could potentially be extended to other heavy surfacing applications such as mining equipment, cement kiln liners, and power plant boiler tube repairs. The key insight is that grain refinement in high-heat-input welding processes can be achieved through external electromagnetic means, offering a pathway to enhanced mechanical properties without the compositional changes that might compromise other material characteristics such as ductility or weldability. For engineers involved in surfacing technology development, this work underscores the importance of understanding and controlling the solidification dynamics within the molten pool, and suggests that external field manipulation represents an underexploited degree of freedom in process optimization.