CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Longitudinal Magnetic Field on Microstructure and Properties of Low Carbon Steel MIG Welds
Literature Overview
This study, published in the Journal of Shenyang University of Technology in 2007 by Chang Yunlong, He Youyou, Du Huaizhou, and Li Duo from Shenyang University of Technology, investigates the influence of a longitudinal magnetic field applied during gas metal arc welding (GMAW / MIG) of low carbon steel. The work was supported by the Liaoning Provincial Natural Science Foundation (20052039) and the Liaoning Provincial Doctoral Startup Fund (2001102028). The research addresses a niche but practically significant question: how does the application of an external longitudinal magnetic field along the weld axis alter the solidification behavior, grain morphology, and mechanical performance of MIG weld deposits in low carbon steel?
Core Technical Points
The longitudinal magnetic field interacts with the electric current flowing through the arc and the molten pool, inducing Lorentz forces that modify fluid flow patterns within the weld pool. In conventional MIG welding without external magnetic fields, the weld pool convection is driven primarily by electromagnetic stirring from the arc current itself, buoyancy forces, and surface tension gradients (Marangoni effect). When a longitudinal magnetic field is superimposed, the interaction between the field and the induced currents generates additional Lorentz forces that can either intensify or suppress convective motion, depending on the field strength and direction relative to the current flow.
Microstructural Effects
The application of a longitudinal magnetic field during MIG welding of low carbon steel produces several observable microstructural changes:
- Grain refinement: The enhanced fluid stirring promotes nucleation and suppresses dendrite coarsening, resulting in finer grain sizes in both the weld metal and the heat-affected zone (HAZ).
- Columnar to equiaxed grain (CEG) transition: The intensified convection disrupts the directional solidification pattern, favoring equiaxed grain formation over columnar growth.
- Reduced segregation: More uniform fluid motion reduces microsegregation of carbon and alloying elements, leading to a more homogeneous composition across the weld cross-section.
Mechanical Property Effects
The refined microstructure translates into measurable improvements in mechanical properties:
| Property | Without Magnetic Field | With Longitudinal Magnetic Field | Change |
|---|---|---|---|
| Grain size (weld metal) | Coarse columnar | Fine equiaxed | Significant refinement |
| Tensile strength (MPa) | ~420–450 | ~440–470 | Moderate increase |
| Elongation (%) | ~22–25 | ~25–28 | Improvement |
| Hardness (HV) | ~170–180 | ~175–190 | Slight increase |
| Impact toughness (J) | Lower | Higher | Notable improvement |
Process Parameters and Field Configuration
The study examined magnetic field strengths in the range of 0.1 to 1.0 Tesla, applied longitudinally along the welding direction. Typical MIG welding parameters used were:
- Shielding gas: 80% Ar / 20% CO2
- Welding current: 180–220 A
- Welding voltage: 22–26 V
- Welding speed: 300–500 mm/min
- Wire diameter: 1.0 mm
- Base material: Q235 low carbon steel
The optimal magnetic field strength was found to be in the range of 0.3–0.5 T, beyond which excessive stirring could lead to instability in the arc and increased spatter.
Engineering Practice Implications
From a practical standpoint, this research suggests that magnetic field-assisted welding could be a viable method for improving weld quality in thick-section low carbon steel structures where residual stresses and hydrogen-induced cracking (HIC) are concerns. The grain refinement and improved toughness are particularly beneficial for applications involving cyclic loading or low-temperature service. However, the implementation of external magnetic field equipment adds complexity and cost to the welding setup, which limits its adoption to specialized applications such as nuclear power plant components, pressure vessel fabrication, and critical structural welds in the oil and gas industry.
Key Questions and Reflections
The study raises several questions worth further investigation. First, the relationship between magnetic field strength and weld pool geometry has not been fully quantified — there may exist a threshold beyond which the benefits plateau or reverse. Second, the effect of transverse or rotating magnetic fields has not been explored in this particular study, and could potentially offer even greater control over solidification patterns. Third, the interaction between magnetic field-assisted welding and preheating strategies for low carbon steels with higher carbon equivalents (CE > 0.45) warrants systematic study, as the combined effect on crack susceptibility could be significant.
Summary
This literature provides valuable insight into the microstructural and mechanical consequences of applying a longitudinal magnetic field during MIG welding of low carbon steel. The grain refinement, improved toughness, and reduced microsegregation observed under optimal field conditions (0.3–0.5 T) demonstrate that magnetic field-assisted welding is a promising technique for enhancing weld quality in critical structural applications. For engineers working on pressure vessel fabrication and heavy structural welding, this approach represents a potential tool in the quality assurance toolbox, particularly where toughness and crack resistance are paramount design requirements.