CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Magnetizing Current of External Magnetic Field on 316L Stainless Steel TIG Weld Joints
Literature Overview
This paper, published in Hot Working Technology (2025), investigates the effect of the magnetizing current of an external magnetic field on the microstructure and mechanical properties of 316L stainless steel TIG weld joints. The research was conducted by a team from the Hunan Provincial Special Equipment Inspection and Testing Research Institute and Xiangtan University, supported by the Hunan Provincial Administration for Market Regulation Science and Technology Project and the Guangxi Science and Technology Major Special Project. The study explores the application of magnetic field-assisted welding, a technique that uses externally applied magnetic fields to influence the welding process and improve weld quality.
Magnetic Field-Assisted Welding Principles
The application of external magnetic fields to welding processes is based on the interaction between the magnetic field and the electric current flowing through the arc and weld pool. The Lorentz force (F = J × B) acts on the current-carrying plasma and molten metal, producing several effects:
- Arc deflection and stabilization: The magnetic field can deflect the arc, improving arc stability and allowing for better control of the heat input distribution.
- Weld pool stirring: The electromagnetic force stirs the molten metal, promoting mixing and reducing segregation. This can homogenize the chemical composition and microstructure of the weld.
- Grain refinement: The electromagnetic stirring can fragment dendrites and promote the formation of equiaxed grains, leading to finer grain structure and improved mechanical properties.
- Reduction of porosity: The stirring action can help gas bubbles rise to the surface of the weld pool more efficiently, reducing porosity.
The magnetizing current is the current supplied to the electromagnet that generates the external magnetic field. Increasing the magnetizing current increases the magnetic field strength, which in turn increases the electromagnetic force acting on the arc and weld pool. However, excessive magnetizing current can lead to arc instability, excessive weld pool stirring, and other adverse effects.
Effect on Microstructure
The microstructure of 316L stainless steel TIG welds is typically characterized by the formation of austenite (γ) and δ-ferrite phases. The ratio of these phases depends on the welding parameters and the thermal cycle experienced by the weld. The following table summarizes the expected microstructural changes with increasing magnetizing current:
| Magnetizing Current (A) | Arc Stability | Weld Pool Stirring | Grain Size | δ-Ferrite Content |
|---|---|---|---|---|
| 0 (no magnetic field) | Baseline | None | Coarse columnar | Low (2–5%) |
| 10–20 | Improved | Mild | Moderate refinement | Moderate (5–10%) |
| 30–50 | Optimal | Strong | Fine equiaxed | Optimal (10–20%) |
| 60–80 | Decreased | Excessive | May coarsen | High (>20%) |
| >80 | Unstable | Turbulent | Irregular | Very high (>30%) |
The optimal magnetizing current range is expected to be around 30–50 A, where the electromagnetic stirring is sufficient to refine the grain structure and promote an optimal δ-ferrite content without causing arc instability. The δ-ferrite content is important because it provides resistance to hot cracking (by interrupting the continuous austenite grain boundary network) but excessive δ-ferrite can reduce ductility and promote 400-temperature embrittlement.
Effect on Mechanical Properties
The mechanical properties of the weld joints are expected to improve with the application of an external magnetic field, up to an optimal magnetizing current. The following table presents the expected mechanical property ranges:
| Property | No Magnetic Field | Optimal Magnetic Field | Excessive Magnetic Field |
|---|---|---|---|
| Tensile strength (MPa) | 480–520 | 500–550 | 470–510 |
| Yield strength (MPa) | 280–320 | 300–350 | 270–310 |
| Elongation (%) | 35–40 | 38–45 | 30–35 |
| Hardness (HV) | 180–220 | 200–240 | 170–210 |
The improvement in mechanical properties is attributed to grain refinement, reduced porosity, and homogenization of the weld microstructure. However, excessive magnetizing current can lead to arc instability, which may introduce defects such as lack of fusion, excessive spatter, and irregular weld geometry, ultimately degrading mechanical properties.
Engineering Practice Implications
The application of magnetic field-assisted welding to 316L stainless steel TIG welding has several practical implications for engineering practice:
- Weld procedure qualification: The welding procedure specification (WPS) must include the magnetizing current as a essential variable. Changes in magnetizing current beyond the qualified range may require requalification of the procedure.
- Equipment requirements: The welding setup must include an electromagnet assembly capable of generating the required magnetic field strength. The electromagnet must be positioned to produce a magnetic field perpendicular to the arc axis for maximum effect.
- Process monitoring: The magnetizing current must be monitored and controlled during welding to ensure consistent results. Fluctuations in magnetizing current can lead to variations in weld quality.
- Non-destructive testing: The NDT requirements for magnetic field-assisted welds should be comparable to conventional TIG welds, but the specific defect modes may differ. For example, arc instability caused by excessive magnetizing current may lead to lack of fusion or irregular weld geometry, which may require specific RT or UT examination techniques.
The following table compares the expected NDT requirements for conventional and magnetic field-assisted TIG welds:
| NDT Method | Conventional TIG | Magnetic Field-Assisted TIG |
|---|---|---|
| RT (radiographic) | Standard | Standard, with attention to lack of fusion |
| UT (ultrasonic) | Standard | Standard, with attention to porosity reduction |
| PT (penetrant) | Standard | Standard |
| MT (magnetic particle) | Not applicable (austenitic) | Not applicable (austenitic) |
| Visual inspection | Standard | Additional check for arc stability indicators |
Study Insights and Reflections
This research contributes to the growing body of knowledge on magnetic field-assisted welding, a technique that offers the potential to improve weld quality without modifying the base material or filler metal. The key finding is that the magnetizing current must be carefully optimized to achieve the desired balance between beneficial effects (grain refinement, porosity reduction, improved mechanical properties) and adverse effects (arc instability, excessive stirring).
From an engineering practice perspective, the implementation of magnetic field-assisted welding requires careful consideration of the equipment design, process parameter optimization, and quality control procedures. The electromagnet assembly must be designed to produce a uniform magnetic field in the arc region without interfering with the shielding gas flow or introducing magnetic particles into the weld pool. The control system must be capable of maintaining the magnetizing current within a narrow tolerance to ensure consistent weld quality.
One important consideration for pressure vessel fabrication is that the application of magnetic field-assisted welding to clad plate or weld overlay applications may offer additional benefits. The electromagnetic stirring can promote better mixing of the cladding material with the base metal, potentially improving the metallurgical bond between the layers. However, this must be carefully controlled to avoid excessive dilution, which would compromise the corrosion resistance of the cladding layer.
The study also highlights the importance of understanding the fundamental physics of the welding process. The interaction between the magnetic field, the arc plasma, and the molten metal is a complex magnetohydrodynamic phenomenon that is not fully understood. Further research is needed to develop predictive models that can accurately predict the effects of magnetic field parameters on weld quality, which would facilitate the optimization of the process and the development of standardized welding procedures.