Multi-Modal Magnetic Control Power Supply and EH40-304 Dissimilar Steel Magnetic-Assisted K-TIG Welding A Study Note

Literature Overview

This paper published in Hot Working Technology (2026, Vol. 55, No. 13) by Shi Yonghua and colleagues from South China University of Technology presents research on the development of a multimodal magnetic control power supply and its application to K-TIG welding of EH40 high-strength steel to 304 stainless steel dissimilar joints. The research is supported by the Guangxi Key R&D Program, Guangxi Nanning Major Science and Technology Project, and Shenzhen High-Tech Zone Innovation Platform Construction projects.

Dissimilar steel welding is a common requirement in engineering applications where different material properties are required in different sections of a structure. The EH40/304 combination is particularly relevant in applications such as offshore platforms, nuclear power plants, and chemical processing equipment, where high-strength structural steel must be joined to corrosion-resistant stainless steel. The primary challenge in welding this dissimilar pair is the significant difference in magnetic permeability between the ferromagnetic EH40 steel and the austenitic (non-magnetic) 304 stainless steel, which causes asymmetric arc behavior and weld pool flow.

Core Technical Approach and Power Supply Development

The authors developed a multimodal magnetic control power supply capable of outputting stable DC and sinusoidal AC magnetic fields in the frequency range of 10 to 100 Hz. The following table summarizes the experimental conditions and key results:

The fundamental mechanism of the undercut defect in EH40/304 dissimilar steel welding is well explained by the magnetic permeability difference. EH40 steel, being a low-carbon high-strength steel, has a relatively high magnetic permeability, while 304 stainless steel, being fully austenitic, is essentially non-magnetic. When the welding arc is positioned at the interface, the magnetic flux lines concentrate in the ferromagnetic EH40 side, creating an asymmetric magnetic force distribution. This results in the molten metal being deflected toward the EH40 side, leaving the 304 side underfused and creating undercut.

Welding Mechanism Analysis

The application of a DC longitudinal magnetic field generates a Lorentz force that acts on the current-carrying arc plasma. This force can be oriented to counteract the magnetic permeability-induced deflection of the weld pool. The authors demonstrate that the DC field effectively balances the magnetization force difference between the two materials, correcting the molten metal flow and eliminating the undercut.

The DC pulsed magnetic field provides an additional benefit through its time-varying nature. The pulsing creates periodic changes in the Lorentz force magnitude, which enhances the electromagnetic stirring of the weld pool. This stirring promotes more uniform mixing of the weld metal, improves fusion with both base metals, and reduces the risk of hot cracking due to segregation at the weld centerline.

The alternating magnetic field at 5 mT and 20 Hz provides the most comprehensive improvement. In addition to correcting the arc deflection, the periodic electromagnetic stirring effect refines the weld grain structure. The refined grain structure leads to improved impact toughness of the welded joint, which is particularly important for structural applications where toughness is a critical design parameter.

Engineering Practice Relevance

The EH40/304 dissimilar steel joint is relevant to several important engineering applications:

• Offshore platforms: EH40 structural steel is commonly used for primary structural members, while 304 stainless steel is used for corrosion-resistant cladding or secondary structures.
• Nuclear power plants: High-strength steels for reactor pressure vessel internals must be joined to austenitic stainless steel components for coolant circuit applications.
• Chemical processing: High-strength steel pressure vessels with stainless steel corrosion-resistant linings require dissimilar welds at the interface.
• Wind turbine towers: High-strength steel tower sections may require stainless steel bolted connections or welded attachments for access platforms.

The magnetic field-assisted K-TIG welding approach offers several advantages for these applications:

• Eliminates the need for transition filler metals or special welding consumables
• Reduces weld preparation requirements
• Achieves full fusion at the dissimilar interface
• Provides sound weld metal with acceptable mechanical properties

However, engineers must be aware of several practical considerations. The magnetic field equipment must be compatible with the welding position and joint geometry. For vertical or overhead welds, the magnetic field orientation may need to be adjusted to maintain effective arc control. The power supply must be designed to handle the inductive load of the excitation coil without affecting the welding current stability.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the chemical composition and microstructure of the weld metal at the dissimilar interface are not fully characterized. In EH40/304 dissimilar welds, the dilution ratio between the two base metals significantly affects the weld metal composition and properties. The weld metal may exhibit a duplex or austenitic-ferritic structure depending on the dilution, which has implications for corrosion resistance and mechanical properties.

Second, the long-term corrosion behavior of the magnetic field-assisted K-TIG welds in aggressive environments is not addressed. Dissimilar steel welds are susceptible to galvanic corrosion, particularly when the weld metal composition creates a potential difference between the weld and the base metals. The impact of the magnetic field on the weld metal composition and, consequently, on corrosion resistance should be evaluated through immersion testing and electrochemical measurements.

Third, the study does not address the effect of the magnetic field on hydrogen diffusion and hydrogen-induced cracking susceptibility. EH40 steel is susceptible to hydrogen-induced cracking, particularly in the heat-affected zone. The electromagnetic stirring effect of the magnetic field may influence hydrogen distribution in the weld pool, potentially reducing or increasing the cracking susceptibility.

Study Insights and Implications

This research demonstrates the versatility of the magnetic field-assisted K-TIG welding technology, extending its application from homogeneous material welding (as in the companion study on 9% Ni steel) to dissimilar steel welding. The development of a practical multimodal magnetic control power supply that can output various field configurations represents a significant engineering achievement.

The key insight is that the magnetic field serves as a universal tool for addressing welding challenges related to asymmetric arc behavior, regardless of whether the asymmetry is caused by material property differences (magnetic permeability, thermal conductivity, electrical resistivity) or geometric factors (joint configuration, welding position). This universality makes the technology attractive for industrial adoption.

For practitioners in the steel pipe and fitting industry, the magnetic field-assisted K-TIG technology may find applications in welding of composite pipes with dissimilar material layers, welding of pipe-to-plate connections where different materials are used for structural and corrosion-resistant purposes, and welding of repair patches where dissimilar materials are used for localized reinforcement.