Effect of Excitation Current on Microstructure and Properties of 316L Stainless Steel TIG Welded Joints Under External Magnetic Field

Literature Overview

The study by Jiao Shaobin and colleagues from the Hunan Provincial Special Equipment Inspection and Testing Research Institute, published in the journal Hot Working Technology (2025, Vol. 54, No. 21, pp. 41-46), investigates the influence of external magnetic field excitation current on the microstructure and mechanical properties of TIG welded 316L stainless steel joints. The research was supported by the Hunan Provincial Market Supervision Administration Science and Technology Project (2022KJJH82) and the Guangxi Science and Technology Major Project (Guike AA23023028). This work addresses a relatively novel approach to arc welding process enhancement — the application of an external magnetic field during TIG welding — and provides quantitative data on how excitation current levels affect weld penetration, bead geometry, grain size, and hardness distribution.

Core Technical Findings

The experimental design employed three excitation current levels: 1 A, 2 A, and 3 A. The researchers utilized optical microscopy (OM), scanning electron microscopy (SEM), and microhardness testing to characterize the welded joints. The key findings are summarized below.

The results clearly demonstrate that increasing the excitation current improves penetration depth, which is attributed to the Lorentz force effect on the electric arc — the magnetic field deflects the arc plasma, increasing arc pressure and concentration of energy density at the weld pool. However, this benefit comes with trade-offs: weld collapse becomes more pronounced, the weld zone width increases, and grain coarsening accelerates significantly, particularly when the current rises from 2 A to 3 A.

Microstructural Analysis

The weld zone microstructure consists of austenite and worm-like ferrite, which is characteristic of 316L stainless steel welds. The presence of worm-like ferrite is beneficial for crack resistance, as it interrupts the continuous austenite network and provides a pathway for hydrogen diffusion, thereby reducing solidification cracking susceptibility. Distinct transition bands and heat-affected zones (HAZ) were observed, indicating the typical thermal gradient distribution in TIG welding.

The grain coarsening observed at 3 A excitation current — from 170 μm to 350 μm — represents a critical threshold beyond which mechanical property degradation becomes unacceptable. This coarsening is attributed to the increased thermal input and extended liquid phase lifetime caused by the enhanced arc penetration. The larger grains reduce dislocation density and yield strength according to the Hall-Petch relationship, which explains the increased hardness reduction rate at higher excitation currents.

Engineering Practice Implications

From a practical standpoint, the 2 A excitation current is identified as the optimal parameter. This finding has significant implications for industries that routinely weld 316L stainless steel, including nuclear power, chemical processing, food processing, and pharmaceutical equipment manufacturing. The external magnetic field approach offers a non-contact method to improve weld penetration without increasing the primary welding current, which could reduce heat-affected zone width and minimize distortion.

However, several engineering considerations must be addressed before implementation:

• The magnetic field apparatus adds complexity and cost to the welding setup
• The magnetic field may interfere with nearby magnetic sensors or instrumentation
• Operator safety must be evaluated regarding electromagnetic field exposure limits
• The method's applicability to thick-section welding requires further investigation

Key Questions and Reflections

The most compelling aspect of this research is the identification of a clear optimum at 2 A, suggesting that magnetic field-assisted welding operates within a relatively narrow process window. The dramatic jump in grain size from 170 μm to 350 μm between 2 A and 3 A indicates a threshold phenomenon — likely related to the transition from arc concentration to arc instability. This threshold behavior warrants further investigation into the interaction between magnetic field strength, arc geometry, and weld pool fluid dynamics.

A question that remains unanswered is whether the hardness reduction observed at 2 A (10.4%) is acceptable for all service conditions. For pressure vessels and piping systems governed by ASME or GB standards, the hardness ratio between the weld and base metal must meet specific criteria. A 10.4% reduction may be within acceptable limits for many applications, but for high-stress or creep-critical applications, further optimization may be required.

Summary

This study provides valuable quantitative data on magnetic field-assisted TIG welding of 316L stainless steel, identifying 2 A as the optimal excitation current that balances improved penetration against grain coarsening and hardness degradation. The findings suggest a promising auxiliary technique for enhancing weld penetration in austenitic stainless steel, though practical implementation requires addressing equipment complexity and process window limitations. The clear threshold behavior observed between 2 A and 3 A excitation current highlights the sensitivity of weld pool metallurgy to even modest changes in magnetic field intensity, reinforcing the importance of precise parameter control in advanced welding processes.