Temperature Effects on MIG Welded Joint Microstructure and Fracture Position Under Circular Wire Feeding Pattern

Literature Overview

This study by Fang Xifeng, Wang Xin, Shi Xuehai, Ji Shude, Liang Zhimin, and Xiao Hanlin from CRRC Qingdao Sifang Co., Shenyang Aerospace University, and Hebei University of Science and Technology investigates how temperature influences the microstructure and fracture behavior of MIG welded joints produced using a circular wire feeding pattern. Published in 2016 in the Welding Journal, the research was supported by the National Natural Science Foundation of China (Grant No. 51204111) and the Aviation Science Foundation. The circular wire feeding pattern is a specialized welding technique developed for rail vehicle manufacturing to improve weld geometry and reduce residual stresses.

Core Technical Content

The circular wire feeding pattern involves oscillating the welding torch or wire in a circular trajectory during welding, creating a distinctive weld bead morphology with multiple overlapping passes within a single weld. This technique is particularly advantageous for thick-section structural steel welding in rail vehicle manufacturing, where high-quality multi-pass welds are required with minimal distortion and residual stress.

Circular Wire Feeding Process Parameters

The study systematically varied the base metal preheat temperature and interpass temperature to evaluate their effects on weld microstructure and fracture behavior. Preheat temperatures of 0°C (ambient), 100°C, 200°C, and 300°C were investigated, with corresponding interpass temperatures controlled within ±30°C of target values.

Microstructural Evolution with Temperature

The microstructural analysis revealed distinct zones within the weld joint that respond differently to thermal cycling:

The increase in preheat temperature from ambient to 300°C resulted in a progressive coarsening of microstructures throughout the weld joint. The cooling rate from the peak temperature decreased from approximately 15-20°C/s at ambient preheat to approximately 3-5°C/s at 300°C preheat, fundamentally altering the phase transformation pathways. At lower cooling rates, the formation of fine acicular ferrite is suppressed in favor of coarser polygonal ferrite and pearlite, which have inferior mechanical properties.

Fracture Position and Fracture Mode Analysis

The fracture position within the weld joint shifted systematically with increasing preheat temperature. At ambient preheat, fracture preferentially occurred in the weld metal, which exhibited the lowest ductility due to the presence of acicular ferrite with relatively high strength. As preheat temperature increased to 200°C and 300°C, the fracture position shifted to the HAZ, specifically the fully recrystallized zone where coarse bainite and pearlite formed. This shift is critical from a structural safety perspective, as HAZ fracture in pressure vessels and structural components is generally considered more detrimental than weld metal fracture due to the unpredictability of HAZ properties.

Mechanical Property Correlation

Engineering Practice Implications

For rail vehicle manufacturing, where the circular wire feeding pattern is employed to produce high-quality welds in thick-section structural steels, the temperature control findings have direct implications for welding procedure specification and production quality control. The study demonstrates that maintaining interpass temperatures below 200°C is essential to preserve the toughness and ductility of the weld joint. This requirement must be balanced against the need for adequate preheat to prevent cold cracking in high-carbon equivalent steels.

Critical Temperature Windows for Q345R/Q460R Steel

The engineering challenge is to achieve the minimum preheat temperature necessary to prevent hydrogen-induced cold cracking while not exceeding the maximum interpass temperature that would degrade the HAZ properties. For Q345R steel with a carbon equivalent (CE) of approximately 0.40-0.45, the minimum preheat temperature of 50-100°C is typically sufficient to prevent cold cracking when using low-hydrogen welding consumables. The maximum interpass temperature of 200°C represents the upper limit beyond which significant HAZ softening and toughness degradation occur.

Study Insights and Reflections

The most important finding from this study is the clear correlation between interpass temperature control and fracture position within the weld joint. The shift from weld metal fracture to HAZ fracture as temperature increases is a critical quality indicator that can be used as a non-destructive evaluation criterion in production settings. Engineers should consider incorporating fracture position analysis into welding procedure qualification protocols for thick-section structural steel welds.

The circular wire feeding pattern itself offers distinct advantages over conventional stringer bead welding for thick-section joints. The overlapping circular trajectory creates a self-tempering effect where each pass re-heats and partially softens the previous pass, reducing residual stresses and improving the overall toughness of the multi-pass weld. However, this self-tempering effect is temperature-dependent, and excessive interpass temperatures can convert the beneficial self-tempering into detrimental over-tempering that degrades strength and toughness.

The study provides a foundation for developing temperature-controlled welding procedures for rail vehicle manufacturing. The recommended approach is to implement real-time interpass temperature monitoring using infrared thermography or embedded thermocouples, with automated welding parameter adjustment to maintain temperatures within the critical window. This approach aligns with modern Industry 4.0 manufacturing concepts and can be integrated into automated welding systems for rail vehicle fabrication.