Microstructural and Mechanical Characterization of Narrow-Gap TIG Welds in Q235 Steel

Literature Overview

Published in Ordnance Materials and Engineering (2025, Vol. 48, No. 6, pp. 37-42), this paper by Gao Hui et al. from Beijing University of Chemical Technology investigates narrow-gap TIG welding of thick-walled Q235 steel pipe with a deep U-shaped groove preparation. Funded by the National Key R&D Program of China (2023YFB3407705), the research addresses the weldability and performance of narrow-gap TIG (NG-TIG) in carbon steel, a process that offers significant productivity advantages for thick-section fabrication.

Core Technical Findings

The study conducted comprehensive mechanical property testing including tensile, bending, and impact tests, combined with metallographic examination using optical microscopy, electron microscopy, electrochemical workstation analysis, and hardness measurements. The results demonstrate that Q235 carbon steel exhibits good weldability under narrow-gap TIG conditions, though certain performance differentials between weld zones and heat-affected zones warrant attention.

Test Parameter	Weld Zone	HAZ	Base Metal
Tensile strength (average)	450 MPa	—	—
Elongation (average)	26.3%	—	—
Impact energy (average)	91.1 J	60.1 J	—
Fracture morphology	Dimpled (ductile)	Quasi-cleavage	—
Microstructure	Fine equiaxed recrystallized grains	Coarse ferrite + pearlite	Ferrite + pearlite
Hardness	Higher	Moderate	Baseline
Corrosion resistance	Good	Significantly lower	Good

Microstructural Analysis

The weld zone microstructure undergoes complete recrystallization during the multi-pass welding process, forming fine equiaxed grains that contribute to elevated hardness and tensile strength. This is a direct consequence of the repeated thermal cycling inherent in narrow-gap welding, where each subsequent pass acts as a localized annealing treatment for the previously deposited metal. The fine grain structure resulting from this thermal history is beneficial for strength but must be evaluated for long-term service stability.

The heat-affected zone presents a more concerning picture. The coarse ferrite and pearlite structure indicates that the peak temperatures in the HAZ were sufficient to cause significant grain growth but not high enough to promote complete recrystallization. This coarse microstructure is responsible for the reduced impact energy (60.1 J versus 91.1 J in the weld zone) and the quasi-cleavage fracture mode observed in impact specimens. The quasi-cleavage morphology suggests a mixed-mode fracture with features of both ductile tearing and brittle cleavage, indicating that the HAZ represents the weakest link in terms of fracture resistance.

Corrosion Behavior Analysis

A particularly noteworthy finding is the significantly reduced corrosion resistance in the HAZ compared to both the weld zone and the base metal. This degradation is attributed to the disruption of material uniformity caused by the coarse, heterogeneous microstructure in the HAZ. The coarse ferrite-pearlite structure creates preferential paths for corrosive attack, with the pearlite regions acting as cathodic sites and the ferrite regions as anodic sites, establishing microgalvanic couples that accelerate localized corrosion.

Zone	Corrosion Resistance	Primary Cause
Weld zone	Good	Fine, homogeneous recrystallized structure
HAZ	Significantly reduced	Coarse ferrite-pearlite, microgalvanic couples
Base metal	Good	Uniform original microstructure

Engineering Practice Considerations

For thick-walled Q235 pipe fabrication, narrow-gap TIG welding offers substantial productivity advantages. The deep U-groove preparation combined with the narrow-gap approach reduces the number of passes required compared to conventional V-groove preparation, significantly decreasing welding time and filler metal consumption. However, the HAZ concerns identified in this study must be addressed in practical applications.

Process Consideration	Recommendation	Rationale
Groove geometry	Deep U-groove with controlled root land	Minimize number of passes while ensuring fusion
Interpass temperature	Controlled and monitored	Prevent excessive grain growth in HAZ
Post-weld treatment	Consider low-temperature PWHT	Refine HAZ microstructure if service conditions require
Corrosion protection	Enhanced coating or cathodic protection	Compensate for reduced HAZ corrosion resistance

Key Questions and Reflections

The reduced corrosion resistance in the HAZ raises important questions about the long-term service life of narrow-gap TIG welded Q235 pipe, particularly in aggressive environments such as chemical processing or marine applications. While Q235 is not typically specified for highly corrosive service, the localized degradation in the HAZ could lead to premature failure if not properly addressed through protective measures.

Additionally, the quasi-cleavage fracture mode in the HAZ at room temperature is concerning for applications involving cyclic loading or impact events. Engineers should consider whether the HAZ impact energy of 60.1 J meets the specific acceptance criteria for the intended service conditions, and whether additional thermal treatment or process modification is required.

Study Insights and Implications

This research confirms that narrow-gap TIG welding is a viable and productive process for Q235 steel thick-section fabrication, delivering acceptable overall mechanical properties with good weld zone toughness and ductility. The critical engineering challenge lies in managing the HAZ microstructure to ensure adequate corrosion resistance and fracture toughness for the intended service life. For practical implementation, process qualification should include specific HAZ evaluation protocols, and design considerations should account for the localized performance degradation in the heat-affected region.