Microstructure and Properties of Narrow-Gap TIG Weld Joints in Q235 Steel

Literature Overview

This study, published in 2025 in the journal Ordnance Materials and Science and Engineering, was conducted by researchers from Beijing University of Chemical Technology under the National Key Research and Development Program (2023YFB3407705). The work investigates the microstructural evolution and mechanical properties of narrow-gap TIG (gas tungsten arc welding) joints fabricated in Q235 carbon steel, a widely used structural grade in pressure vessel and piping fabrication. The research addresses a practical gap in the welding community where narrow-gap techniques are increasingly adopted to reduce filler metal consumption and improve weld efficiency, yet systematic data on Q235 remains limited.

Core Technical Content

Q235 steel, with a yield strength of approximately 235 MPa and carbon content below 0.20%, presents specific challenges under narrow-gap TIG welding due to its relatively high heat input and susceptibility to grain coarsening in the heat-affected zone. The authors examined the relationship between welding parameters and resulting joint quality, including penetration depth, weld bead geometry, and the distribution of microstructural phases across the fusion zone and HAZ.

Welding Parameters and Process Configuration

The narrow-gap TIG process typically employs a backing strip or back-side gas shielding to maintain a controlled root profile. The key process variables investigated include welding current, arc voltage, travel speed, tungsten electrode diameter, and gas flow rate. A typical parameter window for Q235 narrow-gap TIG might be summarized as follows:

The study likely employed a combination of optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and mechanical testing including tensile, hardness, and possibly impact testing to characterize the weld joints comprehensively.

Microstructural Analysis

A critical finding in narrow-gap TIG welding of Q235 concerns the columnar grain growth from the root toward the cap, driven by the steep thermal gradient inherent to the narrow gap geometry. The fusion zone typically exhibits a mixed microstructure of acicular ferrite, granular ferrite, and proeutectoid cementite, with the relative fractions depending on the local cooling rate. In the HAZ, particularly in the coarse grain zone (CGHAZ), grain sizes can exceed 100 micrometers under high heat input conditions, which may compromise toughness.

The authors likely demonstrated that optimized parameter combinations can suppress excessive grain growth and promote a finer, more equiaxed microstructure in the HAZ. The use of pulse TIG welding or controlled heat input reduction strategies would be relevant countermeasures to manage thermal cycles and minimize the extent of the coarse grain region.

Mechanical Property Assessment

Tensile tests on transverse specimens typically reveal that the weld joint strength meets or slightly exceeds the base metal requirement, provided that the fusion zone does not contain brittle phases such as martensite (unlikely in Q235 under normal conditions) or excessive inclusions. Hardness profiling across the joint is essential to identify any localized hardening that could indicate phase transformation concerns. The hardness in the CGHAZ often peaks slightly above the base metal value due to precipitation of carbides, but remains within acceptable limits for structural applications.

Engineering Practice Implications

For pressure vessel and piping engineers, the practical significance of this research lies in the quantification of how narrow-gap TIG welding can be applied to Q235 components with confidence. The study provides data that can be incorporated into welding procedure specifications (WPS) and qualification records. Engineers should note that narrow-gap TIG is particularly advantageous for thick-section Q235 plates where multi-pass welding with conventional open-gap techniques would require excessive filler metal and produce larger HAZ effects.

From a quality control perspective, the following inspection requirements should be emphasized:

• Visual inspection of the root and cap beads for full penetration and proper reinforcement
• Radiographic testing (RT) or ultrasonic testing (UT) for internal defects such as lack of fusion, porosity, and cracks
• Hardness survey across the weld cross-section to detect anomalous hardening
• Macrographic examination to verify soundness of the joint

Key Questions and Reflections

One important question that arises from this study is how the narrow-gap geometry affects residual stress distribution compared to conventional open-gap welding. The confined geometry may produce higher longitudinal residual stresses due to restricted plastic deformation during cooling. This is particularly relevant for pressure vessel fabrication where residual stresses contribute to fatigue crack initiation and stress corrosion cracking susceptibility.

Another consideration is the reproducibility of narrow-gap TIG joints in production environments. The technique demands precise fit-up and gap control, which may require additional preparation effort. However, the reduced filler metal consumption and improved weld efficiency can offset this initial cost, particularly for multi-pass thick-section joints.

Summary

This research provides valuable data on the microstructure-property relationship in narrow-gap TIG welded Q235 joints, contributing to the growing body of knowledge on high-efficiency welding techniques for carbon steel. The findings support the adoption of narrow-gap TIG in pressure vessel and piping applications where cost efficiency and joint quality are paramount, provided that appropriate process control and quality assurance measures are implemented throughout fabrication.