Microstructure and Mechanical Properties of Narrow-Gap TIG Weldments in Q235 Carbon Steel

Literature Overview

This paper, published in Ordnance Materials and Science (2025, Vol. 48, No. 6, pp. 37-42) by Gao Hui et al. from Beijing Institute of Petrochemical Technology, investigates the microstructure, mechanical properties, and corrosion behavior of narrow-gap TIG weldments in Q235 carbon steel thick-walled pipe. The research was supported by the National Key Research and Development Program (2023YFB3407705).

Q235 is the most widely used carbon structural steel in China, specified in GB/T 700 for general structural applications including piping systems. The study addresses a practical fabrication challenge: achieving sound welds in deep U-groove configurations with narrow gaps, which is a common configuration for pipe-to-pipe and pipe-to-fitting joints in thick-walled piping systems.

Core Technical Findings

Welding Process Configuration

The authors employed a deep U-groove preparation for Q235 thick-walled pipe, welded using the narrow-gap TIG process. This process is characterized by a narrow root gap (typically 3-6 mm) and a deep groove geometry, which allows for single-pass or minimal-pass welding of thick sections while maintaining good penetration control.

The narrow-gap TIG process differs from conventional TIG in that the groove geometry constrains the arc and molten pool, promoting deeper penetration per pass and reducing the number of passes required. This is particularly advantageous for thick-walled pipe where multi-pass welding can lead to excessive heat input, distortion, and reduced mechanical properties due to multiple heat cycles.

Mechanical Properties

The mechanical properties of the weldment were evaluated through tensile testing, bending tests, and Charpy impact testing:

The average tensile strength of 450 MPa and elongation of 26.3% indicate good mechanical performance. The tensile fracture surface exhibited a clear dimple morphology, confirming ductile fracture behavior. The weld zone impact energy of 91.1 J is acceptable for room temperature applications, while the HAZ impact energy of 60.1 J, though lower, remains within acceptable limits for Q235 applications.

The impact fracture surface of the HAZ was characterized by quasi-cleavage fracture, indicating a mixed-mode fracture mechanism. This is typical for the coarse-grained HAZ region where the microstructure consists of coarse ferrite and pearlite with limited grain refinement.

Microstructural Analysis

The microstructural evolution across the weldment was characterized using optical microscopy and electron microscopy:

• Weld zone: The weld metal underwent complete recrystallization during solidification, forming fine equiaxed grains. This grain refinement is attributed to the high cooling rates associated with the narrow-gap geometry, which promotes a high nucleation rate and suppresses grain growth. The fine grain structure contributes to the elevated hardness and tensile strength observed in the weld zone.
• Heat-affected zone (HAZ): The HAZ microstructure consists of coarse ferrite and pearlite. The coarse grain structure is a result of the high peak temperatures experienced in the HAZ, which promote grain growth. The coarse grain structure is associated with reduced impact toughness and increased susceptibility to brittle fracture.
• Base metal: The base metal Q235 exhibits a typical ferrite-pearlite microstructure with moderate grain size.

Corrosion Behavior

A notable finding of this study is the degradation of corrosion resistance in the HAZ region:

The authors attribute the reduced corrosion resistance of the HAZ to the destruction of material uniformity. The coarse grain structure in the HAZ creates preferential corrosion paths along grain boundaries, and the non-uniform distribution of phases (ferrite and pearlite) creates galvanic couples that accelerate localized corrosion.

Engineering Practice Implications

Groove Design Considerations

The study highlights the importance of groove design in narrow-gap TIG welding. The deep U-groove geometry is effective for achieving full penetration of thick-walled pipe, but the groove dimensions must be carefully controlled:

• Root gap: Should be maintained within a narrow range (typically 3-5 mm for Q235) to ensure consistent penetration without excessive burn-through.
• Groove angle: A narrower angle promotes deeper penetration but requires more precise fit-up and alignment.
• Groove depth: Should be sufficient to allow the arc to reach the root while minimizing the number of passes.

Weld Procedure Development

When developing a welding procedure for Q235 thick-walled pipe using narrow-gap TIG, the following parameters should be optimized:

Quality Control Recommendations

Based on the findings of this study, the following quality control measures are recommended for narrow-gap TIG weldments in Q235 pipe:

1. Dimensional inspection: Verify groove geometry, root gap, and alignment before welding.
2. Visual inspection (VT): Check for surface defects including undercut, porosity, and excessive reinforcement.
3. Radiographic testing (RT): Verify full penetration and absence of internal defects. The narrow-gap geometry can produce characteristic radiographic appearances that require experienced interpretation.
4. Impact testing: Conduct impact tests at the design temperature, with particular attention to the HAZ region where toughness is lowest.
5. Corrosion testing: For applications in corrosive environments, conduct accelerated corrosion testing to evaluate the long-term performance of the HAZ region.

Key Questions and Reflections

One important question raised by this study is the long-term durability of the HAZ in corrosive service. While the mechanical properties are acceptable, the reduced corrosion resistance of the HAZ could lead to premature failure in aggressive environments. For piping systems exposed to corrosive media, additional surface treatment or coating of the HAZ region may be necessary.

Another consideration is the effect of multiple heat cycles on the HAZ microstructure. In multi-pass welding, the HAZ adjacent to later passes experiences additional heating, which can further coarsen the grain structure and reduce toughness. The study focuses on a single-pass or minimal-pass configuration, but in practice, thicker sections may require multiple passes, and the cumulative thermal effect should be evaluated.

The study also does not address the effect of welding position on the narrow-gap TIG process. In vertical or overhead positions, the molten pool behavior differs significantly from the horizontal or flat position, and process parameters may need adjustment.

Study Insights and Implications

This paper provides valuable insights into the microstructure-property relationships in narrow-gap TIG weldments of Q235 carbon steel. The findings demonstrate that while the narrow-gap TIG process produces welds with good mechanical properties, the HAZ region is the weakest link in terms of both toughness and corrosion resistance.

For engineering practice, the key takeaway is that narrow-gap TIG is a viable process for welding Q235 thick-walled pipe, but the HAZ region requires careful attention during design and quality control. The coarse grain structure and microstructural heterogeneity in the HAZ create vulnerabilities that should be addressed through appropriate welding procedures, post-weld treatments, and inspection protocols.

The study also highlights the importance of comprehensive characterization of weldments, including not only mechanical properties but also corrosion behavior. In many engineering applications, the corrosion performance of the weldment is as important as the mechanical performance, and both should be evaluated during procedure qualification.