CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Narrow-Gap TIG Welded Joints in Q235 Steel
Literature Overview
This paper, published in Ordnance Materials Science and Engineering (2025) by Gao Hui, Lu Yu, Cheng Jincai, Lin Yuanhao, and Wang Wei from Beijing Institute of Petrochemical Technology and the Beijing Advanced Connection Technology Center for Energy Engineering, investigates the microstructural evolution and mechanical properties of narrow-gap TIG welded joints in Q235 carbon steel. The research was supported by the National Key R&D Program (2023YFB3407705), indicating its significance within the context of advanced energy equipment manufacturing.
Core Technical Content
Narrow-gap welding is a solid-state welding process that uses a specially designed groove with a small root gap (typically 2–5 mm) and relies on multiple passes to fill the joint. Unlike conventional V-groove welding, narrow-gap welding requires significantly less filler metal and reduces welding time, making it economically attractive for thick-plate fabrication.
Microstructural Analysis
The study examines the microstructure across the entire weld zone, including the fusion zone, heat-affected zone (HAZ), and base metal:
Fusion Zone:
- The fusion zone microstructure consists primarily of ferrite and pearlite, consistent with the hypoeutectoid composition of Q235 steel.
- Grain size in the fusion zone ranges from 20–50 μm, smaller than the base metal due to the constrained solidification conditions within the narrow gap geometry.
- Segregation patterns are observed along grain boundaries, with slight enrichment of manganese and phosphorus at prior austenite grain boundaries.
Heat-Affected Zone:
- The HAZ exhibits a gradient of microstructural transformations from the base metal to the fusion zone.
- The coarse-grained HAZ (CGHAZ) region, located adjacent to the fusion line, shows grain growth up to 80–120 μm, representing a potential weakness for hydrogen-induced cracking.
- The fine-grained HAZ (FGHAZ) region displays refined ferrite and pearlite structures due to partial recrystallization during the thermal cycle.
Base Metal:
- The base metal microstructure remains essentially unchanged, consisting of equiaxed ferrite grains (30–60 μm) with dispersed pearlite colonies.
Mechanical Properties
| Property | Base Metal | Fusion Zone | HAZ (CGHAZ) | HAZ (FGHAZ) |
|---|---|---|---|---|
| Tensile strength (MPa) | 375–460 | 350–420 | 340–390 | 380–450 |
| Yield strength (MPa) | 235–280 | 220–270 | 210–260 | 240–290 |
| Elongation (%) | 26–32 | 22–28 | 20–26 | 24–30 |
| Hardness (HV) | 120–150 | 110–140 | 100–130 | 130–160 |
| Impact energy at 20°C (J) | 45–60 | 35–50 | 30–45 | 40–55 |
Process Parameters and Their Influence
The study evaluates the effect of welding parameters on joint quality:
| Parameter | Range Studied | Optimal Range | Effect on Properties |
|---|---|---|---|
| Welding current (A) | 100–180 | 130–160 | Higher current increases dilution and grain size |
| Travel speed (mm/min) | 40–100 | 60–80 | Higher speed reduces heat input and HAZ width |
| Shielding gas flow (L/min) | 8–20 | 12–15 | Insufficient flow increases porosity |
| Interpass temperature (°C) | 100–250 | 150–200 | Controls HAZ grain growth and residual stress |
| Number of passes | 3–8 | Depends on plate thickness | More passes increase thermal cycles |
Defect Analysis and Quality Control
The paper identifies several defect modes specific to narrow-gap TIG welding of Q235 steel:
- Porosity: Gas porosity is the most common defect, caused by inadequate shielding gas coverage in the narrow groove geometry. Countermeasures include using a trailing shield nozzle and maintaining gas flow above 12 L/min.
- Lack of fusion: Insufficient heat input at the root of the narrow gap can result in incomplete fusion between the base metal and the first weld pass. This is particularly problematic in thick plates where heat dissipation is significant.
- Cracking: While Q235 steel has relatively low carbon equivalent (CE ≈ 0.35–0.45), hydrogen-induced cracking can still occur in the CGHAZ region under unfavorable conditions. Preheating to 100–150°C and post-weld heat treatment are recommended for thick sections.
- Undercut: The narrow groove geometry makes undercut detection more difficult. Visual inspection supplemented by dye penetrant testing (PT) is essential for quality assurance.
Engineering Practice Implications
Narrow-gap TIG welding offers significant advantages for pressure vessel and structural fabrication:
- Material savings: Compared to conventional V-groove welding, narrow-gap welding reduces filler metal consumption by 60–80%, which is particularly significant for large-diameter pressure vessels.
- Reduced welding time: The smaller groove volume and optimized heat input result in 30–50% reduction in total welding time.
- Lower residual stress: The reduced heat input and multiple thermal cycles promote stress relief through thermal cycling, potentially reducing the need for post-weld heat treatment.
However, several challenges must be addressed in production:
- Groove preparation precision: The narrow root gap requires precise beveling and fit-up, with tolerance typically maintained within ±0.5 mm.
- Welding position control: The narrow gap geometry makes it difficult for the welder to maintain consistent arc position, particularly in horizontal and overhead positions.
- Inspection accessibility: Non-destructive testing of narrow-gap welds is more challenging due to the confined geometry, requiring specialized UT probes and techniques.
Study Insights and Reflections
This research provides valuable data on the microstructural evolution and mechanical behavior of narrow-gap TIG welded joints in a widely used structural steel grade. The findings confirm that narrow-gap welding can produce joints with properties comparable to or better than conventional groove welds, provided that appropriate process parameters are selected and maintained.
From an engineering practice perspective, the key takeaway is that narrow-gap welding is not simply a cost-reduction technique but a fundamentally different welding approach that requires careful consideration of process parameters, operator skills, and quality control procedures. The microstructural refinement observed in the fusion zone, despite the lower heat input, suggests that narrow-gap welding may offer improved fatigue resistance compared to conventional methods, although long-term fatigue testing would be needed to confirm this hypothesis.
The research also highlights the importance of understanding the interaction between groove geometry and thermal cycling in determining final weld properties. For pressure vessel fabrication, where long-term reliability is paramount, the reduced HAZ width and refined microstructure associated with narrow-gap welding represent genuine advantages that should be considered in process selection.