CLADDING TECHNOLOGY SHANXI CO., LTDManual TIG Large Gap Single-Side Double-Side Forming Process
Literature Overview
Published in 1991 by Lin Chengde from Ansteel Construction Electromechanical Company, this paper addresses an advanced GTAW technique for welding joints with large root gaps using single-side access while achieving double-sided forming. Large gap welding is particularly relevant in pressure vessel fabrication where fit-up tolerances are relaxed due to thermal distortion, large component sizes, or field assembly constraints. The technique bridges the gap between conventional butt welding and plug welding, offering flexibility in joint preparation while maintaining structural integrity.
Core Technical Content
Definition and Application of Large Gap Welding
Large gap welding in this context refers to root gaps exceeding the conventional 2–3 mm range, typically 4–8 mm for manual GTAW. This technique is applicable in:
- Field fabrication: Where precise fit-up is difficult due to wind, temperature variations, or component weight
- Thick section welding: Where multiple passes would require excessive heat input for conventional narrow-gap preparation
- Repair welding: Where damage removal creates irregular gap profiles
- Clad vessel nozzle attachments: Where the gap between nozzle and vessel shell varies due to cladding thickness differences
Process Parameters for Large Gap Welding
| Parameter | Small Gap (2–3 mm) | Large Gap (4–8 mm) | Adjustment Rationale |
|---|---|---|---|
| Welding current | 80–120 A | 120–180 A | More heat for larger volume |
| Travel speed | 30–40 mm/min | 15–25 mm/min | Slower to fill larger volume |
| Filler wire diameter | 1.6–2.0 mm | 2.4–3.2 mm | Higher deposition rate |
| Arc length | 2–3 mm | 3–4 mm | Maintain pool stability |
| Wire angle | 20–30° | 30–45° | Direct filler into gap |
| Root pass technique | Single pass | Multi-pass (2–3) | Fill gap progressively |
Multi-Pass Root Filling Technique
The key innovation described in this paper is the progressive filling of the large gap through multiple root passes:
- First pass: Establish penetration through the gap using low current (80–100 A) and high travel speed, creating a thin root bead
- Second pass: Increase current (100–130 A) and deposit filler to build up approximately 50% of gap height
- Third pass: Complete gap filling with final current (120–150 A), achieving full section
Each pass must be allowed to cool to below 200°C before the next pass begins, particularly for materials susceptible to hydrogen cracking.
Connection to Bimetallic Pressure Vessel Fabrication
Large gap GTAW has specific applications in bimetallic vessel fabrication:
- Clad-plate vessel shell joints: Where the cladding layer thickness varies (typically 3–6 mm), the gap between base metal edges may exceed conventional limits, requiring large gap techniques for the base metal weld while maintaining clad layer integrity.
- Nozzle-to-shell attachment: The gap between a nozzle and the vessel shell is often larger than conventional butt joints due to the need for reinforcement pad attachment, making large gap GTAW an economical alternative to full penetration SAW.
- Repair of clad components: When localized damage requires removal of material from a clad vessel, the resulting gap may be irregular and large, requiring flexible GTAW techniques.
Quality Control Requirements
| Inspection Method | Acceptance Criteria | Frequency |
|---|---|---|
| Visual examination (VE) | No cracks, porosity > 2 mm, undercut > 1 mm | 100% |
| Dye penetrant testing (PT) | No linear indications | 100% |
| Radiographic testing (RT) | Per ASME IX Table 7 (Level II) | 20–100% |
| Ultrasonic testing (UT) | Per JB/T 4730 | Where RT impractical |
| Hydrostatic test | 1.25× design pressure | 100% |
Common Challenges and Solutions
| Challenge | Cause | Solution |
|---|---|---|
| Inconsistent back-side profile | Variable gap width | Use backer plate with controlled gap |
| Excessive reinforcement | Over-deposition in large gap | Plan passes, measure bead height |
| Lack of fusion at gap edges | Insufficient heat at edges | Focus arc at edges, use drag technique |
| Excessive distortion | High heat input | Use backing plate, intermittent welding |
| Back-side porosity | Insufficient shielding from back | Use backer plate with gas port |
Engineering Practice Insights
In my experience with large pressure vessel fabrication, particularly hydrogenation reactors and ammonia synthesis loops, large gap GTAW has been increasingly adopted as a cost-effective alternative to conventional narrow-gap preparation for joints up to 12 mm thick. The technique reduces fit-up labor by 30–50% while maintaining weld quality, provided the operator has been properly qualified and the process is controlled through documented procedures.
The critical success factor is the qualification of the welding procedure. ASME IX requires that the WPS qualification coupon demonstrate adequate penetration and fusion across the entire gap width. For large gap joints, I recommend qualification at the maximum anticipated gap width plus 25%, ensuring that the procedure is robust against field variations.
Study Reflection
This 1991 paper represents an early Chinese contribution to the development of large gap welding techniques, which have since been formalized in various international standards. The fundamental insight — that GTAW can successfully fill gaps significantly larger than conventional practice if the operator employs a multi-pass progressive filling approach — has been validated by decades of subsequent engineering practice. In the context of bimetallic pressure vessels, where joint preparation is often constrained by cladding layer considerations, large gap GTAW provides the flexibility needed to produce sound, code-compliant welds without compromising the corrosion-resistant overlay.