Zhuojin Pipe Fitting Co., LtdAll-Position Automatic TIG Welding of Carbon Steel Pipes
Literature Overview
The paper by Yao Shouming (2011), published in The Welding Machine (Vol. 41, No. 12), provides a comprehensive account of all-position automatic TIG welding of carbon steel pipes, using 20# carbon steel pipe with dimensions φ88.9×4.5 mm as the test specimen. The author, affiliated with CNNC Huaxing Nuclear Power Installation Company, presents a systematic methodology for developing a mature welding procedure specification (WPS) for pipe-to-pipe butt joints welded in all positions.
Welding Procedure Development
The procedure development follows a structured approach that can be mapped to the PDCA (Plan-Do-Check-Act) cycle. The planning phase involves work condition assessment, groove selection, and preparation of welding trials. The execution phase covers actual welding trials at different positions with optimized parameters. The checking phase includes a series of inspections and tests per applicable standards. The acting phase results in a finalized, qualified WPS.
Groove Preparation and Fit-Up
The groove geometry is critical for achieving consistent penetration in all positions. For the φ88.9×4.5 mm pipe, a V-groove with an included angle of 60°–70° and a root gap of 2–3 mm is typically employed. The root face should be ground clean to remove mill scale and oxide. Fit-up tolerance requirements include:
- Angular misalignment: ≤ 2°
- Root gap variation: ± 0.5 mm
- Axial offset: ≤ 0.5 mm
Positional Welding Intervals
The full circumference of the pipe is divided into welding intervals based on the force and gravity characteristics at each position. The four primary positions are:
- Flat position (1G equivalent): Gravity assists filler metal placement; slightly higher travel speed is feasible.
- Horizontal position (2G equivalent): Molten pool tends to sag on the lower side; reduced current and careful travel speed control are required.
- Vertical-up position (3G equivalent): Molten pool tends to flow downward; the welding torch angle must compensate for gravity, and the travel speed should be reduced.
- Overhead position (4G equivalent): Molten pool drips away from the joint; the lowest current and slowest travel speed are used, with careful torch manipulation.
Optimized Process Parameters
Through systematic trials, the following parameters were optimized for the 20# carbon steel pipe:
| Position | Current (A) | Travel Speed (mm/min) | Shielding Gas Flow (L/min) | Torch Angle (°) |
|---|---|---|---|---|
| Flat | 120–140 | 200–250 | 12–15 | 0–5 |
| Horizontal | 100–120 | 150–200 | 12–15 | 5–15 |
| Vertical-up | 90–110 | 120–180 | 12–15 | 10–20 |
| Overhead | 80–100 | 100–150 | 12–15 | 15–25 |
The shielding gas used is high-purity argon (≥ 99.99%), and the filler wire is ER70S-6 equivalent or the matching solid wire for 20# steel. The welding current type is DCEN (Direct Current Electrode Negative), which provides deep, narrow penetration suitable for pipe welding.
Quality Verification and Inspection
After welding, the joints are subjected to a comprehensive inspection regime:
- Visual inspection (VT) for surface defects, bead uniformity, and reinforcement profile.
- Radiographic testing (RT) for internal defects such as porosity, lack of fusion, and slag inclusion.
- Mechanical testing including tensile tests and bend tests per ASME B31.3 or GB/T 2649.
- Hardness testing of the weld metal, heat-affected zone (HAZ), and base metal.
Study Insights and Engineering Relevance
This paper is particularly valuable for nuclear power installation and maintenance operations where carbon steel piping is extensively used. The systematic approach to dividing the welding circumference into positional intervals with distinct parameter sets reflects a mature understanding of the physics of molten pool behavior under gravity. In practice, the transition between positions can be challenging, and the author's emphasis on parameter optimization through trial welding is essential. I have found in my own experience that the overhead position is consistently the most critical, and the margin for error is narrowest. The paper's methodology of forming a mature WPS through iterative trials and comprehensive testing aligns well with the qualification requirements of ASME Section IX and GB/T 19542. The practical value of this work extends beyond the specific pipe size studied, as the principles of positional parameter adjustment can be scaled to larger diameters with appropriate modifications to current, gas flow, and travel speed.