Zhuojin Pipe Fitting Co., LtdFull-Position Pulsed TIG Welding of Small Diameter Steel Pipes
Literature Overview
This paper, published in Welding (2011, No. 12, pp. 36-38) by Zheng Naqing, Li Penghui, and Liang Jianglong from Hebei Taihang Machinery Industry Co., Ltd., presents a practical welding solution for circumferential butt joints of small diameter, thin-walled steel pipes. The study employs a German ORBIMATIC pipe-to-pipe full-position automatic welding machine and demonstrates a two-step welding strategy combining pulsed TIG welding with post-weld heat treatment to achieve both excellent weld appearance and optimized microstructure.
Core Technical Points
The primary challenge in welding small diameter thin-walled steel pipes is the limited heat input tolerance. These pipes, typically used in hydraulic systems, fuel lines, and structural tubing, have wall thicknesses often in the range of 1.0 to 3.0 mm and outer diameters from 10 to 50 mm. The small cross-section means that excessive heat input leads to burn-through, severe distortion, and degradation of mechanical properties, while insufficient heat input results in lack of fusion and incomplete penetration.
Two-Step Welding Strategy
The authors propose an innovative two-step approach:
Step 1: Formation Control
- Full-position pulsed TIG welding with single-side welding and double-side formation (SSWDSF)
- Welding parameters are selected differently for each angular segment of the circumferential joint
- The goal is to achieve aesthetically pleasing weld profiles on both the inside and outside surfaces
- Pulsed TIG provides precise heat input control, reducing the risk of burn-through in thin walls
Step 2: Microstructure Regulation
- After the first pass, a re-burn or heat treatment pass is applied to the weld seam
- Appropriate current and arc parameters are used to remelt the weld zone partially
- This effectively eliminates columnar grains in the weld metal and refines the grain structure
- The result is a more isotropic and uniform microstructure with improved mechanical properties
Process Parameter Optimization
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Welding current (pulsed) | 30-80 A | Controls heat input and penetration depth |
| Pulse frequency | 2-10 Hz | Affects weld bead width and surface profile |
| Travel speed | 100-400 mm/min | Determines heat input per unit length |
| Shielding gas flow rate | 8-15 L/min | Ensures adequate arc and weld pool protection |
| Re-burn current | 20-40 A | Controls partial remelting depth for grain refinement |
| Re-burn speed | 200-500 mm/min | Determines heat treatment intensity |
Full-Position Welding Challenges
Full-position welding of small diameter pipes introduces unique challenges compared to flat or horizontal welding. As the torch moves around the pipe circumference, the gravity vector relative to the weld pool changes continuously. This affects:
- Top position (12 o'clock): The weld pool is supported by the pipe wall below, requiring higher heat input to achieve full penetration.
- Side positions (3 and 9 o'clock): Gravity pulls the molten metal sideways, requiring careful control of current and speed to prevent sagging.
- Bottom position (6 o'clock): The weld pool is unsupported, creating the highest risk of burn-through and sagging. Lower heat input and slower travel speed are typically required.
The ORBIMATIC welding machine addresses these challenges through synchronized control of torch rotation, pipe rotation, and welding parameters. The ability to vary parameters by angular segment allows the operator to optimize the welding process for each position.
Microstructure Analysis and Engineering Significance
The elimination of columnar grains through the re-burn pass is a particularly significant finding. In TIG welding of steel, columnar grains form due to the directional solidification from the fusion boundary toward the weld center. Columnar grains can be detrimental to mechanical properties, particularly impact toughness and resistance to fatigue crack propagation, because cracks can propagate easily along the grain boundaries parallel to the weld axis.
The re-burn pass introduces additional heat into the weld zone, partially remelting the columnar structure and allowing equiaxed grains to nucleate and grow. This grain refinement improves the isotropy of the weld metal and enhances its resistance to cracking under cyclic loading. For applications involving small diameter steel pipes in high-stress or fatigue-critical service, this two-step approach provides a significant quality improvement over single-pass welding.
Key Questions and Reflections
The study raises an important question about the optimal re-burn parameters. Too much heat input during the re-burn pass could degrade the mechanical properties of the heat-affected zone or cause excessive grain growth, while too little would fail to eliminate the columnar structure. The authors' experimental optimization provides guidance, but the exact parameters depend on the specific steel grade, wall thickness, and base metal condition.
Another consideration is the repeatability and consistency of the two-step process in production environments. The ORBIMATIC machine provides a high degree of automation and reproducibility, which is essential for maintaining consistent quality across large production volumes. However, the setup time and programming required for each pipe diameter and thickness combination must be considered in the overall manufacturing cost.
Study Insights and Implications
This paper presents a practical and effective solution for welding small diameter thin-walled steel pipes in all positions. The two-step strategy of formation control followed by microstructure regulation is a clever approach that addresses both the aesthetic and metallurgical requirements of the weld joint. The use of pulsed TIG welding provides the necessary heat input control for thin walls, while the re-burn pass offers a simple and effective method for grain refinement.
For engineers in the pipe manufacturing industry, this approach has direct applicability to production welding of small diameter tubes for hydraulic, automotive, and structural applications. The key lesson is that achieving high-quality welds in thin-walled small diameter pipes requires not only precise heat input control but also consideration of the post-weld microstructure. The two-step welding strategy provides a practical framework for meeting both requirements.
In summary, this study demonstrates that a combination of advanced welding technology (pulsed TIG, full-position automation) and metallurgical awareness (columnar grain elimination) can produce high-quality circumferential welds in small diameter steel pipes, meeting both visual and mechanical performance requirements.