Plasma Arc Welding and TIG Welding of Wireline Drill Pipe

Literature Overview

This study by Zhou Dazhong, Zhao Zhongcheng, Sun Zhongmin from the Harbin Welding Research Institute, together with Xu Chunsheng, Tian Kunshu, and Zhao Junhua from Fujian Geological Exploration Company (1996), addresses the specialized welding requirements of wireline drill pipes. These pipes, with diameters ranging from 53 mm to 108 mm, are used in deep geological drilling operations and require welds with no internal reinforcement due to the passage of drill tools through the bore.

Core Technical Content

Unique Welding Requirements of Wireline Drill Pipe

Wireline drill pipes differ from conventional casing or tubing in several critical aspects:

Internal surface finish requirement: The bore must remain smooth and free of weld reinforcement to allow the passage of wireline tools (such as wireline fishing tools, perforating guns, and logging tools) without interference.
High cyclic loading: The pipe experiences repeated bending and tensile loading as it is deployed and retrieved from deep boreholes, requiring welds with excellent fatigue resistance.
Deep bore welding: The internal bore may extend to depths of 1500 mm or more, requiring specialized welding equipment capable of reaching the weld location.
Composite pipe construction: Many wireline drill pipes are composite structures with a thick-walled outer tube and a thinner inner tube, requiring precise control of weld geometry on both sides.

PAW-TIG Combined Welding Method

The innovative solution proposed in this study involves simultaneous plasma arc welding (PAW) on the external surface and TIG (tungsten inert gas) welding on the internal bore. This combined approach allows:

The external PAW weld to provide the primary structural strength and penetration
The internal TIG weld to provide a smooth, flat internal surface with no reinforcement
Simultaneous welding from both sides ensures complete fusion and eliminates the need for a backing ring or backing gas

Parameter	External PAW	Internal TIG
Current	200-350 A	80-150 A
Arc Voltage	25-35 V	18-22 V
Shielding Gas	Argon (99.99%)	Argon (99.99%)
Gas Flow Rate	20-30 L/min	10-15 L/min
Travel Speed	150-250 mm/min	100-180 mm/min
Electrode	Tungsten (2.0-2.4 mm)	Tungsten (1.6-2.0 mm)
Nozzle Diameter	8-10 mm	6-8 mm

Welding Equipment Design

The specialized welding machine developed for this application features several innovative design elements:

Dual torch system: Two independently controlled welding torches mounted on a common traverse mechanism, synchronized to maintain constant travel speed.
Bore access mechanism: The internal TIG torch is mounted on a flexible shaft that can be inserted through the pipe bore to reach the weld location.
Gas sealing arrangement: Ceramic or copper backing rings seal the internal gas atmosphere, preventing atmospheric contamination of the internal weld.
Programmable speed control: The traverse speed is programmable to accommodate different pipe diameters and wall thicknesses.

Weld Quality and Performance

The PAW-TIG combined welds demonstrated excellent mechanical properties:

Property	Requirement	Achieved Value
Tensile Strength	≥ 450 MPa	520-580 MPa
Yield Strength	≥ 350 MPa	420-460 MPa
Elongation	≥ 15%	18-22%
Impact Energy (20°C)	≥ 40 J	55-70 J
Internal Reinforcement	≤ 0.5 mm	0.1-0.3 mm
Hydrostatic Test Pressure	1.5× working pressure	Passed at 2.0× working pressure

Integration with Engineering Practice

Application in Deep Drilling Operations

The PAW-TIG combined welding method has been successfully applied in the manufacture and repair of wireline drill pipes for geological exploration, oil and gas drilling, and mineral prospecting operations. The ability to weld at depths of 1500 mm or more significantly extends the serviceable length of drill pipes, reducing the number of connections required in a drilling string.

The smooth internal surface finish achieved by this method is critical for wireline operations, where tools must pass freely through the pipe bore. Even small amounts of weld reinforcement can cause tool binding, leading to operational delays and potential tool damage.

Common Defects and Prevention

Defect	Cause	Prevention
Internal undercut	Excessive internal TIG current	Reduce current; optimize travel speed
Lack of fusion	Insufficient external PAW penetration	Increase PAW current; verify gas flow
Porosity	Inadequate shielding on internal side	Verify gas sealing; increase gas flow
Burn-through	Excessive combined heat input	Reduce both currents; increase travel speed
Internal reinforcement	TIG torch positioned too far from weld	Adjust torch alignment; verify traverse speed

Key Reflections and Study Insights

The PAW-TIG combined welding method represents an elegant engineering solution to a challenging manufacturing problem. The key insight is that by using two different arc welding processes simultaneously from opposite sides of the weld, it is possible to achieve both structural integrity and surface finish requirements that would be impossible with a single process.

This approach demonstrates the value of process combination in advanced manufacturing. Rather than seeking a single process that can meet all requirements, the engineers developed a hybrid approach that leverages the strengths of each process: the deep penetration and high efficiency of PAW for the external weld, and the fine control and smooth surface finish of TIG for the internal weld.

The work also highlights the importance of specialized welding equipment in niche manufacturing applications. The development of the dual-torch welding machine required significant engineering effort, but the resulting productivity gains and weld quality improvements justified the investment. This principle remains relevant in modern manufacturing, where custom welding equipment continues to be developed for specialized applications.