Manual Welding Overlay Process for Inner Wall of Stainless Steel Tubes

Literature Overview

This paper by Sun Yong, published in Oil and Gas Field Surface Engineering in 2008, documents the development and application of a manual welding overlay process for the inner wall of stainless steel tubes used in water supply engineering at the Daqing Petroleum Administration Bureau. The technical challenge addressed is the fabrication of overlay-lined tubes for water distribution systems where corrosion resistance and hygiene requirements demand a stainless steel internal surface on a more economical carbon steel or alloy steel base tube.

Core Technical Content

Engineering Background and Requirements

The Daqing Petroleum Administration Bureau operates extensive water supply networks serving oil production facilities. The water chemistry in these systems often contains chlorides, sulfides, and other corrosive species that can attack carbon steel pipes. The solution adopted was to manufacture composite tubes with carbon steel outer walls (for mechanical strength and cost efficiency) and stainless steel inner walls (for corrosion resistance and potable water compatibility). The inner diameter of the tubes typically ranges from 200 mm to 600 mm, with wall thicknesses of 6–12 mm.

Tube Specification	Value
Outer diameter	219–630 mm
Base tube material	20# steel or 16Mn
Overlay material	304 or 316L stainless steel
Overlay thickness	2.0–3.0 mm
Overlay process	Manual GTAW (TIG)
Tube length	6000–12000 mm
Service condition	Water supply, temperature 20–60 °C

Manual GTAW Overlay Process Development

The manual GTAW overlay process for tube interiors presents unique challenges due to restricted access, limited visibility, and the need for uniform coverage over the entire inner circumference. The process development involved the following key steps:

Root pass preparation: A tungsten electrode (2.0–3.2 mm diameter, LaB6 or WC type) is used with pure argon shielding (flow rate 8–12 L/min). The welding current is set at 80–150 A depending on overlay thickness and electrode diameter.
Fill pass deposition: Subsequent passes are deposited using ER308L or ER316L filler wire (1.6–2.4 mm diameter). Each pass is deposited with a slight overlap (25–30% of wire diameter) to ensure full coverage and avoid lack of fusion between passes.
Travel speed control: Manual travel speed is maintained at 30–60 mm/min, with the welder adjusting speed to maintain a consistent weld bead width of 12–18 mm.
Positional welding: The tube is rotated mechanically (using a tube rotating fixture) while the welder maintains a fixed position, ensuring uniform weld bead geometry around the entire circumference.

Process Parameters and Quality Control

Parameter	Specification	Rationale
Shielding gas	100% Ar, 8–12 L/min	Prevents oxidation of austenitic weld metal
Back purge	Ar, 2–4 L/min	Protects root side of overlay
Electrode stick-out	8–12 mm	Balances arc stability and visibility
Weave width	10–15 mm	Ensures full substrate coverage
Interpass temperature	<150 °C	Prevents excessive grain growth
Post-weld treatment	None (solution anneal at 1050 °C optional)	Maintains as-welded properties

Engineering Practice Considerations

Welder Qualification and Skill Requirements

The manual GTAW overlay of tube interiors is highly dependent on welder skill. The following qualification criteria are recommended:

Welder must hold a valid GTAW qualification per NB/T 47014 or ASME IX for the relevant material combination.
Practical qualification test should include welding on a representative tube section with internal access limited to a 60–90° window.
Qualification weld specimens should be subjected to metallographic examination to verify overlay thickness uniformity (±0.5 mm tolerance), absence of unmelted particles, and sound bond at the interface.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Porosity	Inadequate shielding, moisture in filler wire	Increase gas flow, use oven-dried filler wire
Lack of fusion	Low current, excessive travel speed	Increase current by 10–15%, reduce travel speed
Undercut	Excessive current, poor weave technique	Reduce current, maintain consistent weave pattern
Excessive dilution	High current, wide weave	Reduce current, narrow weave width
Cracking	High residual stress, hydrogen pickup	Reduce interpass temperature, use low-hydrogen filler

Economic and Practical Evaluation

The manual GTAW overlay process, while labor-intensive, offers several advantages for the specific application of water supply tube lining:

Low capital investment (no specialized equipment beyond standard TIG welding station and tube rotator)
Flexibility to handle various tube diameters and lengths
Acceptable deposition rate for the required overlay thickness (2–3 mm)
Good weld quality achievable with skilled welders

The primary disadvantage is the relatively low deposition rate (approximately 0.5–1.0 kg/h) compared to automated processes such as PTA or laser cladding. For high-volume production, automated processes should be considered. However, for batch production of small quantities or repair applications, manual GTAW remains a practical and economical solution.

Study Insights and Outlook

This paper demonstrates the practical application of manual welding techniques in solving real engineering problems with limited resources. The process development approach—starting with root pass optimization, followed by systematic parameter adjustment and quality verification—is a model for welding procedure development in industrial settings. The key insight for engineers is that the most appropriate welding process is not always the most advanced one; rather, it is the one that best balances quality, cost, and practicality for the specific application. For future work, the integration of semi-automated GTAW (with mechanized tube rotation and electrode feed control) could improve consistency and productivity while maintaining the flexibility of manual welding.