Zhuojin Pipe Fitting Co., LtdTIG Repair Welding Technology for Small-Diameter Spiral Welded Steel Pipes
Literature Overview
The paper by Jiang Yongfeng and colleagues from Jiangsu Yulong Steel Pipe Technology Co., Ltd., published in Steel Pipe (2024, Vol. 53, No. 6, pp. 40–45), addresses a long-standing engineering bottleneck in the spiral-welded steel pipe (SSAW) manufacturing industry: the repair welding of internal weld defects in small-diameter pipes. The authors report the successful application of Tungsten Inert Gas (TIG) welding as a repair technique, achieving single-sided welding with double-sided formation, and demonstrating compliance with GB/T 9711—2023 PSL2 requirements.
Core Technical Challenge
Small-diameter spiral-welded pipes present a unique geometric constraint. Unlike large-diameter pipes where internal access permits conventional Shielded Metal Arc Welding (SMAW) repair, the narrow bore of small-diameter SAW pipes prevents the insertion of welding electrodes and associated equipment. This spatial limitation has historically forced manufacturers to either scrap defective pipes or accept marginal quality, both of which are economically and reputationally unacceptable. The core question the authors address is whether TIG welding, with its compact torch geometry and precise heat input control, can substitute for SMAW in this confined environment.
Process Analysis and Key Parameters
The authors selected TIG welding specifically because the torch diameter is significantly smaller than an SMAW electrode holder, allowing approach to the internal spiral weld seam through the pipe bore. The following process window was optimized:
| Parameter | Typical Range | Rationale |
|---|---|---|
| Shielding gas | Pure Ar (99.99%) | Stable arc, minimal oxidation for carbon steel |
| Backing gas | Ar or N₂/Ar mixture | Prevents internal oxidation during single-sided welding |
| Welding current | 80–160 A | Matches pipe wall thickness and defect geometry |
| Travel speed | 5–10 cm/min | Ensures full penetration without excessive HAZ |
| Tungsten electrode | CeLa₂O₂ or ThO₂, 2.4–3.2 mm | Longevity and arc stability in intermittent duty |
| Filler wire | ER70S-6 or matching grade | Mechanical property compatibility |
Single-Sided Welding with Double-Sided Formation
The most technically demanding aspect is achieving full penetration from a single side while maintaining a smooth, defect-free weld root on the opposite side. The authors describe the use of backing gas to maintain a reducing atmosphere on the root side, combined with precise control of arc travel speed and torch angle. The torch is held at approximately 10–15° from perpendicular to the pipe axis, allowing the weld pool to pool toward the root with controlled solidification.
Welding Materials Selection
The choice of filler metal is critical for maintaining the mechanical properties required by GB/T 9711—2023 PSL2. For Grade B and L245 pipes, ER70S-6 was selected; for higher grades (L290, L360), ER80S-G or ER80S-D were matched. The filler wire composition must ensure that the weld metal tensile strength meets or exceeds the minimum specified yield strength of the base pipe material.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Incomplete penetration | Insufficient current or excessive travel speed | Increase current by 10–20%, reduce speed |
| Internal oxidation | Inadequate backing gas coverage | Increase backing gas flow rate; use confined gas shroud |
| Porosity | Contamination of weld zone | Thorough pre-cleaning; dry shielding gas supply |
| Undercut on root side | Excessive arc drag angle | Correct torch angle to 10–15° from perpendicular |
| Distortion | Excessive heat input | Reduce current; use pulsed TIG; interpass temperature control |
Standards Compliance and Quality Verification
The repaired pipes were subjected to the full inspection regime required by GB/T 9711—2023 PSL2, which includes hydrostatic testing, radiographic testing (RT) of the repair weld, and mechanical property verification. The authors report that all repaired samples passed RT inspection with no indication of internal defects exceeding acceptance criteria, and tensile specimens from the repair zone achieved minimum tensile strength values exceeding the PSL2 requirements.
Engineering Practice Insights
From a manufacturing perspective, this work represents a significant productivity improvement. Previously, a single internal weld defect in a small-diameter SSAW pipe could result in the scrapping of an entire length, with material and processing costs easily exceeding several thousand yuan per meter. The TIG repair technique converts this into a controlled, cost-effective repair operation. However, the technique demands a high level of operator skill. The authors emphasize that the welder must maintain consistent torch angle, travel speed, and filler wire feed rate throughout the repair, particularly when working in the confined geometry of a small-diameter pipe. I would add that welder qualification testing under conditions that replicate the actual pipe geometry—rather than flat or horizontal test coupons—is essential for ensuring transferability of the technique to production.
Study Reflection
This paper is a practical contribution that bridges the gap between welding technology and production constraints. The authors did not merely propose a technique; they validated it through production trials against a current national standard. The selection of TIG over alternative processes such as laser welding or electron beam welding is well-justified by the equipment availability and cost considerations in the Chinese steel pipe industry. The technique could potentially be extended to other small-diameter welded pipe geometries, including small-diameter ERW pipes and spiral-welded stainless steel pipes, where similar internal access limitations exist.