Zhuojin Pipe Fitting Co., LtdApplication of Automatic Pulsed TIG Backfill and Submerged Arc Capping on Small Diameter Pipes
Literature Overview
This 1994 paper by Li Zhenlun from Jinan Boiler Works, published in Welding Technology (Vol. 23, No. 4, pp. 22–23), addresses a practical and enduring challenge in boiler and pressure vessel fabrication: achieving high-quality, efficient butt welds on small-diameter steel pipes. The author proposes a hybrid welding sequence combining automatic pulsed TIG for root backfill followed by automatic submerged arc welding (SAW) for capping passes. The central thesis is that this combination significantly improves weld quality, multiplies production efficiency, and reduces welder fatigue. The paper is classified under TG444, reflecting its focus on arc welding processes.
Core Technical Approach
The hybrid process described here leverages the complementary strengths of two welding methods. Pulsed TIG (GTAW) provides excellent arc stability, precise heat input control, and superior penetration characteristics for root passes on thin-walled or small-diameter pipes. SAW, on the other hand, delivers high deposition rates, deep penetration, and excellent mechanical properties for fill and cap passes. By assigning the root to pulsed TIG and the remaining passes to SAW, the process exploits the precision of TIG where it matters most and the productivity of SAW where volume deposition is required.
Process Parameters and Critical Variables
The author identifies two factors as particularly critical to weld quality: welding linear energy (heat input) and electrode offset (weld wire deflection). These parameters directly influence weld geometry, HAZ microstructure, and residual stress distribution.
| Parameter | Typical Range for Small Diameter Pipes | Effect on Weld Quality |
|---|---|---|
| Pulsed TIG current | 80–160 A (DC) | Controls root penetration and reinforcement |
| Pulse frequency | 2–8 Hz | Influences arc stability and bead width |
| Pulse-on time | 20–60% of cycle | Governs heat input per pulse |
| SAW current (capping) | 350–550 A | Determines deposition rate and penetration |
| SAW voltage | 28–36 V | Affects arc length and bead profile |
| Travel speed | 0.5–1.5 m/min | Controls heat input and weld shape |
| Wire offset | 0–5 mm | Prevents burn-through on thin walls |
Heat Input Management
For small-diameter pipes (typically DN25 to DN100, with wall thicknesses of 3–8 mm), the allowable heat input per pass is tightly constrained. Excessive heat input causes excessive HAZ grain growth, potential softening in the base metal, and distortion. The pulsed TIG root pass typically operates at a linear energy of 0.3–0.8 kJ/mm, while SAW capping passes may reach 0.8–1.5 kJ/mm depending on the number of passes and the required reinforcement. The author emphasizes that matching these heat inputs to the pipe diameter and wall thickness is essential for avoiding defects such as burn-through, lack of fusion, and excessive undercut.
Engineering Practice Insights
From a production standpoint, the hybrid approach described here represents a significant departure from the all-TIG or all-SAW methods that were common in the early 1990s for small-bore pipe welding. The productivity gains are substantial: SAW capping passes deposit metal at rates 3–5 times higher than TIG, and the automated nature of both processes eliminates the variability associated with manual welding. However, the setup time for automatic TIG and SAW equipment on small-diameter pipes can be considerable, and the approach is most economical for batch production rather than single-piece fabrication.
Defect Analysis and Countermeasures
Based on the process description and the author's emphasis on linear energy and wire offset, the following defect-prone scenarios and countermeasures can be inferred:
| Defect | Likely Cause | Countermeasure |
|---|---|---|
| Burn-through | Excessive root heat input | Reduce pulse-on time; lower peak current |
| Lack of fusion (root) | Insufficient penetration | Increase pulse frequency; optimize gas flow |
| Excessive undercut (cap) | Wire offset too large | Adjust wire guide; reduce offset to 2–3 mm |
| Porosity | Inadequate shielding | Increase flow rate; check gas purity |
| Weld reinforcement too high | Excessive SAW current | Reduce current; increase travel speed |
Standards and Code Considerations
The welding sequence described here would need to comply with applicable pressure vessel and boiler codes. In the Chinese standard system, this would involve GB/T 150 (pressure vessels) or SY/T 0407 (oilfield welding procedures). Internationally, ASME Section IX would require qualification of the combined WPS covering both the pulsed TIG root and SAW fill/cap passes. The weld procedure specification must define the heat input range, interpass temperature limits, and the minimum number of qualification coupons representing the pipe geometry.
Study Reflections
Reading this paper as a retrospective document from 1994, I find it remarkable for its forward-looking approach. The concept of hybrid welding—combining different processes for different weld zones—has since become standard practice in modern pipe fabrication. Today, robotic pulsed TIG root with FCAW or SAW fill/cap is routine in line pipe and pipe fitting production. The key insight that has aged well is the recognition that heat input control and wire positioning are the two dominant variables in automated pipe welding. These remain the primary optimization targets in modern welding systems, whether the process is TIG, SAW, FCAW, or even laser-hybrid welding. The paper's brevity (two pages) is a limitation, but the technical content is sound and directly applicable to modern engineering practice.
Summary and Implications
The paper presents a pragmatic and effective solution to the challenges of welding small-diameter steel pipes: use pulsed TIG for precise root penetration and SAW for high-productivity capping. The emphasis on linear energy and wire offset as critical control variables remains valid decades later. For engineers designing welding procedures for small-bore pipe assemblies, this literature provides a foundational reference for hybrid process design, and its principles are directly transferable to contemporary automated welding systems.