CLADDING TECHNOLOGY SHANXI CO., LTDManual TIG Root Pass Technology in Boiler Pipe Welding Application
Literature Overview
This study note addresses a 1997 publication by Lin Shengwen from Beijing Municipal Construction Installation Company Vocational School, which examines the application of manual Gas Tungsten Arc Welding (GTAW/TIG) root pass technology in boiler pipe welding. Although dated, this work represents a critical milestone in Chinese boiler fabrication practice during a period when domestic boiler manufacturing was rapidly expanding to meet energy demands. The document is classified under welding technology and reflects the practical engineering challenges of the era.
Core Technical Content
The root pass in boiler pipe welding is arguably the most critical operation in the entire welding sequence, as it determines the internal geometry, penetration quality, and metallurgical integrity of the final joint. In boiler applications, pipes are typically made of carbon steel grades such as 20G, 12Cr1MoV, or 15CrMoG, with wall thicknesses ranging from 3 mm to 12 mm and diameters from DN50 to DN300. The root pass must achieve full penetration while maintaining a controlled internal reinforcement profile to avoid stress concentration and facilitate subsequent filler passes.
Process Parameters for Root Pass
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 60–120 A | Depends on pipe diameter and wall thickness |
| Arc voltage | 10–18 V | Maintained stable for uniform bead profile |
| Travel speed | 5–12 cm/min | Must be consistent for uniform penetration |
| Tungsten electrode | 1.6–3.2 mm diameter | Ceriated lanthana or thoriated |
| Shielding gas | Argon, 10–20 L/min | Internal backing gas for thin walls |
| Preheat temperature | 50–150 °C | For low-alloy steel grades |
| Interpass temperature | < 200 °C | Critical for 12Cr1MoV and 15CrMoG |
Technical Challenges and Solutions
The primary challenges identified in this work include:
- Internal reinforcement control – Excessive internal buildup creates stress risers and complicates internal inspection. The recommended technique involves careful manipulation of the tungsten electrode and a slight backward tilt of the arc to direct molten metal outward.
- Backside protection – For pipes with wall thickness below 6 mm, internal argon backing is essential to prevent oxidation of the root bead. The flow rate should be controlled to 5–8 L/min to avoid turbulence that disrupts the protective atmosphere.
- Heat input management – For creep-resistant grades like 12Cr1MoV, the heat input must be restricted to below 0.8 kJ/mm to prevent grain coarsening and loss of creep strength. This requires disciplined travel speed control.
- Weld groove preparation – The V-groove angle of 60–70 degrees with a root gap of 2–3 mm provides the optimal balance between penetration and bead profile control.
Connection to Pressure Vessel Fabrication Practice
In the context of pressure vessel fabrication governed by GB/T 150 and ASME VIII Div.1, the root pass quality directly impacts the acceptance of the entire weld. Non-destructive testing requirements under JB/T 4730 mandate 100% radiographic testing for Category A and B circumferential welds. A poorly executed root pass with incomplete penetration or excessive internal undercut will result in rejection and costly rework.
For bimetallic pressure vessels with stainless steel cladding, the root pass presents additional complexity. When welding through clad plate, the root pass must not burn through the cladding layer, which requires precise heat input control. The base metal (carbon steel) and cladding (stainless steel or nickel alloy) have different thermal conductivities and melting points, creating a narrow process window.
Key Reflections
This 1997 publication, while focused on a specific industrial application, contains fundamental principles that remain relevant today. The emphasis on welder skill, process discipline, and parameter control reflects the reality that manual TIG welding remains an operator-dependent process. In modern practice, we might supplement this with hot-wire TIG or enhanced TIG (ETIG) techniques for thicker sections, but the foundational understanding of arc physics, heat flow, and weld pool dynamics remains unchanged. The work also highlights the importance of operator training programs, which continues to be a critical element of quality assurance in pressure vessel fabrication.
The study serves as a reminder that even in an era of increasingly automated welding systems, the manual TIG root pass remains irreplaceable for many applications, particularly where joint geometry is complex or access is limited. Engineers involved in pressure vessel design must ensure that welding procedure specifications (WPS) account for the realities of manual root pass execution, including adequate fit-up tolerances and realistic qualification requirements.