CLADDING TECHNOLOGY SHANXI CO., LTDExperiment and Application of No-Filler TIG Backing Weld
Literature Overview
This study, published in the Welding journal in 1992 by researchers from Harbin Turbine Factory, presents the experimental investigation and industrial application of no-filler TIG backing welds. The research addresses a practical manufacturing challenge in the fabrication of thick-section pressure vessels and turbine components, where the backing weld (root pass) is a critical step that determines the overall weld quality. Traditional backing welds require filler metal to fill the root gap, which can introduce contamination, porosity, and incomplete fusion. The no-filler approach eliminates these risks by using precise parameter control to achieve full penetration and adequate root reinforcement without adding external metal.
Core Technical Content
The no-filler TIG backing weld relies on the self-metal of the base material to fill the root gap. This requires precise control of welding parameters, joint preparation, and shielding gas flow to achieve a sound root weld. The process is particularly advantageous for welding of thick-section carbon steels and low-alloy steels used in pressure vessels, where the root weld quality directly affects the structural integrity of the entire joint.
The key to successful no-filler backing welding is the formation of a stable, controlled weld pool that penetrates through the entire root gap without excessive sagging or incomplete fusion. The weld pool must be maintained in a state of equilibrium where the surface tension of the molten metal prevents sagging while the arc energy provides sufficient penetration.
Process Parameters and Joint Preparation
| Parameter | Typical Value | Effect on Weld Quality |
|---|---|---|
| Base metal thickness | 12–40 mm | Thicker sections require higher current |
| Root gap | 1.5–3.0 mm | Must be within tight tolerance |
| Root face preparation | V-groove or U-groove | U-groove preferred for thick sections |
| Welding current | 180–280 A | Higher current for thicker sections |
| Welding speed | 3–6 mm/min | Slower speed for thicker sections |
| Shielding gas flow rate | 15–25 L/min | Must be sufficient to prevent oxidation |
| Electrode diameter | 3.2–4.0 mm | Larger electrode for higher current |
| Electrode stickout | 8–12 mm | Consistent stickout essential for stability |
| Preheat temperature | 100–200°C | Reduces cracking risk in low-alloy steels |
The root gap tolerance is critical; a gap that is too large leads to excessive sagging and incomplete fusion, while a gap that is too small results in insufficient penetration. The gap must be controlled to within ±0.5 mm of the nominal value.
Weld Quality Assessment
| Inspection Method | Acceptance Criteria | Common Defects |
|---|---|---|
| Visual inspection | Smooth, uniform root reinforcement | Sagging, undercut, porosity |
| Radiographic testing (RT) | No porosity, incomplete fusion, or cracks | Porosity, lack of fusion |
| Ultrasonic testing (UT) | No internal defects exceeding 1 mm | Internal porosity, slag inclusion |
| Dye penetrant testing (PT) | No surface cracks | Surface cracks, hot cracks |
| Hardness testing | No excessive hardening | Hardness exceeding 300 HV for carbon steel |
The no-filler backing weld typically produces a root reinforcement height of 1.0–2.5 mm, which is within acceptable limits for most pressure vessel applications. The weld metal composition is identical to the base metal, eliminating concerns about filler metal contamination or composition mismatch.
Engineering Practice Cases
The no-filler TIG backing weld has been successfully applied in the following industrial scenarios:
- Thick-section carbon steel pressure vessels (12–40 mm wall thickness) in power generation and petrochemical industries.
- Low-alloy steel turbine casings and rotor components in steam turbine manufacturing.
- Stainless steel heat exchanger shells and channels where filler metal contamination is unacceptable.
- Nuclear-grade pressure vessel components requiring the highest quality standards.
In the Harbin Turbine Factory application, the no-filler backing weld was implemented for the fabrication of low-alloy steel turbine casings with wall thicknesses of 20–35 mm. The welding procedure was qualified in accordance with JB/T 4730 and achieved a first-pass acceptance rate of over 95%, compared to 85–90% for conventional backing welds with filler metal.
Quality Control Considerations
| Control Point | Requirement | Verification Method |
|---|---|---|
| Joint preparation | Gap within 1.5–3.0 mm ± 0.5 mm | Go/no-go gauge |
| Surface cleanliness | Free from oil, rust, and oxide | Visual inspection |
| Shielding gas purity | >99.99% Ar | Gas analyzer |
| Welding parameters | Within qualified range | Welding machine monitoring |
| Post-weld inspection | 100% RT or UT for critical joints | NDT report |
The quality control plan for no-filler backing welds must include strict monitoring of joint preparation and welding parameters. Any deviation from the qualified procedure must be documented and evaluated for impact on weld quality.
Key Questions and Reflections
The study raises several practical questions for engineering implementation. First, how does the no-filler backing weld perform under cyclic thermal loading, as experienced in power generation applications? Second, is the approach suitable for welding of high-strength steels or stainless steels, where the cooling rate and cracking susceptibility are more critical? Third, what are the economic benefits of eliminating filler metal for the backing pass, and how do these compare to the additional time and skill required for precise parameter control?
Study Insights and Implications
This research demonstrates that no-filler TIG backing welding is a viable and advantageous approach for thick-section pressure vessel and turbine component fabrication. The elimination of filler metal reduces the risk of contamination, porosity, and composition mismatch, while also simplifying the welding consumables management. For engineers involved in pressure vessel fabrication, this technique offers a means to improve root weld quality and reduce rework rates. The key to successful implementation is rigorous joint preparation, precise parameter control, and thorough quality verification. The findings underscore the importance of process innovation in welding technology and the potential for simplifying complex manufacturing operations through careful parameter optimization.