Radiographic Testing of Butt Welds on Equal-Tee Fittings

Literature Overview

This paper by Dong Jiyuan, published in the journal Nondestructive Testing (Volume 30, Issue 1, 2008, pages 64-65), addresses a highly practical yet technically challenging problem in the inspection of steel pipe fittings: the radiographic examination of the intersecting-line (interference-line) welds on equal-diameter tees. The author, affiliated with Zhejiang Nondestructive Testing Engineering Technology Company, draws upon field experience to describe the specific difficulties encountered when performing radiography on these complex geometries and proposes practical solutions.

Technical Challenges in Radiography of Tee Intersecting-Line Welds

The intersecting-line weld on an equal-diameter tee is a circumferential weld formed at the junction where the branch pipe meets the run pipe. Unlike a simple butt weld on a straight pipe, this weld presents several unique challenges for radiographic testing:

• Geometric complexity: The weld follows a hyperbolic curve on the surface of the tee, making it difficult to achieve uniform irradiation across the entire weld length.
• Variable wall thickness: The effective thickness seen by the X-ray beam varies significantly along the weld path, from the thinnest point at the branch opening to the thickest point where the branch pipe walls overlap with the run pipe wall.
• Film coverage: A single film may not adequately cover the entire intersecting-line weld, necessitating multiple exposures or special film positioning techniques.

Key Technical Points and Solutions

Source-to-Film Geometry

The paper emphasizes the critical importance of the source-to-film distance (SFD) and the source-to-object distance (SOD) configuration. For tee intersecting-line welds, the recommended approach involves:

Film Positioning and Coverage

The author discusses several practical techniques for achieving adequate film coverage:

1. Single-film technique: For small-diameter tees (DN50 and below), a single large-format film can sometimes cover the entire intersecting-line weld when positioned correctly.
2. Multi-film overlap technique: For larger tees, multiple films are arranged with overlapping edges to ensure no portion of the weld is missed. The overlap should be at least 25 mm to allow for image matching.
3. Source positioning: The X-ray source should be positioned perpendicular to the weld line at each exposure position, with careful attention to the angle of incidence to minimize distortion.

Density (Blackness) Control

The paper highlights that the variable wall thickness along the intersecting-line weld creates a challenge for achieving uniform film density. The recommended approach is:

• Use a step-wedge or density gauge at each exposure to verify that the density falls within the acceptable range.
• For regions with significantly different thicknesses, separate exposures may be required with different exposure times.
• The minimum acceptable density should be 1.8 and the maximum should not exceed 4.0, per GB/T 3323 or equivalent standards.

Engineering Practice Insights

From a practical standpoint, this paper raises several important points that warrant careful attention in field inspection:

1. Pre-inspection planning: Before performing radiography on tee intersecting-line welds, a detailed inspection plan should be developed that considers the tee geometry, weld size, and available radiographic equipment. The plan should specify the number of exposures, film sizes, and source positions.
2. Weld preparation: The weld surface should be cleaned of any slag, spatter, or coating that could interfere with the radiographic image. Any significant weld reinforcement or undercut should be documented, as these features can affect the interpretation of the radiograph.
3. Image interpretation: The intersecting-line weld appears as a curved band on the radiograph. Defects such as lack of fusion, incomplete penetration, porosity, and cracks must be distinguished from the normal geometry of the weld. The hyperbolic shape of the weld can sometimes create misleading shadow patterns that may be misinterpreted as defects.
4. Quality assurance: The radiographic examination should be performed by a qualified Level III or Level II inspector in accordance with the relevant standards (GB/T 3323, NB/T 47013, or ASME Section V). The quality of the radiograph should be verified by examining the image quality indicator (IQI) or wire gauge at each exposure position.

Study Reflections

This paper, while brief, addresses a genuinely challenging aspect of radiographic testing that is often overlooked in standard training. The intersecting-line weld on a tee is one of the most difficult welds to inspect radiographically due to its complex geometry and variable thickness. The practical solutions proposed by the author, including multi-film overlap techniques and careful source positioning, represent valuable field experience that can significantly improve inspection reliability.

One area that could be further developed in future research is the application of computed radiography (CR) or digital radiography (DR) to tee intersecting-line welds. These technologies offer advantages such as adjustable image contrast, digital image processing, and the ability to combine multiple exposures into a single panoramic image, which could greatly simplify the inspection process.

In summary, this paper provides a concise and practical guide to the radiographic examination of equal-diameter tee intersecting-line welds, emphasizing the importance of careful planning, proper source-to-film geometry, and thorough image coverage. The techniques described are directly applicable to field inspection and represent a valuable contribution to the nondestructive testing community.