What is this method?
In NDT, radiography is one of the most important and widely used methods. Radiographic testing (RT) offers several advantages over other NDT methods, however, one of its major disadvantages is the health risk associated with the radiation. This method of weld testing makes use of X-rays, produced by an X-ray tube, or gamma rays, produced by a radioactive isotope. The basic principle of radiographic inspection of welds is the same as that for medical radiography. Penetrating radiation is passed through a solid object, in this case a weld rather that part of the human body, onto a photographic film, resulting in an image of the object’s internal structure being deposited on the film. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object will cause exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film while inclusions of high density, such as tungsten, will appear as light areas. All discontinuities are detected by viewing shape and variation in density of the processed film.
Why is this method?
This testing method is usually used and mostly suited to have access to both sides of the welded joint (except for double wall signal image techniques used on some pipe work). Although this is a slow and expensive method of nondestructive testing, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of welds. It is essential that qualified personnel conduct radiographic interpretation since false interpretation of radiographs can be expensive and interfere seriously with productivity. There are obvious safety considerations when conducting radiographic testing. X-ray and gamma radiation is invisible to the naked eye and can have serious health and safety implications. Only suitably trained and qualified personnel should practice this type of testing.
When is this method used?
The part to be inspected by Radiographic testing, is placed between the radiation source and a piece of radiation sensitive film. The radiation source can either be an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases Cs-137). The part will stop some of the radiation where thicker and more dense areas will stop more of the radiation. The radiation that passes through the part will expose the film and forms a shadowgraph of the part. The film darkness (density) will vary with the amount of radiation reaching the film through the test object where darker areas indicate more exposure (higher radiation intensity) and lighter areas indicate less exposure (lower radiation intensity). Radiographic Testing is widely used in the industries like, Aerospace industries, Military defense, Offshore industries, Marine industries, Power-gen industries, Petrochem industries, Waste Management, Automotive industries, Manufacturing industries, Transport industries.
- This applies only to girth and miter groove welds and to fabricated branch connection welds unless otherwise specified in the engineering design.
- Random Radiography – This applies only to girth and miter groove welds.
- Spot Radiography – This requires a single exposure radiograph in accordance with para. 344.5.1 at a point within a specified extent of welding. For girth, miter, and branch groove welds the minimum requirement is
- for sizes ≤ DN 65 (NPS 21 ⁄ 2), a single elliptical exposure encompassing the entire weld circumference
- for sizes > DN 65, the lesser of 25% of the inside circumference or 152 mm (6 in.) For longitudinal welds the minimum requirement is 152 mm (6 in.) of weld length.
Roles and responsibilities:
A radiographer must visually examine materials, structures, or components using tools and equipment such as endoscopes, closed circuit television systems, and fiber optics for signs of corrosion, metal fatigue, cracks, or other flaws. And Inspect finished products to locate flaws. He/she must supervise or direct the work of non-destructive testing (NDT) trainees or staff and supervise engineering or other technical personnel.