Radiography (X-ray and Gamma Ray)

Radiography is one of the most useful NDT, which can be applied for inspection of cracks, porosity, blowholes, etc. in welds of all types and thickness ranging from minute welds in electronic components to welds up to half metre thick employed in heavy fabrications. The principle of the technique is based on exposing the components to short wavelength radiation in the form of X-rays (wavelength less than 0.001 × 10- 8cm) or gamma rays (wave length about 0.005 × 10- 8 to 3 ×10- 8 cm) from a suitable source such as an X-ray tube or cobalt-60. The position of the weld where defects are suspected is exposed to X-rays. During exposure, X-rays penetrate the welded object and thus affect the X-ray film kept behind. Since most defects (such as blowholes, porosity, cracks, etc.) possess lesser density than the parent metal, they transmit X-ray better than the surround parent metal does. Therefore the film appears to be darker where defects are in line of the X-ray beam.

Gamma rays are utilized for detecting defects in welded plates thicker than those inspected by X-rays. Gamma rays can inspect a number of welded objects at a time. Radium and its salts (now days decompose at a constant rate, emitting gamma rays which are of shorter wave length and more penetrating than ordinary X-rays. Unlike X-rays, gamma rays from its source are emitted in all directions; therefore a number of separate welded objects having cassette containing film, fastened to the back of every object, are disposed in a circle around the source placed in a central position.

A comparison chart of all the above techniques is specified in Table for quick reference with their advantages, limitations, while and where to apply. The student is advised to collect details for other NDT techniques, which are not discussed here.