Radiography Testing

A variety of NDT techniques are available for detection and characterisation of defects in welds. All NDT techniques are based on physical principles. Nearly every form of energy is used as probing medium in NDT. Likewise nearly every property of the materials to be inspected has been made the basis for some method or technique of NDT. In general, NDT methods involve subjecting the material (being examined) to some form of external energy source (X-rays, ultrasonic, thermal wave, electromagnetic fields etc.) and analysing the detected response signals (refracted energy, induced voltage and diffracted energy).

Inspection of welds

The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

RADIOGRAPHY
As the X-ray absorption coefficient depends strongly on material density, radiography is particularly effective at detecting volumetric defects, which contain either extra mass or missing mass (such as slag inclusions or porosity). The benchmark for radiographic inspection of welds is still high-quality film radiography and good radiographic practice is now enshrined by a series of national standards, covering factors such as choice of voltage, film–source distances, intensifiers, image quality indicators, film density, film processing, etc. There have been a number of advances in radiography over the past 10–15 years including more reliable microfocus tubes, real-time radiography and the application of image processing techniques to sharpen the image and to increase the contrast. For better definition of defects and delectability of small defects like micro-cracks in thin components and complex geometries, high resolution micro-focal X radiography has an edge over the conventional radiography. One of the important applications of micro-focal radiography is evaluation of tube to tube sheet weld joints of PFBR steam generators (made by welding between pull out of tube sheet and the tube).

The most significant recent development in radiography has been the real-time radiography. Real time radiography or fluoroscopy differs from conventional radiography in that the X ray image is observed on a fluorescent screen rather than recorded on a film. Fluoroscopy has the advantages of high speed and low cost of inspection. Present day real time systems use image intensifiers, video camera and monitor. The principal advantages of real-time radiography are that it is well suited to automation and the images of the component under inspection are available directly without time delays due to film exposure and processing. Furthermore, as the images are provided in digital form, image processing and automatic defect interpretation softwares can be readily incorporated into the inspection system. On-line monitoring of welding is another possibility by real time radiography. Direct examination of the welds in real time saves films and time and is found to be cost effective in the long run [5]. The use of microfocal units in conjunction with image intensifying system greatly enhances the versatility and sensitivity of the real time radiography, by way of zooming or projection magnification.

With the advent of image processing systems, the sensitivity that can be achieved is comparable to film sensitivity. The stored or digitized X-ray image can be subjected to image processing and enhancement techniques such as contrast stretching, edge enhancement, special filtering, differentiation, averaging, and pattern recognition for enhanced detection of defects and also for obtaining quantitative information. The versatility of image processing is that this can be performed in real time as well as on film images. Figures 1(a) and 1(b) show typical radiograph of a weld joint. Figure 1(a) gives the raw image wherein penetrameter wires are not clearly seen. After contrast stretching and image enhancement (Fig. 1(b)), the lack of penetration can be seen and the wire penetrameters can be identified thereby increasing the sensitivity.