Radiography (Metrology)

22.6.
When we talk about radiographic inspection we include X-ray, gamma ray and variations of these methods. Basically, method involves passing rays through materials to be tested. Rays impinge on a film or screen, reveal and record the internal structure of test object. Thus radiography records the transmitted X-ray radiation on a film to produce an image or shadow picture termed as “radiograph”. Applications of radiography in industry fall into two basic categories : (1) Foundry products ; (2) Welded products.
Defects that occur in castings and which can be detected by radiographic examination are : shrink, hot tears, porosity, lack of fusion around chaplets, inclusion of sand and tramp metal, coarse dendritic structure and cracks. Shrink resulting from certain foundry practice faults is characterised by its location and appearance. As the radiographic image gives a true projection of this condition, the foundaryman can use this information in making remedial changes. Under certain circumstances shrink produces cracklike void commonly called “hot tears”. These flaws generally occur at the junction of heavy and thin section of the casting and can easily be identified from the X-ray film.
Gases not liberated from the mould create various forms of porosity in the casting and this will be revealed by radiographic inspection. Similarly other defects are also inspected. In all these cases the sensitivity limit is about 2 per cent. (Radiographic sensitivity is a measure in percentage of the minimum thickness change evident in a radiographic image. Standard
practice requires that radiographs show evidence of a minimum sensitivity of 2 per cent. Thus if a 25 mm thick plate of steel is radiographed, a section change in the form of an increase or decrease in thickness equal to 0.5 mm or 2 per cent must be visible on the film).
Radiography has been extensively used to examine welds of pressure vessels, storage containers, aircraft landing gears and, propellers. When faulty welding is located by a radiography the weld metal is chipped out and the seam rewelded. X-ray examination is then made to determine the quality of repair. Most common flaws detected are: gas porosity, trapped slag, lack of fusion or incomplete penetration, cracks etc. Radiography is not reliable for the detection of extremely fine flaws originating at the weld or plate surface.