Radiographic Testing
Principles
Radiography is carried out using x-ray machines or artificial gamma sources (radio-isotopes).
X-rays or gamma rays pass through the object to be radiographed and record an image on a radiographic film on the opposite side. The radiation reaching the film will be determined by the object’s thickness and density, e.g. lack of root penetration in a weld will increase the amount of radiation falling on the film in that area due to a reduction in thickness.
For x-radiography 150-300 kV is typically used on steel weldments up to approximately 20mm total thickness.
Cobalt 60 (Co60) has a very high penetrating power – very short wavelength – and can be used on materials up to 200 mm thick. Iridium 192 (Ir192) is commonly used on steel weldments up to 50 mm thick; this therefore produces radiation with a longer wavelength than Co60.
It is the wavelength of the radiation which governs its penetrating power; this is governed by the kilovoltage (kV) when using x-rays, and isotope type with gamma rays. The intensity of the radiation is governed by the milli-amperage (mA) when using x-rays, and by the activity of the specific isotope with gamma. Activity is measured in Curies (Ci) or gigabecquerels (GBq).
A negative is produced when the film is processed. The thin areas of an object will be darker than the thicker areas, therefore most weld defects will show up dark in relation to the surrounding areas; exceptions are excess weld metal, spatter, tungsten and copper inclusions.
Radiographic quality
An overall assessment of radiographic quality is made by the use of image quality indicators (IQI’s); these usually consist of seven thin wires decreasing in thickness. IQI(s) are pre-placed on the weld being examined and therefore show on the radiographic image. The more wires visible the better the flaw detection sensitivity is likely to be.
The density – degree of blackness – of a radiograph is also measured by using a densitometer to ensure it lies within a specified range for optimum quality.
Advantages and disadvantages
X-radiography requires bulky and expensive machinery in comparison with gamma radiography, but x-radiography generally produces better quality radiographs and is safer. X-ray machines can be switched on and off, unlike gamma sources which permanently produce radiation and therefore require shielding when not in use.
A major disadvantage with radiography is that it will only detect defects which have significant depth in relation to the axis of the x-ray beam – roughly over 2% of the wall thickness in the same axis as the x-ray beam, i.e. radiography will not usually detect plate laminations, lack of inter- run fusion or cracks perpendicular to the x-ray beam.
A major advantage of radiographic testing is that a permanent record is produced, i.e. the radiograph.