Varian Linatron High-Energy X-ray Applications 2007

When radiographing curved sections, the concave side
should face the source and, when possible, flexible film
holders should be formed to the casting contour. While
this practice tends to improve film/screen contact, it may
increase the image distortion at the edges of the curved
film. To control for this, image quality indicators should
be placed at the extreme widths of the film, and there
should be adequate overlap in adjacent exposures to
ensure complete coverage. While an oblique angle may
be required in some regions, when possible, the X-ray
beam should enter the casting at right angles to the front
surface, and the film should be set up perpendicular to
the central ray.
When the casting thickness varies more than 1/2-inch
(12.7 mm) equivalent steel, the multiple-film technique
can be employed to cope with the varying section
thicknesses. These variations may also create significant
scatter, which can be minimized using special procedures
(see the discussion on scatter control).
Layout and Marking. Radiographic assignments can
involve fairly simple techniques. This can be as simple as
placing the film cassette under or behind the casting and
making the exposure. But, for large, heavy castings such as
turbine bodies, the film placement and radiographic
projections can be complex and may require careful
planning and fixturing.
Crane mounted Linatron used to examine large objects.
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The radiographer should be furnished with either a marked
print of the casting, showing the projections and areas to
be covered, or a rough sketch showing the film locations,
markers and other relevant information. The marked print
or sketch should remain with the casting radiographs. The
radiographer should retain a copy to add information
about the film and screens used, image quality indicator
selection and placement, filtration, exposure and resulting
film density, and other pertinent facts about the job.
Casting Radiographic Procedures. When compared with
lower energy systems, Linatron radiography of castings can
often produce significantly more readable film images.
The increased latitude and reduced wide angle scatter
provide major advantages not found in lower energy
radiography. However, high-energy radiographs will have
less contrast, and the imaging of fine cracks and
filamentary porosity can be difficult. Cracks usually do
not progress in a straight line and do not present welldefined
edges to the X-ray path. This usually causes
images to have penumbral edges, which makes detection
difficult. Cracks which lie at angles to the X-ray beam can
be seen if their width exceeds the limit imposed by the
overall radiographic sensitivity, as determined from the
equation;
Varian Linatron applications
Δx = W/sin ø (see Figure 5-1)
where Δx = the crack width in the direction of the radiation
W = the crack width
ø = the angle of the crack to the x-ray beam