Varian Linatron High-Energy X-ray Applications 2007

Casting Radiography
Radiography of castings can reveal defects which can be
repaired before expensive machining operations are
performed. Therefore, it can be a very effective tool to
determine the suitability of the casting. Radiography also
reveals how well casting techniques are working.
Most castings inspected by high-energy radiography are
produced in sand mold flasks or in metal-framed
containers. The upper section of the mold is referred to as
the cope, the bottom section as the drag, and the junction
of the two as the parting line. The mold of a large,
complex casting may be assembled from a number of parts
and cores. Radiography may also be required for large
ingots that are cast in permanent molds and for parts
suitably shaped for casting by the centrifugal process.
Listed below are common casting defects that radiography
can reveal.
• COLD SHUTS are areas between two quantities of cast
material that have cooled below the molten state before
coming into contact with each other, thereby causing an
unfused interface.
• HOT TEARS show as crackline indications usually
appearing singly near the surface and at hot spots or
where temperature gradients are high.
• CRACKS show as sharply defined indications, sometimes
with penumbral shadows, caused by metal parting from
itself. These are frequently caused by improper stress
and temperature gradients. Cracks are often created
when a core or mold resists the compression of the
casting as it shrinks on solidification.
• PIPING shows as elongated, cylindrical-shaped
indications located at or near the center of the casting.
These are caused by risers and feeders not being kept hot
and liquefied, which causes a lack of metal flow to the
center of the casting mold.
• SHRINKAGE is generally associated with improper
feeding of metal into the casting mold. Shrinkage
manifests itself in the following three forms:
- Sponge Shrinkage, found in heavier sections, appears
on the radiograph as a dark, lacy area with a diffused
outline.
- Feather Shrinkage, found in thinner sections, appears
on the radiograph as a sponge, with a feathery outline.
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Varian Linatron applications
- Linear Shrinkage, a continuous structure of connected
lines with branches of variable lengths, widths, and
densities.
• GAS HOLES AND VOIDS are caused by trapped air or
mold gases. They appear on the radiograph as round or
elongated, smooth edged, dark spots and may appear
individually, in clusters, or distributed throughout the
casting.
• DROSS represents oxidized metals. This appears as a
series of lines in a swirl pattern sometimes combined
with agglomerated irregular indications.
• SEGREGATION is the result of certain elements of a
metal to concentrate in the last liquid portion of the
metal. This results in an uneven distribution of the
chemical constituents. These can appear as uneven,
irregularly shaped, dark areas on the radiograph.
• MOLD MATERIAL, SAND, AND OTHER
INCLU SIONS
• UNFUSED CHAPLETS AND CHILLS
• INCOMPLETE DEFECT REMOVAL
• DEFECTIVE REPAIR WELDS
Radiographic Coverage. The coverage on castings may be
specified by the customer or it may be given by casting
specifications. The radiographer is required to select
optimum projections and techniques in the latter case.
Visual inspection of rough-cleaned castings reveals where
risers and gates have been removed. Pattern numbers are
usually on the drag side. Areas typically radiographed
include heavy sections in the drag, the flanges, the
junctions of diaphragms and casting walls in the valve
areas. Among the critical areas are valve seats, steam chest
areas, and other parts where extensive machining is
performed.
Shrinkage defects usually lie in the centerline of cast
sections and, unless they are extensive or in areas to be
machined, are less critical than hot tears or cracks. Both
hot tears and cracks tend to be perpendicular to cooling
stress directions and are often difficult to examine unless
the radiographic projection is parallel to the crack or tear
direction.