Interpretation - Castings
The major objective of radiographic testing of castings
is the disclosure of defects that adversely affect the strength
of the product. Castings are a product form that often receive
radiographic inspection since many of the defects produced by
the casting process are volumetric in nature, and are thus relatively
easy to detect with this method. These discontinuities of course,
are related to casting process deficiencies, which, if properly
understood, can lead to accurate accept-reject decisions as well
as to suitable corrective measures. Since different types and
sizes of defects have different effects of the performance of
the casting, it is important that the radiographer is able to
identify the type and size of the defects. ASTM E155, Standard
for Radiographs of castings has been produced to help the radiographer
make a better assessment of the defects found in components. The
castings used to produce the standard radiographs have been destructively
analyzed to confirm the size and type of discontinuities present.
The following is a brief description of the most common discontinuity
types included in existing reference radiograph documents (in
graded types or as single illustrations).
RADIOGRAPHIC INDICATIONS FOR CASTINGS
porosity or blow holes are caused by accumulated gas or air
which is trapped by the metal. These discontinuities are usually
smooth-walled rounded cavities of a spherical, elongated or flattened
shape. If the sprue is not high enough to provide the necessary
heat transfer needed to force the gas or air out of the mold,
the gas or air will be trapped as the molten metal begins to solidify.
Blows can also be caused by sand that is too fine, too wet, or
by sand that has a low permeability so that gas cannot escape.
Too high a moisture content in the sand makes it difficult to
carry the excessive volumes of water vapor away from the casting.
Another cause of blows can be attributed to using green ladles,
rusty or damp chills and chaplets.
inclusions and dross are nonmetallic oxides, which appear on
the radiograph as irregular, dark blotches. These come from disintegrated
portions of mold or core walls and/or from oxides (formed in the
melt) which have not been skimmed off prior to the introduction of
the metal into the mold gates. Careful control of the melt, proper
holding time in the ladle and skimming of the melt during pouring
will minimize or obviate this source of trouble.
Shrinkage is a form of discontinuity that
appears as dark spots on the radiograph. Shrinkage assumes various
forms, but in all cases it occurs because molten metal shrinks
as it solidifies, in all portions of the final casting. Shrinkage
is avoided by making sure that the volume of the casting is adequately
fed by risers which sacrificially retain the shrinkage. Shrinkage
in its various forms can be recognized by a number of characteristics
on radiographs. There are at least four types of shrinkage: (1) cavity; (2)
dendritic; (3) filamentary; and (4) sponge types. Some documents
designate these types by numbers, without actual names, to avoid
shrinkage appears as areas with distinct jagged boundaries.
It may be produced when metal solidifies between two original
streams of melt coming from opposite directions to join a common
front. Cavity shrinkage usually occurs at a time when the melt
has almost reached solidification temperature and there is no
source of supplementary liquid to feed possible cavities.
shrinkage is a distribution of very fine lines or small elongated
cavities that may vary in density and are usually unconnected.
Filamentary shrinkage usually occurs as a
continuous structure of connected lines or branches of variable
length, width and density, or occasionally as a network.
shrinkage shows itself as areas of lacy texture with diffuse
outlines, generally toward the mid-thickness of heavier casting
sections. Sponge shrinkage may be dendritic or filamentary shrinkage.
Filamentary sponge shrinkage appears more blurred because it is
projected through the relatively thick coating between the discontinuities
and the film surface.
are thin (straight or jagged) linearly disposed discontinuities
that occur after the melt has solidified. They generally appear
singly and originate at casting surfaces.
Cold shuts generally appear on or near a
surface of cast metal as a result of two streams of liquid meeting
and failing to unite. They may appear on a radiograph as cracks
or seams with smooth or rounded edges.
are nonmetallic materials in an otherwise solid metallic matrix.
They may be less or more dense than the matrix alloy and will
appear on the radiograph, respectively, as darker or lighter indications.
The latter type is more common in light metal castings.
shift shows itself as a variation in section thickness, usually
on radiographic views representing diametrically opposite portions
of cylindrical casting portions.
Hot tears are linearly disposed indications
that represent fractures formed in a metal during solidification
because of hindered contraction. The latter may occur due to overly
hard (completely unyielding) mold or core walls. The effect of
hot tears as a stress concentration is similar to that of an
ordinary crack, and hot tears are usually systematic flaws. If flaws
are identified as hot tears in larger runs of a casting type,
explicit improvements in the casting technique will be required.
Misruns appear on the radiograph as prominent
dense areas of variable dimensions with a definite smooth outline.
They are mostly random in occurrence and not readily eliminated
by specific remedial actions in the process.
Mottling is a radiographic indication that
appears as an indistinct area of more or less dense images. The
condition is a diffraction effect that occurs on relatively vague,
thin-section radiographs, most often with austenitic stainless
steel. Mottling is caused by interaction of the object's grain
boundary material with low-energy X-rays (300 kV or lower). Inexperienced
interpreters may incorrectly consider mottling as indications
of unacceptable casting flaws. Even experienced interpreters often
have to check the condition by re-radiography from slightly different
source-film angles. Shifts in mottling are then very pronounced,
while true casting discontinuities change only slightly in appearance.
Radiographic Indications for Casting Repair Welds
Most common alloy castings require welding either
in upgrading from defective conditions or in joining to other
system parts. It is mainly for reasons of casting repair that
these descriptions of the more common weld defects are provided
here. The terms appear as indication types in ASTM E390. For additional
information, see the Nondestructive Testing Handbook, Volume 3,
Section 9 on the "Radiographic Control of Welds."
Slag is nonmetallic solid material entrapped
in weld metal or between weld material and base metal. Radiographically,
slag may appear in various shapes, from long narrow indications
to short wide indications, and in various densities, from gray
to very dark.
Porosity is a series of rounded gas pockets
or voids in the weld metal, and is generally cylindrical or elliptical
Undercut is a groove melted in the base metal
at the edge of a weld and left unfilled by weld metal. It represents
a stress concentration that often must be corrected, and appears
as a dark indication at the toe of a weld.
Incomplete penetration, as the name implies,
is a lack of weld penetration through the thickness of the joint
(or penetration which is less than specified). It is located at
the center of a weld and is a wide, linear indication.
Incomplete fusion is lack of complete fusion
of some portions of the metal in a weld joint with adjacent metal
(either base or previously deposited weld metal). On a radiograph,
this appears as a long, sharp linear indication, occurring at
the centerline of the weld joint or at the fusion line.
Melt-through is a convex or concave irregularity
(on the surface of backing ring, strip, fused root or adjacent
base metal) resulting from the complete melting of a localized region
but without the development of a void or open hole. On a radiograph,
melt-through generally appears as a round or elliptical indication.
Burn-through is a void or open hole in
a backing ring, strip, fused root or adjacent base metal.
Arc strike is an indication from a localized
heat-affected zone or a change in surface contour of a finished
weld or adjacent base metal. Arc strikes are caused by the heat
generated when electrical energy passes between the surfaces of the
finished weld or base metal and the current source.
Weld spatter occurs in arc or gas welding
as metal particles which are expelled during welding. These particles do not form part of the actual weld. Weld spatter appears as many
small, light cylindrical indications on a radiograph.
Tungsten inclusion is usually more dense than
base-metal particles. Tungsten inclusions appear very light radiographic images. Accept/reject decisions for this
defect are generally based on the slag criteria.
Oxidation is the condition of a surface
which is heated during welding, resulting in oxide formation on
the surface, due to partial or complete lack of purge of the weld
atmosphere. The condition is also called sugaring.
Root edge condition shows the penetration
of weld metal into the backing ring or into the clearance between
the backing ring or strip and the base metal. It appears in radiographs
as a sharply defined film density transition.
Root undercut appears as an intermittent
or continuous groove in the internal surface of the base metal,
backing ring or strip along the edge of the weld root.