Testing the Casting Quality
Inspection and testing of castings encompasses five main
categories: casting finishing, dimensional accuracy, mechanical
properties, chemical composition and casting soundness.
The surface finish of a metal casting can be influenced by the
type of pattern or molding sand, mold coating, and method of
cleaning. So far, instrumentation for measuring surface
roughness has not provided a useful evaluation, so it is
performed largely through simple visual comparison using a
series of test panels with increasing surface roughness.
Variation in the dimensions of a casting can be the result of
mold cavity expansion caused by the heat and head pressure of
molten metal, the contraction of the metal as it cools and heat
treatment. These expansions and contractions are predicted by
the patternmaker who will compensate for the variations in the
pattern’s design. For large volumes of castings, casting
facilities may measure the critical dimensions of the castings
more often to check for possible drift, particularly drift due
to pattern wear. If a casting requires tight tolerances that are
critical to the part’s application, those tolerances should be
specified by the customer.
Casting customers should check how their casting supplier will
verify the dimensions of the parts they produce. The accuracy of
the measuring tools is just as important as the dimensional
accuracy of the castings. In many instances, the gauges or
fixtures needed to routinely check the dimension are supplied to
the casting facility by the customer.
Expecting exact dimensions over the course of a production run
will result in frustration. The dimensions of each casting will
vary slightly, so castings are specified by setting a range of
values that the dimensions can fall within. The range between
the lower tolerance limit and upper tolerance limit can be set
by the supplier, but the narrower the range, the more difficult
to produce and test and therefore more costly the casting will
Mechanical testing gives an evaluation of the metal and the
casting to determine whether the properties are in compliance
with the specified mechanical requirements. Following are common
mechanical tests used in metalcasting facilities.
Hardness testing—the most commonly used procedure for mechanical
property testing, it provides a numerical value and is
nondestructive. Hardness values generally relate to an alloy’s
machinability and wear resistance. The Brinell hardness test
uses a 10-mm diameter carbide ball to indent a 3,000-kg load.
The impressions are large enough to provide a dependable average
hardness. Rockwell hardness tests make smaller indented
impressions, which also can be satisfactory if the median of
several values is used.
Tensile and impact testing—conducted on test specimens of
standardized dimensions, the two most commont types are tensile
and Charpy impact. Tensile testing provides ultimate tensile
strength, yield strength, elongation and reduction of area data.
Charpy impact testing determines the amount of energy absorbed
during fracture and is used to gauge ductility and strength.
Service load testing—usually conducted on the entire casting to
evaluate its properties, it can be conducted in a number of
ways. Castings that must carry a structural load can have a load
applied in a fixture while the deflection and the load is
measured. Pressure-containing parts can be hydraulically tested
to a proof load or destruction. Rotating parts can be spin
tested. These types of tests check the soundness of the casting,
as well as its properties.
The chemical composition of an alloy has a significant bearing
on its performance properties. Chemical composition can be
further affected by minor alloying elements added to the
material. Casting alloys are typically specified accorded to
ASTM, SAE and AMS alloy specifications. Depending on how
susceptible an alloy is to variation of its chemical
composition, chemical analysis may be required to verify the
proper composition is present to achieve a certain set of
Chemical analysis often involves a sample of molten metal poured
in to a special mold and evaluated by spectrographic atomic
absorption or x-ray fluorescence analysis. Many metalcasting
facilities check the chemical composition of the alloys they are
pouring throughout the course of a day, so melt shop personnel
can make required adjustments to the alloy composition as
The performance of metal components can be notably affected by
internal and surface defects that can not be detected through
the regular course of visual inspection. Several nondestructive
methods can be employed to inspect castings for these
“invisible” flaws. Nondestructive tests determine the integrity
of a casting without causing physical damage, so once it passes
the tests, it can be used for its intended application. Below is
a detailed list of nondestructive tests.
NON-DESTRUCTIVE TESTING METHODS
Non-destructive testing gives the metalcasting facility the
capability of assuring the quality of a casting without
destroying it. A metalcasting facility may have internal
standards regarding nondestructive testing, but it is up to the
customer to specify specific tests or frequency of testing.
While various methods of nondestructive testing exist to measure
mechanical properties, chemical composition, casting soundness
or maximum service loads, a single test that encompasses all
these factors does not exist. A combination of nondestructive
methods may be required to document the soundness and quality of
a casting. The most common methods available are described
Visual inspection is based on the use of the human eye to
identify surface defects, improper filling and molding errors.
Casting defects that can be detected via visual inspection
include sand holes, excessively rough surface, surface
shrinkage, blowholes, misruns, cold shuts, and surface dross or
To ensure a part meets dimensional requirements, such as
tolerances, a metalcasting facility can check the dimensional
accuracy of a part manually or with a coordinate measuring
machine (CMM). Checking the dimensional accuracy of a part helps
guarantee the customer won’t have to perform further costly
machining on a part to meet the specified dimensions.
CMM has improved the speed and accuracy of measuring casting
dimensions, and computerization has made it repetitive and able
to be used as a statistical tool.
Dye Penetrant and Fluorescent Powder Testing
For tiny cracks, pores or other surface glitches that are hard
to detect by the human eye, dye penetrant testing is used for
both ferrous and nonferrous materials. In this method, a colored
dye solution is applied to the surface of the casting. The dye,
which is suspended in penetrating oil, will find its way into
the surface defects. When a special developer is applied, the
defects are clearly indicated.
A similar method involves fluorescent powder suspended in
penetrating oil. Again, the solution penetrates the defects, so
when the casting is dusted or sprayed drying powder, the
solution is drawn from the defect and glows under an ultraviolet
light where defects have occurred. Fluorescent powder testing
only detects surface cracks and flaws but is more effective and
economical than radiographic testing.
In general, dye-penetrant techniques identify defects on the
surface of the casting and do not detect internal porosity or
shrinkage that is not open to the surface. But it can detect
rounded indications for porosity or gas on the casting surface.
Magnetic Particle Inspection
Magnetic particle inspection is quick, inexpensive and sensitive
to defects, particularly shallow (0.003 in.) surface cracks and
other lineal indications.
It detects small cracks on or near the surface of ferrous alloys
that can be magnetized (basically any ferrous alloy except
austenitic material). A high-amperage, low-voltage current is
passed through the casting, which establishes a magnetic field.
Cracks and defects have magnetic properties different than those
of the surrounding material, so their presence will interrupt
the magnetic field, causing distortion. Small magnetic particles
show the path of the flux line that spreads out in order to
detour around the distortion, thereby indicating the shape and
position of the crack or void.
Internal defects that are detected by radiography may also be
detected by sound. Sound waves have been used by fisherman to
locate hot fishing spots and depth of water and by the U.S. Navy
to identify approaching objects. In casting inspection,
ultrasonic testing uses high frequency acoustic energy that is
transmitted into a casting. Because ultrasonic testing allows
investigation of the cross-sectional area of a casting, it is
considered to be a volumetric inspection method.
The high frequency acoustic energy travels through the casting
until it hits the opposite surface or an interface or defect.
The interface or defect reflects portions of the energy, which
are collected in a receiving unit and displayed for the analyst
to view. The pattern of the energy deflection can indicate the
location and size of an internal defect, as well as wall
thickness and the nodule count of ductile iron.
Ultrasonic testing requires a high does of knowledge and
experience for an accurate interpretation of the results, which
will affect the cost added to the part for the inspection.
Another method used to detect internal defects is radiographic
inspection. When done correctly, radiographic inspection is the
best nondestructive method for detecting internal defects, such
as shrinkage and inclusions.
In this method, a casting is exposed to radiation from an x-ray
tube. The casting absorbs part of the radiation, and the
remaining portion of the radiation exposes the radiographic
film. Dense material withstands the radiation penetration, so
the film is exposed to a lesser degree in those areas, giving
the film a lighter appearance. Less dense materials allow more
penetration and correlates to darker areas on the film. Any
hole, crack or inclusion that is less dense than the casting
alloy is revealed as a dark area.
When done correctly, radiographic inspection is the best
nondestructive method for detecting internal defects, such as
shrinkage and inclusions, and the radiograph serves as a
permanent record of the casting quality that can be reviewed by
multiple personnel. Casting thickness and density will limit the
rang of inspection possible, depending on the energy level of
Radiographic inspection also can be performed without film.
Instead, the x-ray image is viewed on a video screen.
Computerized axial tomography (CAT scanning) also is being used
to develop 3-D computer imagery to inspect a casting’s
Eddy Current Inspection
The eddy current inspection method is applied to the detection
of cracks at or near the surface. An electrically charged coil
carrying an alternate current causes an eddy current to flow in
any nearby metal. The eddy current may react on the coil to
produce substantial changes in its reactivity and resistance,
and that reaction is used to pinpoint small cracks or defects.
Eddy current inspection is accurate for the detection of small
flaws or material changes that may not be detected with other
inspection methods, and the discontinuities in the casting will
give an immediate response on the monitoring equipment. However,
it requires a vast amount of knowledge and experience to
properly interpret the results, which will affect the added cost
to the part. The test only can be used with electrically
Pressure Leak Testing
When a casting is specified to be pressure tight or leak-proof,
it is often tested by sealing openings in the casting and
pressurizing it with air, inert gas or water.
When water, or hydrostatic, pressure is used, water seeping
through the casting wall indicates leaks. If air or gas pressure
is used, the pressurized casting is put into a tank of clear
water. The appearance of bubbles indicates the air has
penetrated through the casting wall and a leak is present.
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