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casting 101 C

Nondestructive Testing: More than Meets the Eye AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT

that may not be apparent with a quick once-over. In these cases, nondestruc- tive testing (NDT) can determine the integrity of a casting without caus- ing physical damage and leaving the casting useless. Whether the customer, industrial regulation or a metalcaster’s internal standards require NDT, the process provides the metalcaster and buyer a measure of quality assurance. Five NDT methods commonly are used in the metalcasting industry. Each method has advantages and limitations. Often, a combination of NDT meth- ods may be required to document the soundness and quality of a casting. Here are brief descriptions of these methods: Magnetic particle testing detects


linear surface and near surface discon- tinuities in ferromagnetic materials using magnetization. Typically, a high- amperage, low-voltage current is passed through the casting, which creates a magnetic fi eld. If a discontinuity (crack or other type of linear indication) is present, it will disrupt the magnetic fi eld and result in a fl ux. Magnetic particle testing typically

involves four steps: • Magnetize the casting to be inspected.

• Apply an inspection medium of fi ne iron particles while the casting is magnetized.

• Inspect the casting surface for any fl ux leakage fi elds.

• Clean the casting of any inspection residue and demagnetize. Magnetic particle testing is quick and simple. It’s highly sensitive to the detection of shallow (0.003 in.) surface cracks and other linear indications. But magnetic particle testing is limited to ferrous materials, with limited abilities to detect subsurface defects. Liquid penetrant testing can detect

surface discontinuities in both ferrous and nonferrous castings using the

ife would be easier if the eye test was 100% accurate, but metal cast- ings can be aff ected by internal and surface fl aws

principle of capillary action, or the ability of liquids to travel to or be drawn into surface openings. T e fi rst and most critical step in this process is pre-clean- ing the casting so the applied penetrant can physically enter any discontinuity. Liquid penetrant testing is highly

sensitive to surface defects and round- ed indications. However, the disconti- nuities must be open to the inspection surface, so subsurface discontinuities cannot be detected. Ultrasonic testing is a method that uses high-frequency sound waves to detect discontinuities in both ferrous and nonferrous castings. It also can be used to gauge the thickness of a casting. In the ultrasonic testing method, an ultrasonic transducer transforms electri- cal energy into mechanical energy in the form of sound pressure waves. T e generated sound pulse travels through the casting and is refl ected by both the back wall of the casting and any internal discontinuities. Transit time, amplitude and shape of the sound wave are moni- tored and measured. Ultrasonic inspection also is accurate

for material thickness measurements. It can be performed through coatings on a casting and requires access to only a single surface of the casting. Ultrasound can be bounced or refl ected internally inside the casting to achieve 100% in- spection coverage for discontinuities. Ultrasonic testing can also be used

to determine if ductile iron cast- ings have the proper nodularity. T is testing method can be used to both control the manufacturing process and ensure correct graphite shape in the fi nished castings. As with other test- ing methods, good process control of the testing is required. T e ultrasonic testing method re-

quires extensive knowledge and experi- ence. T e surface roughness of a casting and dimensional variation may scatter the sound pulse, making defect detection or thickness measurement more diffi cult. Radiographic testing uses X-ray or gamma energy to pass ionizing radiation through a casting to reveal

internal discontinuities. Available for both ferrous and nonferrous castings, this inspection method uses radiation to penetrate a casting’s cross-sectional area and expose a piece of radiographic fi lm, a concept similar to an X-ray of a broken bone or tooth cavity. When discontinuities such as

cracks and shrinkage are present in a casting, it absorbs less radiation and increases film exposure to the radia- tion, which produces an image of the defect on the film. Radiographic testing is advanta-

geous because it can detect internal defects. T e inspection process requires access to both sides of the casting, and the discontinuities in the casting must be parallel to the radiation beam for the best detection probability. Casting thickness and density also will limit the range of inspection possible. Eddy current testing observes the

interaction between a low energy elec- trical current and a conductive ferrous or nonferrous casting with electronic equipment designed to measure the inspection method variables. In prin- ciple, an AC current is applied through coil windings to create an expanding and collapsing magnetic fi eld. T e magnetic lines of force extend into the casting, which in turn induce the fl ow of eddy currents (low energy electrical currents). T e induced eddy currents generate their own magnetic fi eld, which interacts with the test coil magnetic fi eld. When a discontinuity is present, it alters the characteristics of the eddy current magnetic fi eld, which then alters the interaction between the two magnetic fi elds. Eddy current testing is accurate for the detection of small fl aws or materi- al changes and can be readily adapted to high-speed automatic scanning equipment. It’s highly sensitive to the detection of shallow (0.003 in.) surface cracks and other linear indications. However, the eddy current inspection method requires signifi cant knowl- edge and experience. It’s also limited for use with conductive materials. ■


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