casting 101 C
Non-Destructive Testing Can Ensure Quality AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT
has other options, methods that can assure the quality of a casting without destroying it. T e presence of both internal
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and surface fl aws, which may not be readily apparent in a simple visual inspection, can impact the performance of castings. Non- destructive testing (NDT) consists of a variety of physical inspection methods that are used to determine the integrity of a casting without damaging it. Whether or not the customer, an
industrial regulation or the metalcasting facility’s internal standards require NDT, the process provides the metalcasting facility and the buyer with a measure of quality assurance. Five NDT methods are
commonly used in the metalcasting industry: magnetic particle testing, liquid penetrant testing, ultrasonic testing, radiographic testing and eddy current testing. Each method has advantages and limitations, but none can provide complete assessment of mechanical properties, chemical composition, casting soundness or proof tests for maximum service loads. Because no single method can be 100% foolproof, a combination of NDT methods may be required to document the soundness and quality of a casting. Magnetic Particle Testing: Magnetic particle testing detects
ne easy way to check the quality of a casting is to cut it up and see what’s inside. But your metalcasting supplier
Shown are the types of nondestructive testing.
linear surface and near surface discontinuities in ferromagnetic materials using the principles of magnetization. Typically, a high- amperage, low-voltage current is passed through the casting, which in turn establishes 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. Liquid Penetrant Testing: Liquid penetrant testing can detect surface discontinuities in both ferrous and nonferrous castings. T is method uses the principle of capillary action—the ability of liquids to travel to or be drawn into surface openings. T e most critical step in this process is pre-cleaning the casting. Because the penetrant physically enters the discontinuity, the opening of the discontinuity must be free of any material that could block the penetrant’s movement. Grease, oil, sand, welding slag or painted/anodized surfaces can inhibit the penetrant from exposing an imperfection. Ultrasonic Testing: Ultrasonic testing uses high-frequency sound waves to detect surface and
subsurface discontinuities in both ferrous and nonferrous castings. It also can be used to gauge the thickness of a casting. Because ultrasonic testing can investigate the cross-sectional area of a casting, it’s considered a volumetric inspection method. (T e frequency of the sound is not audible to the human ear.) Radiographic Testing:
Radiographic testing uses X-ray or gamma energy to pass ionizing radiation through a casting to reveal internal discontinuities on a fi lm medium. X-rays are electronically produced ionizing radiation. Gamma rays are the product of a nuclear disintegration from a radioactive isotope that produces ionizing radiation. T e radiographic testing inspection method can be used on both ferrous and nonferrous castings. Eddy Current Testing: Eddy
current testing is an NDT method that utilizes an induced low energy electrical current in a conductive ferrous or nonferrous casting to observe the interaction between the casting and the current. T e testing is performed with electronic equipment designed to measure the inspection method variables. ■
Liquid penetrant testing involves the use of a dye that can be drawn into cavities and surface openings to highlight defects. Mar/Apr 2016 | METAL CASTING DESIGN & PURCHASING | 47
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