Application: This note contains a general overview of ultrasonic nondestructive testing applications in the foundry industry, including thickness gaging, flaw detection, and nodularity testing.
(b) Ultrasonic Flaw Detection
Figures 3 and 4 show a typical test for porosity in a 40 mm (1.6 in.) iron casting, using an Epoch XT with a DHC709-RM (5 MHz, 0.5 in. diameter) dual element transducer. Figure 3 shows the backwall echo from the casting at the right side of the screen, along with typical low-level surface noise and grain noise along the baseline. Figure 4 shows an indication from a void defect, which is readily identifiable against the background noise.
While the most common flaw detection application in castings involves voids, porosity, and inclusions, some users also need to check for cracking or fractures. Crack tests must always be developed with respect to the specific geometry of the casting, and the location, size, and orientation of suspected cracks, utilizing appropriate reference standards containing known or artificially induced defects. Straight beam transducers are used when the crack face is parallel to the transducer coupling surface, and angle beams are used when the crack is perpendicular or tilted with respect to the coupling surface. Note that because of the lower sound velocity in cast iron and nonferrous castings, the actual refracted angles of wedges designed for use on steel will be lower. These angles should be recalculated by means of Snell's Law whenever conventional steel wedges are used on other materials.
(c) Nodularity Testing
The recommended instruments for nodularity testing are the Model 38DL PLUS and 45MG with Single Element software thickness gages, all of which can provide a direct readout of sound velocity based on an entered part thickness. The High Penetration software options for the 38DL PLUS and 45MG are recommended if metal thickness exceeds approximately 0.5 in. or 12.5 mm. It is also possible to use any Olympus NDT Epoch series flaw detector and obtain velocity information via a velocity calibration procedure. This important subject is discussed in detail in the Application Note " Measuring Nodularity in Cast Iron". Please refer to that document for further detailed information regarding equipment and procedure.
The Olympus NDT Applications Lab is available to provide assistance with transducer selection and instrument setup for this or any other ultrasonic test application.