Application: Measuring degree of nodularity in cast iron, or distinguishing nodular iron from gray iron.

Problem: Carbon in the form of graphite is often used as an additive in the production of cast iron, amounting to 2 to 4 percent by weight or 6 to 10 percent by volume in typical castings. The microstructure of graphite within cast iron has major effects on the casting's mechanical properties. When graphite arranges itself as thin flakes the result is gray iron, which is hard and brittle. When graphite takes the form of spherical nodules the result is nodular iron, which is soft and malleable.
Both gray and nodular iron are made by mixing carbon, silicon, and other additives into molten iron, and often part of the mixing is done in the final mold. If the mixing is non-uniform or the casting process is otherwise imperfect, it is possible to make a casting with variations in nodularity, or pockets of gray iron within a nodular iron casting. Because this will significantly change the mechanical properties of the metal, foundries need to check nodular iron for uniformity. It is important both that the distribution of graphite in the casting be uniform, and that graphite inclusions be of the right form (nodules rather than flakes).
Microscopic examination and tensile strength tests are effective for checking nodularity, but for quick and nondestructive evaluation of a casting the preferred method is ultrasonic testing based on the fact that nodular iron and gray iron have different sound velocities.

Equipment: Nondestructive testing can be done with any ultrasonic instrument that is capable of measuring sound velocity, which includes thickness gages, flaw detectors, and pulser-receivers. For dedicated velocity measurement, we offer the Model 35 series thickness gages (Models 35, 35DL, 35HP, and 35DL-HP), which can provides a direct readout of sound velocity in cast iron and other materials based on an entered thickness calibration. A simple key press enters thickness data, and sound velocity is then calculated automatically from this distance and time information. A high/low alarm function can be used to identify out-of-tolerance conditions. The gage is used with a transducer appropriate for the thickness range being measured, frequency an M1036 2.25 MHz contact transducer.

The Model 38DL PLUS gage is recommended for the most challenging nodularity applications involving complex geometries or very thick castings (thicker than approximately 2 in. or 50 mm). The 38DL PLUS, like the 35 series, offers direct readout of measured sound velocity based on an entered part thickness. The large waveform display allows the operator to adjust gain, blanking, and other setup parameters to optimize echo acquisition and verify measurements.

These gages can be used as a velocimeter by coupling the transducer to a sample of known thickness and performing a velocity calibration via the keypad. The velocity set into the instrument is then the velocity of the test material. Accuracy is typically on the order of one tenth of one percent.
Any of the EPOCH series flaw detectors can be used similarly to measure an unknown velocity. With the instrument calibrated for an appropriate low frequency transducer, obtain a backwall echo from a section of known thickness and adjust the EPOCH's velocity setting until the thickness readout shows the correct material thickness. The entered sound velocity then corresponds to the velocity of the test material.
Sound velocity in cast iron can be measured with any pulser-receiver that is compatible with low frequency transducers (Models 5072PR and 5077PR), along with a transducer that produces a backwall echo from the part in question. Measure round-trip pulse transit time from the oscilloscope waveform, and compute velocity by dividing the thickness of a part by the one-way sound transit time. (Velocity = Distance/Time)

Procedure: There is a consistent difference in sound velocity between pure iron, nodular cast iron, and gray cast iron. Typically, pure elemental iron has a velocity of approximately 0.232/uS (0.59 cm/uS), nodular iron has a velocity of approximately 0.222/uS (0.56 cm/uS), and gray iron has a velocity of approximately 0.192/uS (0.48 cm/uS). Exact velocities for a given application vary depending on alloy composition, grain structure, and other process variables. Exact velocities should always be verified on calibration standards made from the material to be tested. We recommend that you establish your own chart of velocity versus percent of nodularity for each application. Note that published research indicates that the relationship between velocity and percent of nodularity is not linear (1). However, it is possible to have a substantial difference in sound velocity between two otherwise identical castings containing the same percentage of graphite, one with the graphite in flake form (gray iron) and the other with spherical graphite (nodular iron).

Another situation that can be detected ultrasonically is the presence of gray iron inclusions in a nodular iron casting. Because the sound velocity of gray iron is lower than that of nodular iron, the pulse transit time through a casting containing a gray iron inclusion longer than a casting made entirely of nodular iron, and the measured sound velocity is lower. Again, given the complex variables affecting actual castings it is recommended that you set up the test based on calibration standards of known composition. However, in general a localized drop in sound velocity is a nodular iron casting is a sign of probable trouble.

Bibliography
(1) ASM International, Metals Handbook, Volume 17, Nondestructive Evaluation and Quality Control, Metals Park, Ohio 1989 (Ninth Edition), pp. 531-535.