Application: Measuring the degree of nodularity in cast iron, or distinguishing nodular iron from gray iron.
Background: 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 nodularity 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, all Olympus precision thickness gages can provides a direct readout of sound velocity in cast iron and other materials based on an entered thickness calibration. These include models 38DL PLUS and 45MG with Single Element software. For thicknesses greater than approximately 25 mm (1 in.), the High Penetration software option for these gages will often be recommended.
A simple key press enters thickness data, and sound velocity is then calculated automatically from this entered thickness and measured pulse transit time. 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. Velocity measurement accuracy is typically on the order of one tenth of one percent.
Any of the EPOCH series flaw detectors (EPOCH 600, EPOCH 650, EPOCH LT) 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 perform a velocity calibration, which will calculate material velocity based on the detected echoes.
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 in/uS (5.9 mm/uS), nodular iron has a velocity of approximately 0.222 in/uS (5.6 mm/uS), and gray iron has a velocity of approximately 0.192 in/uS (4.8 mm/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).
Note that measurement of sound velocity requires that the thickness of the test piece be known. Typically, ultrasonic nodularity testing is performed at points on a casting where the thickness can be measured mechanically by a micrometer or caliper. Accurate velocity measurement is not possible if the thickness at the test point is unknown.
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.
Note: Nodularity and other aspects of graphite microstructure in cast iron can also be analyzed microscopically, using interpretive software developed by Olympus. You can find further details here .
For further information:
(1) ASM International, Metals Handbook, Volume 17, Nondestructive Evaluation and Quality Control, Metals Park, Ohio 1989 (Ninth Edition), pp. 531-535.
(2) Nondestructive Characterization of Cast Irons by Ultrasonic Method http://www.ndt.net/article/wcndt00/papers/idn035/idn035.htm