arbon 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.