APPLICATION: Measurement of the degree of nodularity in cast iron or differentiation between ductile and flake cast iron.
BACKGROUND: Carbon in the form of graphite is often used as an additive in the production of cast iron, typically in a proportion of 2-4% by weight and 6-10% by volume. The micro-structure of graphite in cast iron has important effects on the mechanical properties of die-cast components. When graphite is ordered in thin flakes, the result is flake cast iron, which is hard and brittle. When graphite is arranged in spherical nodules, the result is ductile cast iron, which is soft and malleable.
Both types of cast iron are obtained by combining carbon, silica and other additives in molten iron. Part of the combination of elements is often done in the final mould. If the result of the combination is not uniform or if the die casting process is not optimal, the die casting may have been carried out with nodularity variations or with inclusions of flake iron present in the ductile iron. Since the type of result significantly changes the mechanical properties of the metal, foundries should check the uniformity of ductile iron. It is important that the distribution of graphite in the die casting is uniform and also that the graphite inclusions are present in the correct form: nodules rather than flakes.
Microscopic analysis and voltage checks are effective in verifying nodularity. However, for a rapid and non-destructive evaluation of a die casting, the preferable method is the ultrasonic test based on the different propagation speed of the sound wave in flake cast iron and ductile cast iron.