Spheroidal Graphite Iron
Ductile iron castings, also known as nodular cast iron or spheroidal graphite iron (SG iron), are a type of cast iron named due to their spherical (nodular) shape of the graphite particles contained within the microstructure. This nodular graphite is achieved by alloying the iron with small amounts of magnesium or cerium during the casting process, which transforms the graphite formation during solidification to form rounded nodules rather than flakes/rosettes. The spherical shape of the graphite significantly improves the mechanical properties of the material compared to traditional grey cast iron.
Consequently, the ductile iron mechanical performance is primarily influenced by the microstructural matrix. Therefore, fully ferritic SG iron behaves very differently from a pearlitic or bainitic SG iron, even if the graphite nodule structure is similar.
Selection of Material Grade:
DCI has been accepted by many countries across the globe as a suitable material for transport containers that contain nuclear waste. By nature, these containers have very thick wall thicknesses of up to 300mm, but this very large thickness is for radiation shielding rather than for strength, and as such, the tensile properties and yield strength properties take second priority to fracture toughness properties. It is essential that under severe shock loading conditions, an existing flaw or crack in the material does not propagate. It is the level of fracture toughness that determines whether a crack will propagate rather than the Charpy impact properties that do not correlate well to fracture toughness. It is the fracture toughness in thick-walled DCI that is the key to making a robust containment vessel. Having said this, steps to maximise the stress rupture properties of DCI have little detrimental effect on the yield or UTS of this material.
Strain rate enhancement:
It is a common observation that with ferritic alloys, an increase in strain rate often results in an increase in tensile strength. Smith, Salzbrenner, Sorenson, and McConnel [2] tested sacrificial DCI castings and, during tensile testing, found that strain rate did have only a small effect on tensile strength. Charpy V-Notch (CVN) testing is a common method of determining resistance to brittle fracture in steel. However, due to the high strain rate of the test, it does not necessarily reflect the ductile behaviour of the DCI in actual application. This is because the hammer of the CVN machine is approximately four orders of magnitude greater than the train rates encountered in severe applications. Also, castings are nearly always large enough to be loaded in plane strain conditions as opposed to plane stress conditions (thin sections) where contraction restraint is absent.
These differences (stress state and strain rate) may increase the brittle-to-ductile transition temperature, and so impact energy (J) for DCI should not be used in design calculations. Therefore, CVN impact results cannot directly correlate to fracture toughness, but their results are useful for comparing relative toughness and the ductile-to-brittle transition temperatures of different heats and section sizes.
