When talking about the classification of cast iron, the combination of the various microstructural constituents and their morphology must be considered into order to determine which class cast iron belongs to.
White cast iron
In white cast iron, when the eutectic solidification takes place, the metastable path is followed and austenite and cementite are formed.
The austenite undergoes the evolutions described above while the cementite remains until room temperature. The presence of cementite leads to high hardness in white iron, but also to fragility and a lack of ductility.
White iron is typically used for mechanical components, such as cart wheels or rolling cylinders which must ensure excellent resistance to wear.
Grey cast iron
Grey cast iron is often interesting more for its thermal than for its mechanical characteristics and is used in the automotive, boiler and radiator, machine tool base and valve industry.
In grey cast iron, the eutectic solidification has a stable path, leading to the formation of austenite (which generally evolves towards a pearlite structure) and graphite with lamellar morphology.
This type of morphology is particularly negative in terms of the ductility and toughness of the cast iron.
Grey cast iron
Malleable cast iron
When white cast iron undergoes a malleabilisation treatment (about 950°C, for several hours), this causes the decomposition of the cementite together with a solubilisation of the carbon in the austenite.
Subsequently, the carbon re-aggregates as graphite, with an equiaxed and relatively regular morphology. This results in malleable cast iron, in which the specific morphology of graphite ensures good ductility (hence the name).
Malleable iron is used in products with a small cross section and which must be able to resist low temperatures, in railways and the automotive industry in general, for valves, taps, fittings, electrical assemblies, hand tools, washers, brackets, and agricultural tools.
Spheroidal graphite iron
Spheroidal graphite iron, on the other hand, is derive from a eutectic solidification of cast iron, therefore, with the formation of austenite and graphite.
An appropriate amount of magnesium allows the kinetic mechanism of the increase in graphite to be controlled which, when solidified, takes on a spherical form.
The austenite can, however, evolve towards a ferritic, pearlite or mixed ferritic-pearlite structure depending on the composition and cooling conditions.
The spherical morphology of graphite determines high ductility and toughness (hence its definition in English of “ductile iron”). Moving instead from a fully ferritic matrix, to a mixed matrix and lastly, to a fully pearlitic matrix, an increase in yield and fracture stresses, and a decrease in ductility can be observed.
Spheroidal graphite iron casts are applied in a variety of fields. They are used to produce mechanical components for farm machinery and earth-moving machinery, for off- and on-road vehicles, railway carriages, and hydraulic and oil-hydraulic applications.
Austempered ductile iron (ADI)
Austempered ductile iron (ADI) is derived from a treatment (austempering) of the spheroidal graphite iron: austenization, salt bath hardening and successive isothermal permanence at temperatures between 250 and 350°C.
This treatment causes a complete transformation of the matrix (which is usually pearlitic at the beginning), resulting in an ausferritic microstructure, ensuring an excellent compromise between strength and ductility.
Austempered ductile iron (ADI) is used to produce components for the farming and earth-moving industry, as well as for the railway and forestry industry, in addition to applications such as gearboxes, suspension of off- and on-road vehicles, and hydraulic and oil-hydraulic components.
Isothermal Ductile Iron (IDI)
Isothermal Ductile Iron (IDI), patented by Zanardi Fonderie, is also derived from an isothermal treatment, carried out, however, in the “inter-critical” region of coexistence of ferrite and austenite.
The result is a matrix consisting of ferrite and pearlite, with an interesting compromise between strength and ductility.
IDI can be used in various sectors, for example in Zanardi Fonderie IDI has proved interesting for the manufacture of both distributor bodies and docking systems for the construction & mining sector.
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