The microstructural constituents of cast iron
Competing cast iron microstructures
The application features of cast iron depend on its microstructure, which develops and stabilises depending on the composition, the cooling conditions dictated by the process, the use of certain technological operations such as inoculation and spheroidization, as well as possible heat treatments.
In some case, there is a real competition between the different types of microstructure. When the eutectic solidification occurs, graphite can be combined with the austenitic structure (the most commonly desired result) or with cementite (only in certain specific types of cast iron). Graphite, moreover, can assume different morphologies (spheroidal, compacted, lamellar), that significantly influence the final behaviour of the cast iron.
When austenite decomposes, at temperatures of around 740 °C, microstructural constituents can form such as ferrite or pearlite (or both), determined by the specific composition. Lastly, during the austempering heat treatment, in which the goal is to generate the ausferritic structure, the formation of carbides and bainitic structures must be avoided.
Solidification and eutectic structure: cementite and graphite
In the eutectic solidification of a Fe-C system, the stable eutectic structure (austenite + graphite can be produced, at the nominal temperature of 1153 °C), or the metastable eutectic structure (austenite + cementite, at the nominal temperature of 1147 °C).
In reality, such structures are not formed at the nominal temperature, but at a lower temperature, determined by the transformation kinetics which can be assessed in terms of subcooling.
The stable eutectic, with a rather complex solidification mechanism (since it requires the formation of carbon-only zones), requires high subcooling.
The metastable eutectic is, however, “easier” to form and is associated with a subcooling of a few degrees centigrade. If there is no external action, the solidification of cast iron is thus associated with the formation of a metastable eutectic.
This scenario can, however, be changed by various factors including:
- The presence, in the cast iron composition, of graphitizer elements (especially silicon), which can raise the nominal temperature of the metastable eutectic;
- Slow cooling, which assists the aggregation of carbon-rich regions;
- The inoculation treatment, which involves the insertion in the liquid cast iron of fine powdered substances that can act as a “platform” for the nucleation of graphite.
Austenite is the solid solution of carbon in the face-centered cubic iron structure. Due to the shape of this lattice and the atomic size of iron and carbon, the solubility of carbon itself in the austenite is quite high, reaching values of just over 2% in weight under the most favourable temperature conditions.
In hypoeutectic cast iron, the austenite is the first phase to solidify. In any case, the austenite is one of the structural constituents formed in the eutectic solidification (both stable and metastable) of cast iron.
At the eutectoid temperature, the austenite ceases to be stable and transforms generating either ferrite or pearlite, or both.
Rapid cooling can lead to the presence of austenite even below the eutectoid temperature: in this case, an isothermal treatment (or “isothermic”), at a constant and appropriately defined temperature can lead to the formation of ausferrite.
Ferrite and Pearlite
At the eutectoid temperature, the austenitic phase is no longer in conditions of thermodynamic stability, and thus tends to transform.
This transformation initially determines the formation of ferrite (which tends to nucleate and develop at the interface between the graphite and the austenite) and the release of carbon, which is deposited in the pre-existing graphite.
Later, when the ferrite has “surrounded” the graphite, the formation of pearlite, i.e. an alternating lamellar structure of ferrite and cementite becomes much easier from a kinetic point of view.
The continued decomposition of the austenite towards a ferritic or pearlitic or mixed structure depends on the composition of the cast iron (silicon is a ferritising element; copper and tin are pearlitising elements) and the cooling rate (if it is high, pearlite will be formed).
A spheroidal graphite iron with a ferritic matrix has good ductile properties, a pearlitic matrix has greater strength. Controlling the amounts of ferrite and pearlite, in the matrix means being able to vary the strength and ductile properties of the cast iron.
The so-called ausferritic structure is developed during the austempering treatment.
At the austempering temperatures (approximately between 250 and 400 °C) the austenite is not stable and tends to transform.
The first result of the transformation is the development of acicular ferrite and the enrichment in carbon by the still untransformed austenite. The structure thus described (acicular ferrite closely mixed with a carbon-rich austenite matrix) is called ausferrite.
This structure shows an excellent combination of toughness and strength and gives the cast iron mechanical properties that are quite comparable to those of the best steel.
If the isothermal treatment continues, the excess carbon in the austenite gradually leads to the formation of iron carbides and a weakening of the mechanical features and toughness.