What is isothermal ductile iron (IDI)

Ferritic – pearlitic cast irons are characterised by a significant sensitivity to thickness, which determines the cooling rate in the mould after solidification, therefore the hardness and mechanical properties. In the case of isothermal ductile iron (IDI), an austenitic structure in equilibrium with a ferritic fraction (this is achieved by austenitization in the intercritical interval), cooled much faster than what happens for example in the sand mould (minutes instead of hours), causes a transformation of austenite into pearlite characterised by a significantly lower sensitivity to thickness and a favourable interconnection with the ferritic phase (thanks to the reduced carbon mobility imposed by fast cooling). IDI ductile iron is therefore particularly suitable for use in the manufacture of castings of high thickness, significant and non-uniform thickness, for which a favourable combination of strength and ductility is desired.

Special characteristics of isothermal ductile iron (IDI)

The term “isothermed” is a neologism that must be kept distinct in its meaning from the term “isothermic”. An isothermic transformation takes place, by definition, at a constant temperature (this is the case of the austempering of the ADIs). The meaning to be attributed to the term “isothermed” is that of a structural transformation that takes place in continuous cooling regime imposed by the rapid immersion of the casting in a salt bath at a constant temperature higher than the martensitic transformation temperature. The temperature of the salt is isothermic, but not that of the casting during transformation, whose cooling rate is imposed by the salt and guided by the thickness of the casting. The thermal profile at all points of the casting (temperature vs time) is identical to that which would occur in the case of an austempering treatment. What changes is the fact that, in the case of IDI, the casting does not contain alloying elements, so it crosses the pearlitic nose and finally reaches equilibrium with the temperature of the salt when the starting austenitic structure has already been transformed into pearlite.

Quenching in salt bath

What are the microstructural characteristics of isothermal ductile iron (IDI)?

With regard to the microstructural characteristics of isothermal ductile iron (IDI) it should be noted that the austenitization stage takes place at a temperature within the intercritical range (between AC1 and AC3), as happens with intercritical ADI ductile irons (for example SAE J 2477 AD750), and before quenching the structure is composed of austenite and proeutectoid ferrite. Passing through the pearlitic nose, the austenite turns into pearlite, while the proeutectoid ferrite remains untransformed. In the case of castings of low thickness, the cooling rate imposed by the salt may be high enough to avoid the pearlitic nose, even if the casting has not been added with alloying elements. On these thicknesses we could find ausferritic structures, the transformation taking place at the isothermal temperature of the salt bath (as happens in austempering). In any case, the formation of martensite is avoided, since the temperature of the salt bath is higher than martensite start (Ms).

This is the main reason why IDI ductile irons are suitable for castings of sufficiently high thickness and also non-uniform. The structure that is obtained consist in ferrite and pearlite. The distribution of the phases differs from the traditional as cast ductile irons for the greater degree of refinement and the unprecedented interconnection between the phases themselves. This structure is called “perferritic”: a neologism coined to summarise the pearlitic-ferritic nature of the material with a forecast of better characteristics compared to traditional pearlitic-ferritic cast irons. The perferritic structure, thanks to the interconnection of the phases, evokes a greater structural perfection.

IDI – Perferritic Matrix (x200)

What are the mechanical characteristics of isothermal ductile iron (IDI)?

The mechanical behaviour of IDI cast irons is similar in nature to that of pearlitic-ferritic spheroidal cast irons, being in all cases structures based on the alpha phase (cubic with centred body), but with an appreciable improvement of the quality index (Rm2 x A5), thanks to the refinement and interconnection of the phases that increase the resistance (Rm, Rp0.2) with the same elongation (A5) guaranteed by the ferritic fraction. Since the salt bath causes a drastic cooling of the casting, the differences in cooling rates at different thicknesses are attenuated, so the differences in microstructures and consequently in the mechanical characteristics are also attenuated, for which the main cause of differentiation could be attributed to the different nodularities rather than to the cooling rate.

Possible microstructure evolution with different cooling rates

Table 1 – Static properties

Table 2 – Un-notched impact strength sample

Characteristics of use of isothermal ductile iron (IDI) and comparison with ADI ductile irons

Isothermal ductile irons are used in various fields and applications. One of the most advantageous is associated with castings of sufficiently high thickness, especially in the presence of significant dimensional non-uniformity. In the case of castings with thin and uniform walls (typically automotive castings such as differential boxes) it is more advantageous to obtain good results with quality as-cast foundry processes (Fiat 52215 Gh 60.38.10, +GF+ SiBoDur). Wanting to obtain the best possible performance for castings of not excessive thickness, the golden standard is evidently represented by the applicable grade of austempered ductile iron (ADI). However, ADI castings are limited in relation to the thickness of the casting. In fact, by increasing the quantity of alloying elements necessary to avoid the pearlitic nose, production costs increase proportionally, introducing at the same time causes of deterioration of the mechanical characteristics due to the segregation of these elements, either because of the high content or because of the distance between the graphite nodules.

IDI ductile irons do not present this type of limitation, being also in the best conditions for obtaining a sound casting in the as cast state (ferritic). In this way, increasing the thickness of the casting and possibly also the not-uniformity of thickness, reduces the convenience to the use of ADI ductile irons and increases the convenience of using IDI ductile irons. When comparing the various options for choosing materials and processes, illusions must be avoided. For example: increasing the austenitization temperature in the ADIs, thereby reducing the need for alloying elements, does not solve problems correctly, even if, apparently, the tensile test has met the specifications. In fact, areas of unstable austenite could occur, with detriment to the machinability and for some stress situations. It is also necessary to move away from the idea that high-silicon ductile irons are the modern efficient solution: what appears in the uniaxial tensile test is not representative of the triaxial state of stress behaviour.

Below, an example of an application in IDI produced by Zanardi Fonderie S.p.a.:

Hydraulic motor components

Sector: Industrial
Application: Hydraulic motor
Distributor body