Mechanical features of spheroidal graphite irons
Ductile irons are materials for structural application that have interesting mechanical properties. The foundry process thanks to which they are obtained makes it possible to obtain components even of complex geometry and with minimum thicknesses close to those of the design (net-to-shape).
A significant improvement in mechanical properties can be achieved by resorting to quenching heat treatment in molten salts. A first possibility consists in obtaining ausferritic microstructures by subjecting pearlitic cast iron with adequate alloying content (C, Mn, Cu, Ni, Mo) to austempering heat treatment, dictated by the significant thickness of the casting.
The treatment parameters, whose choice is guided by the alloying content, can be managed in order to optimise static mechanical characteristics, as well as fatigue strength or wear resistance. Austempered ductile irons (ADI) therefore represent an alternative to forged or welded structural steels in some applications, even very heavy ones, such as: suspension systems, crankshafts, connecting rods and transmissions.
If, on the other hand, we austenitize an unalloyed ferritic ductile iron in the intercritical interval AC1-AC3, only a fraction of ferritic matrix will turn into austenite.
With the subsequent quenching in a salt bath, the prior austenite will evolve into pearlite while the remaining matrix fraction will consist of proeutectoid ferrite.
The structure thus formed is called “perferritic” and is characteristic of IDI ductile iron. Although the heat treatment is very similar to that of austempering, IDI ductile iron is substantially different from intercritical austempered ductile irons (IADI): for IDI ductile iron the action of the salt bath is only to increase the cooling rate or impose a rapid transformation of austenite into pearlite above the martensite start temperature.
The properties of IDI ductile iron are therefore comparable to those of pearlitic-ferritic ductile irons and not to those of ADIs.
The main advantage of IDI ductile irons compared to conventional pearlitic-ferritic ductile irons lies in the new perferritic structure: the low mobility of carbon at high cooling rates produces a pearlitic-ferritic matrix of interconnected type and not the classic bull’s-eye, a characteristic that translates into an increase in coupled mechanical strength, thanks to the presence of an important quantity of proeutectoid ferrite, to a higher ductility.
Fatigue properties of ADI and IDI ductile irons
As a consequence of the diffusion of structural applications employing these materials, extensive experimental activity has been carried out for fatigue characterisation in order to develop specific design models.
In general, the response to cyclic stresses of austempered ductile irons depends on the chemical composition, heat treatment parameters, the geometry of the component (presence of notches and thickness), the surface state (rough or machined).
Referring to more details in the following paragraphs, it is known that fatigue strength is substantially driven by the hardness of the material matrix: fatigue strength increases with increasing hardness until it reaches a peak for HBW Brinell hardness around 380 (about HV30 400). Among ADI ductile irons, the material that represents the best compromise between fatigue strength and static resistance is ADI 1050: it is obtained by austenitizing above the critical AC1-AC3 range and quenching in above the ausferritic nose (hence the definition of Upper ADI).
This ADI grade also has good wear resistance, impact resistance and machinability. Focusing on the lowest grades, namely ADI 800 and ADI 900, there is improved impact resistance and machinability at the expense of decreased fatigue strength, static and wear resistance.
Moving instead to the ADI 1200 grade, obtained by quenching below the ausferritic nose (hence the lower ADI definition) and which represents the upper limit for fatigue structural applications, static and wear resistance are privileged, giving up a share of fatigue strength, impact resistance and machinability.
Even higher grades among the lower ADIs, namely ADI 1400 and ADI 1600, are mainly aimed at applications where wear resistance is the prevailing requirement. As far as IDI ductile iron is concerned, it has a fatigue strength comparable to that of a pearlitic-ferritic cast iron of equal hardness.
However, given the absence of alloy pearlitising elements, it is possible to obtain uniformity of the perferritic matrix with the variation of the wall thickness. As described above, with the same hardness and static resistance of a conventional pearlitic-ferritic cast iron, it offers higher impact resistance.
It should be considered that both classes of materials, being obtained by heat treatment, have a limit of use at high temperatures: ADI ductile irons are substantially limited to temperatures approximately not higher than those of austempering (therefore different depending on the ADI grade considered); IDI ductile irons, presenting a pearlitic-ferritic matrix, are limited to temperatures not higher than the eutectoid that follows the high cooling rate imposed by the quenching in a salt bath.