Spheroidal cast irons of different types and having different structures are currently known and used particularly to provide different types of mechanical components.
Spheroidal cast iron has, as its main characteristic, the shape of the graphite, which is indeed spheroidal, differently from what occurs in conventional gray cast irons with lamellar graphite; the spheroidal structure of the graphite gives the material high ductility.
Spheroidal cast irons subjected to a thermal treatment for normalization have a completely pearlitic matrix. In this case, the material is characterized by a higher wear resistance, although ductility is quite reduced and fatigue strength does not increase due to the thermal treatment. Indeed, with reference to the ISO 1083 standard, pearlitic spheroidal cast iron without thermal treatment, classified by the code JS/800-2/S, has a minimum HBW hardness of 245, a minimum tensile strength of 800 MPa, and a typical fatigue strength of 304 MPa.
Pearlitic spheroidal cast iron subjected instead to a thermal treatment for normalization has a minimum HBW hardness of 270, a minimum tensile strength of 900 MPa, and a typical fatigue strength which is unchanged, i.e., equal to 304 MPa.
Spheroidal cast irons subjected to thermal treatment for hardening in water or oil have a bainitic or martensitic structure. They can optionally be subjected, at the end of the cooling process, to a thermal tempering treatment. Such cast irons are generally characterized by a very low ductility accompanied by high surface hardness and consequently are not used in applications which require a certain fatigue strength.
From what has been described above briefly, it can be seen that if a pearlitic spheroidal cast iron is subjected to a thermal treatment in a classic manner, an increase in fatigue strength is not observed.
In order to try to devise a material which would have improved mechanical strength characteristics and especially improved fatigue strength characteristics, the austempered spheroidal cast iron known commercially as ADI (Austempered Ductile Iron) has been devised.
The thermal treatment required to obtain this type of cast iron consists of a complete austenitizing treatment, keeping the component at a temperature which is higher than the upper limit austenitizing temperature (commonly referenced as Ac3), followed by hardening in a bath of molten salts.
The final structure thus obtained, technically known as ausferritic structure, is composed of acicular ferrite and austenite. This particular structure gives the material high mechanical characteristics and most of all a superior fatigue strength, with lower machinability than traditional spheroidal cast irons.
Since it is essential to avoid the forming of pearlite during cooling, it is necessary to alloy the material with alloying elements such as nickel and/or molybdenum.
In the mid-1980s, the company applying for the present patent developed, under license from Dr. Horst Muehlberger, a particular thermal treatment which allowed to obtain an austempered cast iron known as GGG 70 B/A: this thermal treatment consists of austenitizing at a temperature lower than Ac3 (the upper austenitizing limit temperature) and higher than Ac1 (lower austenitizing limit temperature), followed by hardening in a bath of molten salts.
The resulting final structure, technically known as ausferritic structure with proeutectoid ferrite, is composed of proeutectoid ferrite, acicular ferrite and austenite. Since it is essential to prevent the formation of pearlite during cooling, and since the austenitizing temperature used during the first step of the thermal treatment is also relatively low, in this case also it is necessary to alloy the material with alloying elements such as nickel and/or molybdenum in percentages which are higher than in austempered spheroidal cast irons, which as explained earlier have no proeutectoid ferrite.
This particular type of cast iron has been introduced, in the ISO 17804 standard, with the designation JS/800-10 and more recently in SAE standard J2477 May 2004 revision, with the designation AD750. The fatigue strength of this particular type of cast iron is typically equal to 375 MPa.
Recently, spheroidal cast irons known commercially by the acronym MADI (Machinable Austempered Ductile Iron) have also been proposed; this type of cast iron also is obtained as a consequence of a thermal treatment for partial austenitizing at a temperature which is lower than Ac3 and higher than Ac1 and subsequent hardening in a bath of molten salts. The resulting final structure is different from the structure of the type classified as GGG70 B/A and/or ISO 17804/JS/800-10 and/or SAE J2477 AD750 due to the presence of finally dispersed martensitic needles. However, even MADI cast irons are characterized by the high content of alloying materials such as nickel and molybdenum.
ADI or MADI cast irons ultimately have definitely higher static mechanical characteristics and fatigue limits, but since they are obtained by hardening in salt, as mentioned, they require alloying materials such as nickel and molybdenum in order to ensure their hardenability without the risk of forming pearlite. Currently, therefore, due to the high cost of such alloying elements, these materials, despite being valid in terms of mechanical characteristics, are scarcely competitive on an economical level.