The metallurgical meaning of the word ‘inoculation’ is to provide the melt with ‘germs’ or ‘cores’ on which solid phases grow during solidification. Nowadays, inoculation of cast iron is one of the most important metallurgical treatments carried out on the liquid metal, immediately prior to the casting phase, with a direct effect on the primary structures (austenite, carbides, eutectic cells, graphite particles) and can be considered as a process of graphitization of the liquid metal necessary to obtain an as-cast structure free of carbides and with the highest quality of shape and distribution of graphite.
ITACA Inoc INOCULANTS AND PRECONDITIONERS
Thanks to the experience gained with the ITACA thermal analysis software, Proservice has developed and created a series of inoculants and preconditioning agents with high yield and reliability.
ITACA Inoc is mainly FeSi-based alloy in granular form with which the inoculation treatment is carried out. In addition to Fe and Si, other active elements (such as Al, Ca, Ba, rare-earth elements, etc.) may also be present in precise percentages, depending on the characteristics to be achieved in the final cast iron, while the particle size changes depending on the type of plant and the stage of the melting process in which the inoculation is performed.
ITACA Inoc is also suitable to be added to the base cast iron in the melting furnace or during the tapping phase and acts as preconditioners. This step is important because it prepares the base cast iron for the subsequent spheroidization treatment in the case of spheroidal cast iron production. However, preconditioning with ITACA Inoc can also be carried out in the production of grey cast iron to increase the response to the subsequent inoculation treatment and thus reduce the amount of inoculant required immediately before casting.
Assessing the effectiveness of an inoculation treatment is extremely complex, if not impossible, to rely solely on checking the chemical composition of the metal. Clearly, analytical control is essential to verify the introduction of Si and achieve the correct target value, and to keep under observation the level of trace elements present (such as Al, Ca, Ba, etc.) which, if excessively high, can lead to serious problems related to the quality of the castings (among the most important: carbides, pinholes, dross).
The most suitable instrument for verifying the effectiveness of the inoculation treatment is undoubtedly ITACA software. The introduction of ITACA Inoc, in fact, has a considerable impact on the eutectic temperature, and in particular the minimum eutectic temperature (TeMin), i.e. the lowest temperature reached by the metal during precipitation of the eutectic phase.
The effect of inoculation is to raise the value of the minimum eutectic temperature to above the metastable eutectic temperature and thus ensure graphitic solidification. From the cooling curve, it can therefore be seen whether the carbon in the liquid cast iron precipitates totally in the form of graphite (grey cast iron) or in the form of iron carbide (cementite or white cast iron) or intermediate structures (‘mottled’ structures).
ITACA, A POWERFUL ALLY FOR THE INOCULATION TREATMENT
ITACA thermal analysis software can also provide important information on the microstructural quality, for example, numbers of nodules and nodularity in the case of spheroidal graphite cast iron. One of the most useful parameters is the VPS, i.e. the angle formed by the curve of the first derivative (the curve representing the instant-by-instant cooling rate of the specimen subjected to thermal analysis) at the point of absolute minimum. In general: the higher the TeMin, the lower the recalescence, the lower the VPS angle, and the better the microstructural quality of the cast iron concerning the shape and distribution of the graphite nodules.
ITACA Inoc Inoculants Purposes:
- promote solidification in the stable system and reduce the formation of carbides (grey and spheroidal)
- promote the formation of type A graphite (grey)
- increase the number of nodules and improve the degree of nodularity (spheroidal)
- improve machinability (grey and spheroidal)
- improve mechanical properties and reduce variability caused by different casting thicknesses (grey and spheroidal)
- reduce defects caused by volumetric shrinkage during solidification (grey and spheroidal);
- reduce defects caused by gas inclusions of metallurgical origin (grey and spheroidal)
An important aspect to focus on when choosing inoculation treatment practice is the previously mentioned phenomenon of evanescence. In fact, the effect of added inoculants is not permanent but is temporary, and after a certain period their effect fades (hence the term ‘evanescence’). As soon as inoculants are added to the liquid metal, they begin a dissolution process, active elements (such as Si, Al, Ca, etc.) begin to bond with oxygen and other elements dissolved in the cast iron to form oxides or complex intermetallic compounds that are very stable at the maintenance temperatures of the molten metal. Such compounds act as effective substrates for the nucleation and growth of graphite particles, having a crystal lattice that is consistent or semi-coherent with the hexagonal lattice of graphite. These particles, however, have a lower specific weight than the liquid metal and can therefore, over time, move toward the surface and agglomerate with the slag floating on the bath. According to this mechanism, over time, all the particles that can act as nucleants for the graphite are gradually lost within the liquid mass, and thus there is a reduction in the state of nucleation of the cast iron: a decrease in TeMin, an increase in recalescence and an opening of the VPS angle can then be seen.
As we can see in the example shown, it can take only a few minutes of waiting time to drastically reduce the number of nodules, and as a result, excessively penalize the mechanical characteristics or find porosity defects on the castings. Here again, ITACA software proves to be the main ally for verifying this type of phenomenon without necessarily resorting to micrographic analysis, which would require much more time and resources not normally available.
LATE INOCULATION WITH ITACA Stream
Precisely because of the phenomenon of evanescence, the late addition of inoculant directly into the cast iron flow during the pouring phase has become one of the most effective techniques used by foundries. Proservice created ITACA Stream as the decisive solution to a precise and optimal in-stream inoculation treatment. In addition to the very high dosing precision of the in-stream ITACA Inoc, ITACA Stream can be combined with the ITACA Vision camera to ensure perfect and constant alignment between the cast iron and the inoculant. In this way, the maximum inoculation yield is accompanied by the least waste of material.
If ITACA Stream is connected to the ITACA software, then it is possible to activate a “dynamic inoculation” function based on the real metal nucleation potential provided by the thermal analysis, making the inoculation process even more efficient.
PRECONDITIONING WITH ITACA Inoc & ITACA Stream XL
As mentioned earlier, whether the inoculation treatment is carried out to the base cast iron into the melting furnace or in the ladle during the tapping phase is referred to as preconditioning.
Historically, preconditioning originated in spheroidal cast iron production to try to improve the efficiency and yield of the spheroidization treatment as much as possible. Preconditioning can be carried out using appropriate ITACA Inoc ferroalloys to deoxidize the melt as much as possible and form stable nuclei at high temperatures that act as substrates for the subsequent nucleation and growth of the graphite spheroids.
Preconditioning was then also successfully introduced into the production of lamellar cast iron to improve the effectiveness and response of the subsequent inoculation steps as much as possible.
Proservice also designs and manufactures ITACA Stream XL, a fully automated dosing equipment to provide the ITACA Inoc preconditioning to the liquid metal, weighing the material with very high precision and dosing it in the most effective way to achieve the highest possible yields.
ITACA Inoc Preconditioning Purposes:
- promote stable cores at high temperatures (grey and spheroidal)
- improve the efficacy and response of the subsequent inoculation (grey)
- improve the yield of the spheronization treatment and reduce the residual Mg fork (spheroidal)
- the possibility of producing castings with low to medium residual Mg values (spheroidal)
- reduce oxide formation during the casting of cast iron (grey and spheroidal)
Many other inoculants in Proservice‘s portfolio can be tested according to the needs of the foundry, and our technicians can provide full support in choosing the most suitable type and quantity for the casting process and the various cast irons to be produced.