Inoculation of Cast Iron
Inoculation is an important process in the production of cast iron. Essentially, inoculation means fast cooling of the cast iron production to enhance its qualities. This is done by the introduction of fine nuclei (less than or equal to 4 micron) of desired element that influences the structural formation of the casting in a specific way. Manufacturers of cast iron from India and elsewhere use the inoculation for industrial quality castings. Without any other alloy additions, an extremely slow cooling rate would result in the precipitation of carbon as graphite, which reduces its quality. A low number of modules in ductile cast iron results in large and poorly shaped spheroids. Secondly, the lower the nodule counts, the higher the tendency of the cast iron to form pearlite, carbides and microporosities. Inoculation allows fast cooling and favours the formation of metastable carbide formation, which is desirable. It also has a multiplying effect on the formation of nuclei and solidifying of cells in ductile cast iron. And lastly, as the inoculating alloy is rich in silicon, it further contributes to the solidification of the iron.
A few of the benefits of inoculation of cast iron include counteracting the effects of variation in raw materials and melting practices and in avoiding chill formation in thin sections.
Elements such as Ba, Ca and Sr, which are usually introduced to a bath in ferrosilicon, are the most important inoculants of cast iron. Ferrosilicon that contains these elements is treated as a complex inoculant. Inoculation does not have a permanent effect. The efficiency of the inoculant starts to fade as soon as the inoculant is added. In ductile iron, each graphite nucleus gives rise to a single graphite nodule. These nuclei require a high surface energy to attract carbon atoms. As the solidification process continues, the austenite shell nucleates directly on the graphite nodule and the eutectic transformation begins. Since small particles have a high surface energy, the melt is flooded with suitable sized particles after initial inoculation. Over time, these particles coalesce and grow, reducing their surface energy and nucleation potential. This is also called as inoculation fade. When this starts to happen, the number of nodules that are formed decreases and the tendency to produce chill and mottle increases. When inoculating ductile cast iron therefore, the inoculant must be added after the magnesium flare has subsided.
The Inoculation Process
There are several stages in which inoculation occurs while producing cast iron. However, it is most effective just before or during pouring. Depending on what kind of nuclei is added and the conditions under which the cast iron is cooled, inoculation occasionally takes place in the furnace or in the intermediate vessel as well. the more stable a nucleus, more uniform the nodule count in the iron casting. Therefore, it is necessary to choose a material that has a high degree of nucleation potential. Careful inoculation offers two advantages - one, it makes the ductile cast iron free from carbides and two, it makes heat treatment redundant.
There are four steps involved in inoculation of cast iron:
- Preconditioning: in which a particular inoculant is added to the clean, deslagged surface of a furnace melt prior to the nodulizing treatment process.
- Pre-inoculation: in which an inoculating grade ferrosilicon is added either to the stream of iron filling the treatment ladle, or as part of the alloy.
- Ladle inoculation: In this step, the inoculant is added during tapping or pouring of the melt. If the metal is transferred to the mould via a pouring furnace, the selection of the inoculant(s) and the quantity added should be determined experimentally to avoid build up in the furnace area.
- Late inoculation: is a very expensive process. It is however the most effective step in the inoculation series. Late inoculation counters the effect of inoculant fading as well as magnesium treatment. In this process, the inoculant is added to the iron stream directly during pouring. In most cases pouring stream inoculation devices are used, which enable a quantitatively uniform addition of the inoculant to the pouring stream over the entire casting process.
Cast iron casting inoculation can be defined by the process as well. The types of inoculation include ladle inoculation, pouring steam inoculation, wire inoculation and mould inoculation.
To summarize, inoculation is an essential practice used by ductile and gray cast iron producers worldwide, including those in India. Late inoculation is the most ideal way to improve the structure of the cost alloy and augment its mechanical properties.