This defect occurs in all gray solidified casting materials. Due to the necessity of second, late inoculation to be performed shortly before or during casting of high-quality nodular graphite cast iron.
The defect occurs on the inside of the casting, regardless of the molding process, and can be identified by metalographic cuts of the structure. Accurate quantification is performed by means of scanning electron microscopy and EDX analysis (Figures 1 to 3).
According to their composition, inoculant inclusions are slags and/or hart spots (silicoferrite). They are present in streak-like shape and are often accompanied by graphite degeneration and gas blisters in their immediate proximity or pin holes underneath the surface. In some parts, these structural anomalies are also referred to as slag inclusions, similar to dross; however, EDX analyses reveal significant differences in composition.
These inclusions weaken the structural bonds and thus may significantly impair the mechanical and dynamic properties of the material. The fact that this defect only becomes visible after processing or, in the worst case, after component damage makes it a problematic one.
The following modes of formation are viable:
Inoculation is generally defined as the addition of substances with an effect on nucleation to the melt directly in the mold in order to strategically influence the solidification behavior and structure formation in the as-cast condition in a targeted manner. Inoculation is predominantly performed for cast iron melts shortly before or during casting to promote gray solidification and prevent ledeburitic whithe solidification (edge hardness). The greatest effect is achieved by inoculation alloys when added at possibly low temperatures (this often also represents the main cause of origination of this defect).
The inoculation additive primarily causes increase of the number of nuclei and thus proliferation of eutectic grains so that supercooling below the metastable equilibrium temperature and potential formation of supercooled graphite and ledeburitic carbides is prevented.
In nodular graphite cast iron, inoculation also has another effect:
Since the inoculation additives increase the number of nuclei, i.e. it creates as many nucleation centers as possible for graphite formation, many small graphite nodules of accordingly small diameter and rather uniform distribution are formed during solidification, which is an advantage with regard to mechanical properties.
Inoculants generally are alloys, composites, or mixtures on silicon and/or carbon basis with special additives included depending on the desired effect. All constituents of the inoculants have in common that they have an affinity to oxygen and are capable of reducing carbon solubility in iron due to local oversaturation. With correct handling, the inoculants will evenly distribute within the melt, not change or manipulate the chemical composition of the melt, and not have any side effects, e.g. on graphite formation.
The inoculation quantity permanently emitted by the inoculant is a function of the iron temperature, the inoculant quantity and the chemical composition which determines the melting interval or the melting interval of the inoculant and the heat tonality of the inoculant solution, e.g. its Al-, Sr-, Zr-contents, etc. Consequently and also owing to reoxidation processes, the inoculation effect is mainly time- and temperature-dependent so that type and quantity of inoculant, time of inoculation, temperature of the melt to be inoculated, and addition method must also be accurately adjusted to one another.
Poor dissolving capacity (inoculant type), too low melt temperature, incorrect method of addition (uneven distribution), unsuitable casting system and too coarse grain size are frequent causes for occurrence of this defect. Also, excessive inobulation, slagging of the inoculant (inoculant type), and too early oxidation of the inoculant may be causes of this defect.
Too high quantities of elements with oxygen affinity in the inoculant (e.g. Al, Mg, Ca, Zr, Ti), carbon monoxide formation (mainly in green sand molds in connection with moisture), and nitrogen composites in C-containing inoculants increase the tendency for pin holes and gas blister formation.
Measures for prevention:
1. Ensure sufficiently high temperature of the melt to be inoculated; hot pouring causes immediate and fast dissolving of the inoculant.
2. Adjust inoculant quantity, casting weight and casting temperature to one another so that the inoculation effect prevails up to the last mold (with ladle inoculation) or the last iron quantity to be cast from the respective ladle (with mold inoculation).
3. Selection of the correct inoculant composition; product-specific effective additives have influence on dissolution rate (melting point and melting interval), among others. In this way, low silicon contents, for example, delay dissolving of mold inoculants and thus extend their “life span”.
4. Performance of multiple inoculation processes with smaller individual inoculation quantities to ensure reliable dissolving of the substances. Moreover, multiple inoculation may be required if final values or variables are required that are inherently contrary. Gray solidification, high grain count, no porosity.