Nodular graphite degeneration in the post-inoculated ductile iron

Abstract

Post-inoculation during the casting of iron is known as one of the techniques most often used in the foundry. The aim of this operation for the cast iron with nodular graphite is the control of structure formation, including nodular graphite, and the prolongation of Mg advantageous action during the spheroidisation disappearance effect. The influence of post-inoculation on the decreasing of undercooling level during solidification and the counteraction the metastable crystallisation as well as counteraction the formation of primary carbides is known. On the other hand we have observed that the life duration of these artificial nucleus of crystallisation is not the same as for the original heterogeneous nucleus remaining after the fusion in a furnace. For the heavy castings with important thickness of wall, which solidification time exceed one hour (until a few hours or more for massive cast iron unitary castings), the nucleus of post-inoculation origin, cause an irreversible degeneration of cast iron primary structure, “chunky”—the degenerate graphite was evidently present.

The experimental study of ductile iron solidification was executed using thermal analyse method (cooling curve) for the test casting with different thickness (until 150 mm). The different kinds of post-inoculants, containing for example aluminium and zirconium, were used. The structure and mechanical properties of the post inoculated nodular iron test casting, cut on the defined distances from mould-casting interface (cooling surface of casting) are different with relation to the predicted quality, obtained as results of simulation code.

Keywords:

• Ductile iron
• inoculation
• degeneration graphite
• heavy section castings