Formation and Prevention of Chunky Graphite in Slowly Cooled Nodular Irons

An experimental device has been designed in order to simulate the solidification of large castings by means of a small-sized sample (1 kg). Once the spheroidizing and inoculation processes have been completed, this sample is slowly cooled at controlled rate and under protective atmosphere in an insulated crucible placed in a laboratory resistance furnace. The maximum freezing time is about 90 min. These tests relate to a base iron having a good degree of purity. SEM, optical microscopy and automated electron microprobe have been used for studying structure and mechanism of chunky graphite formation. Results of over forty tests confirm several data abstracted from literature and provide some more information. For instance, there are different ways of preventing chunky graphite formation : acceleration of solidification, use of Ni-Mg or pure Mg as spheroidizers and, in any case, an antimony addition (0,02%). A rather low carbon equivalent is also a favourable factor. However, for a given carbon equivalent, a final relatively high Si content improves graphite spheroidizing, chunky graphite appearing only if this content results from Fe-Si-Mg additions and/or from overinoculation. Besides, confirmation is given of the detrimental effect of useless rare earth additions (e.g. 0,02%). The matrix surrounding chunky graphite is on average more rich in Si than the one surrounding spheroidal graphite. An oxidizing etching reveals that chunky eutectic has formed at first. Aiming an interpretation of the results, structure examination keeps going on, in order to find out if there could be several kinds of chunky graphite.