Experimental Evidence of Metallurgical Modification Associated To Chunky Graphite in Heavy-Section Ductile Iron Castings

摘要

Heavy-section castings made of spheroidal graphite cast irons are known to be prone to graphite degeneracy. Upon the various degenerate forms of graphite reported in the literature, chunky graphite is the most common and detrimental one. A great deal of efforts has been made since long to prevent its formation but no convincing description of the factors controlling its appearance has been given so far. In large-size castings, chunky graphite appears generally away from the surface and its formation is made evident on a metallographic section by a darker contrast. Because of this sharp and clear transition between the non-affected and the affected zones, several authors looked for chemical heterogeneities at the scale of the part but all concluded to the absence of macrosegregation. This paper presents experimental evidence that metallurgical differences do exist in samples machined out from the affected and non-affected zones of one single cast block. These results are then discussed in the light of a review of defect formation during graphite growth in cast irons.