Experimental Evidence for metallurgical modification associated to chunky graphite in heavy-section ductile iron castings

Abstract

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 effort has been made for many years 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 generally appears away from the surface and its occurrence 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 based on differential thermal analyses that metallurgical differences do exist in samples machined out of the affected and nonaffected zones of a single cast block.