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ExpErimEntal EvidEncE for mEtallurgical modification associatEd to chunky graphitE in hEavy-sEction ductilE iron castings


J. Lacaze CIRIMAT, Université de Toulouse, Toulouse, France


I. Asenjo, S. Méndez, J. Sertucha, P. Larrañaga and R. Suárez Engineering and Foundry Department, Azterlan, Durango (Bizkaia), Spain


Copyright © 2012 American Foundry Society 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


introduction


Heavy-section castings made of spheroidal graphite cast irons are known to be prone to graphite degeneracy.1


Upon


the various degenerate forms of graphite reported in the literature, chunky graphite (hereafter denoted as CHG) is the most common and detrimental. A great deal of effort has been made to prevent its formation and it has been found that use of chills as well as minute additions of Sb are amongst the most efficient.2


Both actions have how-


ever their own drawbacks, chills being limited to sections a few centimetres thick while the addition of Sb (and other “counter-active” elements) is complicated because the nec- essary amount lies in a window that depends on the casting section and on other trace elements present and because Sb is a pearlite promoter.


CHG is a degenerate form of graphite that is mainly observed in the thermal centre of large spheroidal graphite (SG) cast- ings,3-5


castings containing high Ce and Si contents,6


but may also appear in thin sections of ductile iron besides austen-


itic Ni-alloyed cast irons.7 In large-size castings, CHG ap-


pears away from the surface and its formation becomes clear on metallographic sections by a darker contrast as illustrated in Figure 1. Numerous attempts have been made to describe the metallurgical process leading to the formation of CHG, but no convincing theory has been established until now.


International Journal of Metalcasting/Winter 2012


Figure 1. Photograph of the whole axial section of a cubic block 30 cm in size. The white arrows show the limit between the outer non-affected zone and the inner with- CHG area.


35


This paper presents differential thermal analysis (DTA) ex- periments performed on samples machined from affected and non-affected zones of a single cast block. The DTA re- cords show that there are in fact metallurgical differences between these two zones.


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 non- affected zones of a single cast block.


Keywords: cast iron, heavy section, graphite degeneracy, chunky graphite


Because of the sharp and clear transition seen in Figure 1 between the non-affected and the affected zones, several au- thors looked for chemical heterogeneities but all concluded to the absence of macrosegregation.8-11


It should be recog-


nized however that the analysis methods used are obscured by detection limits and accuracies that are not always clearly stated. Moreover, they all estimate global compositions and are not able to differentiate if a particular species exists in an atomic state or bound within a compound.


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