Chapter 2. Background Studies
2.4 Matrix of cast iron
It is well known that the cast iron is composed of the graphite and the matrix. If the graphitization during the cooling process is not sufficient, the graphite and the cementite will co-exist in the matrix. Two typical kinds of the matrix of ferrite and pearlite will be introduced here in detail.
Ferrite is called the Fe (the solid solution) and very soft, and contains a small amount of C. As the cast iron often contains some Si, ferrite is actually Silicon-ferrite, and is harder and more brittle. Therefore, the property of the matrix is affected by the content of Si.
Pearlite is a mixed morphology of cementite and ferrite, which is obtained by the eutectoid transformation of austenite, and is usually in lamellar structure. The morphology of pearlite after the heat treatment is spheroidal. In general, pearlite and ferrite coexist in the matrix of the cast iron. In particular, the coexistence morphology of pearlite and ferrite in the spheroidal graphite cast iron is called the “bull’s eye”.
Research on effect of elements on matrix of cast iron
There have been a lot of investigations on the effect factors on the matrix morphology. Meanwhile, a significant number of studies 61-69) on the influence of Cu on the matrix of cast iron were carried out, showing that Cu was a pearlite-forming element both in the flake and spheroidal graphite cast irons. However, in the book of ‘Cu Addition in Cast Iron’ 61), Fig. 22 on p. 42 was misquoted for the flake graphite cast iron, while in fact, it is
the data for the spheroidal graphite cast iron 70). Therefore, the actual influence of Cu on the matrix of the flake graphite cast iron was not clear, and furthermore, the ASM Handbook 65) reported that a Cu content of less than 0.3% does not significantly affect the matrix of the flake graphite cast iron, while for the spheroidal graphite cast iron, it is a pearlite-stabilizing element 64,66,68). According to the research done by Karsay 67), in the spheroidal graphite cast iron, the pearlite-stabilizing effect of Cu is much more obvious than that of Mn. Nevertheless, in his earlier research 69), he did not mention the influence of Cu, while the optimum value of Mn in stabilizing the pearlite in the matrix was reported.
Mn, Cr and Mo are the cementite-stabilizing elements, while Cu and P are the graphitizing elements for solidification. Concerning the pearlite-stabilizing effect of Cu, Ishiguro 71) reported that the matrix of the spheroidal graphite cast iron for the 2.8% Cu addition is completely pearlite, and succeeded in observing the existence of the Cu film around the spheroidal graphite on the fracture surface for the first time. Therefore, we considered that the pearlitization mechanism could be explained by the existence of the Cu film around the spheroidal graphite. However, Igarashi
72) observed the ferrite matrix and the precipitated secondary graphite on the spheroidal graphite surface in the B-contained 0.5%Cu-alloyed spheroidal graphite cast iron, and indicated that it is due to the existence of B. He also estimated the existence of the Cu film on the spheroidal graphite surface. Furthermore, Tsujikawa 73) also tried to find the Cu film based on TEM observations, but failed to confirm its existence.
There have been few recent reports about the influence of Cu on the flake graphite cast iron and the influence is considered to be very weak. In this study, the cast iron with almost the same C and Si contents was used, and the influence of Cu on the spheroidal graphite cast iron and the flake graphite cast iron was discussed. In the flake graphite cast iron, because the pearlite-stabilizing effect of S is very strong 74), then the same effect of Cu is difficult to detect. Therefore, the effects of S and Cu on the flake graphite cast iron are separately discussed.
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