Kinetic Effects on the Austenite Carbon Equivalent and Eutectic Carbon Equivalent of Silicon Cast Irons

Abstract

The so-called carbon equivalent of austenite liquidus (CEL) and eutectic carbon equivalent (CE) that are used in cast iron metallurgy are known as linear functions of the composition. This paper first reminds how CEL and CE expressions are obtained from the knowledge of the relevant equilibrium phase diagram, emphasizing that they are not the same quantities. To account for the observed differences between these expressions and the experimental ones, it has been proposed in the literature to add kinetic terms to the equilibrium expressions. This is first discussed by considering austenite liquidus data for cast irons with 1–3 wt% silicon, from which the appropriateness of the CEL is confirmed while also evidencing the role of austenite growth undercooling. Then, experimental results on the solidification onset of near eutectic (slightly hypo- and mildly hyper-eutectic) and strongly hyper-eutectic cast irons are considered for discussing the significance of CE. For this second part, alloys that solidify in the stable system were considered whose results show again the role of austenite growth undercooling, but also that of graphite growth undercooling. The effects of addition of magnesium and of inoculation are discussed.

Appendices

Appendix 1

Tables 1 and 2 list the data from Chaudhari et al. on base alloys (Table 1) and on Ni–Mg spheroidized alloys (Table 2).¹² This is the same data as selected by Stefanescu⁵ with a couple of more results.

Appendix 2: Effect of Magnesium on the Equilibrium Liquidus

Yamane et al.²³ investigated in situ using synchrotron X-ray tomography the solidification of two hyper-eutectic alloys, one with 0.002 wt% residual Mg (3.69 C, 2.71 Si, 0.45 Mn, wt%) and one spheroidized with 0.05 wt% Mg (3.73 C, 2.57 Si, 0.45 Mn, wt%). The CE₉₉ value of both alloys is 4.45 wt%. After melting of the alloys, the authors observed during cooling at 0.17 °C/s that nucleation and growth of primary graphite proceeded in a temperature interval of about 20 °C for the alloy without Mg while this temperature interval was limited to 3 °C for the spheroidized alloy. The author mentioned that there could be two explanations to this effect of adding magnesium: 1. It inhibits graphite nucleation (or rather growth); 2. It affects phase equilibria. Yamane et al. disregarded the first possibility and investigated further the second one, concluding that 0.05 wt% Mg depresses the graphite liquidus by 45 °C. Such an effect means that the graphite liquidus depression is about 900 °C/wt% Mg which is more than significant and rather doubtful. In fact, the effect of Mg on the graphite liquidus may be estimated on the C-rich side of the C–Mg phase diagram and the slope is slightly lower than 30 °C/wt% Mg, i.e., 30 times lower than the value implied by Yamane’s assumption. This conclusion was further substantiated by calculation performed with the TCFE-8 database of Thermocalc illustrated in Figure

Figure 8

Isopleth Fe–C section at 2.5 wt% Si calculated with no Mg and with 0.06 wt% Mg using the TCFE-8 databank.⁹

8. This figure shows isopleth Fe-C sections at 2.5 wt% Si with 0 and 0.06 wt% Mg, and it is seen that they are superimposed (only the second digits in temperature are modified with the addition of Mg). On the contrary, the 45 °C difference mentioned above is of the order of the shift between LG and SG lines in Figures 2 or 3, suggesting that the effect of Mg on growth of graphite was in fact the reason for the observations by Yamane et al.