Growth of eutectic austenite in free graphite cast iron

GRACIELA RIVERA; ROBERTO BOERI; JORGE SIKORA

KEY ENGINEERING MATERIALS

Trans Tech Publications

Año: 2011 vol. 457 p. 67 - 72

1013-9826

Free graphite cast irons are alloys of great metallurgical complexity. As a result, there is no universally accepted description of its solidification process. Although there is a general agreement on the dendritic nature of the proeutectic austenite, noticeable discrepancies appear regarding the growth of the eutectic austenite, for both eutectic and hypereutectic alloys. The authors developed a method to reveal the solidification macrostructure of free graphite irons, called DAAS (Direct Austempering After Solidification). The validity of DAAS was proved recently by using Electron BackScattering Diffraction (ESBD). Special color metallography techniques were also used to reveal the microsegregation. The objective of the present research is to apply these techniques to investigate the growth of the eutectic austenite of free graphite cast irons of different graphite morphology. The study was based on the analysis of several samples obtained from a highly hypereutectic cast iron melt, to ensure the absence of proeutectic austenite. In order to obtain samples of different graphite morphology but almost identical chemical composition, the melt was nodularized by using a standard procedure, and different samples were obtained after various fadding periods. Through this procedure, spheroidal, vermicular and flake graphite samples were obtained Round bars of 30 mm in diameter and 90 mm in length were cast in resin bonded sand molds. The DAAS technique was applied on the samples, and the main parameters characterizing the solidification structure were measured. The same samples were later etched to reveal the microsegregation patterns. The different observed features were related to the graphite morphology and the austenite growth pattern. The results show that the eutectic austenite grows dendritically in all samples investigated. Nevertheless, significant differences were found in the size of the solidification units (grains) and in the microsegregation patterns as the graphite morphology changes from spheroidal to flake.