Searching for Graphite Nodule Nuclei Using Automated SEM/EDX Analysis
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00418-1
Abstract Nonmetallic inclusions play a vital role for graphite nodule nucleation in cast iron with spheroidal graphite (SGI). Therefore, knowledge about composition of heterogeneous nuclei is practically important and can be used to control solidification in SGI castings. Thermodynamic simulations were performed to predict types of nonmetallic precipitates formed in the entire SGI processing, including melt nodularization and inoculation as well as followed melt cooling and casting solidification. To verify simulation results, the experimental SGI heats were performed with variations in nodularization and inoculation practices. Two types of specimens were collected. The first one was direct-quenched from the inoculated melt, and the second one was taken from the castings. An automated SEM/EDX inclusion
analysis was used for evaluation of families of nonmetallic inclusions located in the matrix and inside graphite nodules. Nucleation activity of different precipitates was determined from the partitioning of active elements (Mg, Al, Si, Ca, Ce, La, Ti, Zr) between the graphite nuclei and the matrix. Simulation and experimental data were used to discuss a nucleation mechanism of graphite nodules in inoculated SGI. Engineering of nonmetallic inclusions can be used to control SGI solidification.
Heterogeneous Nucleation of Graphite Nodules
were wavy and distorted close to the nucleus and very straight further away from it, while the mechanism behind this phenomenon is still unclear.
Cast iron with spheroidal graphite (SGI) has a unique solidification structure with an unusually high volume density of solidification cells consisting of graphite nodules each surrounded by an austenite shell. A volumetric number of prime grains precipitated directly from the melt in SGI casting is larger than in other technical alloys solidified at a similar cooling rate (Figure 1). Mg-treatment blocks the growth of small graphite nodules in the Fe–C–Si melt, and this undercools melt and activates massive nucleation. This mechanism is not well understood. Qing et al.1 studied the crystal structure of a graphite spheroid in the vicinity of its nucleus using high-resolution TEM. Crystal orientation maps of the graphite spheroid revealed misorientations and twist boundaries. High-resolution lattice fringe images showed that the basal planes of graphite
This paper is an invited submission to IJMC selected from presentations at the 6th Keith Millis on Ductile Iron held October 23–26, 2018 at the Sonesta Resort, Hilton Head Island, SC. It is published in the IJMC by permission of the DIS (Ductile Iron Society).
International Journal of Metalcasting
Keywords: solidification, nucleation, cast iron, graphite nodules, nonmetallic inclusions
In an ideal case, the homogeneous liquid–solid transformation upon undercooling (DT) below the equilibrium solidification temperature (Ts) begins with randomly formed clusters. Clusters that are too small to survive are
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