NOVEL APPROACHES TO ANALYZE STRUCTURE OF DUCTILE IRON Simon Lekakh and Mark Harris
Missouri University of Science and Technology, Rolla, MO, USA Copyright © 2014 American Foundry Society
A version of this paper was previously published in the 2013 Keith Millis Symposium Proceedings Abstract
The suggested approaches, including an automated SEM/ EDX (Scanning Electron Microscopy/Energy Dispersive X-ray) analysis of graphite nodule nuclei and a special al- gorithm to convert two-dimensional to three-dimensional graphite nodule size distribution, were tested. The “soft” quenching technique was applied to develop small graph- ite nodules and increase probability to reveal non-metallic heterogeneous nuclei using automated SEM/EDX analysis. Ternary diagrams present experimental statistics of the graphite nuclei chemistry. A special algorithm for the con- version of diameters of two-dimensional sections of graph- ite nodules to the real three-dimensional distribution of
Importance of Understanding of Ductile Iron Inoculation
The necessity of inoculation of different types of irons is determined by the thermodynamic nature of the iron-carbon equilibrium diagram, which has stable (iron-graphite) and metastable (iron-cementite) liquid-solid types of transfor- mations. Only a small temperature gap divides the stable from metastable solidification. In addition, various factors can lead to undercooling of the melt and promote cementite formation – the so-called “chill tendency.” Homogeneous nucleation will not occur in industrial alloys, and the nucle- ation event requires the presence of substrate that will initi- ate graphite solidification.
The nucleation of the graphite eutectic has been discussed1 from the point of view of changing of the nucleation ener- gy. A possible ranking of nucleants is as follows: graphite (highest – least energy required), silicate, oxides, sulfides, carbides, nitrides, and austenite (lowest). Skaland2
described
different categories of non-metallic inclusions in iron after Mg treatment. Angular, faceted sulfide (Mg,Ca)S and oxide MgO·SiO2 particles were found in the center of the graphite nodules, as well as in the matrix. Igarashi and Okada,3
who
studied the structure of spheroidal graphite in thin plates, found a spherical sulfide (Mg,Ca)S of about 1µm diameter near the center of graphite spheroids and it is thought to be a nucleus for the graphite. A spherical MgO of about 0.2 µm diameter was found inside the sulfide and is considered to be a nucleus for the sulfide. They explained nucleus com-
International Journal of Metalcasting/Volume 8, Issue 2, 2014
spherical particles was developed. This algorithm is based on inverse simulation. The developed program calculates a three-dimensional nodule diameter distribution curve, the real average diameter, and volumetric nodule number. The examples of practical applications of these methods for spherical graphite characterization in different ductile iron castings are provided. Heterogeneous nucleation of graphite nodules is discussed based on the novel experi- mental data.
Keywords: ductile iron, graphite, heterogeneous nucleation, 3-D quantitative technique
positions from a thermodynamic point of view. The MgO formed first because it has the higher value of free energy of formation and that functions as a nucleation site for the MgS that forms later.
The mechanism of heterogeneous nucleation of ductile iron could be more complicated and consisted of several steps, including formation of non-metallic substrates and their activation by graphite during dissolution of ferro- silicon-based additives. When the ferrosilicon additive dissolves, regions in the form of nearly concentric rings with different silicon concentration will arise around the particles.4
The melt composition in these regions varies from homogeneous melt to hypereutectic Fe-C-Si alloy, where the liquid composition is highly supersaturated with respect to the equilibrium.1,5
In these regions, favor-
able conditions for the formation of carbon-containing substrates are created. The strong inoculation effect of ferrosilicon-based additives is determined by the forma- tion of carbon-containing substrates on non-metallic in- clusions during dissolution of inoculants in the iron melt. Submicron non-metallic inclusions covered by a graphite layer can nucleate eutectic graphite during solidification with minimal melt undercooling.
A magnesium treated and ferrosilicon inoculated ductile iron melt is a complex thermodynamic system with multiple possible reaction products, which could initiate graphite nucleation or exist as undesirable inclusions and degraded mechanical properties. This paper introduces novel experi-
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