mental/simulation approaches to statistically evaluate the nature of heterogeneous nuclei and link the real 3D graphite nodules structure in castings to heterogeneous nucleation and casting solidification kinetics.
Approaches to Analyze Structure of Ductile Iron
The graphite phase, developed during heterogeneous solidi- fication, influences the physical and mechanical properties of the final castings. Shape, size, quantity, and distribution of the graphite phase in high carbon iron alloys (cast irons) are some of the most important microstructural parameters. In this article, two experimental/modeling approaches, in- cluding an automated SEM/EDX analysis of graphite nod- ule heterogeneous nuclei and a special algorithm to convert two-dimensional to three-dimensional graphite nodule size distribution were suggested and described.
Statistics of Heterogeneous Nucleus
Heterogeneous nucleation plays an important role in stable graphite eutectic solidification to avoid metastable cementite formation and associated shrinkage defects. A vast variety of FeSi-based inoculants are used in ductile iron industrial practices. An assessment of inoculation efficiency can be done on the basis of knowledge of heterogeneous nuclei chemistry. A suggested technique for evaluation of hetero- geneous nuclei chemistry statistic includes two steps: (i) “soft” melt quenching to develop small graphite nodules and increase probability to reveal non-metallic heterogeneous nuclei and (ii) using automated SEM/EDX analysis of the center of small graphite nodules.
The melt (3.7%C, 1.7%Si) was prepared in a 200 lb induc- tion furnace using 30% pure induction iron, 70% foundry ductile iron return, and high purity carbon riser. The melt was Mg-treated in the ladle by 1.6% alloy (Fe-46%Si- 6%Mg-1%Ca) and inoculated by 0.3% of Fe-75%Si-1%Al- 1%Ca. Samples were collected directly from the melt in the ladle after treatment using a submerged core sampler with two internal steel chillers at 4 mm apart. A typical microstructure contained 4-6 µm diameter graphite nodules (Fig. 1) in carbidic metal matrix.
Statistics of non-metallic inclusions were studied using an automated SEM/EDX inclusion analyzer (ASPEX) system. Two-step automated analysis includes search and measure of structure features by back-scattered electrons (BSE) im- age contrasting and energy dispersive spectroscopy analy- sis of the feature center. The applications of the automated analysis for control of non-metallic inclusions in iron matrix are described elsewhere.6,7
In this study, the special search
routine and rule files were developed to separately analyze non-metallic inclusions in the metal matrix and inside graph- ite nodules. Fig. 2 shows individual examples of different observed chemistries of heterogeneous nuclei inside small graphite nodules: (a) complex silicate with MgS, (b) Si-Mg
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oxide, and (c) complex Mg-Ca sulfide with Mg-Si oxide. Typically, 1000 – 2000 structure features were analyzed in each sample.
Ternary diagrams were used to present experimental statis- tics of the graphite nuclei chemistry (Fig. 3) and for com- parison to chemistry of inclusion located in the metal matrix (Fig. 4). It could be observed that the majority of hetero- geneous nuclei contained a complex of Si-Mg-Ca oxides and Mg-Ca sulfides. A limited amount of mono- Mg and Ca oxides and sulfides were found inside graphite nodules. Alumina was not present inside the graphite nodules. The majority of inclusions in the metal matrix were complex Mg-Si-Ca oxide with a significantly less quantity of sulfides when compared to heterogeneous nuclei chemistry. Analy- sis showed significant partitioning of non-metallic inclusion between metal matrix and graphite nucleus.
Combination of “soft” quenching sampling technique and automated SEM/EDX showed that heterogeneous graphite nuclei have: i) a variety of chemistry composition and ii) higher concentration of Ca and S when compared to inclu- sions located in the matrix.
Three Dimensional Quantitative Analysis of Ductile Iron Structure
A simple, practical way of graphite structure evaluation is based on a comparison of the microstructure viewed at a particular magnification (typically 100x) with standardized tabulated microstructures. More advanced quantitative opti- cal metallography generates data including total quantity (% area), number per unit area, size distribution and graphite particle shape factor, which are calculated based on several
Figure 1. “Soft” quenched Mg-treated ductile iron. International Journal of Metalcasting/Volume 8, Issue 2, 2014
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