Problem
There is an ongoing debate on the relationships between the alloying/inoculating elements and the graphite structure in cast irons. Since properties of cast irons are dependent on graphite morphology, establishing correlations between process variables to graphite morphology is vital to develop cast irons with optimized properties. Limited by typical industrial 2-D imaging techniques or time-consuming 3-D laboratory studies, researchers have been unable to pinpoint the exact processing parameters needed to elicit the ideal properties for each cast iron application.
R&D Analysis
As part of a program funded by the U.S. Department of Energy’s Vehicle Technologies Office, researchers from Caterpillar and Argonne National Laboratory used high-energy X-ray tomographyat the 1-IDX-ray beamline at the U.S. Department of Energy’s Advanced Photon Source at Argonne to take 2-D and 3-D images at 2-micrometer resolution. Large volumes (in mm3 size) of the sample were analyzed in a relatively short time and provided results with high accuracy. These measurements can be readily extended to in situ studies of microstructural evolution at elevated temperatures, to provide further understanding of cast iron solidification process.
Result
3D structural analysis revealed that the compacted graphite (CG) can grow to a coral-tree-like morphology as large as a few millimeters in the iron matrix.The study results showed that high-energy X-ray tomography can reveal previously unknown behaviors of graphite in cast iron, such as the growth of nodules, as it undergoes various treatments.The analysis of the 3-D structure reveals information, which can either not be seen or possibly misinterpreted with standard 2-D analysis.
Benefit of Working with Argonne
Synchrotron X-ray analysis has several advantages over the current techniques used to evaluate graphite microstructure. Three-dimensional imaging of the structure of graphite, its spatial arrangement in the alloy, and its phase connectivity are key factors that determine the properties of cast iron. These parameters cannot be attained reliably by the current industry standard 2-D test. Less frequently used, but more effective, is the use of focused ion beams (FIB) and transmission electron microscopy (TEM), which can provide high-resolution 3-D images, but is labor-intensive and time consuming and destroys the sample. High-energy X-rays penetrate inhomogeneous samples up to a centimeter thick under real operating conditions. This avoids the challenges of FIB and TEM techniques while also providing a better statistical representation of parameters in bulk material. The 3-D characterization of the material enables greater insight into the structure formation and structure-property relationships.
More Information
“3D Quantitative Analysis of Graphite Morphology in High Strength Cast Iron By High Energy X-ray Tomography,” Scripta Materialia, 106:5-8 (2015). DOI: 10.1016/j.scriptamat.2015.03.017
“Application of X-ray computed tomography for the characterization of graphite morphology in compact-graphite iron,” Materials Characterizations, August (2016). DOI: 10.1016/j.matchar.2016.08.007