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Cast iron can be modified to optimize its mechanical and physical properties Researchers at DOErsquos Argonne National Lab used ldquosynchrotron Xray analysisrdquo to gain a deeper understanding the materialrsquos structural formation

High-Energy X-Rays Raise Possibilities for Engineering Microstructures

Feb. 18, 2016
3D characterization gives metalcasters a deeper understanding of the behavioral habits of cast iron Argonne National Lab team Addressing unknown material behavior Beyond FIB and TEM

Foundries know much about the characteristics and behavior of cast iron, and how to modify it during melting and alloying, and how to handle it during casting, in order to achieve the optimal mechanical and physical properties (e.g., material strength, or corrosion resistance) for a specific product application. This, among other factors, makes iron a material of choice in transportation and machinery industries, wherein cast iron's different qualities offer many options for design, production, and application.

But, many aspects of iron casting rely as much on skill or experience as science. As Tona Kunz of the U.S. Dept. of Energy’s Argonne National Laboratory points out, “producing good results yet not capturing cast iron’s full potential. Controversy still exists over the correlation between manufacturing casting parameters and desirable properties.”

More specifically, neither the standard industrial 2D imaging techniques nor the more intensive research efforts that take in 3D analysis have been able to document the exact processing parameters that will produce the ideal properties for any specific cast iron application.

Finding an easier way to look deep into the material structure would seem to be the way to answer such questions, and in that way could give manufacturers a new competitive advantage. According to a study released last fall in the peer-reviewed journal Scripta Materialia, high-energy synchrotron X-rays can provide that insight.

"By understanding the structure, it will be possible to develop alloys with improved mechanical and thermal properties. This implies that for applications such as vehicle engine and engine components, one could use less material and reduce overall vehicle weight, which would translate into fuel savings," stated Dileep Singh, group leader of thermal-mechanical research at Argonne National Laboratory's Center for Transportation Research, and the study’s technical leader.

Argonne National Lab is a science and engineering research center near Chicago, operated by UChicago Argonne LLC for DOE. Its programs cover basic science research, energy storage and renewable energy, environmental sustainability, supercomputing, and national security.

For auto, truck, and heavy-equipment designers and builders, the ability to modify manufacturing processes for high-performance materials has obvious advantages for developing more fuel-efficient engines or driveline parts that withstand heat and pressure more effectively, promoting longer service life.

"Researchers at Caterpillar are actively seeking to improve our understanding of cast iron alloys in order to provide innovative product solutions to our customers," added Richard Huff, a technical team leader with Caterpillar Inc., which supplied iron castings in typical alloys used in heavy-duty engines for use in the proof-of-principle study.

The results showed that high-energy X-ray tomography reveals previously unknown behavior by graphite in cast iron, including the growth of nodules, as it undergoes various treatments. The X-rays can also unambiguously classify the particle type involved in the behavior, which is critical to identifying the structure-process relationship. These insights are critical to the effort to manipulate the atomic structure of the graphite through manufacturing treatments, such as changing the chemistry of the melt or adding inoculants to the molten metal.

Along with Argonne’s Singh and Caterpillar’s Huff the research team included ANL’s Chihpin Chuang, John Hryn from the Energy Systems Division and Jon Almer and Peter Kenesei from the X-Ray Science Division. The Advanced Photon Source, a DOE Office of Science User Facility based at Argonne National Laboratory was used as part of this research. ANL’s Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, and U.S. Department of Energy, supported the work, and the Office of Science supported the use of the APS.

They found that synchrotron X-ray analysis has several advantages over current standard industrial 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 critical factors that determine the properties of cast iron. These parameters cannot be attained reliably by the current industry-standard 2D test. Less frequently used, but more effective, are focused ion beams (FIB) and transmission electron microscopy (TEM), which can provide high-resolution 3D images, but are labor-intensive and time consuming. Those methods also destroy a 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.

Argonne’s research team found that the synchrotron characterization methods enable new insight into why compacted graphite iron, which Caterpillar uses for heavy-duty engine components, can conduct heat better than ductile iron while maintaining good ductile strength. The reason for this is the shape, size, and distribution of the graphite particles in the cast iron, the concluded. The study results were published in Scripta Materialia.

"The 3D characterization of the material enables greater insight into the structure formation and structure-property relationships," Huff said.

Tona Kunz is a public information officer who covers X-ray science research at Argonne National Lab's Advanced Photon Source.