Synchrotron analysis reveals new insights into structure-process relationship.

A recent study demonstrated that high-energy x-ray tomography can reveal previously unknown graphite behavior in cast iron.

A sample 2-inch casting block and a 2-mm rod cut from the sample via electrical discharge machining. (Image courtesy of Argonne National Laboratory.)

As iron undergoes various treatments, graphite nodules form in ways that can be difficult to predict.

However, synchrotron analysis can be used to observe this behavior, as well as unambiguously classifying the particle types involved.

This classification is critical to identifying the structure-process relationship in cast iron manufacturing.

(b) A typical slice of tomographic image obtained in the study. (c) An unetched metallography image of the same region in (b) under optical microscope. (Image courtesy of Argonne National Laboratory.)

The insights from this new method of analysis are key to manipulating the atomic structure of graphite through manufacturing treatments. These methods include changing the chemistry of the melt and altering the alloying and refining elements added to the liquid iron.

Synchrotrons: A New Tool for Quality Testing?

The key factors that determine the properties of cast iron are the graphite’s structure, its spatial arrangement in the alloy and its phase connectivity.

Industry-standard 2D tests cannot reliably determine these key factors. More effective tests, such as those involving focused ion beams (FIB) or transmission electron microscopy (TEM) come with drawbacks of their own.

3D model of the graphite particles in (b), showing that the 2D features observed in (b) and (c) belong to a coral tree-like structure with flat, rounded branches that span ~200 µm in the iron matrix. (Image courtesy of Argonne National Laboratory.)

Although FIB and TEM testing both provide high-resolution 3D images, they are also labor-intensive, time-consuming and destroy the samples they test. High-energy x-ray tomography avoids these challenges while also provided a better statistical representation of the parameters in bulk metal.

For example, the study found that synchrotron characterization methods yielded new insight into why compacted graphite iron conducts heat better than ductile iron while maintaining good ductile strength. The answer lay in the shape, size and distribution of graphite particles in the cast iron.

“By understanding the structure, it will be possible to develop alloys with improved mechanical and thermal properties,” said Dileep Singh, technical lead of the study at Argonne National Laboratory.

Left: the slicing surface and the graphite structure beneath the surface. Right: the same graphite structure sliced in a different orientation, showing that 2D analysis could identify the same feature (red arrow) as either nodular or compact, depending on where it is sliced. (Image courtesy of Argonne National Laboratory.)

“Researchers at Caterpillar are actively seeking to improve our understanding of cast iron alloys in order to provide innovative product solutions to our customers,” said Richard Huff, technical team leader with Caterpillar Inc., which supplied engine alloy castings for use in the study.

Synchrotron analysis certainly provides useful information for cast iron manufacturers, but it is not yet clear how this method could be implemented on an industrial scale.

The results of the study were published in the journal Scripta Materiali and can be viewed here.