White Cast Iron

This is a continuation of the article by Janina Radzikowska, Senior Metallographer, The Foundary Research Institute (Instytut Odlewnictwa) Kraków, Poland . It was originally published by Buehler in Tech-Notes, Volume 2, Issue 2 and is reproduced here with the kind permission of the Editor, Mr. George Vander Voort, former Director, Research and Technology, Buehler, who granted it while still at Buehler.

White Cast Iron
The microstructure of white cast iron is best observed after etching. Figure 17 shows a typical example after etching with 4% nital. Note the interdendritic cementite (white) which sometimes has a Widmanstiitten ('spiky') appearance. Austenite formed as the proeutectic constituent before the eutectic reaction (liquid transforms to austenite and cementite) and later transforms to pearlite and cementite upon cooling below the eutectoid temperature, about 723°C.

white cast iron w/massive cementite and pearlite 100x

Figure 17. Microstructure of white cast iron containing massive cementite (white) and pearlite etched with 4% nital, 100x.

Figure 18a shows a higher magnification view of this specimen etched with 4% nital. The massive cementite particles are clearly visible appearing to be outlined by the etch. Actually, the cementite is not attacked by the etch, while the surrounding structure is. The 'outline' around the cementite particles is due to light being scattered from the height difference or 'step' around the particles. Note that ferrite surrounds each cementite particle due to local decarburization. The pearlite is colored.

white cast iron etched w/4% nital 400x Figure 18a. High magnification view (400x) of the white cast iron specimen shown in Figure 17, etched with 4% nital.

Figure 18b shows the effect of etching this specimen with alkaline sodium picrate which colors the massive cementite brown. The cementite in the pearlite constituent is colored tan and blue. Ferrite is not colored.

white cast iron etched w/ alkaline sodium picrate 400x Figure 18b. High magnification view (400x) of the white cast iron specimen shown in Figure 17, etched with alkaline sodium picrate.

	Figure 18b. High magnification view (400x) of the white cast iron specimen shown in Figure 17, etched with alkaline sodium picrate.

Conclusions
Modern polishing materials and procedures can be employed very effectively to reveal the microstructure of cast iron specimens. Graphite retention, always a problem with these metals, can be accomplished with a minimum of difficulty. Crossed polarized light is very useful for observing graphite substructure. Etching brings out the matrix constituents.

Selective etching with color producing films, briefly discussed here, is a highly informative tool. This will be discussed in more detail in a future issue.

Acknowledgments
Specimens for Figures 3, 16, 17, and 18 were provided by St. Fuksa and W. Wierzchowski, Foundry Research Institute, Kraków, Poland.

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