metallography dot com logo 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. It is reproduced here with the kind permission of the Editor, Mr. Vander Voort, who granted it while still with Buehler.

Etching
To see details of the matrix microstructure, specimens must be etched. The Polish standard for cast irons (PN-61/H-0503) recommends three etchants. The first is 4% alcoholic nitric acid ("nital") used at room temperature to reveal the ferrite grain boundaries and reveal phases and constituents such as cementite and pearlite. The second is alkaline sodium picrate (25g NaOH, 2g picric acid and 75 mL distilled water) used at 60 to 100°C for up to 30 minutes (1-3 minutes is usually adequate). This is used to color cementite yellow to brown (ferrite is not colored). The third is the standard version of Murakami's reagent (10g KOH, 10g potassium ferricyanide and 100 mL distilled water) used at 50°C for 3 minutes to color iron phosphide dark yellow or brown (cementite and ferrite not colored).

Gray Iron
As an example, Figure 11 shows a flake graphite specimen (see Table 1) etched with 4% nital. The matrix is predominantly pearlitic (colored tan, blue, and brown) and shows patches (arrow) of the ternary eutectic (ferrite, cementite, and phosphide).
pearlitic gray iron with ternary eutectic 250x Microstructure of gray iron containing a pearlitic matrix and the ternary eutectic (4% nital, 250x). The larger white particles (arrow-C) are cementite while the adjacent area (arrow-fp) contains ferrite and iron phosphide.

In comparison, Figure 12 shows the matrix structure of the specimen shown in Figure 2. The ternary phosphide is not present. The matrix is all fine pearlite.
pearlitic gray iron 100x Pearlitic matrix of the gray iron specimen shown in Fig. 2 etched with 4% nital, 100x.

Besides the ternary ferrite-cementite-iron phosphide eutectic and the previously mentioned binary eutectics (austenite and cementite and austenite and graphite), it is possible to obtain a binary ferrite-iron phosphide eutectic in cast iron. Figure 13 shows the binary ferrite-iron phosphide psuedo- eutectic in the specimen previously shown in Figure 10, after etching with hot Murakami's reagent which colors the phosphide brown but does not color ferrite.
gray iron w/binary ferrite-iron phosphide eutectic 100x Gray iron specimen containing a binary ferrite-iron phosphide eutectic with the phosphide colored by the hot Murakami's etch, 100x.

Nodular Iron

Go to: [FAQ] [Home] [Beginning of Article]