The basics of cast iron metallography.
The metallographic examination of cast iron reveals the metal's microstructural constituents. As more sophisticated alloys such as austempered ductile iron and compacted graphite iron become commonplace in metalcasting, foundries will turn to metallography as their link to quality assurance in their melting practices. Whether the examination is a qualitative assessment to identify and define the type and size of the graphite phase and other constituents such as nitrides and inclusions or a quantitative assessment of the amounts of phases, graphite shapes and nodule density, the results provide a base for comparison of microstructures. This comparison is the key as it is an affirmation of melt quality and control. It ensures that during solidification, castings have achieved desired microstructures and mechanical properties.
The following recommended procedure provides a framework for the preparation of cast iron specimens for metallographic examination. The goal is to remove surface deformation using a series of graded abrasives so that the true microstructure of the cast iron can be seen and analyzed.
This procedure is based on mounted (or unmounted) specimens placed in a holder for multiple specimen preparation. If an individual specimen is to be examined, divide the force values by six to determine the individual force to use.
1. Use 120-grit silicon carbide (sic) paper at 300 revolutions/min (rpm) with 22.5 lb of force until all surfaces are coplanar;
2. Use 240-grit SiC paper at 300 rpm with 22.5 lb of force for 2 min;
3. Use 9-[[micro]meter] diamond paste on a napless, polyester, hard-woven pad at 150 rpm with 22.5 lb of force for 3 min;
4. Use 3-[[micro]meter] diamond paste on a napless, non-woven, synthetic, chemotextile pad at 150 rpm with 27 lb force for 3 min;
5. Use 1-[[micro]meter] diamond paste on a napless, non-woven, synthetic, chemotextile pad at 150 rpm with 22.5 lb of force for 2 min;
6. Lightly etch specimens with 2-4% nitric acid (HN[O.sub.3]) in alcohol;
7. Use a 0.05-[[micro]meter] alumina polishing suspension on a low-nap, synthetic nylon, polishing cloth that is wet with water at 150 rpm with 20-27 lb of force (depending on the degree of etch) for 1.5-2 min.
After each polishing step, the specimens are washed with alcohol and blown dry with compressed air. Washing with water can result in corrosion stains on the surface so only alcohol should be used for the final cleaning step.
After preparation, the first step with cast iron is to examine the as-polished specimen before etching to properly view the graphite phase. This can be accomplished using brightfield vertical illumination. Since cast iron is not a simple binary iron-carbon (Fe-C) alloy, the carbon-equivalent (CE) value [total carbon content plus 33% of the sum of silicon (Si) and phosphorous (P) contents] also can be calculated. If the CE is greater than 4.3, the iron is hypereutectic; if it is less then 4.3 it is hypoeutectic.
In the Fe-C system, carbon (C) may exist as either cementite ([Fe.sub.3]C) or as graphite. As a result, the eutectic reaction is either liquid transforming to austenite and cementite at 2066F (1130C) or liquid transforming to austenite and graphite at 2074F (1135C). Slow cooling rates and the addition of elements such as Si promote graphite formation, while higher cooling rates promote cementite. The eutectic grows cellularly, varying with the cooling rate.
The microstructures of each type of iron vary according to graphite size and shape, and it important to recognize these differences to determine sound microstructures from sound casting practices. Following is the examination of gray and ductile iron micrographs before etching.
Figure 1a shows interdendritic flake graphite in a hypoeutectic gray iron alloy where proeutectic austenite forms before the eutectic reaction. This type of graphite is referred to as Type D, or undercooled graphite, and it always freezes into a weak interdendritic network.
Figure 1b shows regularly-shaped graphite flakes in a hypoeutectic alloy with high C content. While the flakes in Fig. 1a are 15-30 [[micro]meter], flake lengths in Fig. 1b are 60-120 [[micro]meter]. Figure 1c shows coarser flakes (250-500 [[micro]meter]) in a higher C content gray iron. Figure 1d shows a mix of graphite flakes (B- and D-type).
The addition of magnesium desulfurizes the iron and causes the graphite to grow as nodules rather than flakes, however, the nodule size and nodularity can vary depending upon composition and cooling rate. Figure 2a shows fine nodules 15-30 [[micro]meter] in diameter while Fig. 2b shows coarse nodules 30-60 [[micro]meter] in diameter. In addition, the comparison of nodules by area is 350/sq mm to 125/sq mm, respectively.
Crossed Polarized Light
Metallographic examination of graphite in crossed polarized light provides a definitive look at the specimen's microstructure. But, this observation method requires a well-prepared specimen, otherwise, the matrix exhibits a heavy scratch pattern and the graphite growth pattern will not be visible.
The polarizer and analyzer filters are placed in the crossed position (which produces the darkest matrix position), and a sensitive tint plate (lambda plate) may be inserted to further enhance coloration as in the ductile iron sample in Fig. 3a. Working at higher magnifications, as in Fig. 3b, allows for more structural detail, however, it also requires high-quality, stress-free objectives.
Figure 3c illustrates flake graphite under polarized light. The color varies with the crystallographic orientation of the graphite, illustrating that some internal details of the flake structure are better revealed with the color.
To see details of the matrix microstructure, specimens must be etched, with most standards recommending three etchants. The first is 2-4% alcoholic nitric acid (nital) used at room temperature to reveal the ferrite grain boundaries and phases as well as constituents such as cementite and pearlite. The second is alkaline sodium picrate (25g sodium hydroxide, 2g picric acid and 75 mL distilled water) used at 140-212F (60-100C) for up to 30 rain (1-3 min is adequate). This is used to color cementite yellow to brown. The third is the standard version of Murakami's reagent (10g potassium hydroxide, 10g potassium ferricyanide and 100 mL distilled water) used at 122F (50c) for 3 min to color iron phosphide dark yellow or brown and leave the cementite and ferrite colorless.
As an example, Fig. 4a shows a flake graphite specimen etched with 4% nital. The matrix is predominately pearlitic (colored tan, blue and brown) and shows patches of the ternary eutectic (ferrite, cementite and phosphide). In comparison, Fig. 4b shows the matrix structure of Fig. 1b in which ternary phosphide is not present and the matrix is all fine pearlite.
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 4c illustrates this with an etching of hot Murakami's reagent which colors the phosphide brown and doesn't color the ferrite.
Ductile iron specimens can have a wide range of matrix structures depending upon composition and as-cast cooling rate. Figure 5a shows a specimen with a fully ferritic matrix after etching with 4% nital. Figure 5b shows a specimen with a pearlitic matrix and ferrite surrounding the nodules.
The microstructure of white cast iron is best observed after etching. Figure 6a shows a typical example after etching with 4% nital. As can be seen, the interdendritic cementite (white) has a spiky appearance. Austenite formed as the pro-eutectic constitute before the eutectic reaction and later transforms to pearlite and cementite upon cooling below the eutectoid temperature of 1334F (723C).
Figure 6b shows a higher magnification of a specimen etched with 4% nital. The massive cementite particles are visible with the outline made by the etch. The outline around the cementite particles is due to light being scattered from the height difference or "step" around the particles. In addition, ferrite surrounds each cementite particle due to local decarburization. Figure 6c shows the effect of etching this specimen with alkaline sodium picrate, which colors the large cementite brown.
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|Author:||Vander Voort, George F.|
|Date:||Dec 1, 1998|
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