Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
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MACROSTRUCTURE 15
Figure 1-3 Macrostructure of directionally solidified nickel-base eutectic alloy (etchant, solution of
1 mL H202 and 99 mL HC1). (Courtesy ofW. Yankausas, TRW, Inc.)
The presence of a coarse columnar grain structure can impart useful properties
to a material that is to be used at high temperature. Considerable effort has
been made to preferentially grow such grains in high-temperature alloys used in
turbines. Figure 1-3 shows the macrostructure of a directionally solidified nickelbase
eutectic alloy in several product forms.
1-3.2 Billet and Bloom Macrostructures
In general, the steelmaker uses the hot-acid etch on discs cut, with respect to the
ingot location, from the top and bottom or the top, middle, and bottom of billets
or bloomst rolled from the first, middle, and last ingots teemed from the heat. If a
disc reveals a rejectable condition, billet material is rejected until the condition is
removed.
Figure 1-4 shows "dirty" corners, a lap, several small seams, and freckle-type
segregation in a hot-acid etched disc of bearing steel. The inclusion present in the
dirty corner (lower right) is a Mn-Fe-Al silicate. Figure 1-5 shows ingot pattern
and pits from inclusions in alloy steel. In Figure 1-6 the standard hot etching has
revealed entrapped gas, heavy segregation, voids, and ingot pattern in a disc of
AISI4140 alloy steel. Figure 1-7 shows the microstructure near the center of this
disc (longitudinal plane through the disc). The center of the disc is coarse and
exhibits an open pipe condition and associated segregation.
t Blooms are rolled sections larger than 6 by 6 in, while billets are smaller than this.