Metallography: Principles and Practices - ASM International
Metallography: Principles and Practices - ASM International
Metallography: Principles and Practices - ASM International
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<strong>Metallography</strong>: <strong>Principles</strong> <strong>and</strong> Practice (#06785G)<br />
Author(s): George F. V<strong>and</strong>er Voort<br />
Copyright © 1984 <strong>ASM</strong> <strong>International</strong> ®<br />
All rights reserved.<br />
www.asminternational.org<br />
MACROSTRUCTURE 13<br />
/. Pinch-roll cracks. Cracks that can be caused by excessive roll pressure<br />
applied when the center is still liquid or above 1340°C.<br />
/. Longitudinal midface cracks. Surface cracks observed on slabs.<br />
k. Longitudinal corner cracks. Cracks at the corners of billets <strong>and</strong> blooms<br />
that are due to compositional <strong>and</strong> operating factors.<br />
/. Transverse, midface, <strong>and</strong> corner cracks. Surface cracks that occur at the<br />
base of oscillation marks. Steel composition is a critical factor in their<br />
formation.<br />
m. Star cracks. Surface cracks that occur in clusters, each having a starlike<br />
appearance. They are generally fairly shallow <strong>and</strong> are usually caused by<br />
copper from the mold walls.<br />
4. Macroetch features of consumable electrode remelted steels<br />
a. Freckles. Circular or nearly circular dark etching spots due to concentration<br />
or carbides or carbide-forming elements.<br />
b. Radial segregation. Radially or spirally oriented dark etching elongated<br />
spots generally located at midradius. These areas are usually enriched with<br />
carbides.<br />
c. Ring pattern. Concentric rings (one or more) which etch differently than<br />
the bulk of the disc as a result of minor variations in composition.<br />
d. White spots. Globular light-etching spots due to a lack of carbide or<br />
carbide-forming elements.<br />
1-3 APPLICATIONS OF MACROETCHING<br />
The various imperfections or defects just described can be detected by hot-acid<br />
etching. Since the cross section usually provides more information than the<br />
longitudinal section, the general practice is to cut discs transversely, i.e., perpendicular<br />
to the hot-working axis. To facilitate h<strong>and</strong>ling, disc thickness should<br />
generally be 1 in or less. Longitudinal sectioning is used to study fiber, segregation,<br />
<strong>and</strong> inclusions.<br />
1-3.1 Solidification Structures<br />
The structure resulting from solidification can be clearly revealed by macroetching.<br />
Figure 1-1 shows the macrostructure of a transverse disc cut from a small<br />
laboratory-size steel ingot that was etched with 10% HN03 in water. At the mold<br />
surface, there is a small layer of very fine equiaxed grains. From this outer shell,<br />
large columnar grains grow inward toward the central, equiaxed region.<br />
Figure 1-2 shows the macrostructure of a 99.8% aluminum centrifugally cast<br />
ingot after a minor degree of reduction. There is a thin b<strong>and</strong> of fine grains around<br />
the edge, which is considerably thicker in the area near the left side of the<br />
photograph. Rather coarse columnar grains are observed growing from the outer<br />
surface, merging at a spot which is off center.