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 />
24 METALLOGRAPHY<br />
Copyright © 1984 <strong>ASM</strong> <strong>International</strong> ®<br />
All rights reserved.<br />
www.asminternational.org<br />
Figure 1-15 Hot-acid etching of this disc from an electroslag-remelted tool steel billet revealed light<br />
freckle segregation <strong>and</strong> a faint, discontinuous ring pattern. (Courtesy of M. H. Lasonde, Bethlehem<br />
Steel Corp.)<br />
Dendrites grow initially in the form of rods. However, growth perturbations<br />
or minor changes in the liquid around the growing dendrite occur. These<br />
temperature <strong>and</strong> compositional perturbations in the liquid cause bumps to form<br />
on the side of the rods, which grow outward into the liquid forming the secondary<br />
arms. In a like manner, tertiary arms can form on the secondary arms <strong>and</strong> so forth.<br />
Later, the liquid between the primary, secondary, <strong>and</strong> tertiary arms freezes. The<br />
planes containing the primary stalk <strong>and</strong> a secondary arm are called primary<br />
sheets. These planes are parallel to the direction of heat flow. Planes perpendicular<br />
to the primary sheets containing secondary <strong>and</strong> tertiary arms are called<br />
secondary planes. In the examination of dendrite structures, low magnifications<br />
(10X for example) are much more useful than high magnifications (100X <strong>and</strong>
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