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 25<br />
Figure 1-16 Hot-acid etching of this disc from an electroslag-remelted tool steel billet revealed a welldeveloped<br />
ring pattern <strong>and</strong> a few r<strong>and</strong>omly dispersed pits. (Courtesy of M. H. Lasonde, Bethlehem<br />
Steel Corp.)<br />
higher). Figure 1-17 shows dendrites observed on a broken tensile bar from a<br />
casting. The primary <strong>and</strong> secondary arms are readily visible, <strong>and</strong> tertiary arms can<br />
be detected occasionally.<br />
The primary <strong>and</strong> secondary arm spacings have been measured in solidification<br />
studies. The secondary arm spacing has been shown to be a sensitive measure of<br />
solidification phenomena. While most studies have measured the secondary arm<br />
spacing, Weinberg <strong>and</strong> Buhr measured the primary dendrite spacing because it<br />
changes more rapidly with freezing distance than the secondary arm spacing [14].<br />
The basic difference between the primary <strong>and</strong> secondary arm spacings can be<br />
viewed in terms of nucleation <strong>and</strong> growth mechanisms. The primary dendrite<br />
stalks develop from grains that nucleate at the chill surface. Only those grains with
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