Metallography: Principles and Practices - ASM International

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Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort 42 METALLOGRAPHY breaking. Face-centered cubic metals can be difficult to fracture by these methods, especially if the section thickness is appreciable. Some of the uses of fracture examination include: • Identification of specimen composition • Detection of inclusion stringers • Detection of degree of graphitization • Assessment of grain size • Assessment of depth of hardening • Detection of overheating • Evaluation of quality These items are discussed in the sections that follow. A general review of some of these topics is provided in the book by Enos [29]. 1-5.1 Composition In the identification of unknown metals in the field [30], fracture examination can provide a clue to the identity of the material. Along with other visual features, such as color and apparent density, the fracture appearance can be used to provide a rough separation of metals. Ostrofsky provides the following guidelines for use with iron-base alloys [30]: Metal Fracture appearance Gray cast iron Coarse grain, gray Malleable iron Fine grain, black Wrought iron Fibrous, light gray Low-carbon steel Fine grain, light gray Tool steel Very fine grain (silky), light gray Prior to the development of rapid chemical analysis procedures, the steel melter followed the progress of the refining process by fracture examination. A small sample was poured and cooled rapidly. The sample was broken, and the fracture "read." Obviously, some experience was required. The approximate carbon content was assessed by the degree of brittleness or toughness of the fracture. High oxygen content could be detected by observation of blowholes in the specimen. 1-5.2 Inclusion Stringers Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org Inclusion stringers of macroscopic size can be readily detected on a fractured specimen after heat tinting in the blue heat range. This procedure is described in
Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org MACROSTRUCTURE 43 Figure 1-34 Macrograph of a hardened, fractured, and blued macroetched disc revealing inclusion stringers (white streaks). ASTM Specification E45. There are ASTM specifications that provide fracture test limitations for acceptance for a number of specific materials. Figure 1-34 shows an example of inclusion stringers revealed by use of this procedure. In most cases where the test is applied, the macroetch discs from the ingots tested are hardened, fractured, and blued on a hot plate or in a laboratory furnace. In general, heating to 500 to 700°F (260 to 371°C) is adequate, although slightly higher temperatures may be required for high-chromium grades. The fracture is always broken so that the fracture plane is longitudinal. The ASTM specifications describe both qualitative and quantitative methods for assessment of inclusion severity. The qualitative assessment can be conducted by comparing the samples to the ten standards in ISO Specification 3763 and noting the distribution of the stringers (core, surface, or uniform). Quantitative assessment requires measuring the length and/or thickness and counting the number in each size range. 1-5.3 Degree of Graphitization Chill and wedge tests have both been widely employed in the manufacture of gray and white irons to reveal the carbide stability, or the tendency of an iron to solidify as white iron rather than gray iron. These tests show the combined effect of melting practice and composition on carbide stability but are not a substitute for chemical analyses.
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Metallography: Principles and Practices - ASM International

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