Metallography: Principles and Practices - ASM International

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Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort 12 METALLOGRAPHY Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org ingot during solidification. Many inclusions will appear as pits after hot etching. /. Metallic segregates. Also concentrated toward the center of the ingot during solidification. g. Internal cracks. Flakes and cooling cracks due to excessive hydrogen content. h. Dendrites. Results from the solidification process and are present in most cast metals. /. Pattern effect {"ingot pattern"). A result of the solidification characteristics of the ingot and not a cause for concern, unless inclusions have segregated to the pattern interface. /. Decarburization. Occurs at the surfaces of steel ingots and billets during processing and shows up as a light etching rim. k. Carburized surfaces. Surfaces that etch darker than the interior of the disc due to enrichment of carbon content. /. Hardness patterns and soft spots. Revealed by etch contrast. m. Flow lines. Result from hot working and are revealed on longitudinal samples. The inclusions and segregates elongated by hot working are preferentially attacked by etching. 3. Macroscopic features in continuously cast metals a. Axial porosity. Porosity exhibited by continuously cast metals (as-cast) along the centerline that is due to incomplete feeding during solidification. b. Large inclusions. Oxidation of the pouring stream, generally between the tundish and the mold, that produces large oxide inclusions. c. Segregation streaks. Stressing (mechanical or thermal) of the solidifying steel that produces internal cracks which are immediately filled by metal enriched with sulfur from the interdendritic regions. d. Segregation bands. Light and dark etching bands that are sometimes observed on transverse sections. These bands are produced by excessive or uneven secondary water spray cooling. They are also referred to as halfway or midway cracks, radial streaks, or ghost lines. e. Triple-point cracks. Cracks that occur in continuously cast slabs. When observed on a transverse section, they are perpendicular to the narrow side of the slab within the V-shaped region where the three solidification fronts meet. These cracks are caused by bulging of the wide slab face, which results from inadequate containment of the solid shell. /. Centerline cracks. Cracks that form in the center area of the cast section near the end of solidification. The cracks are caused by bulging of the wide slab face or by a sudden drop in centerline temperature. g. Diagonal cracks. Cracks that occur in billets as a result of distortion of the billet into a rhomboid section. The distortion may be cause by nonuniform cooling, such as when two adjacent faces cool more rapidly than the other faces. h. Straightening or bending cracks. Cracks that occur during straightening or bending procedures if the center of the section is still liquid or above 1340°C.
Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org MACROSTRUCTURE 13 /. Pinch-roll cracks. Cracks that can be caused by excessive roll pressure applied when the center is still liquid or above 1340°C. /. Longitudinal midface cracks. Surface cracks observed on slabs. k. Longitudinal corner cracks. Cracks at the corners of billets and blooms that are due to compositional and operating factors. /. Transverse, midface, and corner cracks. Surface cracks that occur at the base of oscillation marks. Steel composition is a critical factor in their formation. m. Star cracks. Surface cracks that occur in clusters, each having a starlike appearance. They are generally fairly shallow and are usually caused by copper from the mold walls. 4. Macroetch features of consumable electrode remelted steels a. Freckles. Circular or nearly circular dark etching spots due to concentration or carbides or carbide-forming elements. b. Radial segregation. Radially or spirally oriented dark etching elongated spots generally located at midradius. These areas are usually enriched with carbides. c. Ring pattern. Concentric rings (one or more) which etch differently than the bulk of the disc as a result of minor variations in composition. d. White spots. Globular light-etching spots due to a lack of carbide or carbide-forming elements. 1-3 APPLICATIONS OF MACROETCHING The various imperfections or defects just described can be detected by hot-acid etching. Since the cross section usually provides more information than the longitudinal section, the general practice is to cut discs transversely, i.e., perpendicular to the hot-working axis. To facilitate handling, disc thickness should generally be 1 in or less. Longitudinal sectioning is used to study fiber, segregation, and inclusions. 1-3.1 Solidification Structures The structure resulting from solidification can be clearly revealed by macroetching. Figure 1-1 shows the macrostructure of a transverse disc cut from a small laboratory-size steel ingot that was etched with 10% HN03 in water. At the mold surface, there is a small layer of very fine equiaxed grains. From this outer shell, large columnar grains grow inward toward the central, equiaxed region. Figure 1-2 shows the macrostructure of a 99.8% aluminum centrifugally cast ingot after a minor degree of reduction. There is a thin band of fine grains around the edge, which is considerably thicker in the area near the left side of the photograph. Rather coarse columnar grains are observed growing from the outer surface, merging at a spot which is off center.
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