Metallography: Principles and Practices - ASM International

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Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort 26 METALLOGRAPHY Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org Figure 1-17 Dendrites observed on a broken sec- 9X tion of cast iron. the proper crystallographic orientation will grow an appreciable distance into the liquid. In body-centered cubic metals, such as iron, the direction of dendrite growth is always the cube axis, i.e., . The primary dendrite spacing depends upon the initial freezing conditions in the chill zone and thus is controlled by the nucleation rate. However, the secondary arm spacing is not a function of the nucleation rate at the chill surface but is controlled by the growth rate away from the surface. Thus, the critical factor for the secondary arm spacing is the rate of heat removal from the casting. The degree of heat removal changes constantly during most casting processes, and therefore measurement of the secondary arm spacing is of considerable value in the study of solidification. Although some qualitative information regarding dendritic spacing was known, Alexander and Rhines performed the first quantitative study of the solidification process [15]. In this report Alexander and Rhines discussed the problem of making measurements of dendrite spacing. In order to eliminate the need for corrections for orientation effect, they first made spacing measurements only in grains where the major dendrite axis was nearly in the plane of polish. Since this is a matter of judgment, some error can be introduced. They observed that not all of the secondary arms were well developed, and chose to measure only spacings between fully developed secondary arms. Since this is also a matter of judgment, error can result and the degree of repeatability of measurements suffers. They also observed that two characteristic spacings were present in the center of some of their ingots; they chose to record the larger spacing rather than the smaller or an average of the two. In some metals, such as aluminum castings, it is difficult to measure dendrite arm spacings. In these metals, one can measure the dendrite cell size, which is the width of the individual cells, or the dendrite cell interval, which is the center-tocenter distance between adjacent cells. In alloys with a small amount of interden-
Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort MACROSTRUCTURE 27 dritic material, the cell size and cell interval are equal. However, as the amount of interdendritic matter increases, the cell interval becomes greater than the cell size, and the dendritic cell size is usually the preferred measurement. If the amount of interdendritic material is small, the line intercept method can be used to compute the number of cells per unit length and the average cell size. 1-3.6 Forging Flow Lines Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org Macroetching is widely used to study metal flow patterns due to hot or cold working. Figure 1-I8a shows a disc cut from a close-die-forged steering knuckle made from AISI 4140 steel. The disc was deep-etched in the standard hot etch of hydrochloric acid and water. The flow lines can be observed, but they are much Figure l-18a Flow lines in closed-die-forged AISI 4140 steering knuckle revealed by hot etching with 50% aqueous HC1 (V2 x).
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Metallography: Principles and Practices - ASM International

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