Grain Size Measurement Methods
Although Committee E-4 was formed in 1916 for the express purpose of establishing standard magnifications for micrographs, its first standard, E 2-17T, Methods of Preparation of Micrographs of Metals and Alloys, was partly devoted to grain size measurement. Two basic approaches to measure grain size were being developed at that time. In the United States in 1894, Albert Sauveur published a "planimetric" approach, which was further developed by Zay Jeffries with two 1916 publications.
This approach measured grain size in terms of the number of grains visible on a cross section within a fixed area, the number per square inch at 100X, or the number per square millimetre at 1X, NA. From this value, the average cross-sectional area of the bisected grains can be computed. This is not an average of the maximum cross-sectional area of each grain because the sectioning plane does not intersect each grain at its maximum width.
Many grain size raters expressed the need for simpler ways to estimate the grain size. In some cases, such as heat clearance, grain size measurement is required. In many cases, it is required that G be 5 or greater (i.e., "fine- grained"). Hence, if the grain size is substantially finer than this, a quick method, which may not be as precise as an actual measurement, is adequate. A comparison chart method with examples of grain sizes meets this need adequately, as long as the grain size distribution is normal. Additionally, the specimens should be etched in the same manner as depicted on the chart. If the grain size is near the specification limit, an actual measurement is preferred due to the improved precision. The first grain size comparison chart was introduced in Methods E 2 in its 1930 revision; this chart was for copper.
In Germany in 1904, Emil Heyn published an intercept approach for measuring grain size. In this method, one or more lines are superimposed over the structure at a known magnification. The true line length is divided by the number of grains intercepted by the line. This gives the average length of the line within the intercepted grains. This average intercept length will be less than the average grain diameter but the two are interrelated.
Note that these methods are applied on the polished surface of the specimen, that is, on a plane that cuts through the three-dimensional grains. Thus, these are planar rather than spatial measures of the grain size. The planimetric, or Jeffries method, defines the grain size in terms of the number of grains per unit area, the average grain area, or the average grain diameter, while the Heyn intercept method defines it in terms of the average intercept length. The comparison chart method expresses the grain size only in terms of G, except for the copper charts, which use d.
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