Different Measures of Grain Size
Another complicating factor is the different measures of grain size. The planimetric method, described below, yield the number of grains per square millimeter area, NA, from which we can calculate the average grain area, A. It is common practice to take the square root of A and call this the grain diameter, d, although this assumes that the cross sectional shape of the grains is a square, which it is not.
The intercept method yield a mean intercept length, L3 ; its relationship to NA, A, or d is not exceptionally well defined. A variety of planar grain size distribution methods have also been developed to estimate the number of grains per unit volume, Nv, from which the average grain volume, V, can be calculated. The relationship between these spatial measures of grain size and the above planar measures is also ill-defined.
It is now common to express grain sizes in terms of a simple exponential equation: (Equation 1)
n = 2 G - 1
where:
n = the number of grains per square inch at 100X magnification, and
G = the ASTM grain size number.
This approach was developed and introduced in 1951 with the premiere of ASTM standard E 91, Methods for Estimating the Average Grain Size of Non-Ferrous Metals, Other Than Copper and Their Alloys. Although the NA, d, or L3, values had been used for many years as measures of grain size, the G values were adopted readily due to their simplicity. As shown in Eq. 1, we can directly relate the number of grains per unit area to G, but the relationship between L3, and G, or NV and G are not as clearly defined. This problem is one of many being addressed by ASTM Committee E4 on Metallography.
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