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QUANTITATIVE MICROSCOPY 4 5 7 14- 13- 12 II «I0 UJ _i 9 a £ 8 < o 7 a E 6 u. o 5 t- z 4 UJ ^ O 0= 3 UJ Q. 2 I INTERCEPT LENGTH DISTRIBUTION-DUPLEX GRAIN STRUCTURE Fine Groin Size rm-h-r Coarse Grain Size n _. CL- IO 15 20 25 INTERCEPT LENGTH,mm 30 35 40 Figure 6-12 Arithmetic frequency histogram of the random intercept measurements of the duplex grain structure shown in Fig. 6-11. If more than a few percent of pearlite is present, the methods just described to measure grain size must be modified to obtain an accurate grain size estimate. The morphology of the ferrite phase can vary substantially depending on the composi- tion and processing. If the second phase is similar in size to the matrix phase, comparison charts can be used to estimate the matrix grain size. If the amount of the second phase is low, and is located primarily at the grain boundaries, chart ratings can be performed. If these conditions are not present or if a more accurate estimate is desired, the planimetric or intercept methods can be used. If the planimetric method is chosen, the number of matrix grains that are within the test area and intersect the perimeter is counted as described previously, but the percentage of the test area containing these grains must also be measured. This can be done by point counting or lineal analysis within the test area. Then the corrected test area is used in the calculations. This method is not very convenient. The intercept method is simpler and yields excellent results. First, the volume fraction of the matrix is determined, generally by point counting or lineal analysis. This measurement is usually made on the second phase, and the V v of the matrix phase is determined by difference. Next, the three-circle test grid, or another linear test grid, is applied to the structure, and the number of matrix grains N a