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Count the grains on the boundaries of the micrograph.
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Then count the grains inside the boundaries of the given photomicrograph.
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The grains on the boundaries i.e., partially visible grains are considered as half grains.
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The total no. of grains =1/2(Grain at boundaries) + Grains inside the boundaries
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Now, the total no. of grains per unit area is calculated by adding all the grains and dividing them by the area of the photomicrograph.
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For calculating ASTM grain size number, we require the no. of grains per square inch at 100x magnification(N100).
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If the magnification of photomicrograph is other than 100x then we need to calculate N100 which is given by N100 = [ Magnification of micrograph at hand/100]2xN(at given magnification)
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Now using formula n=2G-1 we get ASTM grain size number.
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The method with greater grain size number has more grains and smaller grains than the metal with smaller grain size number
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