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What is the average grain diameter (in mm) for the grain size ASTM grain size 12?

According to ASTM standard E1112, ”standard test methods for determining average grain size is

n = 2 G - 1

where:
n = the number of grains per square inch at 100X magnification, and
G = the ASTM grain size number.

Average Grain diameter (d, in mm)

\(\dfrac{1}{{\sqrt {\dfrac{{n \times {{10}^4}}}{{645}}} }}\)

Given ASTM grain size 12, thus,

n = 212 − 1 = 211 = 2048

Therefore, the average grain diameter will be

\(\dfrac{1}{{\sqrt {\dfrac{{2048 \times {{10}^4}}}{{645}}} }}\)

= 0.0056 mm.