An ASTM grain size determination is being made from a photom

An ASTM grain size determination is being made from a photomicrograph of a metal at a magnification of 100x. What is the ASTM grain size number of the metal if there are 64 grains per square inch?

Solution

Number of grains per square inch at 100X magnification s given as-

n = 2 ^{G - 1}

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

Given- n=64

Therefore , 64 = 2^(G-1)

After solving , G= 7

Therefore, ASTM grain size number = 7