Determination of Grain Size

Eng. Saeed Mater Eng. Ahmed Al-Afeefy

Objective To become acquainted with photomicrography

and make use of it in the determination of the grain size for a microsection.

Introduction Metals, except in a few instances, are

crystalline in nature and, except for single crystals, they contain internal boundaries known as grain boundaries. When a new grain is nucleated during processing (as in solidification or annealing after cold working), the atoms within each growing grain are lined up in a specific pattern that depends upon the crystal structure of the metal or alloy. With growth, each grain will eventually impinge on others and form an interface where the atomic orientations are different.

Procedure n100 = Nm * [M / 100]2

n100 : number of grains per square inch at 100x magnification.

Nm : number of grains per square inch at the magnification M of the photomicrograph.

N100= 2N-1

The ASTM grain size number N= [ ln (N100 ) / ln ( 2) ] + 1

How to calculate the are of one grain (actual area of grain at M1)???????????????????????

How to calculate the grain size Let M =200 A = 2*3 = 6 inch2

nm = 200 grain /6 inch2 = 33.3 grain / inch2

n100 = nm * [M / 100]2

=33.3*[200/100]2

n100 =133.2 grain / inch2

N= [ ln ( n ) / ln ( 2) ] + 1

= [ ln ( 133.2) / ln ( 2) ] + 1

= 8.06 grain / inch2