Determination of Grain Size Eng. Ibrahim Kuhail 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. The ASTM grain size number N= [ ln ( n ) / ln ( 2) ] + 1

How to calculate the grain size Let M =200 A = 2*3 = 6 inch2 nm = 6grain /6 inch2 =1grain / inch2 n100 = nm * [M / 100]2 =1*[200/100]2 n100 =4 grain / inch2 N= [ ln (n100 ) / ln ( 2) ] + 1 = [ ln ( 133.2) / ln ( 2) ] + 1 = 3 grain / inch2

How to calculate the diameter of grain To find n1 n100 = n1 * [1 / 100]2 n1 = n100 [100]2 n1= 4[100]2 = 40000 grain / inch2 How many grains are in 1 inch2 grains in1 inch2 = 1/ 40000 grain / inch2

Questions For the pervious three photomicrographs, Calculate the ASTM number of grains size of material assuming that 200x magnification factor. Calculate the diameter of its grains . Which of the three material is the hardest and which of them is the most ductile?