1. Determination of Grain Size
Eng. Ibrahim Aljaish Eng. Ahmed Al-Afeefy

2. Objective
To become acquainted with photomicrography and make use of it in the determination of the grain size.

3. 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 called grain boundaries.

4. Applications of microstructural examination:
1. Understand relation between properties & structure. 2. Predict properties of materials once these relationships have been established. 3. Design alloys with new property combinations. 4. Monitor & control results of heat treatment. 5. Study mode of mechanical fracture.

5. Optical microscopy Used in a reflecting mode
Contrasts in the image produced result from differences in reflectivity of the various regions of the microstructure.
Specimen surface must be ground and polished to a smooth and mirror like finish.
Microstructure is revealed using chemical etching (grain boundaries) .

6. Procedure
1- Take a photomicrograph (using the metallurgical microscope) of one of the specimens you prepared in the Experiment 1. Make more trials in order to reach optimal camera setting that yields clear photographs in which the grain boundaries are evident and well defined. The magnification used should be higher than 100x.
2- Using a sharp pencil, draw a rectangle of known area on the photomicrograph. Count the nominal number of grains as one and each boundary grain intercepted by the perimeter as a half.
3- Calculate the number of grains per square inch at the magnification used to obtain the photomicrograph. Modify the calculation to obtain the number of grains per square inch at 100x magnification, using the following equation(cont. in the next slide).

7. Procedure cont. n100 = nm * [M / 100]2 n100= 2G-1
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= 2G-1
The ASTM grain size number G= [ ln (n100 ) / ln ( 2) ] + 1

8. 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
G= [ ln (n100 ) / ln ( 2) ] + 1
= [ ln ( 133.2) / ln ( 2) ] + 1
= 8.06

9. Grain size calculation
M= 500
n100= 2G-1
find G the grain size number?

10. Questions
What is the relationship between the grain size and ductility and hardness?