This video shows how to determine the ASTM 
grain size number for a metal specimen.
Generally, the strength of metal increases as 
the grain size decreases.
Other properties, such as ductility, change with 
grain size as well.
To begin, a metal specimen is polished and 
etched to expose the grain structure.
Here we have a microscopic photo of a 1045 
steel sample.
Now what can we do with this photo?
Let's start with number 1, Determine the 
Magnification.
In order to begin we must find the print size 
dimensions of the photo.
We can select either the width or the height.
Let's use the width of 165 mm, which is the 
width of the photo.
The actual width that the photo covers on the 
steel sample is 0.215 mm.
Knowing the dimensions we can solve for the 
magnification.
This solves part 1.
Since we have the magnification we can 
determine the Grain size number.
This equation will determine the number of 
grains in the given sample.
A whole grain has a closed visible boundary.
A partial grain is normally found on the outer 
edges where the grain was cut off.
Now that we know the difference let's count the whole and partial grains and insert the values into the equation.
Since we know the number of grains we can 
continue solving for the grain size number.
We start off by solving for the true area,
where W is the print width,
H is the print height,
and M is the magnification we solved for in 
number one.
Now we have the true area that the photo covers 
and the number of grains in that area.
ASTM relates the grain size number, n to the 
number of grains in a one square inch area of a 
specimen photo at 100 magnification.
This example uses a photo magnification of 768, 
so some conversions are required.
We found that there are 140 grains in 0.0351 
square millimeters.
ASTM wants number of grains in a one square 
inch area of a specimen photo at 100 
magnification.
So, one inch on a 100 magnification photo is 
actually 0.254 millimeters on the specimen.
And one square inch on a 100 magnification 
photo is actually 0.0645 square millimeters.
Therefore the actual area per ASTM is equal to 
0.0645 square millimeters.
We now have an equation with all the variables 
known and are able solve for N.
Now that we have a value for N we can plug it 
into the other equation to solve for n.
The grain size number is the n.
In order to bring the unknown n down from the exponent we must Log both sides.
This is our grain size number.  We have now 
completed part two.
The last thing to do is solve for the average 
Grain size diameter.
First by solving for the total true length, and then 
solving for the number of grains intercepted.
Number of grains intercepted refers to the grains 
intercepting two vertical lines .
Plug in the values to solve for the average grain 
diameter.
This concludes number three.
We have now solved for all three problems.
