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American
Standard Of Testing Materials ie. ASTM has found a methodology to
evaluate the quality and characteristics of metals with respect to
the microstructure i.e. metal structure based on the grain size
which is achieved after casting/melting/heat treating the metal.
Normally the structures will not be in perfect arrangements and it
will be oriented in a random basis. These are also called as mosaic
block structure. And this is of great importance since the size of
the blocks considerably affects the properties of the metal. The
grains or crystallites, usually of irregular form please see the
structure below:
On
the boundaries of grains, the atoms have a much less regular
arrangement than in the grain body, due to the influence of atomic
force fields in the neighboring grains. It should also be noted that
in engineering metals, all sorts of impurities are concentrated on
the grain boundaries and they increase disorientation in atomic
arrangement. The atoms in the boundary layers will possess higher
energy due to distorted packing.
Now grain size of a specimen can be evaluated through the following
methods with the help of your metallurgical microscope, scan the
structure through microscope at 100 x mag. i..e your eye piece
should be 10 x eye piece and objective should be also 10x , and try
to measure the individual grain which has an average size , if 1 ½
grain occupies in an inch, Then it is defined by ASTM Grain NO.1
Accordingly
if:
1
½ - 3 grains are there in a Square inch the Grain No is 2
3 - 6 grains are there in a Square inch the Grain No is 3
6 - 12 grains are there in a Square inch the Grain No is 4
12 - 24 grains are there in a Square inch the Grain No is 5
24 - 48 grains are there in a Square inch the Grain No is 6
48 -96 grains are there in a Square inch the Grain No is 7
96 grains and more Grain No is 8
Follow
the above Procedure to arrive at the Grain Size Measurement of a
microstructure as per ASTM standards.
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