|
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.
<<
Back to Resources List
|
