Metallography:: Steps To Reveal Microstructures of A Specimen
Metallography:: Steps To Reveal Microstructures of A Specimen
Metallography:: Steps To Reveal Microstructures of A Specimen
Metallography is the study of metals by optical and electron microscopes. Structures which are
coarse enough to be discernible by the naked eye or under low magnifications are termed
macrostructures. Useful information can often be gained by examination with the naked eye of
the surface of metal objects or polished and etched sections.
Those which require high magnification to be visible are termed microstructures. Microscopes
are required for the examination of the microstructure of the metals. Optical microscopes are
used for resolutions down to roughly the wavelength of light (about half a micron) and electron
microscope are used for detail below this level, down to atomic resolution.
The most commonly used microscope is the conventional light microscope. In principle, optical
microscopes may be used to look through specimens (in transmission) as well as at them (in
reflection). Many materials, however, do not transmit light and so we are restricted to looking
at the surface of the specimens with an optical microscope. Electron microscope can be used in
the transmission e.g. Transmission Electron Microscope (TEM) and to look at the surfaces e.g.
Scanning Electron Microscope (SEM)
Microscopy can give information concerning a materials composition, previous treatment and
properties. Particular features of interest are
grain size
phases present
Chemical homogeneity
distribution of phases
elongated structures formed by plastic deformation
Procedure
1) Grinding and polishing
The specimen is first grounded on anSiC waterproof paper of grit(grit of 80), to produce a
flat surface, it is lubricated with water to keep it cool and to remove the grinding
products.
The sample should be moved forward and backward on thepaper until the whole surface
is covered with unidirectional scratches.
The specimen is then washed with running water to remove debris associated with the
grade of paper used
Then it is moved to a finer SiC waterproof paper of grit (grit of 220) and same procedure
is done again.
The specimen should be placed in a way such that the scratches produced are at right
angles to those formed by the previous paper. (This enables you to easily decide when the
scratches from the coarser paper have been completely removed)
It is then grounded on the next finer SiCpaper (grit of 500) and procedure is repeated
again.
Note :If the sample have some sharp or irregular edged, it might be necessary to remove it by
filing it. This is done in order not to damage the paper grit and also for safety reasons.
2) Diamond polishing
Two separate wheels are used, the first with 6micron diamond paste and the second 1
micron diamond paste.
3) Etching
The specimen is dipped in acid bath for few seconds. ( time will vary depending on the material
of which the specimen is made of)
The specimen is removed and immediately washed with alcohol as soon as we see a slight
blurring on the surface.
The specimen is viewed under the microscope in order to see its microstructures.
Optical Microscope Fitted with 35mm Camera and Digital Camera
4) Hardness
The hardness value of each specimen is determined using the Vickers hardness apparatus and Vickers
hardness number.
Observations
The microscope is used to observe the microstructure of each specimen.
The specimen is mounted on the microscope
Using the lowest magnification the surface is focused
Then, the magnification is increased until we are able to see and distinguish the grains, a slight
focusing is done again.
Dimension
Dimension
Average /
Value for
Average
1/D1 (m)
2/D2 (m)
<D> (m)
Vickers
hardness
Value for
Vickers
hardness
29.5
28.5
29.0
441
439.5
29.0
18.8
29.1
21.2
29.1
20.0
438
927
892
18.8
40.0
22.8
39.0
20.8
39.5
857
238
251
37.5
37.5
37.5
264
number
Conclusion:
Upon comparing the hardness values of the samples from unfailing sources, it was concluded that
specimen 1 is mild carbon steel, specimen 2 is high carbon steel and specimen 3 is stainless steel.
Specimen 1
Specimen 2
Specimen 3