2. Introduction and Purpose
Metallography is the study of the physical structure and components
of metals, typically using microscopy.
Precision metallurgical sample preparation is a key step in
performing reliable metallurgical testing.
Facilitate examination and interpretation of microstructural features.
Ex. To find out various phase present inmaterial.
Imprope preparation methods may obscure features, and even create
artefacts that may be misinterpreted.
3.
4. Microscopic Examination is used to find
Grain Size, shape and distribution
Non Metallic inclusion
Segregation of elements
Heterogenous conditions
5. Selection of piece from main product
It is important to study something that is representative of the
whole specimen.
Cutting of that representative part must also be done very carefully.
Operations such as shearing produce severe cold work, which can
alter the microstructure of a sample.
Abrasive cutting (sectioning) offers the best solution to eliminate these
undesirable features; the resultant surface is smooth, and the sectioning
task is quickly accomplished.
6. Selection of piece from main product
Low-speed cut-off wheels are utilized in cases where the heat created by
standard abrasive cutters must be avoided. Ample coolant and proper speed
control are essential in all sectioning operations.
7. Mounting
When the specimen to be tested is inexpensive and easily available, a
suitable size specimen may be cut and polished for examination.
Mounting of specimens is usually necessary to allow them to be handled
easily. It also minimises the amount of damage likely to be caused to the
specimen itself.
Properties of mounting material
Should not influence the specimen as a result of chemical reaction
Should not impart any mechanical stresses
Should adhere well to the specimen
8. Hot Mounting Press
A typical mounting cycle will compress the specimen and mounting media
to 4,000 psi (28 MPa) and heat to a temperature of 350 °F (177 °C). The
pressing mechanism is achieved by hydraulics.
Specimens can be hot mounted (at around 200 °C) using a mounting
press, either in a thermosetting plastic (e.g. phenolic resin), or a
thermo-softening plastic (e.g. acrylic resin).
9. Mounting
A mounted specimen usually has a thickness of about half its
diameter, to prevent rocking during grinding and polishing.
The edges of the mounted specimen should be rounded to
minimise the damage to grinding and polishing discs.
If hot mounting will alter the structure of the specimen a cold-setting
resin can be used, e.g. epoxy, acrylic or polyester resin.
10. Coarse Grinding
For a perfect observation sample, it must :
Be free from scratches, stains and others imperfections which tend to
mark the surface.
Retain non-metallic inclusions.
Reveal no evidence of chipping due to brittle intermetallic compounds
and phases.
Be free from all traces of disturbed metal.
The purpose of the coarse grinding stage is to generate the initial flat
surface necessary for the subsequent grinding and polishing steps.
11. • Course grinding can be accomplished either
wet or dry using 80 to 180 grit electrically
powered disks or belts.
• Care must be taken to avoid significant
heating of the sample.
• Grinding belt material is usually made of
SiC paper.
• Rotate the specimen by 90⁰ on every grade-
change
12. Fine Grinding
Each grinding stage removes the scratches from the previous coarser
paper.
This is more easily achieved by orienting the specimen perpendicular
to the previous scratches, and watching for these previously oriented
scratches to be obliterated.
Between each grade the specimen is washed thoroughly with soapy
water to prevent contamination from coarser grit present on the
specimen surface.
In general, successive steps are 240, 320, 400 and 600 grit SiC and the
grinding rate should steadily decrease from one stage to the next.
13. ABRASIVE BELT GRINDER
Different grades of SiC paper are
rolled on rollers which rotate at a
specific speed. The specimen is then
moved from one end to another in a
uniform fashion.
When shifting to a different grade,
the specimen is rotated by an angle
of 90⁰.
14. Polishing
Polishing involves the use of abrasives, suspended in a water
solution, on a cloth-covered electrically powered wheel.
In intermediate polishing, SiC paper of different grades are used.
Again, the specimen is rotated while switching from one grade to
another. The operation is carried out on a disc with the sandpaper
stretched across it.
.
15. Double Disc Polishing Machine
For fine polishing, this machine is used with a
napped cloth fixed atop it.
Diamond particles or Al2O3 is suspended in
distilled water in a light slurry.
The disc is rotated and the specimen is held
with mild pressure to absolutely remove
scratches
16. Polishing
Following the final 600 grit fine-grinding stage, the sample must be
washed and carefully dried before proceeding to the first polishing
stage.
Beginning with 25-micron suspended aluminium oxide particles
(suspended in water) on a Nylon-cloth, the final fine-grinding
surface layer resulting from the previous grinding procedure should
be completely removed with a rotation rate of 150-200 rpm.
Wash the specimen and move on to finer suspended particles on
separate cloth.
17. Polishing Cloth
The final polishing stage with 1-micron suspended aluminium oxide or
diamond particles should be carried out on a separate polishing wheel at a
slower speed of 100 - 150 rpm using a napped cloth. After 1 or 2 minutes a
properly polished specimen should have a mirror-like surface free of scratches
There are three types of polishing clothes; Woven, Non-Woven and
Flocked.
Woven cloths offer ‘hard surface’ polishing properties and
guarantee flat pre- polishing, without deterioration of the edges.
18. Non-woven cloths, are used on very hard materials for high precision surface
finishing such as glass, quartz, sapphire and semi-conductors.
The Flocked cloths, guarantee a super-polished finish. The polishing duration
must be as short as possible, to avoid inclusions from being extracted.
19. The grinding and polishing procedure for steel
sample preparation is as given below
20. Etching
Metallographic etching is the process of revealing microstructural details
that would otherwise not be evident on the as-polished sample.
Etching is not always required as some features are visible in the as-polished
condition
such as porosity, cracks and inclusions, for eg, in grey cast iron.
Properties revealed by etching
grain size
Segregation
shape, size, and distribution of the phases
mechanical deformation
21. The specimen after polishing needs to be properly washed and cleaned with
distilled water and after proper drying, the etching reagent is applied by various
methods.
Types of Etching:
Chemical Etching
Electrolytic etching
Heat tinting
22. a) Polished but unetched surface
gives a clean image but no
details about the
microstructure of the
specimen
b) Etched surface: When the
specimen has grains with
same orientations, only the
grain boundaries are visible.
c) Etched surface: When the
specimen has grains oriented
differently, each grain reacts
differently to give varying
colours.
23. Chemical Etching
This typically involves immersing the sample in an etchant such or
swabbing the surface with an etchant. The etchant selectively corrodes
microstructural features.
Immersion time or etching time is highly dependent on the system and in
most cases requires experience.
The reactivity of a grain is dependent on the orientation of its
microstructure.
Deeper etches are preferred for low magnification examinations, while
shallow etches are preferred for higher magnification etches.
24.
25. Sample material Etchant Time
Wrought Iron 5% HNO3 in Alcohol 30 sec – 1 min
Cast Iron
2% HNO3 in
alcohol or 5% picric acid
10 – 30 sec
Tempered high carbon steel 1% in HNO3 alcohol 5 – 15 sec
Low alloy steel 10% HNO3 in alcohol Upto 1 min
Stainless Steel 10% HNO3 in alcohol 5 – 40 min
High Speed Steel
10g of Potassium ferrialdehyde+10g KOHin
10ml water
20 sec – 6 min
Cu and its alloys 10% soln. of Ammonium Sulphate in water -
Magnesium Alloys 2 – 4 % soln. of HNO3 in alcohol -
Aluminum 2% HF (conc.) + 25% HNO3 in water Swab for 15 sec.
30 min 90 min 240 min
26. Final step
After etching process, the specimen needs to be washed again in distilled
water to remove any excess reagent present on it.
If not washed, under microscopic observation, there might be aberrations
in the colour of the sample.
Also, slow and continuous reaction for a long time may take place
because of which we cannot use the sample for proper microscopic
observation.
Cleaning can also be done by placing a drop of spirit and drying it.
After washing, it can be dried using a low power blower.
Finally, the specimen is ready for observation under microscope.
