MATERIALS SCIENCE & METALLURGY

1. PREPARATION OF SPECIMEN FOR
MICROSCOPIC EXAMINATION
MATERIALS SCIENCE & METALLURGY
DEPARTMENT OF MECHANICAL ENGINEERING
B. Tech II, IV Semester
1) MAHAVEER SUTHAR (U15ME084)
2) DIVIJ GAJJAR (U15ME089)
3) VASAVA GAURANG(U15ME098)

2. Preparation of Specimen
for
Microscopic Observation

3. INTRODUCTION
Metallography is the study of the microstructure of all
types of metallic alloy typically using microscopy.
• It can be more precisely defined as the scientific
discipline of observing and determining the chemical and
atomic structure.
•Metallurgical sample preparation is a key step in
performing reliable metallurgical testing.

4. why this preparation?
•To find out the various phases present in the
material.
•To find out Approximate strength, and
Hardness.
•To find out Approximate composition
material.
•To find out the reason of failure of Material.
•To study the constructional details of
Metallurgical Microscope and observe the
micro structure of the prepared specimen.
Purposes ….

5. Selection of piece from given
specimen...
•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.

6. Copper specimen ground with 800 grit paper

7. SAMPLE PREPARATION
Consists of five major steps
• sectioning
• mounting
• grinding
• polishing
• etching

9. MOUNTING
• Small or oddly shaped specimens are mounted to facilitate
easy handling during preparation and examination
• When the specimen to be tested is inexpensive and easily
available, a suitable size specimen may be cut and polished
for examination
• Standard mounts usually measure 25 mm (1 in.), 32 mm
(1.25 in.), or 38 mm (1.5 in.) in diameter
• Bakelite and diallyl phthalate are thermosetting resins
which are most widely used as moulding material
• Transparent methyl methacrylate, polystyrene, polyvinyl
chloride (PVC)are some of the thermoplastic resins used in
moulding
• Both requires heat and pressure during molding.
Thermosetting molds can be ejected from the mould at
the moulding temperature, while thermoplastic resins
must be cooled to ambient temperature under pressure

10. 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

11. Mounting
•Specimens can be hot mounted using a mounting press, either in a
thermosetting plastic (e.g. phenolic resin), or a thermo-softening plastic (e.g.
acrylic resin).
Hot Mounting Press
•A typical mounting cycle will
compress the specimen and
mounting media to (28 MPa)
and heat to a temperature of
350 °F (177 °C). The pressing
mechanism is achieved by
hydraulics.
•If hot mounting will alter the structure of the specimen a cold-setting resin can
be used, e.g. epoxy, acrylic or polyester resin.

12. 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.

13. GRINDING
• To prepare the cut surface suitably for metallographic
examination as optically flat, reflective, smooth and scratch
free
• Machining
– involves the use of tools having cutting edges of controlled
shape like sawing, lathe turning, milling, and filing
– used only for the preliminary stages of preparation
• Grinding and abrasion
– uses abrasive particles whose projecting points act as the
cutting tools
– the abrasive particles are cemented together into a block
whose exposed surface is the working surface

14. GRINDING
– Examples are abrasive cut-off wheels, grinding wheels,
abrasive laps, and abrasive stones
– in another type, a layer of abrasive particles is cemented
onto a cloth or paper backing, creating coated abrasive
products
– Emery papers are typical example for this kind
• Grinding employ high surface speeds were heating of the
surface layers may occur
• Abrasion uses low surface speeds, hence no significant
heating takes place

15. •After removing the scratches with the help of bench grinder then
the specimen through 90 degree and again grind it on belt grinder
to remove scratches formed by the wheel of the bench grinder and
make surface completely flat.
Bench grinder
GRINDING

16. Belt grinder

17. • Turn the specimen through 90 degrees and using emery
paper go on publishing. The specimen consequently with
1/0,2/0,3/0,4/0 papers each time turning the specimen
through 90 degrees while switching over to next emery
paper.
Emery paper
GRINDING

18. Coarse Grinding
• The purpose of the coarse grinding stage is to generate the
initial flat surface necessary for the subsequent grinding and
polishing steps.
•
• 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

19. Fine Grinding
•Each grinding stage removes the scratches.
•This is more easily achieved by orienting the specimen
perpendicular to scratches
•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.

20. 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.
Fine Grinding

21. EMERY PAPER
•A grayish black form of corundum containing iron oxide or
other impurities
•It is used in form of powder form as an abrasive
•Made from milled emery rock, bonded or sized to paper
with an animal glue for water resistance
•Coarse to fine :
40,46,54,60,70,80,90,100,120,180,220,320f,ff

24. 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.

25. Polishing
• Manual polishing is done by rotating specimen by hand
against a cloth that has been charged with a fine abrasive and
an appropriate liquid, and then has been stretched across a
flat backing surface
• Diamond, alumina (Al2O3), and magnesium oxide (MgO) are
the abrasives most commonly used for polishing
• Mechanized processes are less time consuming and laborious
than manual operations
• The paper or cloth is attached to the surface of a wheel that is
rotated at a comparatively low speed in a horizontal plane
• The specimen is held against the working surface of a wheel
and rotated slowly in a direction opposite that of the wheel

26. 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.
• 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

27. •Water-soluble lubricants that have an oily feel are
recommended for most polishing applications. The water-
soluble lubricants are easily washed away in warm tap water.
•Oil based lubricants can be used for polishing but are not as
easy to remove.
•Diamond pastes and slurries are usually available as either
water- or oil-based suspensions.
• velvet is used for final polishing purpose.
Material used for polishing

28. •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.
Polishing(fine polishing)
•The polishing should be continued till a Mirror polished surface is
obtained.

29. The grinding and polishing procedure for steel
sample preparation is as given below
Polishing

30. ETCHING
• Metallographic etching is the process of revealing
microstructural details that would otherwise not be evident
on the as-polished sample.
• Properties revealed by etching
– grain size
– Segregation, etc.
• 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.
• Etching process should be selectivity to the material that
has to be removed.
• Types of Etching:
– Chemical Etching
– Electrolytic etching
– Heat tinting

31. 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.
Etching

32. Chemical Etching
• Oldest and most commonly applied technique
• Etchant reacts with the specimen without the use of an external
current supply
• Etching proceeds by selective dissolution according to the
electrochemical characteristics of the constituents
• Proper selection of etchant and etching time is the most important
criteria which are determined experimentally and is given by ASTM
E407 (Standard Practice for Microetching Metals and Alloys)
• Microstructure obtained is decided by etchant and etching time

33. 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 KOH in 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

34. Electrolytic Etching
• In electrolytic (anodic) etching,
electrical potential is applied to
the specimen using an external
circuit
• During electrolytic etching,
positive metal ions leave the
specimen surface and diffuse
into the electrolyte
• Typical examples are platinum,
graphite and stainless steels.

35. Heat tinting
•Process of oxidizing a sample in a furnace. This
induces oxidation of surface features at different rates,
to reveal various features.
•Coloration of the surface takes place at different rates
according to the reaction characteristics of different
elements.
•The observed interference colours allow the
differentiation of phases and grains.

36. 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.

38. Precaution
• cleanliness : basically in polishing in we want
better polishing, to do so our hand as well as
sample must be free from abrasive.
•Wear Goggles and gloves while handling
chemicals.
•Do not eat or drink while working in the lab.
•Throw away the left over etchants into the
sink immediately after using it.

39. THANK YOU