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Preparation of Specimens
for Metallographic
Examination
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Metallography
INDEX
 Objectives
 Introduction
 Metallography
 Questions
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Objective
 To prepare the specimens surfaces to be examined
for their microstructure study by the microscope .
 To learn and to gain experience in the preparation of
metallographic specimens.
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Introduction
 Metallography is basically the study of the structures and
constitution of metals and alloys, using metallurgical
microscopes and magnifications, so that the physical and
mechanical properties of an alloy can be related to its
observed microstructure.
 It provides information about the specimen under
investigation, including the size and shape of the grains
(crystallites), the presence of micro defects (such as
segregation, hair cracks, and nonmetallic inclusions), and
the nature and distribution of secondary phases.
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Metallography
 Cutting
 Grounding – emery paper (240, 300, 400, 600)
 Polishing (0.5, 0.1, 0.05μ)
 Etching –Nitol/ Kellers Solution
 Microscopy
Grounding
 grinding is a subset of cutting, as grinding is a true metal
cutting process.
 Abrasive :The use of Premium SiC abrasive paper is the
most efficient and practical technique for grinding metallic
metallographic specimens. Although many qualities of
silicon carbide are readily available, only the premium
grade SiC powder provides the most consistent results and
highest grinding rates.
 Each grain of abrasive functions as a microscopic singlepoint cutting edge and shears a tiny chip that is analogous
to what would conventionally be called a "cut" chip (turning,
milling, drilling, tapping, etc.)
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Grinding Soft non-ferrous metals
 Soft non-ferrous metals - Initial grinding is recommended
with 320 grit SiC

abrasive paper followed by 320 400, 600 and , 800 grit SiC
paper. because These materials are relatively soft they do not
easily break down the SiC paper.
 The initial grinding with 320 grit is generally sufficient for
minimizing initial deformation and yet maintaining adequate
removal rates.

 For extremely soft materials such as tin, lead and zinc it is also
recommended that the abrasive paper be lightly coated with a
paraffin wax. The wax reduces
 the tendency of the SiC abrasive to embed into the soft
specimen.
Grinding Ferrous metals
 Ferrous metals - are relatively easy to grind with the depth of
deformation being a major consideration.
120 grit SiC abrasives provide a good initial start with
subsequent use of 240 or 320, 400, 600 and 800 grit SiC.
 Super alloys - are generally of moderate hardness but have
extremely stable elevated temperature characteristics and
corrosion resistance.
 the procedures for preparing super alloys is very similar to that
for most non-ferrous metals.
 Sample Preparation
 The purpose of this practice is to understand how to prepare
and interpret metallographic samples systematically.
 Gather information about chemical composition, heat treatment,
processing, phase diagram.
 Cut representative sample.
 Mount sample, grind and polish.
 Examine un etched sample.
 Etch lightly and examine again.
 Etch further if necessary.
 Compare with microstructure expected from equilibrium phase
diagram
Spheroidal Graphite Cast Iron
Abrasive Material
Materials used for the abrading particles are:
 garnet: commonly used in woodworking
 emery: commonly used to abrade or polish metal
 aluminium oxide: perhaps most common in widest variety of
grits; can be used on metal (i.e. body shops) or wood
 silicon carbide: available in very coarse grits all the way
through to micro-grits, common in wet applications
 alumina-zirconia: (an aluminium oxide–zirconium oxide alloy),
used for machine grinding applications
 chromium oxide: used in extremely fine micron grit (micrometre
level) papers
 ceramic aluminum oxide: used in high pressure applications,
used in both coated abrasives, as well as in bonded abrasives.
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Polishing
 Polishing is the process of creating a smooth and
shiny surface by rubbing it or using a chemical action,
leaving a surface with a significant reflection
 Aluminum Oxide(0.5, 0.1, 0.05μ)
PROCESS
The specimen must
1. Be free from scratches, stains and others imperfections which
tend to mark the surface.
2. Reveal no evidence of chipping due to brittle inter metallic
compounds and phases.
3. Be free from all traces of disturbed metal.
4. The specimen has to be grounded with the help of abrasive papers.
5. Polishing enhances the surface and makes it suitable to observe its
grain structure under Microscope.
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Grain
 The micro structure of many metallic or ceramic materials consists of many
grains.
 A grain
 is portion of the materials within which the arrangement of the atoms is
nearly identical but the orientation or crystal structure of atoms
are different.
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A Grain Boundary
 The surface that separates the individual grains is a narrow
zoon in which the atoms are properly spaced.
 One method of controlling the properties of a material is by
controlling the grain size.
aluminum oxide
and titanium dioxide (Al2O3 . TiO2)
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Surface Defects
 Surface defects are the boundaries, or plans, that separate a
material into regions.
 Each region may have the same crystal structure but different
orientation
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Industrial etching
 The surface is than exposed to chemical attack or ETCHING,
with grain boundaries being attacked more aggressively than
reminder of the grain to reveal the microstructure.
 Light from an optical microscope is reflected or scattered from
the sample surface depending how the surface is etched
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Etching:
Sample
material
Carbon steel
Carbon steel
Aluminum
Etchant
Composition
(usually 2%)
(nitric acid)
HNO3 1-5 ml
Ethyl alcohol 100ml
Few seconds (15
Sec)
Picric acid 4g
Ethyl alcohol 100ml
Few seconds (15
Sec)
Picric Acid
Hydrofluoric acid
HF (conc.) 0.5ml
H2O 99.5ml
Remarks
Swab for 15 sec.
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Surface Finish and Surface Roughness
(Original Magnification 50x)
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REFERENCES
 http://sembach.com/uploads/images/brevier/bild18.gif
 http://www.springerimages.com/img/Images/Springer/JOU=11661/VO
L=2011.42/ISU=9/ART=688/MediaObjects/MEDIUM_11661_2011_68
8_Fig26_HTML.jpg
 http://www.springerimages.com/img/Images/Springer/JOU=11661/VO
L=2011.42/ISU=11/ART=749/MediaObjects/MEDIUM_11661_2011_74
9_Fig1_HTML.jpg
 http://www.sfsa.org/tutorials/uplock/images/Grains.Jpg
 http://ars.els-cdn.com/content/image/1-s2.0-S0043164804002364gr11.jpg
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