ISO/TR 20580:2022
(Main)Preparation of metallographic specimens
Preparation of metallographic specimens
This document presents a list of common practices in preparation methods of metallographic specimens for optical and scanning electron microscopy, including preliminary preparation, grinding and polishing of specimens as well as microstructure revelation methods covering the optical method, etching methods (chemical, electrolytic, constant potential, ion sputtering and high temperature relieving) and the interface layer method [1][2].
Confection des éprouvettes métallographiques
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 20580
First edition
2022-07
Preparation of metallographic
specimens
Confection des éprouvettes métallographiques
Reference number
ISO/TR 20580:2022(E)
© ISO 2022
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ISO/TR 20580:2022(E)
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© ISO 2022
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ISO/TR 20580:2022(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Preliminary preparation . 1
4.1 Selection of metallographic specimens . 1
4.1.1 General . 1
4.1.2 General studies or routine work . 1
4.1.3 Study of failures . 1
4.2 Selection of type of section to be examined . 2
4.3 Size of metallographic specimens . 3
4.4 Cutting of metallographic specimens. 3
4.5 Marking of metallographic specimens . 3
4.6 Cleaning . 3
4.7 Mounting . 3
4.7.1 General . 3
4.7.2 Mechanical mounting . 4
4.7.3 Plastic mounting: . 4
5 Grinding. 5
5.1 Planar or rough grinding . 5
5.2 Fine grinding . 5
5.2.1 General . 5
5.2.2 Manual methods . 5
5.2.3 Automated methods . 6
6 Polishing . 6
6.1 General . 6
6.2 Mechanical polishing . 6
6.2.1 Rough polishing . 6
6.2.2 Fine polishing . 6
6.3 Electrolytic polishing . 6
6.4 Chemical polishing . 7
6.5 Vibratory polishing . 7
7 Microstructure revelation . 7
7.1 General . 7
7.2 Optical method . 7
7.3 Etching method . 7
[13]
7.3.1 Chemical etching . 7
[21]
7.3.2 Electrolytic etching . 7
7.3.3 Constant potential etching . 8
7.3.4 Ion sputtering etching (cathode vacuum etching) . 8
7.3.5 High temperature relieving etching. 8
7.4 Interference layer method . . 8
Annex A (informative) Etchants for metals . 9
Bibliography .16
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ISO/TR 20580:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
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expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 17, Steel, Subcommittee SC 7, Methods of
testing (other than mechanical tests and chemical analysis).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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TECHNICAL REPORT ISO/TR 20580:2022(E)
Preparation of metallographic specimens
1 Scope
This document presents a list of common practices in preparation methods of metallographic
specimens for optical and scanning electron microscopy, including preliminary preparation, grinding
and polishing of specimens as well as microstructure revelation methods covering the optical method,
etching methods (chemical, electrolytic, constant potential, ion sputtering and high temperature
[1][2]
relieving) and the interface layer method .
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Preliminary preparation
4.1 Selection of metallographic specimens
4.1.1 General
Because the metallographic examination serves a specified purpose that differs from case to case,
there is no one single way to select and prepare specimens. However, it is the accepted state of the art
to select specimens that are representative of the material that is being studied. The location, type of
section and number of the specimens to be studied are usually dictated by the manufacture method of
metals, examination intent or purpose, related standards or agreement upon enquiry.
4.1.2 General studies or routine work
Specimens are generally chosen from locations most likely to reveal the maximum variations within
[3]
the material under study . For example, specimens could be taken from a casting in the zones
wherein maximum segregation might be expected to occur as well as specimens from sections where
[4]
segregation could be at a minimum ; in the examination of strip or wire, test specimens are often
[5]
taken from each end of the coils ; heat or surface treated specimens are often taken so as to include
[6]
all the heat or surface treated layers ; welding specimens often incorporate the welding seam, heat
[7]
affected zone and base metal .
4.1.3 Study of failures
In nearly all situations, test specimens are taken as closely as possible to the fracture or to the initiation
of the failure. Before taking the metallographic specimens, study of the fracture surface is completed,
[8]
or at the very least, the fracture surface is documented . In many cases, specimens are taken from a
sound area for a comparison of structures and properties.
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4.2 Selection of type of section to be examined
4.2.1 Having established the location of the metallographic samples, the type of section to be
examined is decided. The locations of surfaces examined are always given in reporting results and in
any illustrative micrographs. A suitable method of indicating surface locations is shown in Figure 1.
Key
A rolled surface
B direction of rolling
C rolled edge
D planar section
E longitudinal section perpendicular to rolled surface
F transverse section
G radial longitudinal section
H tangential longitudinal section
[9]
Figure 1 — Method of designating location of area shown in photomicrograph
4.2.2 Transverse sections or cross sections (Surface F) taken perpendicular to the main axis of the
material are often used to reveal the following information:
a) Variations in microstructure from surface to center;
[10][11][12]
b) Distribution of nonmetallic impurities across the section ;
c) Distribution of carbide net;
d) Depth of surface imperfections;
e) Depth of coatings;
[13][14]
f) Depth of decarburization ;
g) Depth of corrosion;
h) Surface chemical heat treatment and microstructure and thickness of coating.
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4.2.3 Longitudinal sections (Surface D, E, G and H) taken parallel to the main axis of the material are
often used for revealing the following information:
[10][11][12]
a) Inclusion content of steel ;
b) Degree of plastic deformation, as shown by grain distortion;
[15]
c) Banding in the structure ;
d) The microstructure attained with any treatment.
4.2.4 In hot-worked or cold-worked metals, transverse and/or longitudinal sections are studied.
Special investigations require specimens with surfaces prepared parallel to the original surface of the
product. In the case of wire and small rounds, a longitudinal section through the centre of the specimen
proves advantageous when studied in conjunction with the transverse section.
4.3 Size of metallographic specimens
2
Specimens to be polished for metallographic examination are generally not more than 400 mm in area
[16]
for the section to be prepared . The height perpendicular to the section to be prepared is generally
no greater than the transverse size of the specimen and is often dictated by the sample preparation
equipment available.
4.4 Cutting of metallographic specimens
There is no single ideal technique to section specimens. Possible techniques include wheel cutting, linear
cutting, mechanical machining (turning, milling, planing, grinding), sawing, shearing, flame cutting,
hammering for the hard and brittle metal, etc. It is the accepted state of the art to select a technique that
minimizes alterations to the structure of the metal, such as deformation and overheating of specimens.
Aside from the choice of technique, strategies to reduce sample damage include adapting the machine
parameters, using coolant or lubricant, and removal of this damage by subsequent preparation steps
(such as by grinding wheel).
4.5 Marking of metallographic specimens
Marking of metallographic specimens is made right after the cutting in order to trace the specimens
during their preparation. Marking is not made on the observed surface. Care is taken to avoid the
degradation of the marks in the following processes, such as cleaning and heat treatment. The specimen
is re-marked after each step that degrades or obscures the previous marking.
4.6 Cleaning
All foreign material on the specimen, such as greases, oils, coolants and residue from cutoff blades,
are removed by a suitable solvent (such as ethanol, acetone, etc.). Ultrasonic cleaning is effective in
removing the last traces of residues on a specimen surface. Any coating on metal that interferes with
the subsequent treatment, etching, or observation of the base metal is removed before polishing.
4.7 Mounting
4.7.1 General
Mounting of the specimen is usually performed to simplify and improve the sample preparation.
Reasons to mount a specimen can include its handling (small, fragile, soft, or oddly shaped specimens),
a need to fixate separate parts (e. g. fractures), a need for edge retention, and a desire for specimen
standardization (e. g. for automatic grinding and polishing). The mounting method is chosen so as not
to change the microstructure of the specimen. The observed surfaces are generally placed facedown.
Specimens are either mechanically mounted, mounted in a support material (usually plastic), or a
combination of the two.
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4.7.2 Mechanical mounting
4.7.2.1 Mechanical mounting refers to the tight joining of specimens in suitable clamps by bolts
and screws, to prevent absorption and subsequent exudation of polishing materials or etchants (see
Figure 2). Strip and sheet specimens are mounted by binding or clamping several specimens into a pack
held together by two end pieces and two bolts. The clamp is often selected to be of similar hardness and
composition as the specimens to minimize the rounding of the edges of the specimens during grinding
and polishing as well as any galvanic effects that would affect the polishing process or inhibit etching.
Mechanical mounting is often avoided in cases where the clamping pressure presents a risk of specimen
alteration.
4.7.2.2 A common aid to minimize the seepage of polishing materials and etchants is the use of filler
elements of a softer material. Use of filler material is especially advantageous if the specimens have a
high degree of surface irregularities. Filler material is chosen so as not to react electrolytically with the
specimen during polishing or etching. Thin pieces of plastic, lead, or copper are typical materials used
for this purpose. Copper is especially good for steel specimens since the usual etchants for steels do
not attack the copper. Alternatively, the specimens are coated with a layer of epoxy resin before being
placed in the clamp in order to minimize the absorption of polishing materials or etchants.
Key
1 specimen
2 filler material
Figure 2 — Mechanical mounting clamps
4.7.3 Plastic mounting:
4.7.3.1 General
Plastic mounting is the most common method for mounting metallographic specimens. The choice of a
mounting compound influences the extent of edge rounding observed during the grinding and polishing
operations. Strategies to minimize rounding include grouping small pieces of similar hardness around
the specimen, reinforcing the plastic with hard filler (e. g. alumina or glass), and plating specimens with
metals of lower hardness and resistance to electrochemical reaction. Specimens with a thin surface
layer (diffusion layer, metal coating, plating, etc.) are sometimes tilted before mounting to enlarge
the visible area of the layer in a specific direction. Plastic mounting is divided into two classes —
[17]
compression and castable .
4.7.3.2 Compression mounting
Compression mounts are prepared in the mould of a mounting press with the observed surface
facedown. The height of the plastics introduced into the mould exceeds the height of the specimen
before and after mounting. After sealing, heating, pressing, cooling, hardening and opening the mould,
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a compression mount is accomplished. The temperature and pressure curve of compression mounting
are determined based on the plastics selected and the equipment used. Generally, the mounting
temperature is not higher than 180 °C and the mounting press not more than 30 MPa (300 bar). The
cured mount is usually cooled under full pressure to below 30 °C before ejection from the press. There
are 2 types of compression mounting plastics used in the metallographic laboratory:
a) Thermosetting: Diallyl phthalate, Epoxy, Phenolic, etc.
b) Thermoplastic: Acrylic, Polyester acrylate, Epoxy, Polyester, Polystyrene, Polyvinyl chloride,
methylmethacrylate, etc.
4.7.3.3 Castable mounting
Castable mounts are usually prepared for specimens sensitive to being altered by heat or being
deformed through pressure, such as specimens with a specific thermal history or a low melting point,
intricate and porous specimens, or specimens with a large specific surface area. The specimen is placed
in the castable mould with the observed surface facedown, then resin and curing agent are combined
without forming any bubbles just prior to injection or pouring into the castable mould, after which the
mixture cures to form a castable mount at room temperature. Acrylic, Polyester and Epoxy are common
castable plastics in use, and the moulds for castable plastics are often simple cups made of hard
rubber, polytetrafluoroethylene or cardboard, etc. Porous or intricate specimens are often vacuum
impregnated in order to fill voids, prevent contamination and seepage, and prevent loss of friable or
loose components.
In special cases, molten metal is substituted for the polymer mix; the mould material is adapted
accordingly.
5 Grinding
5.1 Planar or rough grinding
Planar or rough grinding (240 grit and coarser) is performed on belts, rotating wheels or stones,
preparing specimens ready for fine grinding. Specimens are usually cooled by water while doing rough
grinding in order to avoid changing the microstructure through heat.
5.2 Fine grinding
5.2.1 General
In fine grinding, damage to the specimens incurred from the planar or rough grinding is removed. The
specimen is either ground on successively finer abrasive papers or foils (using water or another liquid
to wash away grinding debris and to act as a coolant) on a rigid disc usually made of glass or metal, or
on a cloth charged with a suitable abrasive.
NOTE The main difficulty in the metallographic preparation of cast iron is to retain the true shape and size
of the graphite in its flake, nodular or tempered form. In particular, cast irons with a soft ferritic matrix tend to
smear and are prone to deformation and scratching during any stage of the preparation process, as are other
soft metallic materials. In these cases, both the samples and the preparation process are often more thoroughly
checked because of the increased possibility of inducing artefacts.
5.2.2 Manual methods
When grinding manually, the specimen is moved across the abrasive paper to allow for even wear.
Between grinding steps, the specimen is rotated, usually by about 90°. At the end of grinding on each
paper, the surface of the specimen and its mount, if any, are flat with one set of unidirectional grinding
scratches. Each grinding stage is followed by careful cleaning of the specimen (by water or another
liquid or ultrasonic cleaning) to prevent contamination and artefacts from entrained coarser abrasive.
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5.2.3 Automated methods
Major advantages of automated grinding and polishing procedures are the consistent quality of
specimen preparation and the substantial decrease in time. Abrasive papers from coarse to fine are
placed on mechanical grinding device, then the specimen is ground successively. The specimen surface
shows uniform scratches before proceeding to the next step. Cleaning between stages is needed to
prevent carryover of abrasives and contamination of subsequent preparation surfaces.
6 Polishing
6.1 General
During polishing, damage to the specimens incurred from grinding is removed. There are many
polishing methods, such as mechanical polishing, electrolytic polishing, chemical polishing, vibration
polishing, etc. The available equipment and operator skills play a significant role in the polishing result
and are factored into the specific process definition.
6.2 Mechanical polishing
6.2.1 Rough polishing
Rough polishing is often sufficient for routine evaluations such as micro-indentation hardness and grain
size. Typical polishing surface supports include nylon, wool fabric or fine canvas. Abrasives are usually
diamond, alumina, magnesium oxide, chromium oxide, ferric oxide, silicon carbide, etc. of usually 1
to 9 μm in size. They are typically supplied as polishing suspension liquids, spray polishing agents or
polishing pastes. A typical polishing time is 2 min to 5 min, but it depends on the specimen and the
equipment used. The specimens are cleaned by water or another liquid and dried after polishing.
6.2.2 Fine polishing
6.2.2.1 In fine polishing, typical polishing surface supports include nylon silk, velvet, or other
fibre velvet. The size and type of abrasive are typically decided in accordance with the hardness of
the specimen and the desired end result. Other parameters that influence the end result include the
polishing time, the amount of force applied, the rate and direction of movement of the specimen, in order
to avoid edge rounding and relief. The specimen is polished until the scratches are completely removed
and the observed surface presents a mirror effect. After all polishing has been done, the specimen
is cleaned thoroughly by water or another liquid, then cleaned by absolute ethyl alcohol or another
[18]
suitable liquid with a high vapor pressure and dried, to avoid water stains and contamination .
6.2.2.2 Fine polishing can be performed by manual or automated method. When using manual
method, the specimen is lightly pressed on the polishing disc and moved back and forth in the direction
of the diameter of the polishing disc. The typical duration for the specimen and polishing surface
support to be in contact is 10 s to 20 s. The humidity of the support is usually controlled in such a way
as to ensure the surface liquid film completely evaporates in 2 s to 3 s after the supply of liquid stops.
Excess humidity brings out artifacts such as tailing. Lack of humidity causes a temperature rise of the
specimen, decreases lubrication and in certain situations damages the specimen surface. Automated
[19]
polishing devices move the specimen on the polishing disc following a set track . The clamping force,
rotating speed and direction can be adjusted to achieve efficient polishing.
6.3 Electrolytic polishing
For the electrolytic polishing of metal specimens in an appropriate electrolyte, the metal specimens
work as anodes, on the surfaces of which the selective corrosion occurs due to the electrolytic reaction
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and fine polishing is obtained. Satisfactory polishing is the result of a combination of appropriate
voltage, current, temperature and polishing time.
NOTE Electrolytic polishing is not generally used for cast irons and other composite microstructures.
6.4 Chemical polishing
The principle of chemical polishing is to unevenly dissolve the surface of a metal specimen using
chemical reagents and thus to obtain a mirror surface. This polishing method can only make the
specimen surface smooth, but not planar. The chemi
...
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