ASTM E3-95
(Practice)Standard Practice for Preparation of Metallographic Specimens
Standard Practice for Preparation of Metallographic Specimens
SCOPE
1.1 The primary objective of metallographic examinations is to reveal the constituents and structure of metals and their alloys by means of a light optical or scanning electron microscope. In special cases, the objective of the examination may require the development of less detail than in other cases but, under nearly all conditions, the proper selection and preparation of the specimen is of major importance. Because of the diversity in available equipment and the wide variety of problems encountered, the following text presents for the guidance of the metallographer only those practices which experience has shown are generally satisfactory; it cannot and does not describe the variations in technique required to solve individual specimen preparation problems.
Note 1--For a more extensive description of various metallographic techniques, refer to Samuels, L. E., Metallographic Polishing by Mechanical Methods, American Society for Metals (ASM) Metals Park, OH, 3rd Ed., 1982; Petzow, G., Metallographic Etching, ASM, 1978; and VanderVoort, G., Metallography: Principles and Practice, McGraw Hill, NY, 2nd Ed., 1999.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E3–95 An American National Standard
Standard Practice for
Preparation of Metallographic Specimens
This standard is issued under the fixed designation E 3; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3. Significance and Use
1.1 The primary objective of metallographic examinations 3.1 Microstructures have a strong influence on the proper-
is to reveal the constituents and structure of metals and their ties and successful application of metals and alloys. Determi-
alloys by means of the light microscope. In special cases, the nation and control of microstructure requires the use of
objective of the examination may require the development of metallographic examination.
less detail than in other cases but, under nearly all conditions, 3.2 Many specifications contain a requirement regarding
the proper selection and preparation of the specimen is of microstructure; hence, a major use for metallographic exami-
major importance. Because of the diversity in available equip- nation is inspection to ensure that the requirement is met. Other
ment and the wide variety of problems encountered, the major uses for metallographic examination are in failure
following text presents for the guidance of the metallographer analysis, and in research and development.
only those practices which experience has shown are generally 3.3 Proper choice of specimen location and orientation will
satisfactory; it cannot and does not describe the variations in minimize the number of specimens required and simplify their
technique required to solve individual problems. interpretation. It is easy to take too few specimens for study,
but it is seldom that too many are studied.
NOTE 1—For a more extensive description of various metallographic
techniques, refer to Samuels, L. E., Metallographic Polishing by Mechani-
4. Selection of Metallographic Specimens
cal Methods, American Society for Metals (ASM) Metals Park, OH, 3rd
4.1 The selection of test specimens for metallographic
Ed., 1982; Petzow, G., Metallographic Etching, ASM, 1978; and Vander-
Voort, G., Metallography: Principles and Practice, McGraw Hill, NY, examination is extremely important because, if their interpre-
1984.
tation is to be of value, the specimens must be representative of
the material that is being studied. The intent or purpose of the
1.2 This standard does not purport to address all of the
metallographic examination will usually dictate the location of
safety concerns, if any, associated with its use. It is the
the specimens to be studied. With respect to purpose of study,
responsibility of the user of this standard to establish appro-
metallographic examination may be divided into three classi-
priate safety and health practices and determine the applica-
fications:
bility of regulatory limitations prior to use.
4.1.1 General Studies or Routine Work—Specimens from
2. Referenced Documents
locations that are most likely to reveal the maximum variations
within the material under study should be chosen. For example,
2.1 ASTM Standards:
specimens should be taken from a casting in the zones wherein
E 7 Terminology Relating to Metallography
maximum segregation might be expected to occur as well as
E 45 Practice for Determining the Inclusion Content of
specimens from sections where segregation should be at a
Steel
minimum. In the examination of strip or wire, test specimens
E 340 Test Method for Macroetching Metals and Alloys
should be taken from each end of the coils.
E 407 Test Methods for Microetching Metals and Alloys
4.1.2 Study of Failures—Test specimens should be taken as
E 1077 Test Method for Estimating the Depth of Decarbur-
closely as possible to the fracture or to the initiation of the
ization of Steel Specimens
failure. Before taking the metallographic specimens, study of
E 1268 Practice for Assessing the Degree of Banding or
the fracture surface should be complete, or, at the very least,
Orientation of Microstructures
the fracture surface should be documented. Specimens should
E 1558 Guide to Electrolytic Polishing of Metallographic
be taken in many cases from a sound area for a comparison of
Specimens
structures and properties.
4.1.3 Research Studies—The nature of the study will dictate
This practice is under the jurisdiction of ASTM Committee E-4 on Metallog-
specimen location, orientation, etc. Sampling will usually be
raphy and is the direct responsibility of Subcommittee E04.01 on Sampling,
Specimen Preparation, and Photography. more extensive than in routine examinations.
Current edition approved Jan. 15, 1995. Published March 1995. Originally
4.2 Having established the location of the metallographic
published as E3–21T. Last previous edition E3–80 (1986).
samples to be studied, the type of section to be examined must
Annual Book of ASTM Standards, Vol 03.01.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E3
be decided. For a casting, a section cut perpendicular to the
surface will show the variations in structure from the outside to
the interior of the casting. In hot-worked or cold-worked
metals, both transverse and longitudinal sections should be
studied. Special investigations may at times 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 center of the specimen proves advanta-
geous when studied in conjunction with the transverse section.
4.3 Cross sections or transverse sections taken perpendicu-
lar to the main axis of the material are more suitable for
revealing the following information:
4.3.1 Variations in structure from center to surface,
4.3.2 Distribution of nonmetallic impurities across the sec-
tion,
4.3.3 Decarburization at the surface of a ferrous material
(see Test Method E 1077), Symbol in
Suggested Designation
Diagram
4.3.4 Depth of surface imperfections,
4.3.5 Depth of corrosion,
A Rolled surface
4.3.6 Thickness of protective coatings, and
B Direction of rolling
4.3.7 Structure of protective coating.
C Rolled edge
4.4 Longitudinal sections taken parallel to the main axis of
D Longitudinal (or lengthwise) section parallel to rolled sur-
face
the material are more suitable for revealing the following
E Longitudinal section perpendicular to rolled surface
information:
F Transverse section
4.4.1 Inclusion content of steel (see Practice E 45),
G Radial longitudinal section
4.4.2 Degree of plastic deformation, as shown by grain
H Tangential longitudinal section
distortion,
FIG. 1 Method of Designating Location of Area Shown in
4.4.3 Presence or absence of banding in the structure (see
Photomicrograph.
Practice E 1268), and
4.4.4 The quality attained with any heat treatment.
body of the material, care must be exercised to minimize
4.5 The locations of surfaces examined should always be
altering the structure of the metal. Three common types of
given in reporting results and in any illustrative micrographs. A
sectioning are as follows:
suitable method of indicating surface locations is shown in Fig.
6.1.1 Sawing, whether by hand or machine with lubrication,
1.
is easy and fast, and relatively cool. It can be used on all
5. Size of Metallographic Specimens
materials with hardnesses below approximately 35 HRC. It
does produce a rough surface containing extensive plastic flow
5.1 The specimens to be polished for metallographic exami-
that must be removed in subsequent preparation.
nation are generally not more than about 12 to 25 mm (0.5 to
6.1.2 An abrasive cut-off wheel will produce a smooth
1.0 in.) square, or approximately 12 to 25 mm in diameter if the
surface often ready for fine grinding. This method of sectioning
material is round. The height of the specimen should be no
is normally faster than sawing. The choice of cut-off wheel,
greater than necessary for convenient handling during polish-
lubricant, cooling conditions, and the grade and hardness of
ing.
metal being cut will influence the quality of the cut. A poor
5.2 It is not always possible to secure specimens having the
choice of cutting conditions can easily overheat the specimen,
dimensions given in 5.1, when the material to be examined is
producing an alteration of the microstructure. As a general rule,
smaller than the ideal dimensions. For example, in the polish-
soft materials are cut with a hard bond wheel and hard
ing of wire, strip, and other small articles, it is necessary to
materials with a soft bond wheel. Aluminum oxide abrasive
mount the specimens because of their size and shape.
wheels are preferred for ferrous metals and silicon carbide
5.2.1 Larger samples may be mounted or not, as the
wheels are preferred for nonferrous alloys. Abrasive cut-off
available equipment dictates. However, the larger the speci-
wheels are essential for sectioning metals with hardnesses
men, the more difficult it is to prepare, especially by manual
above about 35 HRC. Extremely hard metallic materials and
methods.
ceramics may be more effectively cut using diamond-
5.2.2 Specimens that are too small to be handled readily
impregnated cutting wheels. Manufacturer’s instructions
during polishing should be mounted to ensure a surface
should be followed as to the choice of wheel and speeds.
satisfactory for microscopical study. There are, based on
6.1.3 Flame cutting completely alters the structure of the
technique used, three fundamental methods of mounting speci-
metal at the flame cut edge. If flame cutting is necessary to
mens (see Sections 7-9).
remove the specimen, it should be cut sufficiently large so that
6. Cutting of Metallographic Specimens
it can be recut to the proper size by some other method that will
6.1 In cutting the metallographic specimen from the main not substantially alter the structure. Exercise care to ensure that
E3
the region of interest is not altered by the heat of the cutting layer of phenolic or epoxy resin before being placed in the
flame. clamp in order to minimize the absorption of polishing
6.2 Other methods of sectioning are permitted provided they materials or etchants.
do not alter the microstructure at the plane of polishing. All
8.3.6 The clamp material should be similar in composition
cutting operations produce some depth of damage, which will
to the specimen to avoid galvanic effects that would inhibit
have to be removed in subsequent preparation steps.
etching. The specimen will not etch if the clamp material is
more readily attacked by the etchant.
7. Cleanliness
8.3.7 The clamp should preferably be of similar hardness as
7.1 Cleanliness (see Appendix X1.) during specimen prepa-
the specimens to minimize the rounding of the edges of the
ration is essential. All greases and oils on the specimen should
specimens during grinding and polishing.
be removed by some suitable organic solvent. Failure to clean
8.3.8 Exercise care in clamping the specimen. Excessive
thoroughly can prevent cold mounting castable resins from
clamping pressure may damage soft specimens; however, good
adhering to the specimen surface. Ultrasonic cleaning is
sealing is required to prevent absorption of polishing materials
particularly effective in removing the last traces of residues on
or etchants.
a specimen surface.
8.4 Plastic Mounting:
7.2 Any coating metal that will interfere with the subse-
8.4.1 Specimens may be embedded in plastic to protect
quent etching of the base metal should be removed before
them from damage and to provide a uniform format for both
polishing, if possible. If etching is required, when studying the
manual and automatic preparation. This is the most common
underlying steel in a galvanized specimen, the zinc coating
method for mounting metallographic specimens. Mounting
should be removed before mounting to prevent galvanic
plastics may be divided into two classes—compression mount-
effects. The coating can be removed by digestion in cold nitric
ing and castable.
acid (HNO , sp gr 1.42), in dilute sulfuric acid (H SO )orin
3 2 4
8.4.2 When mounting specimens in plastic, exercise care in
dilute hydrochloric acid (HCl). The HNO method requires
order to avoid rounding of specimen edges during the grinding
care to prevent overheating, since large samples will generate
operation. There are several methods available that prevent
considerable heat. By placing the cleaning container in cold
rounding. The specimens may be surrounded by hard shot,
water during the stripping of the zinc, attack on the underlying
small rivets, rings, etc., of approximately the same hardness or,
steel will be minimized.
when using casting resin, a slurry of resin and alumina may be
7.3 Oxidized or corroded surfaces may be cleaned as
poured around the specimen to prevent rounding. The speci-
described in Appendix X1.
mens may also be plated before mounting (see Section 9).
8.4.3 Compression Mounting—Thermosetting plastics re-
8. Mounting of Specimens
quire the use of a mounting press providing heat (up to
8.1 There are many instances where it will be advantageous
approximately 160°C) and pressure (up to approximately 30
to mount the specimens prior to grinding and polishing.
MPa). The finished mounts can be ejected hot but the best
Mounting of the specimen is usually performed on small,
results are obtained when the finished mount is cooled under
flimsy, or oddly shaped specimens, fractures, or in instances
pressure. There are three types of thermosetting compression
where the specimen edges are to be examined.
mounting plastics used predominantly in the metallographic
8.2 Specimens may be either mechanically mounted,
laboratory. Regardless of the resin used to compression mount
mounted in plastic, or a combination of the two can be used to
specimens, the best results are obtained when (1) the speci-
provide optimum results.
mens are clean and dry, and (2) the cured mount is cooled
8.3 Mechanical Mounting:
under full pressure to below 30°C before ejection from the
8.3.1 Strip and sheet specimens are frequently mounted by
press.
binding or clamping several specimens into a pack held
8.4.3.1 Wood-filled bakelite resins cure in 5 to 10 min, are
together by two end pieces and two bolts. Clamp mounting
relatively inexpensive, can be obtained in several colors, and
generally affords a means of rapid mounting with very good
are opaque. These resins have a tendency to pull away from the
edge retention.
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