major uses for metallographic examination are in failure analysis,and in research and development.
4.3Proper choice of specimen location and orientation will minimize the number of specimens required and simplify their interpretation.It is easy to take too few specimens for study, but it is seldom that too many are studied.
5.Selection of Metallographic Specimens
5.1The selection of test specimens for metallographic examination is extremely important because,if their interpre-tation is to be of value,the specimens must be representative of the material that is being studied.The intent or purpose of the metallographic examination will usually dictate the location of the specimens to be studied.With respect to purpose of study, metallographic examination may be divided into three classi-fications:
5.1.1General Studies or Routine Work—Specimens should be chosen from locations most likely to reveal the maximum variations within 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 segregation could be at a minimum.In the examination of strip or wire,test specimens could be taken from each end of the coils.
5.1.2Study of Failures—Test specimens should be 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 should be complete,or,at the very least, the fracture surface should be documented.In many cases, specimens should be taken from a sound area for a comparison of structures and properties.
5.1.3Research Studies—The nature of the study will dictate specimen location,orientation,etc.Sampling will usually be more extensive than in routine examinations.
5.2Having established the location of the metallographic samples to be studied,the type of section to be examined must be decided.
5.2.1For a casting,a section cut perpendicular to the surface will show the variations in structure from the outside to the interior of the casting.
5.2.2In hot-worked or cold-worked metals,both transverse and longitudinal sections should be studied.Special investiga-tions may require specimens with surfaces prepared parallel to the original surface of the product.
5.2.3In 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.
5.3Transverse sections or cross sections taken perpendicu-lar to the main axis of the material are often used for revealing the following information:
5.3.1Variations in structure from center to surface,
5.3.2Distribution of nonmetallic impurities across the sec-tion,
5.3.3Decarburization at the surface of a ferrous material (see Test Method E1077),
5.3.4Depth of surface imperfections,
5.3.5Depth of corrosion,
5.3.6Thickness of protective coatings,and
5.3.7Structure of protective coating.
5.4Longitudinal sections taken parallel to the main axis of the material are often used for revealing the following infor-mation:
5.4.1Inclusion content of steel(see Practices E45,E768, E1122,and E1245),
5.4.2Degree of plastic deformation,as shown by grain distortion,
5.4.3Presence or absence of banding in the structure(see Practice E1268),and
5.4.4The microstructure attained with any heat treatment.
5.5The locations of surfaces examined should always be given in reporting results and in any illustrative micrographs.A suitable method of indicating surface locations is shown in Fig. 1.
6.Size of Metallographic Specimens
6.1For convenience,specimens to be polished for metallo-graphic examination are generally not more than about12to25 mm(0.5to1.0in.)square,or approximately12to25mm in diameter if the material is cylin
drical.The height of the specimen should be no greater than necessary for convenient handling during polishing.
6.1.1Larger specimens are generally more difficult to pre-pare.
6.1.2Specimens that are,fragile,oddly shaped or too small to be handled readily during polishing should be mounted to ensure a surface satisfactory for microscopical study.There are,based on technique used,three fundamental methods of mounting specimens(see Section
9).
Symbol in
Diagram
Suggested Designation
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
FIG.1Method of Designating Location of Area Shown in
Photomicrograph.
7.Cutting of Metallographic Specimens
7.1In cutting the metallographic specimen from the main body of the material,care must be exercised to minimize altering the structure of the metal.Three common types of sectioning are as follows:
7.1.1Sawing,whether by hand or machine with lubrication, is easy,fast,and relatively cool.It can be used on all materials with hardnesses below approximately350HV.It does produce a rough surface containing extensive plasticflow that must be removed in subsequent preparation.
7.1.2An abrasive cut-off blade will produce a smooth surface often ready forfine grinding.This method of sectioning is normally faster than sawing.The choice of cut-off blade, lubricant,cooling conditions,an
d the grade and hardness of metal being cut will influence the quality of the cut.A poor choice of cutting conditions can easily damage the specimen, producing an alteration of the microstructure.Generally,soft materials are cut with a hard bond blade and hard materials with a soft bond blade.Aluminum oxide abrasive blades are preferred for ferrous metals and silicon carbide blades are preferred for nonferrous alloys.Abrasive cut-off blades are essential for sectioning metals with hardness above about350 HV.Extremely hard metallic materials and ceramics may be more effectively cut using diamond-impregnated cutting blades.Manufacturer’s instructions should be followed as to the choice of blade.Table1lists the suggested cutoff blades for materials with various Vickers(HV)hardness values.
7.1.3A shear is a type of cutting tool with which a material in the form of wire,sheet,plate or rod is cut between two opposing blades.
7.2Other methods of sectioning are permitted provided they do not alter the microstructure at the plane of polishing.All cutting operations produce some depth of damage,which will have to be removed in subsequent preparation steps.
8.Cleanliness
8.1Cleanliness(see Appendix X1)during specimen prepa-ration is essential.All greases,oils,coolants and residue from cutoff blades on the specimen should be removed by some suitable organic solvent.Failure to clean thoroughly can prevent cold mounting resins from adhering to the specimen surface.Ultrasonic cleaning may be effective in removing the last traces of residues on a specimen surface.
8.2Any coating metal that will interfere with the subse-quent etching of the base metal should be removed before polishing,if possible.If etching is required,when studying the underlying steel in a galvanized specimen,the zinc coating should be removed before mounting to prevent galvanic effects during etching.The coating can be removed by dissolving in cold nitric acid(HNO3,sp gr1.42),in dilute sulfuric acid (H2SO4)or in dilute hydrochloric acid(HCl).The HNO3 method requires care to prevent overheating,since large samples will generate considerable heat.By placing the clean-ing container in cold water during the stripping of the zinc, attack on the underlying steel will be minimized.More information may be found in Test Method A90/A90M.
N OTE2—Picral etchant produces little or no galvanic etching effects when used on galvanized steel.
N OTE3—The addition of an inhibitor during the stripping of Zn from galvanized coatings will minimize the attack of the steel substrate.NEP (polethylinepolyamine)or SbCl
3
are two useful inhibitors.
8.3Oxidized or corroded surfaces may be cleaned as described in Appendix X1.
9.Mounting of Specimens
9.1There are many instances where it will be advantageous to mount the specimen prior to grinding and polishing.Mount-ing of the specimen is usually performed on small,fragile,or oddly shaped specimens,fractures,or in instances where the specimen edges are to be examined.
9.2Specimens may be either mechanically mounted, mounted in plastic,or a combination of the two.
9.3Mechanical Mounting:
9.3.1Strip and sheet specimens may be mounted by binding or clamping several specimens into a pack held together by two end pieces and two bolts.
9.3.2The specimens should be tightly bound together to prevent absorption and subsequent exudation of polishing materials or etchants.
9.3.3The use offiller sheets of a softer material alternated with the specimen may be used in order to minimize the seepage of polishing materials and etchants.Use offiller material is especially advantageous if the specimens have a high degree of surface irregularities.
9.3.4Filler material must be chosen so as not to react electrolytically with the specimen during etching.Thin pieces of plastic,lead,or copper are typical materials that are used. Copper is especially good for steel specimens since the usual etchants for steels will not attack the copper.
9.3.5Alternatively,the specimens may be coated with a layer of epoxy resin before being placed in the clamp in order to minimize the absorption of polishing materials or etchants.
9.3.6The clamp material should be similar in composition to the specimen to avoid galvanic effects that would inhibit etching.The specimen will not etch if the clamp material is more readily attacked by the etchant.
9.3.7The clamp should preferably be of similar hardness as the specimens to minimize the rounding of the edges of the specimens during grinding and polishing.
9.3.8Exercise care in clamping the specimen.Excessive clamping pressure may damage soft specimen.
9.4Plastic Mounting:
9.4.1Specimens may be embedded in plastic to protect them from damage and to provide a uniform format for both
TABLE1Cutoff Blade Selection
Hardness HV Materials Abrasive Bond Bond Hardness up to300non-ferrous(Al,Cu)SiC P or R hard
up to400non-ferrous(Ti)SiC P or R med.hard
up to400soft ferrous Al2O3P or R hard
up to500medium soft ferrous Al2O3P or R med.hard
up to600medium hard ferrous Al2O3P or R medium
up to700hard ferrous Al2O3P or R&R med.soft
up to800very hard ferrous Al2O3P or R&R soft
>800extremely hard ferrous CBN P or M hard
more brittle ceramics diamond P or M very hard
tougher ceramics diamond M ext.hard P—phenolic
R—rubber
R&R—resin and rubber
M—metal
manual and automatic preparation.This is the most common method for mounting metallographic specimens.Mounting plastics may be divided into two classes—compression and castable.
9.4.2The choice of a mounting compound will influence the extent of edge rounding observed during the grinding and polishing operations.There are several methods available that minimize rounding.The
specimen may be surrounded by hard shot,small rivets,rings,etc.,of approximately the same hardness or,when using a castable resin,a slurry of resin and alumina may be poured around the specimen.The specimen may also be plated before mounting (see Section 10).Many mounting procedures result in sharp edges on the mount corners.The corners should be beveled to remove any plastic mounting flash.
9.4.3Compression Mounting —There are four types of com-pression mounting plastics used predominantly in the metallo-graphic laboratory (see Table 2).These plastics require the use of a mounting press providing heat (140-180°C)and force (27-30MPa).Thermosetting plastics can be ejected hot but the best results are obtained when the cured mount is cooled under pressure.Thermoplastic compounds do not harden until cooled and therefore should not be ejected while hot.Regardless of the resin used,the best results are obtained when (1)the specimen is clean and dry,and (2)the cured mount is cooled under full pressure to below 40°C before ejection from the press.This will ensure minimal shrinkage gap formation.
9.4.4Castable Plastics —Castable mounts are usually pre-pared at room temperature.Some may require an external heat source or applied pressure in order to cure.These resins consist of two or more components which must be mixed just prior to use.There are four kinds of castable plastics in co
mmon use (see Table 3).
9.4.5The molds for castable plastics are often simple cups that hold the resin until it cures.They may be reusable or not;the choice is a matter of convenience and cost.Handling castable resins requires care.They all can cause dermatitis.Manufacturers’recommendations for mixing and curing must be followed to obtain best results.
9.5Mounting Porous Specimen :
9.5.1Porous or intricate specimens may be vacuum impreg-nated in order to fill voids,prevent contamination and seepage,and prevent loss of friable or loose components.Impregnation is accomplished by placing the specimen in a mold in a vacuum chamber and then introducing the resin into the mold after the chamber has been evacuated.The introduction of the resin into the mold can be accomplished either by having a funnel or stopcock fitted to the vacuum chamber or by having a basin of the resin present inside the chamber.A low-viscosity resin will produce the best results.The pressure in the chamber must remain above the critical vapor pressure of the hardener to avoid boiling away the hardener.After the pressure has equilibrated,the resin is introduced into the mold and the vacuum is released and air admired to the chamber.Atmo-spheric pressure will force the resin into fine pores,crac
ks,and holes.9.5.2If a low-viscosity resin is used,the funnel and stop-cock may be eliminated.The specimen and resin are placed in the mold prior to evacuation.The air in the specimen will bubble out through the resin.Exercise care to ensure the hardening agent is not evaporated during evacuation.Dipping the specimen in the resin prior to placing it in the mold may help in filling voids.9.5.3Vacuum impregnation is an effective method for ensuring optimal results for porous metallographic mounts.It is imperative that the specimens be completely dry prior to impregnation.9.5.4A more rapid technique but less effective method is to lacquer the specimens with one of the formulations used by the canning industry to line food containers.The formulations are highly penetrating and the cure is a short time at low temperatures.After lacquering,the specimens are mounted in the usual fashion.10.Plating of Specimens 10.1Specimens such as fractures or those where it is necessary to examine the edges,are often plated to obtain good edge retention.Plating can be done electrolytically or with electroless solutions.These specimens are invariably mounted prior to the grinding and polishing procedures.Electroless plating solutions can be purchased commercially.10.2Thoroughly clean the specimen surface prior to plating to ensure good adhesion of the plating.Avoid industrial cleaning treatments that are too harsh and may cause damage to the specimen surface.Milder cleaning treatments that involve detergents,solvents,mild alkaline,or acidic solutions are recommended.10.3Chromium,copper,iron,nickel,gold,silver,and zinc may be electrolytical
ly deposited although copper and nickel are predominantly used in metallographic laboratories.10.3.1Ferrous metals are commonly plated electrolytically with nickel or copper.A flash coat in a copper or electroless nickel bath can be first applied for specimens that are difficult to electroplate.10.3.2Nonferrous metals may be plated with silver and the precious metals may be plated with nickel,gold,or silver.10.4The plating material should not react galvanically with the base metal of the specimen during plating,polishing,or etching.10.5Electroless plating is preferred to electrolytic plating
TABLE 2Characteristics of Hot-Compression Mounting Compounds
Type of Compound
Characteristics Acrylic
thermoplastic,cure time 10-15min,optically clear,moderate shrinkage,low abrasion resistance,degraded by hot etchants Diallyl phthalate A
thermosetting,cure time 5-10min,opaque,minimal shrinkage,good resistance to etchants,moderate abrasion resistance Epoxy A
thermosetting,cure time 5-10min,opaque,very low shrinkage,good resistance to etchants,high abrasion resistance Phenolic A (Bakelite)
thermosetting,cure time 5-10min,opaque,moderate shrinkage,degraded by hot etchants,moderate abrasion resistance A These compounds may be filled with wood flour,glass fiber or mineral
particulate.
for specimens with rough,porous,or irregular surfaces,be-cause the electroless solution provides better surface coverage and penetration.
editor evaluating revision10.6Active metals such as zinc and aluminum are difficult to plate.Sometimes a flash cyanide copper plate can be deposited,which then can be followed by normal plating from a sulfate bath.Evaporated coatings of copper,gold,or chro-mium may also be used as starter coatings.
10.7It is recommended that the plating thickness be at least 5µm.
11.Grinding and Polishing
General Information
11.1Many metals and alloys can be prepared using a similar sequence of grinding and polishing.Hard alloys may require greater pressure than soft alloys.The major differences will be in the final polishing.Some metals and alloys will require specific combinations of abrasive and support material,but a surprising number can be handled by the same procedure.Supplies and instructions for grinding,lapping,and polishing are readily obtainable from laboratory supply houses.
11.2Grinding —Grinding can be done in a number of ways,ranging from rubbing the specimen on a stationary piece of abrasive paper to the use of automatic devices.The choice of method depends on the number and type of specimens to be done,financial considerations and requirements such as flat-ness and uniformity.
11.2.1Abrasive grit size designations in this practice are expressed in the ANSI (American National Standards Institute)or CAMI (Coated Abrasives Manufacturers Institute)system units with the corresponding FEPA (European Federation of Abrasive Producers)numbers in parentheses.Table 4provides a correlation between these two systems and the approximate median particle diameter for a given size in micrometres.
11.2.2Grinding should start with the finest paper,platen or stone capable of flattening the specimen and
removing the effects of prior operations,such as sectioning.The subsequent steps should remove the effects of previous ones in a short time.Grinding consists of two stages-planar (rough)and fine.11.2.3Planar or rough grinding [240grit (P220)and coarser]may be performed on belts,rotating wheels or stones.In some methods,diamond abrasives are used on rigid platens.Planar grinding may be used to accomplish the following:11.2.3.1Flatten an irregular or damaged cut surface,
11.2.3.2Remove sectioning damage,scale and other surface conditions prior to mounting,
11.2.3.3Remove substantial amounts of specimen material to reach a desired plane for polishing,11.2.3.4Level the mount surface.11.2.4In fine grinding,damage to the specimen incurred from the planar or rough grinding step must be removed.The specimen is either ground on successively finer abrasive papers (using water to wash away grinding debris and to act as a coolant)or on a rigid disc or cloth charged with a suitable abrasive.11.2.5After all grinding is done,the specimen must be cleaned thoroughly.Ultrasonic cleaning in a water/soap solu-tion containing a corrosion inhibitor may prove beneficial.11.3Polishing —Polishing is usually distinguished from grinding by the use of loose abrasive (#6µm)embedded in an appropriately lubricated supporting surface.The choice of abrasive,lubricant,and polishing surface support is often specific to the metal and the object of the investigation.Polishing can be divided into rough and fine (final)stages.11.3.1Rough polishing is often s
ufficient for routine evalu-ations like microindentation hardness and grain size.11.3.2When fine polishing is required,it may be performed with diamond or an oxide slurry step or both.The choice of final polishing abrasive type and size is dictated by the hardness of the specimen.For instance,a lµm diamond final polish is often sufficient for many grades of steel,however,softer steels and non-ferrous materials often require an addi-tional polishing step with an oxide slurry or suspension of SiO 2or Al 2O 3.Final polishing cloths are generally softer and higher in nap than rough polishing cloths.Therefore,polishing time and force must be kept to a minimum to avoid artifacts such as TABLE 3Characteristics of Castable Mounting Compounds
Type of Compound
Characteristics Acrylic
Cure time 8-15min,moderate shrinkage,peak curing temperature can reach 90-120°C during polymerization,low abrasion resistance,opaque to transparent Polyester-acrylic (quartz-filled)
Cure time 8-15min,very low shrinkage,peak curing temperature can reach 90-120°C during polymerization,high abrasion resistance,opaque Polyester
Cure time 30-60min,high shrinkage,peak curing temperature can reach 90-120C during polymerization,moderate abrasion resistance,transparent Epoxy Cure time 1⁄2-20h,very low shrinkage,good adhesion,low heat generation during polymerization,moderate abrasion
resistance,low viscosity (good for vacuum impregnation),transparent
TABLE 4European/USA Grit Grade Comparison Guide FEPA ANSI/CAMI Grit Number Size (µm)Grit Number Size (µm)P120125.0120116.0P150100.018078.0P22068.022066.P28052.224051.P36040.528042.3P40035.032034.P60025.836027.3P80021.840022.1P100018.350018.2P120015.360014.5P150012.680011.5P200010.310009.5P25008.415008.0P4000A A Not found in the FEPA grading system.ANSI—American National Standards Institute CAMI—Coated Abrasives Manufacturers Institute FEPA—European Federation of Abrasive
Producers
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