METALLOGRAPHICSAMPLE PREPARATIONCUTTING. GRINDING. MOUNTING

@BUEHLER LTD., 1981-AII rights reserved.

INITIAL STAGESOFMETAllOGRAPHICSAMPLE PREPARATION

The purpose of metallographic sample preparation is to produce a polishedsurface from which the true microstructure may be viewed by microscopicexamination. The simplest and most commonly used method of producingthe desired surface is abrasive preparation, consisting of a series of abrasivesteps of increasing fineness which, if correctly performed, will produce asurface that is undamaged, flat, and free of scratches. The basic preparationprocedures are:

Initial StagesSectioningRough GrindingMounting

Final StagesFine GrindingRough PolishingFinal Polishing

All stages of metallographic preparation are important. Errors committed orsteps omitted will contribute to an unacceptable polished surface and maylead to erroneous interpretations or measurements during microscopicanalysis. Since the Initial Stages are characteristically coarser than theFinal, the risk of altering the true microstructure is greater. An altered micro-structure may be caused by mechanical deformation due to incorrectlychosen abrasive material, excessive pressure, or inadequate removal ofnormal damage produced by the previous step. Failure to select the correctmounting technique for heat and pressure sensitive materials or overheatingduring grinding, due to excessive application of pressure or inadequatecooling, are sources of thermal damage. To assure valid microstructuralanalysis, care must be taken at each stage of specimen preparation to avoiddamage which could alter the true microstructure.

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Figure 1. Use of a Polaroid MP-4Macro-Camera to DocumentSample Location BeforeSectioning~g?7~

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~ Figure 2. A SimplifiedCode for MetallographerSample Location (Arrowindicates surface to beprepared)

PREPARATION PROCEDURESAlthough Sectioning is normally the Initial Stage in sample preparation, theproblem of sample identity must be considered first. Once a part has beensectioned and the samples removed and polished, it may be difficult to ascertainthe relationship of the pOlished section to the original location. To circumventthis, a sampling map is recommended. Areas to be cut should be marked withwaterproof ink, showing the location and orientation of the intended plane ofpolish. Figure 2 illustrates one example of sample mapping, and others are pos-sible. The location and orientation of each sample may be recorded manually,with a sketch, or by taking a macrophotograph as shown in Figure 1.

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SECTIONINGSectioning is performed to remove a suitably sized sample for subsequentmounting and polishing. Since the intended plane of polishing is usually de-termined by a sectioning operation, caution must be exercised to avoid exces-sive damage to this surface. Abrasive cutting, the most often recommendedmethod of metallographic sectioning, produces minimal surface deformationand is also the most economical, simple and rapid method available. Whensamples must be removed from large parts by destructive methods such astorches or hack-saws, the cuts should be made at a reasonable distance fromthe area of interest. Subsequent cutting to remove the damaged areas shouldbe performed in the laboratory with an abrasive cutter.

The prerequisites for successful abrasive cutting are shown symbolicallyin Figure 4.

Wheel Selection should be based on the chemical and physical propertiesof the material to be cut. While aluminum oxide abrasive wheels are suggestedfor cutting ferrous alloys, non-ferrous alloys and non-metals should be cut withsilicon carbide wheels. Abrasive wheels are rated according to their hardness.The softer, pressed wheels (paper sided) are used to cut harder materials; theharder, rolled wheels (rough sided) are preferred for softer materials. Specialresin or metal bonded diamond abrasive blades may be required for extremelyhard metals, carbides and ceramics.

Figure 3. The ABRASIMET'M Cutter WHEELSELECTION

Figure 4. Prerequisitesfor SuccessfulAbrasive Cutting

TECHNIQUECOOLANT

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Adequate, uniform coolant is important to prevent heat build-up during thecutting process. Submerged cooling is very efficient, but cutters employing anabundant stream of coolant directed at the cutting area may be equally effective.If a cutter employs adjustable coolant nozzles, the distance from both nozzlesto the work piece must be equal, thereby preventing irregular wear of the abra-sive wheel which may result in curved cuts and possible wheel failure.

Technique is another important aspect of metallographic cutting. Parts mustbe clamped securely to prevent movement during cutting. A vise such as thatshown in Figure 5 ensures positive positioning and gripping of the work pieceand prevents broken wheels and inaccurately cut samples. Firm, but not ex-treme, pressure should be applied to maintain a reasonable cutting action.Excessive pressure could cause burning of the work piece and possible wheelbreakage. Resistance to free cutting could indicate a wrong choice of abrasivewheels for the sample material or insufficient cooling. Drastic slowing down orstalling of the cutter while in operation may indicate that the particular cutteris not suited for the job.

Although conventional abrasive cutting is preferred for sectioning rigidsamples of steel and other common alloys, delicate components must besectioned using a low speed saw.The ISOMET~ shown in Figure 6, is employedwhen cutting materials whose physical shape or microstructure would be altereddue to mechanical forces or heat normally produced by conventional abrasivecutters.

Figure 5. A Work Piece Secured in theMET-KLAMP'IDVise

Figure 6. Cutting with the ISOMETTM LowSpeed Saw

The ISOMETTMutilizes a thin, continuous rim blade which rotates at a lowspeed (up to 300 rpm). Held by a pivoted specimen arm, the sample is gravityfed by a pre-determined, dead-weight load. The cut may be accurately locatedby means of a micrometer cross-feed and the superior quality of the cut sur-face is such that the number of subsequent preparation steps may be reduced.

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Figure7. Using a DUOMEP' Belt Surfacer for Two Stage Rough Grinding

ROUGH GRINDINGRough Grinding is often used to remove coarse deformation produced by shopsawsor heavy oxide layers resulting from heat treatment. If the plane of interestis too near the initial surface, it must be approached by rough grinding ratherthan by abrasive cutting. Burrs and specimen mounting resin flash may also beremoved rapidly by rough grinding.

Equipment used for this Stage includes both belt surfacers and disc grinders.Because abrasive belts normally wear longer than comparable discs, convenientbelt surfacers such as the DUOM~ 11shown in Figure 7 are widely used.CARBIMET<8JMETSPLlCE@(Silicon Carbide) Belts are available in grit sizes50 to 600, however, rough grinding is usually confined to the range of 50-180grit. ZIRMEfTMMETSPLlCE@(Zirconia Alumina Ceramic Composite) Belts offeran alternative, particularly for grinding harder materials such as wt)ite iron.These belts have a higher cutting rate, cooler action, and longer life thancomparable silicon carbide belts.

Dry grinding may be preferred in special cases where coolant might producecontamination. The VACUMETTMDust Removal System, used in conjunctionwith a belt surfacer, removes dust and residue caused by dry grinding.

A higher degree of surface flatness and stock removal may be obtained byusing a disc grinder such as the SUPERMET,@which operates at higher speeds(5700 SFM*) than (1600 SFM) belt surfacers. Regardless of the type of grinderused, the specimen should be continually moved across the available surface toprolong abrasive life and prevent grooving of the belt platen or grinding wheel.

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*Surface feet per minute.

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MOUNTINGMounting provides a safe,convenient means of holding metallographic samplesduring the Final Stages of preparation and protects the sample edge from thedestructive attack of abrasive materials. Encapsulants for metallography fallinto two major categories, Compression Molding and Room TemperatureCuring, as shown in Figure 8. Compression molding resins are dry powders orPREMOLDSTMwhich cure at 3,000 to 4,200 psi (3-4.2 Ksi) pressure and 150°Ctemperature. They are ideally suited for mounting solid materials which are notdamaged by the required heat and pressure. While compression mOlding ismore economical and usually requires less time and effort, room temperaturecuring resins are preferred for samples that are sensitive to damage from heatand pressure.

Selection of a mounting technique must also take into consideration thepossible need of special edge protection, one of the principal functions ofmetallographic mounting. There is a broad variation in the effectiveness ofdifferent edge protecting media. Vital information such as case hardness depth,plated layer thickness and adherence, and surface defects may be preservedby the application of effective edge retention technology. A poorly protectedsample edge becomes a radius rather than a flat plane when attacked byimpinging abrasives. This might cause distortion and loss of important featureswhich may, due to the divergent reflection of light, lead to inaccurate analysisor measurements as shown in Figure 9. If an edge is rounded, a surface layermay appear shallower than it actually is.

Poor edge retention results from low hardness of the mounting materialcompared to the sample and/or excessive shrinkage of the encapsulant from thesample surface.

Mounting Resins for MetallographyHeatandPressure

Thermosetting Thermoplastic

PhenollcsFASTCYCLEECONOMICALEASYTOUSEHIGHSHRINKAGElOWHARDNESSPOOREDGERETEm'ION

DlallylsHIGHHARONESSCHEMICAllY RESISTAm'MODERATESHRINKAGE

EpoxiesHIGHHARDNESSlOWSHRINKAGECHEMICALLYRESISTAm'

AcrylicsTRANSPAROOlOWINITIAlPRESSURESLOWCUREFAIRHARONESSDEFECTPRONElOW CHEMICALRESISTANCE

RoomTemperatureCuring

AcrylicsRAPIDCURE HIGHEXOTHERMTRANSLUCOO HIGHSHRINKAGEMOD.HARDNESSSTRONGOOORDEMOUm'ABlE

EpoxieslOWSHRINKAGESEMITRANSPAROOMOD.HARONESSSOlVOORESISTAmSLOWCURE

POlyestersTRANSPAREm' SLOWCUREFAIRHARDNESSSOLVEm'SENSITIVElOWSHRINKAGESTRONGDOOR

Figure 8. Mounting Resins for Metallography

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Edge rounding may be ideally controlled by choosing a low shrinkagemounting material containing a hard filler (EPOMETTM)or adding a hard fillerto a low shrinkage (Epoxide) Room Temperature Mounting Resin. Anothereffective edge preservation technique utilizes an electroless nickel coating,EDGEMET~)which forms an intimate, hard protective layer on certain samplematerials.

Compression molding resins are the preferred choice for mounting samplesthat can safely withstand pressures up to 4200 psi and temperatures as high as150°C. Although a mounting press such as the SIMPLlMET"" 11is required, thecost per mount is considerably less than for a sample mounted in any of theroom temperature curing resins. Compression molding resins are available inseveral types (Figure 8) depending on the particular sample to be mounted.Phenolics are the most economical for routine work where edge retention isnot critical. EPQMETTMMolding Compound, an epoxy containing a hard filler,provides maximum edge retention. Maximum efficiency and convenience areachievedby using an air activatedmountingpresssuch as the PNEUMET<B>Ishown in Figure 10.

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Edges \ \8

/// Edgesnot protected

protected

C -::J Objective C =:Jlens

Light t tSpecimen

Mount

Rounded Retained

Figure 9. How Edge Rounding Produces Inaccurate Information

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Room temperature curing resins are recommended when metallographicsamples are sensitive to damage from heat or pressure. Epoxies and polyestersare two-liquid systems which must be measured and carefully mixed. Exceptfor EPo-KWICK@which cures in 30-40 minutes, epoxies and polyesters cureslowly (4-8 hours), have virtually no exotherm and adhere well to the samplesurface. Epoxies have higher hardness and greater resistence to solvents andmineral acids than other room temperature curing resins. Acrylics consist of apowder and a liquid which are easy to mix, cure rapidly (SAMPL-KWICKTMin5-6 minutes and PLASTIC KIT in 20-30 minutes) but produce an exotherm ofOO~~~ .

The choice of molds for casting room temperature curing mounts dependson the resin system used, the mount size desired and the subsequent polishing

Figure 10. PNEUMET" Air Activated Press Used for Compression Molding

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Figure 11. SAMPL-KUP;" Phenolic, Metal and Glass Molds for Room Temperature CuringMounting Resins

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techniques employed. Mold types vary from consumable Phenolic Ring Formsto re-usable Aluminum Ring Forms as shown in Figure 11. For further informa-tion consult the BUEHLER ANALYST~ Section 7, pages 12-14.

Prior to further sample preparation (METAL DIGEST,@Volume 20, No. 3-"Final Stages of Metallographic Sample Preparation") sharp edges producedby the molding process should be removed by grinding. If the sample protrudesfrom the surface of the mount due to normal differences in the coefficients ofexpansion of the sample and the resin, a light rough grinding may again berequired.

Although the Initial Stages of metallographic sample preparation are coarserand dirtier than the Final Stages, no less care in their performance should beconsidered. Failure to be thorough and careful in these operations could resultin microstructural damage which may not be evident until the final pOlish iscompleted.

Table I. Guide to Abrasive Cut-OffWheel Selection

9"Dia.(22.9cm)

Aluminum Oxide Abrasive with Resin/Rubber Bond

Aluminum Oxide Abrasive with Rubber Bond

Aluminum Oxide Abrasive with Resin Bond

DryCutting of I 10-413~10Soft Materials

Silicon Carbide Abrasive with Rubber Bond

(Packaged 10 Wheels per Box)

1~" (3.2cm) ARBOR HIGH VOLUME AND PRODUCTION ABRASIVE CUT-OFF WHEELSFOR BUEHLER CUTTERS

RecommendedUses I 9" Dia.(22.9cm)

Aluminum Oxide Abrasive with Rubber Bond

Thickness

(Packaged 10 Wheels per Box)

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Tool Steel Rc 60 & 10-41110 0.070" 1Q-44110 0.100"Above, Carburized Steel (1.8mm) (2.5mm)Hard Steel 10-4112-010 0.070" 10-4412-010 0.100"Rc 50-60 (1.8mm) (2.5mm)Medium Hard Steel 10-4116-010 0.070" 10-4416-010 0.100"Rc 35-50 (1.8mm) (2.5mm)

Soft or Annealed Steel 10-41210 0.063" 1Q-44210 0.063"Rc 15-35 Rb 45-90 (1.6mm) (1.6mm)

Delicate Cutting 10-4127-010 0.032" 10-4427-010 0.045"(Ultra Thin Blade) (0.8mm) (1.1mm)

Hard Non-Metallics, Glass,Rocks and Other Hard 10-4140-010 0.063" 10-4440-010 0.063"Materials (1.6mm) (1.6mm)Medium Hard Non-FerrousMetals; Uranium, 10-4145-010 0.063" 10-4445-010 0.063"Titanium, Zirconium, etc. (1.6mm) (1.6mm)

Soft Non-Ferrous Metals; 10-41510 0.063" 104450-010 0.098"Aluminum, Brass, etc. (1.6mm) (2.4mm)

Tool Steels, Carburized & 10-5110-010 0.063" 10-5410-010 0.063"Hard Steels, Stellite (1.6mm) (1.6mm)Soft or Annealed 10-5112-010 0.063" 10-541 2-01 0 0.063"Steels (1.6mm) (1.6mm)

Burst Too short a cure period.Insufficient pressure.

.engthen cure period.

Apply sufficient pressure duringtransition from fluid state tosolid state.

Unfused Insufficient molqing pressure.Insufficient time at curetemperature.Increased surface area ofpowdered materials.

Use proper molding pressure.Increase cure time.

With powders-quickly seal moldclosure and apply pressure toeliminate localized curing.

TRANSOPTICTM Powder

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Table 11.Abrasive CuttingTrouble Shooting Chart

PROBLEM POSSIBLE CAUSE SUGGESTED REMEDY

Burning Overheated Lighten cutting pressure.(Bluish discoloration) specimen Choose wheel for harder materialRapid wheel wear Wheel bond breaking Choose wheel for softer material

down too rapidly Lighten cutting pressureFrequent wheel breakage Loose specimen Clamp the specimen

fixturing more rigidlyResistance to cutting Slow wheel Choose wheel for harder material

breakdown Reduce coolant flowCutter stalls Cutter capacity Use cutter with greater HP

inadequate Limit sample size

Table Ill. Compression MoldingTrouble Shooting Chart

Phenolic Resins, Diallyl Phthalate and EPOMETTMDEFECT CAUSE REMEDY

Radial Split Too large a section in the given Increase mold size.

@mold area.

Sharp cornered specimens. Reduce specimen size.

Edge Shrinkage Excessive shrinkage of plastic Decrease molding temperature.away from sample. Choose lower shrinkage resin.

Cool mold slightly prior toejection.

Circumferential Split Absorbed moisture. Preheat powder or Premolds.

@ Entrapped gasses during Momentarily release pressuremolding. during fluid state.

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DEFECT CAUSE REMEDY

Cotton ball Powdered media did not reach Increase holding time atmaximum temperature. maximum temperature.Insufficient time at maximumtemperature.

Crazing Inherent stresses relieved upon Allowcooling to a lower

@ or after ejection. temperature prior to ejection.

Temper mounts in boiling water.

BUEHLER SAMPLE PREPARATIONRESEARCH AND TECHNICAL REPORTSCUTTING · GRINDING · MOUNTINGThese reports, published in the open literature by BUEHLER personnel andusers of BUEHLER equipment and supplies, are available as reprints, at nocharge, upon request.

REPRINT NO.2

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Form No. 10-81-99

TITLE/ORIGIN

J. A. Nelson and R. M. Westrich, "Abrasive Cutting in Metal-lography," Metal/ographic Specimen Preparation, Proceed-ings of the 1973 Metallographic Symposium, Beverly Hills,Calif., pp. 41-54, Plenum Press, 1974."Low Speed Saw," Bodine Motorgram, Vo!. 55, No. 3, May-June, 1975.J. A. Nelson, "Modern Methods and Materials for Metallo-graphic Mounting;' Microstructural Science, Vo!. 4, pp.327-338, American Elsevier Publishing, 1976.J. A. Nelson and E.D.Albrecht, "The Basics of Metallography"(Part I), Heat Treating, pp. 19-23, April, 1976.J. A. Nelson and E. D.Albrecht, "The Basics of Metallography"(Part 11),Heat Treating, June, 1976.J. A. Nelson and W. U. Ahmed, "Significance of the Coolant/Lubricant in Low Speed Saw Sectioning;' Praktische Metal-lographie, Vo!. XIII, pp. 297-305, J!Jne, 1976."Diamond Compounds Grind-Porish Steel;' Cutting ToolEngineering, p. 72. May/June, 1978."Diamond Blade Sections Schlitz Ecology Lid;' BrewersDigest, May, 1980.P.Wellner, "Investigations on the Effect of the Cutting Opera-tion on the Surface Deformation of Different Materials;'Praktische Metal/ographie, Vo!.XVII, pp. 525-535, November,1980.

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