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Metallography Principles and Procedures

LECO Corporation3000 Lakeview Avenue St. Joseph, MI 49085 Phone: 800-292-6141 Fax: 269-982-8977

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D

Metallographic Sample Preparation

Mounting Procedures

Specific Grinding and Polishing Procedures

Microetching/Metal Progress Data Book

Macroetching

Table of Contents

A

B

C

D

E

AMetallographic Sample Preparation

Stages of Preparation

Page

Introduction.........................................................................................................1Requisites ............................................................................................................1Stages of Preparation (Definitions) .......................................................................1Methods of Preparation........................................................................................2Surface Deformation............................................................................................2Pressure ..............................................................................................................3Removal Rate.......................................................................................................3Abrasive Sizing ....................................................................................................4

Stage1Sectioning..............................................................................................5-6

Cooling ............................................................................................................5Wheel Speed....................................................................................................5Wheel Edge Wear.............................................................................................5Low Speed, Low Deformation, Precision Sectioning ...........................................6

Stage 2Coarse Grinding ...................................................................................6Stage 3Mounting..............................................................................................6Stage 4Fine Grinding .......................................................................................6-7Stage 5Rough Polishing ....................................................................................7-8

Abrasives .........................................................................................................7Suspension Medium .........................................................................................7Abrasive Selection ............................................................................................8Polishing Cloths................................................................................................8

Stage 6Final Polishing ......................................................................................8-9Abrasives .........................................................................................................8Polishing Cloths................................................................................................9Polishing Vehicle...............................................................................................9Polishing Wheel Wetness ..................................................................................9

Manipulation .......................................................................................................10Fine Grinding ...................................................................................................10Rough and Final Polishing ................................................................................10Pressure ..........................................................................................................10

Cleaning .............................................................................................................11

..........................................................................................11.............................................................................11

Drying..............................................................................................................11

High-Speed Abrasive Sectioning .......................................................................5Abrasive Grain .................................................................................................5Bond ................................................................................................................5

General ...........................................................................................................11Ultrasonic CleaningCold Water and Cotton Balls

ALECO Corporation www.leco.com 1

Metallography Principles and ProceduresMetallography Principles and Procedures

Metallographic SamplePreparation

Introduction

Requisites

Stages of Preparation (Definitions)

Metallographythe study of the microstructureof metals using various techniqueshas been aninvaluable tool for the advancement of scienceand industry for over one hundred years.Metal lography is used to reveal themicrostructure of metals, which is affected byalloy composition and processing conditions;including cold working, heat treatment andwelding. A finished part's environment can alsoaffect its microstructure and cause problems suchas corrosion and decarburization.

Analysis of a material's metallographicmicrostructure aids in determining if the materialhas been processed correctly and is therefore acritical step for determining product reliabilityand/or for determining why a material failed.

The key to obtaining an accurate interpretation ofa microstructure is a properly prepared specimenwhich is truly representative of the material beingexamined.

The definition of a properly preparedmetallographic surface states that the sectionmust meet the following criteria.

To insure achievement of such true surfaces,preparation must be carried out not only withaccuracy, but also with a clear understanding ofwhat must be accomplished during each specificstage.

The most straight-forward approach is to dividethe entire process into a logical series of stagesinvolved and the purpose of same.

The removal of a representative sample from theparent piece.

Producing an initial flat surface.

Embedding the sample in a hot thermosettingpowder or cold castable mounting material forease in manipulation and other factors such asfragility, edge preservation, etc. This stage issometimes omitted for certain methods ofpreparation or in instances where it would serveno purpose.

Removing the zone of deformation caused bySectioning and Coarse Grinding. The depths ofdeformation during this stage need to be limitedby proper abrasive size sequencing.

Further limitation of the deformation zoneproduced by Fine Grinding.

Removal of deformation zone produced duringRough Polishing. Any zone produced at this stageshould be minimal and generally will be removedduring etching.

Stage 1Sectioning

Stage 2Coarse Grinding

Stage 3Mounting

Stage 4Fine Grinding/Pre-Polishing

Stage 5Rough Polishing

Stage 6Final Polishing

Be flat and free from scratches, stains, andother imperfections which tend to marthe surface.

Contain all non-metallic inclusions intact. Show no chipping or galling of hard and

brittle intermetallic compounds. Be free from all traces of disturbed metal.

LECO Corporation would like to thank for his contributions to this project.Cornelius A. Johnson

2 www.leco.com LECO Corporation

Methods of Preparation

Surface Deformation

Any material can be prepared by MECHANICALPREPARATIONhand or semi-automaticmethods. The sequence of stages previouslydefined are necessary in their entirety for thisparticular procedure.

ELECTROLYTIC POLISHING may often be used asan alternate for the Rough and Final Polishingstages, or as an overall improvement after FinalPolishing by other methods.

SLURRY (ETCH-ATTACK) POLISHING willsupplement both Rough and Final Polishing insome instances, and Final Polishing in others.

CHEMICAL POLISHING is usually employed afterFinal Polishing.

The choice of any method is dependent upon thematerial to be prepared, and particularly thephase relationships and distribution within theexistent microstructure.

Details on these alternate and supplementarytechniques are more completely detailed later.

During Sectioning, Coarse Grinding, and (to alesser extent) Fine Grinding, a transitional surfacezone of deformed metal, results from abrasion.Even though this deformation zone is transitional,abrasion has caused the material to exceed theelastic limit and accordingly, permanent plasticdeformation has occurred. See Figure 1, whichshows possible damage traceable to impropersectioning techniques.

According to L.E. Samuels, the abrasion effectscreate a fragmented layer wherein the surfacegrains have been broken down into sub-grainswith a preferred orientation (Figure 2). Many

intermediate strain boundaries extend in raysfrom the "V contours of the scratches.

The strain levels decrease as the plastic elasticboundary is approached. These strain boundariesare not uniformly distributed since the abrasiveaction and resultant scratch depth of each grainvaries due to sizing, shape, hardness, anddynamic strength. A conservative estimate wouldbe that plastic deformation would never be lessthan fifty times the scratch depth.

At the conclusion of Rough and Final Polishing,the thickness of the induced fragmented layerand the accompanying zone of minordeformation have been slightly decreased. Theplastic elastic boundaries will now contour theoriginal scratches.

Scratch depths are dependent upon abrasive sizeand this affects the magnitude of the strainboundary levels. Scratch depth and total zonedeformation can be considered inverselyproportional to an increase in material hardness(Figure 2A).

"

Figure 2

Figure 2A

Figure 1


Pressure

Removal Rate

There are two factors which contribute to pressureapplied against a samplethe externally appliedload, and the adhesive pressure created by thesurface tension of the vehicle (lubricant,extender). These pressures are important as theycontrol the scratch depth and subsequently thetotal depth of deformation.

For the coarser abrasive sizes used in the Coarseand Fine Grinding Stages, the distance betweenthe specimen and lap is relatively large due to theparticle size. In these instances, the principleeffective pressure is the specimen weight and theexternally applied load.

With particle size in general use for polishing, thespecimen/lap distance becomes significantly less,and this distance will approximate the particlediameter. Theoretical considerations show asharp rise in adhesive pressure (Figure 3).

Each abrasive size and type must be consideredby itself in regard to maximum feasible pressure,and this pressure will show an increase with adecrease in particle size. Within reasonablepressure limitations, the depth of damage isslightly affected. Therefore, it is desirable toemploy correspondingly high pressures to obtainmaximum removal rates.

Material removal rate will increase linearly withpressure to a critical point and then taper off(Figure 4).

The coarse size range (50 to 180 grit; 70 to 350microns) of abrasive materials commonly used forCoarse Grinding do not have sufficient dynamicstrength to prevent fracturing. When fracturingdoes occur due to excessive pressure, the surfacemay become impregnated with fragmentedparticles. Such fracturing can account for the lowremoval rates for 120 and 180 grit sizes. Innormal metallographic sequencing, the use ofsuch abrasive sizes is fortunately avoidable. Onecan readily start with a 240 grit which has a muchhigher removal rate, shallower scratch depth,and consequently, a decrease in total depth ofdeformation (Figures 5 and 6).

Figure 3 Figure 5

Figure 6

Figure 4

P24 0.02913 74024 0.02815 71530 0.02512 638 P30 0.02449 62236 0.02106 535 P36 0.02063 52440 0.01685 428 P40 0.01622 41250 0.01382 351 P50 0.01283 32660 0.01055 268 P60 0.01024 260 P80 0.00776 19780 0.00756 192 P100 A200 0.00614 156

100 0.00555 141 P120 A160 0.00500 127

120 0.00457 116 P150 0.00382 97

150 A130 0.00366 93180 P180 A110 0.00307 78220 A85 0.00260 66 P220 A90 0.00256 65 0.00236 60 P240 A75 0.00230 58.5 0.00217 55

240 A65 0.00211 53.5 P280 0.00207 52.5 0.00197 50 P320 A60 0.00182 46.2 0.00177 45 0.00173 44 P360 0.00159 40.5 0.00157 40

320 0.00142 36 P400 A45 0.00138 35 P500 0.00119 30.2 0.00118 30

360 0.00113 28.8 P600 A35 0.00101 25.75 0.00098 25

400 A30 0.00093 23.6 P800 A25 0.00086 21.8 0.00079 20

500 0.00078 19.7 P1000 0.00072 18.3

600 0.00063 16 P1200 A16 0.00060 15.3 P1500 0.00050 12.6

800 0.00048 12.2 P2000 0.00041 10.3

1000 0.00036 9.2 P2500 0.00033 8.4

1200 0.00026 6.51500 0.00012 3.0

CAMI FEPA* Trizact Average AverageDesignation Designation Finishing Particle Size Particle Size

(USA) (Europe) Scale in Inches in Microns

Table IGrit Comparison Guide

*Federation of European Producers of Abrasives

3.0 1.0 0.01

0.05

0.10

0.50

1.00

1.50

2.00

1610

6.5

23

29 3.0

5.0

10.0

15.0

35

54627080

115

190

270

350

Mic

ron

Size

Mic

ron

Size

Mic

ron

Size

Grit

Siz

e

Coa

rse

Grin

ding

Fine

Gri

ndin

g

Rou

gh P

olis

hing

Fin

al P

olis

hin

g

600800

12001500

400

320

280

240220180

120

80

60

50


Abrasive SizingThe abrasive size ranges applicable to thevarious stages of preparation are shown inFigure 7. Comparative sizing values for the morecommonly employed abrasive families appear inTable I through IV.

Table IV BPolishing SuspensionsType Particle Size Application

(m)Levigated Alumina 5.0 Rough PolishingGamma Alumina 0.05 Final PolishingAlpha Alumina 0.3 Rough and/or

Final PolishingChrome Oxide 1.0 Rough PolishingChrome Oxide 0.05 Final PolishingChrome Oxide* 0.05 Final PolishingCerium Oxide** 0.05 Final PolishingColloidal Silica 0.05 Final Polishing*Trade Name CRO; Chrome Oxide/Cerium Oxide Blend**Trade Name Finish-Pol; Cerium Oxide/Aluminum

Oxide Blend

Table IIEmeryGrit No. Particle Size (AV. m)

3 852 701 50

1/0 332/0 303/0 284/0 25

Table IV APolishing PowdersType Particle Size Application

(m)Gamma Alumina 0.05 Final PolishingAlpha Alumina 0.3 Final PolishingAlpha Alumina 1.0 Rough and/or

Final PolishingMagnesium Oxide 2.0 Final Polishing

Table IIIDiamond PastesMicron Size* Size Range Mesh Equivalent

(m) (m) (Approx.)1/10** 0-1/0 1/4** 0-1/2 100,0001/2 0-1 60,0001 0-2 14,0003 2-4 8,0006 4-8 3,0009 8-12 1,80015 12-22 1,20030 22-36 60045 36-54 32560 54-80 230-325

90*** 170-230*NIST**Ultra fine grades, not covered by NIST***Not covered by NIST

Figure 7


Stages of Preparation

Stage 1SectioningSectioning is the removal of a representative areafrom the parent piece. The microstructure mustnot be altered in the process. Heat or coldworking are the two most likely conditions whichwould quickly bring about structure changes.

Quite obviously operations such as sawing orshearing are not preferable due to thedeformation produced. Abrasive cutting offersthe best solution to eliminate or minimize heatand deformation.

To cut properly, a bonded abrasive wheel must bematched to the cutoff machine. Primaryconsiderations are surface speed (SFM) for agiven wheel diameter and the type of coolingsystem employed. Selection must be made fromthe proper family of abrasive wheels to meet therequirements for the vast variety of materials andhardness levels. The principle controlling andguiding variables for wheel selection can beclassified as follows.

Aluminum Oxide is generally recommended forsectioning steels and high tensile strengthmaterials. Silicon Carbide abrasives are used forsome grades of iron, nonferrous materials,Titanium, Zirconium, Uranium, and their alloys.

Regardless of bond hardness, the coarser gritsizes will produce a harder action. However, thecutting action will be more open as the clearanceof cut will be greater. Finer grits result in a softeraction and a smoother surface.

The purpose of the bonding material is to hold theabrasive grains in place. In general, rubberbonded wheels are used for wet operation andare best suited for metallographic specimens.Resinoid bonds are used for dry cutting.

Resin/Rubber can be used wet or dry and mayoffer an economy factor.

To cut clean and fast, the bond must wear away orbreak down rapidly enough to expose the newabrasive grains.

Softer bond wheels are used for sectioning hardmetals and alloys, whereas harder bond wheelsare used for softer materials. As bond hardnessincreases, the wheel wear is decreased.

The rate of bond breakdown is related to severalfactors.

Sufficient and proper cooling is very important.High-volume jet spraying or submerged cuttingare the two major techniques used. Section size,material, and hardness dictate which methodshould be employed. Submerged cutting will tendto make a wheel bond act harder.

Speed (SFM) must be carefully considered both inthe design of a cutter and the selection of wheelsfor a given cutter. In general, a given wheel bondwill act harder as speed is increased.

Wheel edge wear may be used as a very goodguide to indicate whether the proper wheel hasbeen selected.

High-Speed Abrasive Sectioning

Abrasive Grain

Bond

Cooling

Wheel Speed

Wheel Edge Wear

a)Type Al O or SiCGrit Size

b)RubberResinoidResin/Rubber

c)SoftMediumHard

d)OpenDense

Abrasive Grain

Bond

Hardness of Bond

Density (Structure)

2 3

Rounded edges signify thecorrect wheel properly appliedto the cutting of solids.

Concave edges indicate properwheel for sectioning light walltubing of thin wall sections.

Square edges are retained on awheel well suited to cutting solids,sample standards, and tubing ofmedium wall thickness.

Bond hardness Hardness and workability of sample Size and speed of abrasive wheel Power of driving motor Type, amount, and method of coolant application Amount of pressure applied to wheel


Precision Sectioning

The technique of low speed sectioning formetallographic and related type specimens ispatterned after principles from the precious gemindustry.

Small diameter, four- to six-inch diamond or CBNrimmed wheels are used. The speed range is 0 to1000 RPM and the load range is 0 to 1000 grams.The technique is applicable to many types ofmaterials. Response is excellent to metals andnon-metallicssoft, hard, brittle, ductile, porous,simple or complex configurations, composites,etc.

The resultant surfaces are extremely smooth withvery little surface deformation. Tendencies towardcracking at brittle-ductile interfaces in laminatedor deposited materials is nonexistent. Brittlephases within a complex microstructure do nottend to "check" or "pluck".

The purpose of coarse grinding is to removedeformation produced during sectioning andprovide the initial flat surface. A secondarypurpose may often be to remove gross amounts ofsurface material for microsample preparation ormacroetching.

The process is performed on abrasive belts ordisc-covered rotating wheels. The size range is 50to 180 grit. Water is recommended as a coolant to

prevent overheating of the specimen and flushaway the surface removal products, thus keepingsharp abrasive grain exposed at all times.

Abrasive belts and discs are available in siliconcarbide and aluminum oxide with resin bond forwet or dry operation. Garnet coated materials areonly available with glue bond and can only beused dry. Diamond structured fixed abrasives andspot pattern discs are also for coarse grindingapplications.

The abrasive action is very aggressive with this gritrange. With higher speed coarse grinding (i.e.increased surface feet per minute), the resultingsurface finish for a given grit size will approachthat produced by a finer grit size. For example, asurface finished with 60 grit/5200 SFM would beequivalent to one produced by 120 grit/2500SFM.

A surface that appears smooth and bright doesnot necessarily have the least (shallowest)amount of deformation. An apparent improvedappearance can be due to rubbing or smearing ofthe surface by the abrasive particles not cuttingcleanly. Grinding with worn or loaded abrasivesurfaces will produce more extensive surfacedeformation.

The abrasives used for fine grinding are siliconcarbide, emery, and aluminum oxide. Pre-polishing diamond structured discs with 3 to 15 diamond suspensions are also utilized for thisstage preparation. Generally, fixed type abrasivesare used (the abrasive grain is bonded to a paperor cloth backing). The bonding material may beglue, resin, or resin over glue. Silicon carbide andaluminum oxide materials are available witheither a non-waterproof paper backing with glueor resin bond for dry operation; or waterproofcloth or paper backing with resin bond for wet ordry operation. Emery coatings are only fabricatedwith a glue bond.

Preference is for wet operation, which offers aflushing action to prevent the surface frombecoming clogged with removal products.Flushing will also keep the cutting edges of theabrasive grains exposed.

Stage 2Coarse Grinding

Stage 3Mounting (See Section B)

Stage 4Fine Grinding

Glazed edges occur when thebond is not breaking downproperly.

Tapered (chisel) edge iscaused by improperapplication of coolant.

Pointed edges indicatewheel bond is too hard.

Low Speed, Low Deformation,


Silicon carbide abrasive grain, when used wet,has a removal rate twice that of a correspondinggrade of emery, and provides a much shallowerdeformation depth.

Aluminum oxide has a lower hardness value thansilicon carbide, which could exhibit higherdynamic strength and therefore decrease shear orfracture values, which can correspondingly affectdeformation depth.

This stage may be considered the most importantin the entire preparation sequence. The nature ofthe abrasive type employed should permitaccurate sizing and separation by variousmethods into fractions of uniform particle size.

Diamond abrasives fall into the above category.Other contributing properties are high hardness,inertness, and low coefficient of friction. Diamondparticles retain their shape and size during

abrasion and produce a uniform and high rate ofmaterial removal with minimal induced surfacedamage. Removal rates may often exceed thoseproduced during the fine grinding sequence(Figures 8 and 9).

The suspension medium for diamond powders isvery important as it provides particle suspensionand contributes to lubrication and removal rate.Some adjustments in viscosity of the medium mustbe made for various particle sizes to compensatefor possible drastic changes in heat generation.

Oil or water-soluble media promote superiorlubrication and removal rates in comparison toslurry suspensions. The reason being the particlesare uniformly dispersed and held in a definitesuspension. The paste-like material facilitatesconvenient charging of the polishing clothsurface, and the addition of an extendercontributes to even particle distribution over thesurface.

Stage 5Rough Polishing

Abrasives

Suspension Medium

Figure 8

Figure 9

Figure 10


Abrasive Selection

Polishing Cloths

Abrasives

A sufficiently coarse abrasive should be selectedto accomplish this stage in a minimum time. Thetime factor will greatly influence relief effects.However, size selection is greatly dependent uponthe particle sizes and material types to be used insubsequent operations.

Particle sizes in general use are in the overall sizerange of 0 to10 microns. The 6 micron (range 4 to8) classification will produce the highest removalrates for most materials. A sharp decrease inremoval rates under similar conditions is to beexpected for the lower micron and sub-micronranges (Figure 9).

As previously stated, removal rates will increaselinearly with pressure to a critical point(Figure 8).

At the conclusion of rough polishing, the samplesurface will naturally show scratches of visibledimensions and there will be localizeddeformation associated with these scratches.

The type of cloth used for this stage has anextremely important bearing on the end result. Iti s i m p e r a t i v e t h a t r e l i e f b e t w e e nmicroconstituents of varying hardness andsample mount interfaces be held to a minimum.Napless cloths such as nylon, cotton, silk,chemotextile materials, etc., should be used.Cloths of this nature will hold relief andundercutting at interfaces to a minimum, as "pilewhip" is non-existent. Selection should also besuch that the cloth itself does not produce anyabrasive artifacts. The hardness of the materialbeing prepared is the guide point.

As previously stated, the final polishing stageserves to remove any deformation zone resultingfrom rough polishing. Here, a uniformly polishedand scratch-free surface must be produced.

Care must be taken to insure removal of any andall surface deformation. If this is notaccomplished, scratches may still be apparent inthe unetched state. The same artifacts will appearand to an even greater extent if any precedingsteps or stages were not properly accomplished.

Scratches may also be evident after etching. Thissignifies the deformed surface was not completelyremoved, see Figure 10. The etchant attack willbe more severe and preferential along thoseregions of localized deformation, as they possesshigher surface energy levels.

A prolonged series of alternate etching andrepolishing is generally discouraged as a meansto remove deformation. Relief effect tendenciescan accrue with an increase in the number ofetch-repolish cycles. Relief can also beattributable to preferential attack of the localizeddeformed areas, or selective attack of certainphases or grain orientations.

A wide variety of abrasive materials are used forfinal polishing. The most common are AluminumOxide, Chromium Oxide, Magnesium Oxide,Cerium Oxide, Colloidal Silica, and Diamond.

Aluminum Oxide is the most extensively usedmaterial. Two types are availablelevigated andhigh-purity synthetic powders. Preference is forthe synthetic materials in either powder orsuspension form within the 1 micron and sub-micron size range. Particle size and crystallinestructure are dependent upon temperature. Thegamma type, low temperature form, is sized 0.05microns. The particle sizes 1.0 and 0.3 micron,are the high temperature alpha structure.

Stage 6Final Polishing


Particles are grown to size by precisely controllingthe temperature range. The alpha lattice isslightly harder than the gamma form.

Ferrous, copper, titanium, zirconium-basedmaterials, and super-alloys are compatible withalumina abrasives. General preference is thegamma type. However, with some material, thealpha form may be profitably used as anintermediate step.

Other commonly used media are liquidsuspensions of chromium oxide, and chromiumoxide/cerium oxide blends. In many instances,these are unsurpassed for the graphitic irons andferrous materials containing complex inclusionsor gross amounts of inclusions.

Magnesium Oxide (even though the techniquesare somewhat difficult to master), is ideally suitedto many materials. Aluminum, magnesium, andtheir alloys are best prepared with this material.The powder has uniformly well-shaped particlesof considerable hardness and the cutting edgesare well defined. Today's high-temperaturecalcining treatments have eliminated problemsformerly associated with the subsequentformation of hard carbonates. Any trace alkalisare water soluble.

Cerium Oxide slurries are a relatively newinnovation as a final polishing abrasive. Theblends, particularly those with small amounts ofaluminum oxide, are readily adaptable to a largegroup of materials. The extremely fine particlesize is a definite attribute. However, thesesolutions have not been exploited to their fullest.

Colloidal suspensions of Silicon Dioxide (Silica)have been used with remarkable success in theelectronic wafer industry. Colloidal silica is anexcellent final polishing media for ferrous,nonferrous, and electronic components.

Diamond abrasives have several sizing levelsbelow the 4 to 10 micron range (3 micronaverage, 1 micron average, 0.25 micronaverage). The logical choice would be the sub-micron grade as this would not be too great a stepfrom the size used in rough polishing. The 3micron or 1 micron grading would only be used ifan intermediate step were desired. The finishproduced by even the finest sized diamond isgenerally only for routine applications. Resultsare more satisfactory as overall hardness of thematerial increases.

Napped cloths are generally preferred for thisoperation. Unfortunately, the fibers are

compressible, and therefore tend to conform tothe surface of the specimen under the slightestpressure. Due to this type of contact, the softerphases or grains with a certain orientation have ahigher removal rate than the harder phases orgrains with differing orientations. Such effects canbe avoided or minimized with abrasive selectionto shorten the time element.

Distilled or distilled and deionized water isgenerally used as the suspension or extendervehicle media for those materials classified underthe metallic oxide category.

Metallic oxides are basic by nature. Idealpolishing conditions are present when solutionsare neutral pH 7. Precautions are necessary whenconsiderable electrochemical differences arepresent between individual areas or phases of aspecimen. Severe etching of the anodic phasemay occur if the vehicle becomes ionized. Forexample, water must be avoided when polishinggalvanized steel samples.

With highly reactive materials or phaserelationships, it is sometimes necessary to resortto a non-polar vehicle, such as ethylene glycol.However, the polishing rate may be severelyreduced. Therefore, careful observations arenecessary whenever vehicles are altered to offsetany chemical attack that may occur duringpolishing.

The problem is non-existent with diamondabrasives and oil vehicles as the particles are inertand the oil is non-ionizing.

The wetness or "trim" of the cloth with water-typeextenders has a great bearing on the end result. Ifthe cloth is too wet, the sample can show pits; iftoo dry, buffing and/or smearing can result.

To determine proper wetness, remove the samplefrom the wheel and check the time necessary forthe polishing film to dry. In general, this shouldtake no longer than five to eight seconds. To checkfor abrasive addition, note the color andconsistency of the film. The film should not beopaque, but rather sufficiently transparent toreveal the sample shape and luster.

When using diamond abrasives, improvedremoval rates are encouraged by low viscosity oilextenders or ethanol-based extenders (Ultralap).The cloth should always show a slight excess ofvehicle to insure good lubricity and swarfremoval.

Polishing Cloths

Polishing Vehicle

Polishing Wheel Wetness

Step 3

Step 4

Step 2 Step 1

(a)

Polishing WheelRotation

SampleRotation

(b)

Firm Wristand Fingers

AbrasiveSurface


ManipulationFine Grinding

With manual processing, the sample is firmly heldwith the fingers. Movement is in a straight lineacross the abrading surface toward or away fromthe operator (Figure 11). When manual dexterityhas been achieved, motion in both directions maybe employed.

The operator should be positioned to allow freepassage of the elbow past the side of the body forlinear coordination between the shoulder andelbow joints.

Finger and wrist joints should remain rigid, andthe shoulder line fixed to aid in even pressurecontrol and produce a planar surface with nofaceting.

The specimen is rotated 45 between abrasivesteps. The purpose is two-fold; to indicate whenthe abrasive scratches from the previous stephave been removed, and to prevent faceting(Figure 12A).

The rotation of the polishing wheel is normally ina counterclockwise direction. The sample shouldbe moved in a clockwise direction around theentire polishing surface to avoid directionaltraces; "fishtailing" of certain family typeinclusions; and/or "pull-out" of phases poorlyconsolidated within a microstructure. Suchmanipulation also provides equal materialremoval over the entire surface (Figure 12B).

Previous remarks concerning applied pressurehave been made. As an overall generalization,maximum feasible pressures should be used toproduce maximum removal rates.

O

Rough and Final Polishing

Pressure

Figure 12A

Figure 12B

Figure 11

ASolution Levelin Beaker

Solution Level

PositioningCover

Glass Beaker

LECO Corporation www.leco.com 11

CleaningGeneral

Ultrasonic Cleaning

Cleanliness is one of the most importantrequisites in sample preparation. Discouraging orunsatisfactory end results are more oftentraceable to carelessness rather than to faultymaterials. Samples must be carefully cleanedbetween each stage of preparation to preventcontamination by coarser abrasives being carriedover to a finer abrasive stage.

The role of cleanliness also includes operator'shands and equipment. The laboratory layoutshould be such that the extremely coarse abrasivestages are isolated from those stages involvingfiner abrasive material. Polishing wheels shouldbe kept covered when not in use.

A few minutes at the end of each working dayshould be set aside for general clean-up andmonitoring of equipment. The results of the aboveroutine practices are rewarding.

Ultrasonic cleaning is the most effective systemfor the varying dirt problems encountered insample preparation. The higher crystalfrequencies produce better results.

There are many readily available water solubledetergents. Excellent, and sometimes moreeffective, commercial materials are also offered.The use of ammoniated solutions is discouragedas they exhibit etching tendencies with numerousmaterials.

Superior end results are obtained if more thanone sample cleaning step is used. This may be

very simply accomplished through the use of aglass beaker, and positioning the cover to fit thetop of the master tank. Since the glass beaker isacoustically transparent, the ultrasonic energy istransmitted through the tank solution (couplingagent) to the cleaning solution in the beaker(Figure 13).

Proper drying of a surface after cleaning oretching is very important. The specimen must bedried quickly to prevent staining or corrosion.After rinsing, the sample is flooded with a high-quality alcohol and dried in a stream of warm dryair. With porous materials, an additional rinse inhigh-purity acetone after an alcohol rinse will bevery beneficial.

Specimens are generally rinsed with warm water,even after ultrasonic cleaning. However, somematerials may stain or corrode when rinsed inwarm water. In such cases, cooler water isrecommended.

Cold Water and Cotton Balls

Drying

An alternative to ultrasonic cleaning (if notavailable) is cleaning using cold water and acotton ball. Place the sample or sample holderunderneath cold running water, saturate a 100%cotton ball with cold water, and clean by rubbingthe sample or samples. Rinse thoroughly withwater, give a final rinse with ethyl alcohol, anddry. Use of 100% cotton balls will not scratch thesample; .synthetic cotton will

Figure 13

BMounting Procedures PageSpecimen Mounting (Purposes) ............................................................................13

Compression Mounting........................................................................................13-14

Thermosetting Molding Defects.........................................................................13

Thermoplastic Molding Defects .........................................................................14

Cold Mounting ....................................................................................................14-15

Epoxide Molding Defects...................................................................................14

Polyester Molding Defects .................................................................................15

Acrylic Molding Defects.....................................................................................15

Vacuum Impregnation..........................................................................................15-16

Procedure.........................................................................................................15

Curing..............................................................................................................16

Edge Protection....................................................................................................16-17

Steel Shot.........................................................................................................16

Electro or Electroless Deposition........................................................................16

Glass-Filled Mounting Media ............................................................................16

Support Strips...................................................................................................17

Ceramic Fillers .................................................................................................17

BLECO Corporation www.leco.com 13

Mounting Procedures

Specimen Mounting (Purposes)

Compression Mounting

Metallographic samples are mounted primarilyfor ease in manipulation and for edge protectionduring preparation.

Compression molding techniques are used toproduce hard mounts in a minimum amount oftime. The materials used are classified asthermosetting and thermoplastic. Thermosettingmedia requires heat and pressure during themolding cycle and can be ejected at maximummolding temperature. Thermoplastic materialsremain fluid at maximum molding temperaturesand become dense and transparent with adecrease in temperature and an increase inpressure, see Figures 15 and 16.

The variables in compression molding arepressure, temperature, and time. Temperatureand pressure factors can be held constant by thedesign of the mounting press.

With more fragile sections, powdered materialshould be used. Normally powdered mediashould be initiated with the molds at roomtemperature. This practice is recommended aspowdered material has an extremely largeexposed surface area and consequently theindividual grains, upon contact with heatedmolds, have a marked tendency to immediatelycure without fusion.

Pre-molded thermoset preforms can be usedwhen a section will not be damaged as it is forcedinto the mounting material by the initialapplication of pressure.

When transparency is needed for locating aparticular area, Lucite is the best mountingmedium to use. Very light pressures are usedduring the preheating and flow cycles. Eventhough high pressures are normal lyrecommended for the cure cycle, lower pressuresmay be used with no undesirable effects.

The material readily flows into small areas. Thisfactor, plus the allowable pressure variances,make the material very desirable for small, fragilepieces. These possibilities very often offset thelonger times involved in molding transparentmounts.

Section toolarge for mold area. Sharpcorners on specimen. Uselarger mold size. Reducespecimen size and eliminatesharp corners if possible.

Mate r ia l has absorbedmoisture. Gases released bychemica l reac t ion. Usep r e h e a t e d p o w d e r s o rp remo ld s . Momen ta r i l yrelease pressure during flowstage.

Thermosetting Molding Defects

Split (Radial):

Split (Circumferential):

Table VMolding Temperatures and PressuresMolding

Molding PressureMaterial Form Classification Temp ( F) (psi)O

Bakelite Powder Thermosetting 280-320 3800-4400

Bakelite Preform Thermosetting 280-320 3800-4400

Diallyl-Phthalate Powder Thermosetting 300-360 3800-4400

Epoxy Powder Thermosetting 280-320 3800-4400

Lucite (transparent) Powder Thermoplastic 280-320 3800-4400

O

O

1 O

O

O

1 Including Copper Diallyl Phthalate for conductive applications

Figure 15

Figure 16

VeryHard

MediumHard

Medium

Hardness

Epoxide

Polyester (Castoglas)

Polyester (Castolite)

Acrylic

Exotherm

High

Medium

Low

High

Medium

Low

Shrinkage

Extended

Medium

Rapid

Cure Time


Shrinkage (Edge):

Burs t ( Fron t Sur face ) :

Woody (Unfused):

Case Hardening & Blister:

Cottonball:

Crazing:

Cracking:

Pooradhesion to sample surfacewith excessive shrinkage atinterface. Use lower moldingtemperature.

Insufficient pressure and/orinsufficient cure time. Adjustmolding pressure. Increasecure time.

Insufficientpressure and/or insufficientcure time. Curing of powderparticles prior to flow stage.Adjust pressure and/or curetime. Rapidly seal moldclosure and apply pressure toeliminate localized curing.

Excessive mold temperature.Decrease mold temperature.Momentarily release pressureduring flow stage.

If mold temperature is too high, the followingdifficulties may also be encountered.

Center portion ofmedium did not reachmaximum temperature priorto cure stage. Increaseholding time at maximumtemperature.

Inherent stressesrelieved upon or after ejectionof mount. Cool to a lowertemperature prior to ejection.Decrease pressure duringcure stage. Stress relievemounts in boiling water.

Cold mounting techniques offer particularadvantages when a specimen may be too delicateto withstand the pressures and heat involved incompression molding. Large groups of samplesmay also be readily mounted when work flow canbe properly scheduled. However, the timenecessary to process small groups of samples farexceeds that for compression mounting.

The three most common types of materials areEpoxides, Polyesters, and Acrylics. These systemsare all two-component types consisting of a resinand a hardener. Since an exothermic reactionduring polymerization is involved, the mixing byvolume or weight ratios of each system is critical.The epoxides are pale yellow and transparent.The polyesters are also transparent and availablein water clear or a light pink hue. The acrylics areopaque. Other acrylics, when placed and cured ina pressure vessel at 2 bars (28.8 psi) of pressure,produce a semi-transparent mount when trans-parency is needed for locating a particular area.

The characteristics of the common family typesare compared in Figure 17.

Resin-to-hardenerratio incorrect. Exotherm tooextreme. Correct resin-to-hardener ratio. Use forcedcool air to control rate ofexotherm.

General

Thermoplastic Molding Defects

Epoxide Molding Defects

Cold Mounting

Mounts sticking to mold surface regardlessof finish or application of release agent.

Dull surfaces on mounts.

Case hardening of outer mount surfaces.

Excessive flash.

Mold staining.

Figure 17

BLECO Corporation www.leco.com 15

Entrapped Air:

Discoloring:

Soft Mounts:

Cracking:

Discoloration:

Soft Mounts:

Entrapped Air:

Too violentagitation while blending resinand hardener mixture. Blendmixture more gently orremove air with vacuum.

Hardener hasoxidized. Resin-to-hardenerr a t i o i n c o r r e c t . K e e pcontainers tightly sealed.Correct resin-to-hardenerratio.

Res in- to-hardener ratio incorrect.Incomplete blending of resinand hardener mixture. Correctresin-to-hardener ratio.Completely blend mixture.

Resin-to-hardenerratio incorrect. Exotherm tooextreme. Correct resin-to-hardener ratio.

Resin hasoxidized. Resin-to-hardenerratio incorrect. Keep containertightly sealed. Correct resin-to-hardener ratio.

Res in- to-hardener ratio incorrect.Incomplete blending of resinand hardener mixture. Correctresin-to-hardener ratio.Completely blend mixture.

Too violentagitation while blending resinand hardener mixture. Blendmixture more gently orremove air with vacuum.

Many materials, both organic and inorganic, maybe porous, friable, poorly consolidated, have hardand/or soft phase relationships, or otherextremes.

Vacuum impregnation with a suitable liquid epoxysystem will produce a sample which is non-porouswith excellent consolidation and rigidity.Penetration is generally sufficient for sectioning orre-sectioning.

The resultant high density permits preparationwithout plucking, tearing, fracturing, orintroducing other forms of sub-surface damage.

Suggested equipment for using Bakelite ringforms of standardized mount diameters

. Aluminum or tin-coated forms maybe used for larger sections.

Prior to impregnation, samples should bethoroughly cleaned and if necessary, oven dried.

Applying a thin film of release agent to the epoxycontacting surfaces of the specimen forms will behelpful in removing finished mounts from thesame. Do not coat inside diameter of the Bakelitering forms as adhesion of epoxy is highly desirableand necessary.

The sample is placed in the ring form, and theresin-hardener mixture is poured to a level slightlybelow the top surface. With bell jar in place, thesystem is evacuated to 22 inches of mercury for atleast ten minutes total holding time. Activebubbling will occur as air is removed from boththe epoxy and the sample. Intermittent releaseand reactivation of vacuum will indicate when allthe air has been removed. Releasing the vacuumwill force the epoxy into any continuous voidareas. Evacuation below 22 inches of mercurymay produce vaporization in an epoxy system dueto exceeding the boiling point of the mixture.

Polyester Molding Defects

Acrylic Molding Defects

Procedure

Vacuum Impregnation

is shownin Figure 18

Figure 18

SteelShot

Mount

MountSpecimenEdge

PlatedDeposit

Figure 19

Glass-Filled Thermoset

Mount

Specimen

Interface

Figure 20


Curing

S t e e l S h o t :

Electro or ElectrolessDeposition:

Glass-Filled MountingMedia:

The ratio of epoxy resin to hardener is extremelycritical to promote proper curing as this isdependent upon the necessary exothermicreaction for proper polymerization. The supplier'srecommendation should be strictly followed andnever varied.

The choice of system is closely related to thevolume of material being cast. Mounts forgeneralized metallographic sample preparationhave dimensions of 1.0, 1.25, and 1.50 inches indiameter, and are 0.5 to 0.75 inches high. Withthese dimensions, a low exotherm system with anair cure can be successfully used.

With larger sections, higher polymerizationexothermic reactions are involved to promoteproper curing. A controlled curing cycle that maybe programmed with an automatic timer is shownin Figure 19.

The cast samples are placed in front of a small airconditioner. The reduction in temperature retardsthe surface exothermic reaction, preventingshrinkage and stress formation.

A post cure time of 1-1/2 to 2 hours at 150 F willfully develop the physical properties of epoxy. Thisis applicable to either air or force-cure mounts.

Specimen surfaces must be flat to the very edgesfor microscopic observation and properphotomicrography. Unless special techniques areused prior to mounting or in mounting mediaselection edge, rounding will occur at the sample-mount interface, see Figure 20.

The degree of rounding is dependent upon thehardness and abrasion differential between thespecimen and the mounting material.

T h i stechnique serves the samepurpose as the followingsuggestion.

The sectionis plated with a coatingsuf f i c ient l y th ick tocompensate for edger o u n d i n g d u r i n gpreparation. The morec o m m o n m a t e r i a l sinclude Ni and Cu. Nickelwill sometimes peel awayfrom a surface due to itss t r e s s e d c o n d i t i o n .Copper is generally usedfor post-plating electronicgear to preserve edges onsingle or multi-coateddepositions.

Commercially availableelectroless coatings aregenerally stress free.Select ion of plat ingmaterial should be givenprior thought regardingetchant rate and reactionwith the different metalsinvolved.

Thermosettingmounting media withspecial filler additives willoften offer sufficient edgesuppo r t t o p re ven tr o u n d i n g . S t r a i g h tmineral-filled epoxies arealso helpful if allowed topos t cu re a t roomtemperature to increasethe hardness level.

O

Edge Protection

BSupportStrip

Mount

RingForm Ceramic

Filler

Epoxy


Support Strips:

Ceramic Fillers:

Small support strips of a similar hardnessmaterial are spaced to permit flow of the mounting materialbetween section and spacer. Screening and using the "fines"from compression mounting material will increase flow andimprove hardness and density of the finished mount.

Particles of a fine mesh ceramic material(pelletized alumina) are mixed with a liquid epoxy. The pelletizedmaterial is available in several mesh sizes and hardness.Selection can be somewhat adjusted to match sample hardness.A small amount of ceramic material is mixed with the resin-hardener mixture. Due to density differences, the filler will settleand form a layer along the bottom surface.

Grams of mounting media required to produce afinished mount 5/8" (16 mm) tall

Mount Diameter (Inches)

Hot 1.0 1.25 1.5 2.0

Bakelite 11.0 17.1 24.6 43.8Epoxy 15.9 24.8 35.7 63.5

Diallyl Phthalate 14.9 23.2 33.5 59.5Copper Diallyl Phthalate 34.3 53.6 77.2 137.2

Lucite 9.2 14.4 20.7 36.7

Cold 1.0 1.25 1.5 2.0

Long/Quick Cure Epoxy 9.4 14.6 21.1 37.4Lecoset 100 9.6 15.0 21.6 38.4

Lecoset 7007 9.4 14.7 21.1 37.5Lecoset 7000 5.9 9.3 13.4 23.8

CSpecific Grinding and Polishing Procedures Page

Polishing Procedure for Ferrous Materials .............................................................19

Remarks ...........................................................................................................19

Graphitic Cast Irons..........................................................................................19

Galvanized Coatings ........................................................................................19

Stainless Steels, Stainless Steel Casting, Alloys, Heat-Resisting Alloys.................19

Polishing Procedures for Copper-Based Materials .................................................19-20

Remarks ...........................................................................................................20

Suggested Deviations .......................................................................................20

Polishing Procedures for Aluminum and Magnesium-Based Materials....................20

Remarks ...........................................................................................................20

Polishing Procedures for Titanium, Zirconium, Hafnium, and Alloys .......................20-21

Remarks ...........................................................................................................20

Composition.....................................................................................................21

Techniques .......................................................................................................21


Polishing Procedures for Cemented Carbides........................................................21

Remarks ...........................................................................................................21

Polishing Procedures for Lead Alloys, Tin Alloys, and Zinc-Based Die Castings .......21-22

Remarks ...........................................................................................................21

Polishing Procedures for Refractory Alloys and Metals (Nb, Mo, W, V, Ta) ...............22

Remarks ...........................................................................................................22


Polishing Procedures for Plated Sections ...............................................................22

Remarks ...........................................................................................................22

Polishing Procedures for Powder Metals and Alloys ...............................................22-23

Remarks ...........................................................................................................22

General Information.........................................................................................23

Polishing Procedure for Ceramics .........................................................................23

Remarks ...........................................................................................................23

CLECO Corporation www.leco.com 19

Specific Grinding andPolishing Procedures

Remarks

Al O -Coated Products (waterproof)Sameabrasive sequence and lubricant may be used.Abrasive action is less severe on some materials.

240, 320, 400, 600 grit SiC or Al O paper ordisc with water as lubricant. Clean thoroughlyand dry.

Etch rather heavily. Use 4% Picric acid solutionfor pearlitic matrix materials and 4% Nitalsolution for ferritic matrix materials.

Abrade on 1200 grit paper dry. After ashort period of abrasion, clean abrasive surfacewith a cotton swab saturated with alcohol.Repeat etching, abrasion, and cleaning untilthe graphite flakes, nodules, or temper carbonshow definite retention and uniform matrixfinish. Clean samples thoroughly.

Etch sample lightly as recommended above.Precondition synthetic velvet cloth with onemicron diamond paste. A water soluble extenderis recommended as oils may penetrate and staingraphite particles. Carefully observe condition ofgraphite. Use alternate etch and repolish ifnecessary. Clean sample thoroughly.

Etch sample lightly. Precondition synthetic velvetcloth with 0.25 micron diamond paste, use watersoluble extender. Repeat alternate etch andrepolish as necessary. Polarized light will clearlyreveal the condition of the graphite as it isanisotropic. Staining can occur during etching asgraphite can absorb Nital.

Water should not be used as a lubricant at anystage due to staining effect or corrosion effectof the coating. Kerosene or lapping oil are goodalternates.

Silk cloth as lap covering, 0.3 micron AlphaAlumina as abrasive, and filtered kerosene aslubricant.

Synthetic velvet as lap covering, 0.05 micronGamma Alumina as abrasive, and a mixture ofalcohol and glycerine as lubricant.

Superior results can often be obtained bysequencing through 9 micron and 3 microndiamond paste with nylon cloth and lapping oilas lubricant.

Sequencing through 0.3 micron Alpha Aluminaand 0.05 Gamma Alumina, Lecloth , anddistilled or deionized water as lubricant.

Check the possibilities of Electropolishing,particularly with solid solution alloys andtransformed structures.

Check the possibilities of Slurry (Etch-Attack)Polishing, particularly with wrought heat-resisting alloys.

2 3

2 3

Graphitic Cast Irons

Galvanized Coatings

Fine Grinding

Rough Polishing

Final Polishing

Rough Polishing

Final Polishing

Rough Polishing

Final Polishing

Stainless Steels, Stainless Steel CastingAlloys, Heat-Resisting Alloys

Polishing Procedures for Copper-BasedMaterials

Rough Polishing

Abrasive & Size Wheel Covering Lubricant

Diamond Paste Nylon Cloth Lapping(6 m) Oil

Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 m) Deionized

Water

Polishing Procedures for FerrousMaterials

Fine Grinding

SiC 180 grit Paper or Disc (waterproof) WaterSiC 320 grit Paper or Disc (waterproof) WaterSiC 600 grit Paper or Disc (waterproof) Water

Abrasive & Size Lap or Wheel Covering Lubricant

Fine Grinding




Remarks

RemarksRemarks

Al O -Coated Products (waterproof)Sameabrasive sequence and lubricant may be used.Abrasive action is less severe on somematerials.

Check the possibilities of Electropolishing,particularly with microstructure amenable tosame.

Electropolishing may also be used to distinctadvantages with many materials that may havebeen processed through the rough and finalpolishing stages by mechanical methods.Response to extremely short cycles is manytimes advantageous even with those structurescontaining finely dispersed intermetalliccompounds and complex phase relationships.

Check the possibilities of Slurry (etch-attack)polishing as a means to remove deformation orto process multi-material sections.


The conditioning of the wheel covering differsfrom standard procedures. The entire surfaceshould be pre-moistened with distilled ordeionized water. The MgO powder is dispensedin the center of the wheel, moistened, andworked into a heavy, creamy consistency.

The sample is skidded over the surface and theabrasive is moved outward. The sample edgesare slightly beveled to aid hand manipulation.

Light pressure must be used as many opticalidentifications of intermetallic compounds aredependent on standardized oxide film colors.These colors are not reproducible under heavypressure.

Al O -Coated Products (waterproof)Sameabrasive sequence and lubricant may be used.Abrasive action is less severe on some materials.

2 3

2 3

2 3

Suggested Deviations

Polishing Procedures for Aluminum andMagnesium-Based Materials

Polishing Procedures for Titanium,Zirconium, Hafnium, and Alloys

Aluminum polishing wheels are recommendedto eliminate electrochemical reaction betweenthe sample and wheel. A thin insulating plasticmaterial or aluminum foil between the bronzewheel and wheel covering would accomplishthe same.

Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 m) (flocked cotton sateen) Deionized

Water

Rough Polishing



Fine Grinding



Rough Polishing



Final Polishing


Magnesium Lecloth Distilled orOxide (flocked cotton sateen) Deionized

(2.0 m) Water

Fine Grinding



CLECO Corporation www.leco.com 21

This family of materials is extremely susceptibleto surface deformation.

The following etchant also functions as achemical polish.

60 cc Glycerine

20 cc Nitric Acid

20 cc Hydrofluoric Acid (48%)

Swab vigorously with saturated cotton.Reaction is very active at the outset, butdiminishes as deformation is removed.Staining effects on various phases are timedependent. Reaction rate may be varied byheating (increase) or chilling (decrease) thesample or etchant.

Check the possibilities of Electropolishing,Slurry (Etch-Attack) Polishing, or ChemicalPolishing.

For gross surface removal, employ a 74 m(220 mesh) resin-bonded diamond disc todecrease wear on 40 m (280 mesh) disc.

If extremely fine scratches are visible in thebinder material, a short cycle on Lecloth withGamma Alumina (0.05 micron) will removethe same.


There may be some advantage to extend roughpolishing into two steps by incorporating a 0.5micron diamond with Lecloth step. If lappingoil should attack any microconstituents, alcoholor ethylene-glycol may be substituted.

Composition

Techniques

Remarks

Remarks

Caution: Avoidcontact with skin!

Protect yourhands.

Etchant must be fresh each time,stability decreases in a few hours.Caution:


Polishing Procedures for CementedCarbides

Polishing Procedures for Lead Alloys,Tin Alloys, and Zinc-Based Die Castings

Very often the 3 micron diamond paste stepmay be omitted.

2 3

Rough Polishing



Final Polishing


Alpha Alumina Lecloth Distilled or(0.3 m) (flocked cotton Deionized

sateen) Water

Fine Grinding


Diamond Resin-Bonded Water(40 m; 280 mesh) Diamond Disc

Diamond Resin-Bonded Water(15 m) Diamond Disc

Rough Polishing


Diamond Paste Pan W Lapping(6 m) Oil

Diamond Paste Pan W Lapping(3 m) Oil

Final Polishing


Diamond Paste Pan W Lapping(0.1 m) Oil

Fine Grinding



Rough Polishing



Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 m) (flocked cotton Deionized

sateen) Water

The lead alloys lend themselves to mechanicalpreparation rather than electrolytic polishingsince many lead alloys undergo Eutecticformation during solidification. Very oftenEutectic structures will show supercoolingtendencies and instability in the solid solutionzones.

Both tin and lead alloys are inherently soft andvery susceptible to gross surface flow andac company ing de fo rma t i on du r i ngpreparation. Careful etching and repolishingwill remove the disturbed metal.

One should be careful to observe the meltingpoint of the material being prepared and selectmounting methods accordingly.

Extreme caution should be exercised in allpreceding stages to avoid, or at leastminimize, surface deformation.

In the early portion of rough polishing morescratches seem to appear than are beingremoved. The scratches from fine grinding arebeing "opened up". Extending the polishingtime will remove these effects.

Check the possibility of Electropolishing.

Check the possibility of Slurry (Etch-Attack)Polishing.


Edge ProtectionSuggestions for post platingand fluid bedding are described in theMounting Procedures section.

EtchingThe interfaces can be clearlydelineated by etching. Specific details are givenin Table 4 under Microetching.


Polishing Procedures for RefractoryAlloys and Metals (Nb, Mo, W, V, Ta)

Polishing Procedures for Plated Sections

Polishing Procedures for PowderMetals and Alloys

Remarks


Remarks

Remarks

2 3

2 3

During preparation, softer electrodeposits maytend to flow and the interfaces between thevarious layers will not be clearly delineated.

Fine Grinding



Rough Polishing



Fine Grinding



Rough Polishing



Final Polishing


Gamma Alumina Lecloth Distilled or(flocked cotton Deionized

(0.05 m)sateen) Water

Fine Grinding




Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 m) Deionized

Water

CGeneral Information

Remarks

Cleaning

Impregnation

Porosity is generally associated with powder metalsections. The porous area can become filled withforeign products during sectioning andpreparation.

The samples should be very thoroughly cleaned inseveral stages with an ultrasonic cleaner.

Porous sections should be either impregnatedwith a high-temperature wax (350 F) or vacuumimpregnated with epoxy. Such practice willprevent contamination during preparation andalso improve consolidation for mounting andpreparation.

For gross surface removal, employ a 63-74micron (220 Mesh) resin bonded diamond discto decrease wear on 40 micron (80 Mesh) disc.

With some materials the 9 micron diamondpaste step may be omitted. If anymicroconstituents or the mounting media arestained or attacked by an oil extender, ethylene-glycol or alcohol may be used as a lubricant.

Polishing times should be as short as possible toavoid relief polishing.

O

Procedures for both cleaning and impregnationare detailed in the Mounting Procedures section.

Polishing Procedures for Ceramics

Rough Polishing



Final Polishing


Gamma Alumina Lecloth Distilled or(flocked cotton Deionized

(0.05 m)sateen) Water

Rough Polishing


Diamond Paste Pan W Lapping


(9 m) Oil

(6 m) Oil

Fine Grinding


Diamond Resin-Bonded Water

Diamond Resin-Bonded Water

(40 m; 280 Mesh) Diamond Disc

(15 m) Diamond Disc


Final Polishing



Alpha Alumina Silk DistilledDeionized

Water

(3 m) Oil

(0.3 m)

DLECO Corporation www.leco.com 25

Microetching Page

Metal Progress Data Book Page

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Standard Methods for Microetching Metals and Alloys

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Summary of Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Safety Precautions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Miscellaneous Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table IEtching Reagents for Irons and Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28-31

General Reagents for Irons and Steels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

General Reagents for Alloy Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Electrolytes for Polishing and Etching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table II . . . . . . . . . . . . . . . . . 30

. . . . . . . . . . . . . . . . . . . . . . . . . 27-28

Etching Reagents for Aluminum and Aluminum Alloys

Etching Reagents for Copper and Copper Alloys

Etching Reagents for Titanium and Titanium Alloys

Table III . . . . . . . . . . . . . . . . . . . . . 30

Table IV . . . . . . . . . . . . . . . . . . . 31

DMicroetchingDefinition

Standard Method for MicroetchingMetals and Alloys

3. Safety Precautions

A metallographic specimen in the "as-polishedand unetched" state will reveal inclusions,porosity, cracks, intergranular corrosion,surface conditions, etc.Chemical Etching is defined as the process toreveal structural details by preferential attack ofa metal surface with an acid or basic chemicalsolution.The most commonly used etching technique istermed Solution Etching. This may be furtherclassified into two categories.

Acid and Basic Reagents- Immersion- Swabbing

Electrolytic- Direct Current- Alternating Current

For immersion etching, the sample is held withtongs and immersed in a suitable etchingsolution. The specimen is gently agitated toeliminate adherent air bubbles and tocontinually supply fresh reagent to the surface.Swab etching implies the surface is gently wipedwith a soft cotton swab saturated with etchant.The swab should be replenished with freshreagent if etching times are comparatively long.With electrolytic etching, direct currentelectrolysis is usually used. The specimen ismade the anode and a suitable insolublematerial is the cathode. For a few materials(platinum, palladium, and their alloys),alternating current electrolysis is used.

1.1 These methods cover chemical solutionsand procedures to be used in etchingmetals and alloys for microscopicalexamination. Safety precautions andmiscellaneous information are alsoincluded.

2.1 Tables 1 through 4 contain etchantinformation for irons and steels,aluminum, copper, and titanium. Theuses column in each table providesspecific alloy/etchant information

3.1 Special safety precautions are mentioned inTable 2 only for extremely hazardous mixturessuch as the cyanides and those that can formnitrogen dioxide gas. However, ALLC H E M I C A L S A R E P O T E N T I A L L YDANGEROUS, and it is assumed that theperson using any of the etchants is thoroughlyfamiliar with all of the chemicals involved,and the proper procedures for handling andmixing these chemicals.

3.2 Some basic suggestions for the handling ofetching chemicals are as follows.3.2.1 When pouring, mixing, or etching,

always use the proper protective garb(glasses, gloves, apron, etc.).

3.2.2 Use proper devices (glass or plastic)for weighing, mixing, containing, andstorage of solutions.

3.2.3 Wipe or flush any and all spills, nomatter how minute in nature.

3.2.4 Dispose of any and all solutions thatare not properly identified bycomposition and concentration (whenin doubt, throw it out).

3.2.5 Store and handle chemicalsaccording to the manufacturer'srecommendations. Observe printedcautions on reagent bottles.

3.2.6 If not sure about a chemical or itsproper use, contact your Chemical orSafety Department.

3.2.7 Have available and use quickreferences as to toxicity and workingprecautions of various chemicals.

1. Scope

2. Summary of Methods


Picral 4 g picric acid, 100 ml ethylalcohol.

Not as good as nital for revealingferrite grain boundaries. Givessuperior resolution with finepearlite, martensite, temperedmar t en s i t e , and ba in i t i cstructures. Detects carbides.Etching time: a few seconds to 1minute or more.

For all grades of carbon steels:a n n e a l e d , n o r m a l i z e d ,quenched, quenched andt e m p e r e d , s p h e r i o d i z e d ,austempered.

Vilella'sreagent

5 ml HCl, 1 g picric acid, 100 mlethyl alcohol.

Best results obtained whenmartensite is tempered.

For revealing austenitic grain sizein quenched, and quenched andtempered steels.


Table IEtching Reagents for Irons & SteelsGeneral Reagents for Irons and Steels (Carbon, Low, and Medium-Alloy Steels)Etching Reagent Composition Remarks Uses

Nital 2 ml HNO , 98 ml ethyl alcohol. Not as good as picral for high-resolution work with heat-treatedstructures. Excellent for outliningferrite grain boundaries. Etchingtime: a few seconds to 1 minute.

For carbon steels; gives maximumcontrast between pearlite and aferrite or cementite network;reveals ferrite boundaries;di f ferent iates ferr i te frommartensite.

3

Sodiummetabisulfite

Solution A: 8 g Na S O , 100 mldistilled water.

General reagent for steel. Resultssimilar to picral. Etching time: afew seconds to 1 minutes.

D a r k e n s a s - q u e n c h e dmartensite.

Tint etches lath-type or plate-typemartensite in Fe-C alloys.

2 2 5

Solution B: 1 g Na S O , dilute to100 ml with distilled water.

Immerse specimen in the solutionfor 2 minutes or until the polishedsurface turns a bluish-red; do notmount specimen in a steel clamp.

2 2 5

Metal Progress Data Book

4. Miscellaneous Information4.1 Chemicals used should meet USP and NF

specifications or better.4.2 When mixing etchants, always add

reagents to the solvent unless specificinstructions indicate otherwise.

4.3 Where water is given as the solvent, distilledwater is preferred because of the greatvariance of the purity of tap water.

4.4 Etching should be carried out on a freshlypolished specimen.

4.5 Gentle agitation of the specimen or solutionduring etching will result in a more uniformetch.

4.6 The etching times given are only suggestedstarting ranges and not absolute limits.

4.7 In electrolytic etching, DC current is impliedunless indicated otherwise.

4.8 A good economical source of DC current forsmall-scale electrolytic etching is the

standard 6V lantern battery.4.9 In electrolytic etching, the specimen is the

anode unless indicated otherwise.Do not overlook the possibility of multipleetching; that is, etching with more than onesolution in order to fully develop thestructure of the specimen.Microscope objectives can be ruined byexposure to hydrofluoric acid fumes frometchant residue inadvertently left on thespecimen. This problem is very commonwhen the specimen or mounting mediacontain porosity, and when the mountingmaterial (such as bakelite) does not bondtightly to the specimen resulting in seepagealong the edges of the sample. In all cases,extreme care should be taken to remove alltraces of the etchant by thorough washingand complete drying of the specimenbefore placing it on a microscope stage.

4.10

4.11

Marble'sreagent

4 g CuSO , 20 ml HCl, 20 mldistilled water.

Immerse to reveal structure. Structure of stainless steels.4

Vilella'sreagent

5 ml HCl, 1 ml picric acid, 100 mlethyl alcohol.

Immerse to reveal structure. Can etch numerous types of Fe-Cr, Fe-Cr-Ni, and Fe-Cr-Mnsteels. Also attacks the grainboundaries in Cr-Ni austeniticsteels.

Nitric acid 5 to 10 ml HNO , 100 ethylalcohol.

. HNO and ethylalcohol are a dangerous mixtureabove 5% HNO .

General structure of high-speedtool steels.

3 3

3

Use Hood

Hydrochloricacid in alcohol

50 ml HCl, 50 ml ethyl alcohol. More gradual etching can beobtained with less concentratedsolutions (10 to 20%).

Suitable for etching steelscontaining chromium and nickel.

Hydrochloric andnitric acids

10 ml HCl, 3 ml HNO , 100 mlmethyl alcohol.

Etching time: 2 to 10 minutes. To reveal the grain size ofquenched, or quenched andtempered high-speed steel.

3

Chromic acid 10 g CrO , 10 ml H O. Specimen is used as anode;stainless steel or platinum ascathode, 3/4 to 1 inch apart; 6 Vusually used; etching time: 30 to90 seconds.

For various structures except thegrain boundaries of ferrite.Attacks cementite very rapidly,austenite less rapidly, ferrite andiron phosphide very slowly if at all.

3 2

Nitric acidin water

50 ml HNO , 50 ml H O. Room temperature; stainlesssteel cathode; 1.5 V for 2 minutesor more. .

For austenitic or ferritic stainlesssteels; reveals grain boundaries.

3 2

Use Hood

Electrolytes for Polishing and Etching

Hydrochloric acidin alcohol

10 ml HCl, 90 ml anhydrous ethylalcohol.

10 to 30 seconds at 6V. Reveals delta ferrite, and thegeneral structure of chromiumand Cr-Ni steels.

D

Table IcontinuedEtching Reagent Composition Remarks Uses

Klemm'sreagent

50 ml saturated (in H O) Na S Osolution, 1 g K S O .

Etching time: 40 to 120 seconds.Ferrite appears black-brown;while carbides, nitrides, andphosphides remain white. Also,phosphorus distribution can bedetected more sensitively thanwith usual phosphorus reagentsbased on copper salts.

Tint etches pearlite, hardenedstructures of unalloyed steel, andcast iron.

2 2 2 3

2 2 2

Ferricchloride andhydrochloric acid

5 g FeCl , 50 ml HCl, 100 mldistilled water.

Immerse until structure isrevealed.

Reveals structure of austeniticnickel and stainless steels.

3

Kallings 5 g CuCl , 100 ml HCl, 100 mlethyl alcohol, 100 ml distilledwater.

Use cold. For austenitic and ferritic steels;the ferrite being most easilyattacked (carbides and austeniteare not attacked).

2

General Reagents for Alloy Steels (High Alloy, Stainless, and Tool Steels)


Glyceregia Solution A: 10 ml HNO , 20 mlHCl, 30 ml glycerin.

Mix HCl and glycerin thoroughlybefore adding HNO . Beforeetching, heat specimen in hotwater. Best results are obtainedwith alternate polishing andetching.

Etches structures of Fe-Cr alloys,high-speed steels, austeniticsteels, and manganese steels. Foraustenitic alloys.

Reveals the structures of Cr-Niand Cr-Mn steels, and of all Fe-Craustenitic alloys.

3

3

Solution B: 10 ml HNO , 20 mlHCl, 20 ml glycerin, 10 ml H O ..

. Do not store. Actioncan be modified by varying theproportion of HCl.

3

2 2

Use Hood

Table IcontinuedEtching Reagent Composition Remarks Uses

Ammoniumpersulfate inwater

10 g (NH ) S O , 100 ml H O. Use fresh, 6 V for more than 15seconds.

Surface attack occurs on theferrite grains in low-carbonsteels. Reveals the fine structuresof nickel austenitic steels, and oftransformer sheet.

4 2 2 8 2

Sodiumhydroxide inwater

40 g NaOH, 100 ml H O (addslowly).

60 seconds at 1 to 3V. Highlycaustic solution.

Reveals the sigma phase. Colorssuccessively sigma phase, ferrite,and lastly carbides after a longeretching time.

2

Potassiumhydroxide inwater

56 g KOH, 100 ml H O (addslowly).

60 seconds at 1 to 3V. Highlycaustic solution.

Same as above, but sigma phasea n d f e r r i t e a r e r e v e a lsimultaneously.

2

Mixed acidsin alcohol

45 ml lactic acid, 10 ml HCl, 45 mlethyl alcohol.

10 to 30 seconds at 6V. For chromium steels (4 to 30%Cr), or for delta ferrite inaustenitic stainless steels.

Oxalic acidin water

10 ml oxalic acid, 100 ml H O. 5 to 20 seconds at 6V, using aplatinum or stainless steelc a t h o d e . G a p b e t w e e nelectrodes, 3/4 to 1 inch.

For austenitic stainless steels andhigh-nickel alloys. Distinguishesbetween sigma phase andcarbides. Sigma phase is attackedfirst, then carbides; ferrite andaustenite can be attacked slightly.To investigate the carbides,operate at 1.5 to 3 V for a longertime.

2

Electrolytes for Polishing and Etching

Sulfuric acidin water

5 ml H SO , 95 ml H O. Room temperature; stainless steelcathode; 6 V (0.1 to 0.5 amp), 5 to15 seconds. .

For Fe-Cr-Ni alloys.2 4 2

Use Hood


Table IIEtching Reagents for Aluminum and Aluminum AlloysEtching Reagent Composition Remarks Uses

Table IIIEtching Reagents for Copper and Copper AlloysEtching Reagent Composition Remarks Uses

Keller's 2 ml HF, 3 ml HCl, 5 ml HNO ,190 ml H O.

3

2

Immerse 10 to 20 seconds.Rinse in stream of water.

Reveals general structure.

Hatch 1 g NaOH, 100 ml H O.2 Swab 10 seconds. Reveals general structure.

Immerse 15 minutes and rinsefor 10 minutes to form film withcross hatching.

Cross hatching varies with grainorientation.

Barker's 2 to 4% HydrofluorboricAcid in 200 ml H O.2

Electrolytic; 1 Amp for 2 to 3minutes. View under polarizedlight.

Reveals grain structure.

Macro/Weld Etch 1 part HCl1 part HF1 part H O.2

Rinse; immerse in 15% NaOHsolution.

For welds and macro exams.

For Pure Copper 50 ml NH OH, 5 ml H O .4 2 2 Immerse; dilute with wateras needed.

General structure

For Copper/ZincAlloys (Brass)

50 ml NH OH, 5 ml H O ,50 ml H O.

4 2 2

2

Immerse. General structure

For Copper/TinAlloys (Bronze)

1 g NaCl, 2 ml chromic acid (20%),2 ml H SO , 95 ml H O.2 4 2

Immerse. General structure

DLECO Corporation www.leco.com 31

Table IIIcontinuedEtching Reagent Composition Remarks Uses

Table IVEtching Reagents for Titanium and Titanium AlloysEtching Reagent Composition Remarks Uses

For Copper/Beryllium Alloys

5 g Ferric Chloride, 10 ml HCl,95 ml ethyl alcohol.

Immerse. General structure.

Kroll's 6 ml HNO , 2 ml HF,92 ml H O.

3

2

Swab for 2 to 30 seconds. General structure.

Alternative toKroll's

1-3 ml HF, 2-6 HNO100 ml H2O.

3 Immerse or swab 3 to 30seconds.

Adjust mixture to balanceetching with brightness.

Alpha Case Etch 2 ml HF100 ml H O.2

Immerse or swab 2 to 60seconds.

Reveals alpha case in alloys.

EMacroetching PagesDefinition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Standard Methods for Macroetching Metals and Alloys

Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35General Directions and Uses of Macroetching

Applications of Macroetching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Sampling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Specific Preparation Procedures and Recommended SolutionsAluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Beryllium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Cobalt and Cobalt Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Copper and Copper Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Iron and Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Stainless Steel and High Temperature Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 38Lead and Lead Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Magnesium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Nickel and Nickel Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Noble Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Refractory Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Tin and Tin Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Titanium, Zirconium, Hafnium, and their Alloys . . . . . . . . . . . . . . . . . . . . . . . . 39Zinc and Zinc Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Table IMacroetchants for Aluminum and Aluminum Alloys . . . . . . . . . . . . . . . . . . . 39-40Table IIMacroetchants for Beryllium and Beryllium Alloys. . . . . . . . . . . . . . . . . . . . . 40Table IIIMacroetchants for Cobalt and Cobalt Alloys . . . . . . . . . . . . . . . . . . . . . . . . 40Table IVMacroetchants for Copper and Copper Alloys . . . . . . . . . . . . . . . . . . . . . . . 40-41Table VMacroetchants for Irons and Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41-42Table VIMacroetchants for Stainless Steels and High Temperature Alloys . . . . . . . . . 42Table VIIMacroetchants for Lead and Lead Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 42-43Table VIIIMacroetchants for Magnesium and Magnesium Alloys . . . . . . . . . . . . . . . 43-44Table IXMacroetchants for Nickel and Nickel Alloys. . . . . . . . . . . . . . . . . . . . . . . . . 44Table XMacroetchants for Noble Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Table XIMacroetchants for Refractory Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44-45Table XIIMacroetchants for Tin and Tin Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Table XIIIMacroetchants for Titanium, Zirconium, Hafnium, and their Alloys. . . . . . . 45Table XIVMacroetchants for Zinc and Zinc Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

. . . . . . . . . . . . . . . . . . . . . . . . 35-39

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