Metallographic Principles.pdf

Metallography Principlesand Procedures



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Metallographic Sample Preparation

Mounting Procedures

Specific Grinding and Polishing Procedures

Microetching/Metal Progress Data Book

Macroetching

Table of Contents

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B

C

D

E

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Metallographic Sample Preparation

Stages of Preparation

Page

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

Stage1—Sectioning..............................................................................................5-6

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

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

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

Stage 6—Final 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

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LECO Corporation www.leco.com 1

Metallography Principles and Procedures

Metallographic SamplePreparation

Introduction

Requisites

Stages of Preparation (Definitions)

Metallography—the study of the microstructureof metals using various techniques—has 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 1—Sectioning

Stage 2—Coarse Grinding

Stage 3—Mounting

Stage 4—Fine Grinding/Pre-Polishing

Stage 5—Rough Polishing

Stage 6—Final 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 MECHANICALPREPARATION—hand 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

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Pressure

Removal Rate

There are two factors which contribute to pressureapplied against a sample—the 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 a

sharp 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 with

pressure 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 I—Grit 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

Size

Coa

rse

Grin

ding

Fine

Gri

ndin

g

Rou

ghPo

lishi

ng

Fin

alP

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 B—Polishing SuspensionsType Particle Size Application

(µm)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 II—EmeryGrit No. Particle Size (AV. µm)

3 852 701 50

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

Table IV A—Polishing PowdersType Particle Size Application

(µm)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 III—Diamond PastesMicron Size* Size Range Mesh Equivalent

(µm) (µm) (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

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Stages of Preparation

Stage 1—SectioningSectioning 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-metallics—soft, 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 µdiamond 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 2—Coarse Grinding

Stage 3—Mounting (See Section B)

Stage 4—Fine 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,

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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 5—Rough 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 available—levigated 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,

Stage 6—Final Polishing

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are the high temperature alpha structure.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 this

operation. Unfortunately, the fibers arecompressible, 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

A

Solution Levelin Beaker

Solution Level

PositioningCover

Glass Beaker


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 bevery 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

B

Mounting Procedures Page

Specimen 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

B


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 V—Molding 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 of

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

B


exotherm.

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 or

remove 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 due

Entrapped Air:

Discoloring:

Soft Mounts:

Cracking:

Discoloration:

Soft Mounts:

Entrapped Air:

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


to exceeding the boiling point of the mixture.

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 shown

in 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 are

used prior to mounting or in mounting mediaselection edge, rounding will occur at the sample-mount interface, see Figure 20.

The degree of rounding isdependent upon thehardness and abrasiondifferential between thes p e c i m e n a n d t h emounting 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.

Curing

S t e e l S h o t :

Electro or ElectrolessDeposition:

Glass-Filled MountingMedia:

O

Edge Protection

B

SupportStrip

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.

C

Specific 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

C


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 µm) Oil

Final Polishing


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

Water

®

Rough Polishing



Fine Grinding



Rough Polishing



Final Polishing


Magnesium Lecloth Distilled orOxide (flocked cotton sateen) Deionized

(2.0 µm) Water

®

Fine Grinding



C


• 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 µm(220 mesh) resin-bonded diamond disc todecrease wear on 40 µm (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 µm) (flocked cotton Deionized

sateen) Water

®

Fine Grinding


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

Diamond Resin-Bonded Water(15 µm) Diamond Disc

Rough Polishing


Diamond Paste PEFA Lapping(6 µm) Oil

Diamond Paste PEFA Lapping(3 µm) Oil

Final Polishing


Diamond Paste PEFA Lapping(0.1 µm) Oil

Fine Grinding



Rough Polishing



Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 µm) (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 Protection—Suggestions for post platingand fluid bedding are described in theMounting Procedures section.

• Etching—The 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 µm)sateen) Water

Fine Grinding




Final Polishing


Gamma Alumina Lecloth Distilled or(0.05 µm) Deionized

Water

®

C

General 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 µm)sateen) Water

Rough Polishing


Diamond Paste PEFA Lapping


(9 µm) Oil

(6 µm) Oil

Fine Grinding


Diamond Resin-Bonded Water

Diamond Resin-Bonded Water

(40 µm; 280 Mesh) Diamond Disc

(15 µm) Diamond Disc


Final Polishing



Alpha Alumina Silk DistilledDeionized

Water

(3 µm) Oil

(0.3 µm)

D


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 I—Etching 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

D

MicroetchingDefinition

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. The“uses” 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.Some of the many excellentreferences are (1) "Handbook ofDangerous Materials" by N. Irving Sax,(2) "The Chemistry of IndustrialToxicology" by H.B. Elkins, and (3)"Safety in the MetallographyLaboratory" by R.L. Anderson,Scientific Paper 65-1P30-METLL-P2,West inghouse Research Lab.,Pittsburgh, PA.

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 I—Etching 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 thestandard 6V lantern battery.

4.9 In electrolytic etching, the specimen is theanode 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 I—continuedEtching 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

Kalling’s 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 I—continuedEtching 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 II—Etching Reagents for Aluminum and Aluminum AlloysEtching Reagent Composition Remarks Uses

Table III—Etching 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

D


Table III—continuedEtching Reagent Composition Remarks Uses

Table IV—Etching 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.

E

Macroetching 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 I—Macroetchants for Aluminum and Aluminum Alloys . . . . . . . . . . . . . . . . . . . 39-40Table II—Macroetchants for Beryllium and Beryllium Alloys. . . . . . . . . . . . . . . . . . . . . 40Table III—Macroetchants for Cobalt and Cobalt Alloys . . . . . . . . . . . . . . . . . . . . . . . . 40Table IV—Macroetchants for Copper and Copper Alloys . . . . . . . . . . . . . . . . . . . . . . . 40-41Table V—Macroetchants for Irons and Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41-42Table VI—Macroetchants for Stainless Steels and High Temperature Alloys . . . . . . . . . 42Table VII—Macroetchants for Lead and Lead Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 42-43Table VIII—Macroetchants for Magnesium and Magnesium Alloys . . . . . . . . . . . . . . . 43-44Table IX—Macroetchants for Nickel and Nickel Alloys. . . . . . . . . . . . . . . . . . . . . . . . . 44Table X—Macroetchants for Noble Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Table XI—Macroetchants for Refractory Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44-45Table XII—Macroetchants for Tin and Tin Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Table XIII—Macroetchants for Titanium, Zirconium, Hafnium, and their Alloys. . . . . . . 45Table XIV—Macroetchants for Zinc and Zinc Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

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

(Ag, Au, Ru, Rh, Pd, Os, Ir, Pt)(Cr, Mo, W, V, Cb, Ta)


E

MacroetchingDefinitionMacroetching is the procedure used to reveal the quality ofa material by subjecting a gross sample to the corrosiveaction of an etchant.Examinations are limited to visual observation, ormagnification not exceeding ten diameters.

1.1 These procedures describe the methods ofmacroetching metals and alloys to reveal theirmacrostructure.

2.1.1 Macroetching is used to reveal thehe te rogene i t y o f meta l s and a l loys .Metallographic specimens and chemical analyseswill provide the necessary detailed informationabout specific localities, but they cannot give dataabout variation from one place to another unlessan inordinate number of specimens are taken.

2.1.2 Macroetching, on the other hand, willprovide information on variations in (1) structure,such as grain size, flow lines, columnar structure,dendrites, etc; (2) variations in chemicalcomposition as evidenced by segregation, carbideand ferrite banding, coring, inclusions, and depthof carburization or decarburization. Theinformation provided about variations in chemicalcomposition is strictly qualitative, but the locationof extremes in segregation will be shown.Chemical analyses or other means of determiningthe chemical composition would have to beperformed to determine the extent of variation.Macroetching will also show the presence ofdiscontinuities and voids, such as seams, laps,porosity, flakes, bursts, extrusion rupture, cracks,etc.

2.1.3 Other applications of macroetching in thefabrication of metals are the study of weldstructure, definition of weld penetration, dilutionof filler metal by base metals, entrapment of flux,porosity, and cracks in weld and heat-affectedzones, etc. It is also used in the heat treating shopto determine location of hard or soft spots, tongmarks, quenching cracks, case depth in shallowhardening steels, case depth in carburization ofdies, effectiveness of stop-off coatings incarburization, etc. In the machine shop, it can beused for the determination of grinding cracks intools and dies.

2.1.4 Macroetching is used extensively forquality control in the steel industry to determinethe "tone" of a heat in billets with respect toinclusions, segregation, and structure. Forgeshops, in addition, use macroetching to revealflow lines in setting up the best forging practice,die design, and metal flow. For an example of theuse of macroetching in the steel forging industry,see ASTM Method A 317,

. Forging shops andfoundries also use macroetching to determine thepresence of internal faults and surface defects. Thecopper industry uses macroetching for control ofsurface porosity in wire bar. In the aluminumindustry, macroetching is used to evaluateextrusions, as well as other products such asforgings, sheets, etc. Defects such as coring, cracks,and port-hole die welds are identified.

2.2.1 As in any method of examination,sampling is very important. When macroetching isused to solve a problem, the problem itself largelydictates the source of the sample as to the locationon the work piece, and the stage of manufacture.For example, when looking for pipe, the sampleshould represent the top of the ingot, or whenlooking for bursts or flakes, the sample should betaken as soon after hot working as possible.

2.2.1.2 When macroetching is used as aninspection procedure, sampling ought to be donein an early stage of manufacturing so that if thematerial proves faulty, no wasteful unnecessarywork is done. However, the sample should not betaken so early that further working can introduceserious defects. In the steel industry, for example,the sample is usually taken after ingot breakdown,and after most chances of bursts or flakesoccurring have passed. Billets or blooms goinginto small sizes are sampled after initialbreakdown. Material going into forging billets ordie blocks is sampled near finish size. Samplingmay be done systematically or on a random basis.

2.2.2 Samples may be cold cut from the sourceby any convenient fashion; saws and abrasivecutoff wheels are particularly effective. The use oftorch cutting or hot cutting should be used onlywhen necessary to cut a sample from a largepiece. The sample is then sectioned well awayfrom the hot-cut surface. An example ofpermissible use of torch cutting is the excising of apiece from a large plate, and then cutting asample for macroetching 4 to 5 inches away fromthe torch-cut edge.

2.2.3 Some common methods of sampling,listed by source are as follows.

2.2.3.1 Bi l lets , B looms, and Hot-Rol ledProducts—Disks are usually cut from theseproducts near the end. Samples cut too close to theend, however, may have false structures becauseof fish-tailing. Disks from large blooms aresometimes cut into smaller pieces for ease inhandling.

2.2.3.2 Forgings and Extrusions—Disks cuttransverse to the long dimension will show flakes,bursts, etc. Forgings may also be cut parallel to thelong dimension to show flow lines. In complicatedforgings, some thought will have to be given to theproper method of cutting so as to show flow lines.Macroetching of an unprepared specimen willshow surface defects such as shuts, flats, seams,etc. In extrusions, coring and coarse grain aremore commonly found in the back end of theextrusion.

Standard Method for Macroetching Metalsand Alloys1. Scope

2. General Directions and Uses of Macroetching2.1 Applications of Macroetching

2.2 Sampling

Macroetch Testing and

Inspection of Steel Forgings


2.2.3.3 Sheets and Plates—A sufficiently largesample should be taken when looking for surfacedefects. An ideal length would be the circumferenceof the last roll, but this may be inconveniently long.Several samples totaling some given fraction of thecircumference can be used; however, there is alwaysa chance that a defect arising from faulty rolls wouldnot be detected. When seeking information onlaminations, a transverse section is used. In manycases, however, to reduce the size of the specimen,only a section out of the center of the plate may betaken.2.2.3.4 Weldments—A disk cut perpendicular to thedirection of welding will show weld penetration, heataffected zone, structure, etc. Careful preparation isusually rewarded with highly detailed structure givinga large amount of information. Welds involvingdissimilar metals will produce problems in etching.The best method is to etch the least corrosion-resistant portion first, and the more resistant portionafterward. Occasionally an intermediary etchant maybe required. The boundaries between etched andunetched portions will give an idea of weldpenetration and dilution.2.2.3.5 Castings—Cut the specimen to display thedefect or feature being sought.2.2.3.6 Machined and Ground Parts—When lookingfor grinding cracks, etc., the surface itself is used as asample. Because the machined or ground part isoften the finished part, it may be undesirable toimmerse the part in acid. In this case, other methodssuch as dye penetrant methods may be moredesirable.

2.3.1 Sample preparation need not be elaborate.Any method of presenting a smooth surface with aminimum amount of cold work will be satisfactory.Disks may be faced on a lathe or a shaper. The usualprocedure is to take a roughing cut, then a finishedcut. This will generate a smooth surface and removecold work from prior operations. Sharp tools arenecessary to produce a good specimen. Grinding isusually conducted in the same manner, using free-cutting wheels and light finished cuts. When finedetail is required, the specimen should be grounddown through the series of metallographic papers.Where necessary, details are given in the tabulationof procedures.2.3.2 After surface preparation, the sample iscleaned carefully with suitable solvents. Any grease,oil, or other residue will produce uneven attack. Oncecleaned, care should be taken not to touch thesample surface or contaminate it in any way.

2.4.1 The solutions used for macroetching aregiven in the tables listed under each alloy. In mostcases a good grade of reagent should be used, butneed not be chemically pure or of analytical quality.The so-called technical grades are usuallysatisfactory. The solution should be clean and clear,free of suspended particles, scum, etc.2.4.2 Caution must be observed in mixing. Manyof the etchants are strong acids. In all cases, thevarious chemicals should be added slowly to thewater or solvent while stirring. In the cases where

hydrofluoric acid is used, the solution should bemixed and used in polyethylene vessels.Note— Hydrofluoric acid should not beallowed to contact the skin since it can cause painful,serious ulcers if not washed off immediately.

2.5.1 Many of the solutions are aggressive andmay give off irritating and corrosive fumes. Etchingshould be done in a well-ventilated room, preferablyunder a fume hood. The solution should be mixed andplaced in a corrosion-resistant tray or dish andbrought to the operating temperature. The specimenor specimens should be placed in a tray of stainlesssteel screen or on some non-reactive support.Glass rods are often placed on the bottom of the acidcontainer and the specimens laid directly on the rods.When etching is completed, remove the specimensfrom the dish, taking great care not to touch theetched surface. When desmutting is required, dip thespecimen into a second solution. After rinsing thespecimen with hot water, blow dry with clean,compressed air.2.5.2 In the case of large specimens, such as ingotsections, swabbing may be the only practical methodof macroetching. Saturate a large wad of cotton (heldin stainless steel or nickel tongs), with the etchant andsweep over the surface of the specimen. An effortshould be made to wet the entire surface as soon aspossible. After the initial wetting, keep the swabsaturated with solution and frequently sweep over thesurface of the specimen to renew the solution. Whenthe structure has been suitably developed, rinse thespecimen, with either a swab saturated with water, orbetter still, by pouring water over the specimen. Afterrinsing with hot water, blow the specimen dry withcompressed air. Details of the procedure notdiscussed here are covered in the sections for thevarious metals and their alloys.2.5.3 The times given in individual tabulations areonly intended as guides. In fact, the progress ofetching should be closely watched, and etchingstopped when the preferred structural details havebeen revealed. Specimens should be etched todevelop structures. Generally, a light etch is betterthan a heavy etch; over-etching can often lead tomisinterpretation. The actual time to develop astructure properly may be quite different from the onesuggested.

3.1.1 The specimens can be cut using the commoncutting tools: hack saws, band saw, shears, abrasivecutoff wheels, etc. All these methods will cause coldwork at the surface and will generate heat. Thetemperature rise can be enough to cause changes instructure. For these reasons, sharp tools andgenerous lubrication are necessary for sectioning.

3.1.2 The cold-worked surface should be removedby machining the surface. Again, sharp tools andcopious lubrication are required. If fine detail isrequired, the machined surface should be groundusing silicon carbide paper lubricated with water orkerosene.

Caution:

3. Speci f ic Preparation Procedures andRecommended Solutions

2.3 Preparation

2.4 Solutions

2.5 Procedure

3.1 Aluminum


E

3.1.3 Several of the solutions used inmacroetching react vigorously with the metal and canoverheat the specimen. In these cases the specimen isperiodically removed from the solution, cooled inrunning water, and reimmersed in the etchant. Thisprocedure is repeated until the desired degree ofetching is obtained.3.1.4 Macroetchants for Aluminum and AluminumAlloys (Table I).

3.2.1 While beryllium in the massive form is notdangerous, beryllium and its compounds in the finelydivided state are extremely poisonous.Note— Before starting any work involvingberyllium, a review of hazards and plans for handlingshould be made. A number of references on berylliumare available. Particular mention may be made of"Toxicity of Beryllium" (ASD-TR-62-7-667), preparedby the Kettering Laboratory for the Air Force.3.2.2.1 General speaking, beryllium and its alloyshave given difficulty in obtaining good macroetchedspecimens. First, beryllium is a rather brittle metaland sectioning can be difficult. Cut-off wheels withthe designation C46FR70 have been the mostsuccessful. Secondly, beryllium does not grind easily;hence, specimens should be as small as possible tominimize grinding time. Grinding has been mostsuccessful with the entire sequence of wet siliconcarbide papers.3.2.2.2 The etching of fine-grained metal may notalways be entirely successful, and further preparationwill be required. Rough polishing with 16 µm Al Osuspended in water is performed on a low-nap cloth.Light pressure and frequent change of cuttingdirection produce the best results. If further polishingis required, 1 µm green Cr O in tap water onsynthetic suede works best.3.2.3 Macroetchants for Beryllium and BerylliumAlloys (Table II).

3.3.1 Many of the cobalt-base high-temperaturealloys can be etched using the same procedures asthose for iron and nickel-base high-temperaturealloys. Other cobalt alloys, such as the stellites usedas machine tools, require special treatment.3.3.1.2 The cobalt-base alloys, as a group, are noteasily machined. The specimens should be sectionedwith abrasive cutoff wheels, and ground on wetsilicon carbide papers. Because of the rapid work-hardening characteristics of these alloys, fresh paperand copious cooling should be used.3.3.2 Macroetchants for Cobalt and Cobalt Alloys(Table III).

3.4.1 These metals are usually macroetched tobring out the general structure of wire bar and billets,as well as variations in grain size in extrusions andforgings.3.4.2.1 Specimens may be sectioned using commoncutting tools. To minimize cold working, the toolsshould be kept sharp.3.4.2.2 Good results can be obtained by machininga smooth surface in two stages, the first being a heavy

cut to remove the cold work from sectioning, and thesecond a fine cut with a V-shaped tool to remove theremaining cold work. Grinding through the series ofmetallographic papers will give more detailed results.The degree of grinding depends upon the amount ofdetail required. The etching solutions listed in thefollowing table are simple to prepare and their userequires no special technique.Note—It should be pointed out that heavy etching willremove the effect of cold work, but at the expense ofproducing a rough surface. If the specimen is thengiven light regrinding to remove the rough etchedsurface, the second etch will provide good results.3.4.3 Macroetchants for Copper and CopperAlloys (Table IV).

3.5.1 Macroetching has been highly developedand is used extensively in the iron and steel industries.An example of the use of macroetching for inspectionof tool steels is given in the appendix. In hot millproducts such as bars, billets, sheet, and plate, thedisk cut with a parting tool is prepared by facing on alathe or by grinding. In facing, the first cut ismoderately heavy with a sharp tool. The secondfacing is a light cut with a V-shaped tool run at highspeed. Specimens produced in this manner areadequate for general inspection. A better, thoughslower, method is to grind the specimen. Forinspection purposes, finishing on a 120-grit wheelwill be sufficient. If machine grinding is used, thespecimen should be of a size that can be heldconveniently in one hand. The limiting size inmachine grinding is usually 12 inches square, sincelarger specimens are difficult to handle in the etchingbath.3.5.1.1 When the maximum amount of detail isrequired, as in weldments, polishing the specimenwith the series of metallographic papers gives thebest results. When examining for surface defects, thesurface itself should be etched directly without muchpreparation. The only preparation that is advisable isto brush off the loose scale, and then to give thespecimen a light grinding pass with very coarseabrasive to break through the adherent scale. Whenetching in 1:1 HCl for example, this scale will beremoved, exposing the surface underneath. If carehas been exercised in the grinding operations, thegrinding scratches will not interfere with examination.3.5.1.2 The most commonly used solutions formacroetching iron and steel are Solution Nos. 1 and3 in Table V.3.5.2 Forgings—In addition to the examination forinternal structure, surface defects, and structure,closed die forgings are often sectioned to show flowlines. Etching for flow lines requires extremely carefulpreparation to provide a smooth surface with aminimum of cold work. Long pieces, such as crankshafts, are awkward to handle and are best preparedon a grinding machine using successively finergrinding wheels. Sectioning into shorter lengths maybe advisable. Th specimen should be heavily etchedin 1:1 HCl or 20% H SO . Contrast can often beincreased by wiping the surface lightly with very finemetallographic paper after etching. Examination forstructure, defects, etc. is carried out in the samefashion as hotmill products.

3.2 Beryllium

3.3 Cobalt and Cobalt Alloys

3.4 Copper and Copper Alloys

3.5 Iron and Steel

Caution:

2 3

2 3

2 4


3.5.3 Special Tests for Segregation—There are anumber of etchants containing copper salts which willreveal segregation. Careful specimen preparationthrough the metallographic papers is required. Verycareful cleaning after grinding is extremelyimportant. When a specimen is immersed in this typeof solution, copper plates out into the specimen by areplacement reaction. The rate of deposition dependson the composition of the steel. The copper platingwill cover the segregated regions. Sometimes thespecimen can be left in a little longer thanrecommended, and then rubbed lightly withmetallographic papers to increase contrast.3.5.4 Macroetchants for Iron and Steel (Table V).

3.6.1 These alloys are generally more susceptibleto cold working of the surface than the lower alloygrades of steel. The best method of preparation is togrind the specimens as described for iron and steel. Asmut tends to form on the surface of the steel whenimmersed in 1:1 HCl. This can be prevented byadding a small quantity of HNO to the etching bath.It can also be removed by scrubbing the specimenwith a vegetable fiber brush under running warmwater, or by immersion in warm 20% HNO .Scrubbing will provide a higher contrast for detectionof segregation and inclusions. The desmutting, eitherby the addition of HNO to the etching bath, or by thesecondary rinse in HNO , will provide a brightersurface which is suitable for determination of grainsize and structure. High-alloy stainless steels andaustenitic high-temperature alloys, because of theirextreme corrosion resistance, will often give troublein etching. Aqua regia, HCl-H O , and Marble'sreagent are the recommended etchants. All three ofthese require very careful specimen preparation.3.6.2 Macroetchants for Stainless Steels and High-Temperature Alloys (Table VI).

3.7.1 Lead and its alloys are among the mostdifficult metals to prepare for macroetching. They arenot only very soft and cold work easily, but theyrecrystallize readily at temperatures which can beeasily achieved in careless preparation

3.7.2 For best result in the macroetching of lead,all surfaces, other than that to be examined, must bemasked from the macroetch by the use of severalcoats of a plastic spray. The surface to be examinedshould be filed prior to etching. Three 14-inch filesare usually required and used in the following order:(1) aluminum, Type A; (2) hand smooth; and (3) hand-finishing smooth.3.7.3 The file is usually held in a fixture, and thespecimen is drawn over the file proceeding from thepoint of the file to the tang. Remove the filings aftereach pass of the specimen with a few short strokes ofa brass file, brushing in the direction of the last cut ofthe file. The molybdate etch listed below is used in theremoval of worked metal and also to reveal thestructure of lead of low-alloy content. Thepreparation of the solution is of paramountimportance.

3.7.4 Macroetchants for Lead and Lead Alloys(Table VII).

3.8.1 Features detected by macroetching aregrain size, segregation of intermetallic compounds,coring, cracks, porosity, laps, germinations, surfaceburning, and tears.3.8.2 Cast or wrought magnesium alloys areprepared in a similar manner to aluminum or copperand brass. However, a final facing with a 0.005-inchradius V-shaped tool fed at a rate of 0.002 to 0.003-inch is often sufficient. For some applications, thespecimen may be finished on a 400-grit wet wheel. Toresolve small detail, additional polishing with a watersuspension of 600 Alundum, or further with alphaalumina may be necessary.3.8.3 The finely divided magnesium (as chips orswarf) is highly combustible, and precautions againstit catching fire should be taken.3.8.4 Macroetchants for Magnesium andMagnesium Alloys (Table VIII).

3.9.1 Nickel alloys cold work easily and theirpreparation is not always easy. Grinding produces thebest specimens. Low-nickel and cobalt alloys ofbasically pure metal can be etched with Marble'sreagent or strong HNO solutions. The high-temperature alloys are difficult to etch. First of all,they are subject to cold-working problems, andsecondly, the alloys are extremely corrosion-resistant.Best results have been obtained with aqua regia,modified Marble's reagent, or HCl-H O solutions.

3.9.2 Macroetchants for Nickel and Nickel Alloys(Table IX).

3.10.1 These metals are, in general, soft andductile. Because of their expense, specimens formacroetching will usually be small and can behandled the same as microspecimens. Care shouldbe used to avoid cold work. Well-lubricatedmetallographic papers are recommended. Some ofthe platinum group metals, notably osmium andrhodium, are more abrasion-resistant than theirhardness would indicate and, therefore, will requirelong grinding time.3.10.1.1Except for silver, all of these metals arecorrosion-resistant and require the use of strongetchants. Etching should be done under a fume hood,with the proper precautions for the use of HF.3.10.2 Macroetchants for the Noble Metals(Table X).

3.11.1 The above six metals, the refractory metals,are found in Group V-B (V, Cb, Ta) and Group VI-B (Cr,Mo, W) of the periodic table (some tables list them asV-A and VI-A). In general, these metals are soft andductile in the pure state, but in the form usuallyencountered, are hard and brittle. Consequently,these metals and their alloys must be carefully groundbefore macroetching. The abrasives must be ofsufficient hardness, and the particles on the laps mustbe sharp. Wet silicon carbide papers have provedsatisfactory provided they are used with sufficient

3.6 Stainless Steels and High Temperature Alloys

3.7 Lead and Lead Alloys

3.8 Magnesium

3.9 Nickel and Nickel Alloys

3.10 Noble Metals—Ag, Au, Ru, Rh, Pd, Os, Ir, Pt

3.11 Refractory Metals—Cr, Mo, W, V, Cb, Ta

3

3

3

3

2 2

3

2 2

(especiallypure lead).


E

All except highSi castings

HCl (conc) . . . . . . . . . . . . 15 mlHNO (conc) . . . . . . . . . . 5 mlHF (48%). . . . . . . . . . . . . 5 mlH O . . . . . . . . . . . . . . . . 75 ml

1+2 Tucker's; same as above butslower acting.3

2

Immerse specimen at roomtemperature unti l desiredcontrast is developed. Rinse inwarm water and dry.

3XXX4XXX5XXX6XXXHigh Si castings

HCl (conc) . . . . . . . . . . . . 75 mlHNO (conc) . . . . . . . . . . 25 mlHF (48%). . . . . . . . . . . . . 5 ml

Mix fresh before using. Use atroom temperature. May be usedas immersion etch or swabbedover specimen surface. Rinsespecimen in warm water and dry.

Used to develop grain structure.May be diluted with 25% water toslow down etching. Does notrequire fine grinding.

3

pressure to effectively cut the sample and are notused after they are dull or worn. Longer grindingtimes than would be expected from the hardness ofthese metals and alloys are needed. The followingsolutions used for the metals specified will revealdefects, general structure, grain size, andsegregation.3.11.2 Macroetchants for the Refractory Metals(Table XI).

3.12.1.1Tin and its alloys, like lead, are difficult toprepare. Because these metals cold work easily andrecrystallize at room temperature, false structures areeasily produced. The best method of preparation formacroetching is the same as for microetching. Thespecimen should be cut carefully and then groundgently on lubricated silicon carbide paper. This maybe followed by polishing with 6 µm diamond paste ona moderately napped wheel.3.12.1.2Precision cast tin-base bearing alloys may beetched directly without preparation.3.12.2 Macroetchant for Tin and Tin Alloys(Table XII).3.12.3 Preparation of Ammonium Polysulfide—PassH S gas into 200 ml of NH OH (sp. gr. 0.9) untilsaturated. The solution should be kept in an ice bathduring this operation. Add an additional 200 ml ofNH OH (sp. gr. 0.9), and dilute with water to make 1liter. Add 100 g of sulfur. Stir occasionally over aperiod of 1 hour, then filter and use.

3.13.1 Titanium, zirconium, and hafnium, thereactive metals, are macroetched for generalstructure, grain size, and segregation of impurities.They require extreme care in preparation. Sharp toolsand fresh grinding paper are required to prevent coldwork from blurring the structure. The best results areobtained by grinding. These metals grind slowly andrequire sharp, fresh abrasives. The recommendationsof grinding wheel manufacturers should be followedclosely for rough grinding. Silicon carbide papers,usually run wet, will give a fine finish. Papers shouldbe discarded frequently to prevent loading. Chemicalpolishing preparations, while relatively untried, mayhave decided advantages in handling thesematerials.3.13.2 Solutions in the table are not difficult toprepare, but involve the use of hydrofluoric acid. Thisacid can give extremely serious, very painful burns.The HCl-HF solution requires desmutting. Thespecimen should be rinsed between etching anddesmutting (Table XIII).

3.14.1 Zinc and its alloys cold work rapidly andrecrystallize at low temperatures, thus allowing falsestructures to form easily. Coarse-grained zinc andzinc alloys are more prone to cold work than some ofthe fine-grained die-casting alloys. A good test for thepresence of cold work, especially in coarse-grainedsamples, is the appearance of twinning after etching.3.14.2 Samples should be cut with a sharp saw andground on well-lubricated silicon carbide papers atslow speeds.3.14.3 Macroetchant for Zinc and Zinc Alloys(Table XIV).

3.12 Tin and Tin Alloys

3.13 Titanium, Zirconium, Hafnium, and their Alloys

3.14 Zinc and Zinc Alloys

2 4

4

Table I—Macroetchants for Aluminum and Aluminum AlloysAlloy Composition Procedure Comments

All NaOH . . . . . . . . . . . . . . 10 gH O . . . . . . . . . . . . . . . . 100 ml

Immerse sample 5 to 15 min. insolution heated to 60 to 70 C(140 to 160 F). Rinse in water,and remove smut in strong HNOsolution. Rinse and repeatetching if necessary.

Good general-purpose etchant,can be used on almost allaluminum alloys. Does notrequire fine grinding.

2

3

O O

O O

High purity Al1XXX3XXX4XXX5XXX6XXX

HCl (conc) . . . . . . . . . . . . 45 mlHNO (conc) . . . . . . . . . . 15 mlHF (48%). . . . . . . . . . . . . 15 mlH O . . . . . . . . . . . . . . . . 25 ml

Immerse specimen at roomtemperature unti l desiredcontrast is developed. Rinse inwarm water and dry.

Tucker's etch; general purposeetch for revealing microstructureof both cast and wroughtaluminum. Does not require finegrinding.

3

2


2XXXHigh Cu alloys

HCl (conc) . . . . . . . . . . . . 15 mlHF (48%). . . . . . . . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 90 ml

May be used as an immersionetch or swabbed over thespecimen surface. When desiredcontrast is obtained, rinse inwater and remove deposits withconcentrated HNO . Rinse inwarm water and dry.

Flick's reagent; best results areobtained with a ground surface;180 grit will suffice.2

3

Table I—continuedAlloy Composition Procedure Comments

Table II—Macroetchants for Beryllium and Beryllium AlloysMetal Composition Procedure Comments

Table III—Macroetchants for Cobalt and Cobalt AlloysAlloys Composition Procedure Comments

Table IV—Macroetchants for Copper and Copper AlloysAlloys Composition Procedure Comments

Be HCl. . . . . . . . . . . . . . . . . 10 mlNH Cl. . . . . . . . . . . . . . . 4 gH O . . . . . . . . . . . . . . . . 90 ml

Either swab or immerse at roomtemperature for a few minutes,rinse in water, and dry.

Works best on coarse-grained Be.4

2

49Co-49Fe-VSome Co-Cr

alloys

HCl. . . . . . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in hot solution60 to 82 C (140 to 180 F) for30 to 60 minutes.

Rinse in hot water and dry;general structure, porosity.2

O O O O

Cu and allbrasses

HNO . . . . . . . . . . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 90 ml

Immerse specimen in solution atroom temperature for a fewminutes. Rinse in water and dry.

Emphasize grains and cracks.3

2

Cu and allbrasses

HNO . . . . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Brings out grain contrast, pitsresult unless agitated. Aluminumbronzes may form smut which canbe removed by brief immersion inconcentrated HNO .

3

2

3


Cu and allbrasses

HCl . . . . . . . . . . . . . . . . 30 mlFeCl . . . . . . . . . . . . . . . 10 gH O or ethanol . . . . . . . . 120 ml

Good grain contrast.3

2


Cu, high Cualloys,phosphorus,tin bronzes

K Cr O sat . . . . . . . . . . . 2 g

H SOH O

Immerse specimen in solution atroom temperature for 15 to 30minutes then swab with freshsolution. Rinse in warm water anddry.

Emphasizes grain boundaries andoxide inclusions.

2 2 7

2 3

2

solution of NaCl

All HNO . . . . . . . . . . . . . . . 50 mlAgNO . . . . . . . . . . . . . . 5 gH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutiontemperature. Rinse in warm waterand dry.

Brilliant deep etch.3

3

2

25Cr-10Ni-8W21Cr-20Ni-3W-3Mo-1Cb

HCl . . . . . . . . . . . . . . . . 50 mlHNO . . . . . . . . . . . . . . . 10 mlFeCl . . . . . . . . . . . . . . . 10 gH O . . . . . . . . . . . . . . . . 100 ml

Swab until desired contrast isobtained, then rinse in warmwater, and dry.

Grain size, general structure.3

3

2

18Cr-10Ni-14W CuCl •2NH CL•2H O. . . 2 gFeCl . . . . . . . . . . . . . . . 5 gHNO . . . . . . . . . . . . . . . 5 mlHCl . . . . . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 80 ml

2 4 2

3

3

2

Swab until desired contrast isobtained, then rinse in warmwater, and dry.

Grain size, general structure.

Be HCl. . . . . . . . . . . . . . . . . 10 mlNH Cl. . . . . . . . . . . . . . . 2 gPicric acid . . . . . . . . . . . . 2 gH O . . . . . . . . . . . . . . . . 90 ml

An alternative when No. 1 doesnot work. Fine-grained metal maynot give good results in eithercase.

4

2

Either swab or immerse at roomtemperature for a few minutes,rinse in water, and dry.


Brass 20% acetic acid . . . . . . . . 20 ml5% chromic acid . . . . . . . 10 ml10% FeCl in H O. . . . . . . 5 ml

Strain lines.

3 2

Immerse specimen in solutiontemperature. Rinse in warm waterand dry.

E

Silicon brass orbronze

CrO . . . . . . . . . . . . . . . 40 gNH Cl . . . . . . . . . . . . . . 7.5 gHNO (conc) . . . . . . . . . . 50 mlH SO (conc) . . . . . . . . . . 8 mlH O . . . . . . . . . . . . . . . . 100 ml

Immerse specimen in solution atroom temperature, rinse in warmwater, and dry.

3

4

3

2 4

2

Table IV—continuedAlloys Composition Procedure Comments

Table V—Macroetchants for Irons and SteelsAlloys Composition Procedure Comments

Plain and alloysteels, highspeed andtool steels,cutlery (12 to14% Cr), andstainless steels

HCl (conc) . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutionheated to 60 to 82 C (160 to180 F) for 15 to 30 minutes.Desmut by vigorous scrubbingwith vegetable fiber brush underrunning water. Stainless steelsmay be desmutted by dipping in awarm 20% HNO to give a brightfinish.

General purpose.2

3

O O O

O

High alloy steels HCl (conc) . . . . . . . . . . . . 50 mlHNO (conc) . . . . . . . . . . 25 mlH O . . . . . . . . . . . . . . . . 25 ml

Immerse specimen for 10 to 15minutes in solution at roomtemperature. Rinse in warmwater and dry.

Ratio HCl:HNO runs 2:1 to 3:1.3

2

3

Plain and alloysteels, cutlerysteels

HCl (conc) . . . . . . . . . . . . 38 mlH SO (conc) . . . . . . . . . . 12 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen for 15 to 45minutes in solution heated to 60to 82 C (160 to 180 F) Rinse inwarm water and dry.

Works well on 12% Cr steel.2 4

2

O

O O

High alloy steels HNO (conc) . . . . . . . . . . 10 mlHF (48%). . . . . . . . . . . . . 4 mlH O . . . . . . . . . . . . . . . . 87 mltoHNO (conc) . . . . . . . . . . 40 mlHF (48%). . . . . . . . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutionheated to 60 to 82 C (160 to180 F) until desired etch isobtained, and rinse in warmwater and dry.

Ratio HNO :HF varies.3

2

3

2

3O O O

O

Stainless steels,high-alloysteels

HCl (conc) . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 mlH O (30%) . . . . . . . . . . . 20 ml

Mix HCl and water then heat to60 to 77 C (160 to 170 F),immerse specimen, and add H Oin several parts. Do not mix. Makeeach subsequent addition afterfoaming from previous additionhas stopped.

Produces bright finish.2

2 2 2 2

O O O O

Austeniticstainless steels

HCl (conc) . . . . . . . . . . . 50 ml

CuSO in H O . . . . . . . . . 25 ml

Immerse specimen in solutionwhich may be heated or not,depending upon alloy. Time alsodepends on alloy. Rinse in warmwater and dry.

Marble's reagent; light etch, goodfor structure.saturated solution

4 2 2

Plain and alloysteels

CuCl . . . . . . . . . . . . . . . 2.5 gMgCl . . . . . . . . . . . . . . . 10 gHCl (conc) . . . . . . . . . . . 5 mlAlcohol—up to . . . . . . . . 250 ml

Immerse in solution at roomtemperature until a copperysheen appears. Rinse thoroughlyand dry.

Stead's reagent; salts dissolved inHCl with minimum of hot water tobring out P-rich areas andP-banding.

2

2

Mild steel;Bessemer andhigh N steel

CuCl . . . . . . . . . . . . . . . 90 gHCl (conc) . . . . . . . . . . . 120 mlH O . . . . . . . . . . . . . . . . 100 ml

The surface should be rubbedwith cloth soaked in etchingsolution. Wash in alcohol or rinsein HCl (1:1) after etching toprevent deposition of copper.

Fry's reagent; before etching,sample should be heated to 100to 250 C (302 to 482 F) for 5 to30 minutes depending oncondition of steel. To show strainlines due to cold work.

2

2

2

O O O

Plain and low-alloy steels

(NH ) S O . . . . . . . . . . . 10 g

H O . . . . . . . . . . . . . . . . 100 ml

S w a b s o l u t i o n a t r o o mtemperature over specimen.Rinse and dry.

Grain size, weldments.4 2 2 8

2

(ammonium persulfate)


Stainless andhigh Cr steels

HCl . . . . . . . . . . . . . . . . 10 mlAlcohol . . . . . . . . . . . . . 100 mlPicric Acid . . . . . . . . . . . . 1 g

Immerse specimen in solution atroom temperature until desiredcontrast is obtained. Rinseand dry.

Vilella's reagent.

Austeniticstainless steelsand high-temperaturealloys

HCl (conc) . . . . . . . . . . . 50 mlSaturated solution . . . . . 25 ml

of CuSO in H O

Immerse specimen in solutionwhich may be heated up to 77 C(170 F) until desired contrast isobtained. Rinse and dry.

Marble's reagent. Light etch, goodfor structures. Amount of CuSOsolution may be increased to 1:1ratio for difficult alloys.

4 2

4O

O

Table V—continuedAlloys Composition Procedure Comments

Table VII—Macroetchants for Lead and Lead AlloysAlloys Composition Procedure Comments

Table VI—Macroetchants for Stainless Steels and High-Temperature AlloysAlloys Composition Procedure Comments

Plain and alloysteels

CuCl . . . . . . . . . . . . . . . 45 gHCl (conc) . . . . . . . . . . . 180 mlH O . . . . . . . . . . . . . . . . 100 ml

Modified Fry's reagent; same asfor reagent above, but modifiedby Wazau; may give morecontrast; specimen can bewashed in water wi thoutdepositing copper.

2

2

The surface should be rubbedwith cloth soaked in etchingsolution. Wash in alcohol or rinsein HCl (1:1) after etching toprevent deposition of copper.

Stainless steelsand iron-base high-temperaturealloys

HCl (conc) . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutionheated to 60 to 82 C (160 to180 F) for 30 minutes. Desmut byv i go rou s s c r ubb i ng w i t hvegetable brush under runningwater. Stainless steels may bedesmutted by dipping in warm20% HNO to give bright finish.

General purpose.2

3

O O O

O

Iron, cobalt, andnickel-basehigh-temp-erature alloys

HCl (conc) . . . . . . . . . . . 50 mlHNO (conc) . . . . . . . . . . 25 mlH O . . . . . . . . . . . . . . . . 25 ml

Immerse specimen in solution atroom temperature for 10 to 30minutes. Rinse and dry.

Ratio HCl : HNO runs 2:1 to 3:1.3

2

3

Stainless steelsand high-temperaturealloys

HNO . . . . . . . . . . . . . . . 10 mlHF (48%) . . . . . . . . . . . . 3 mlH O . . . . . . . . . . . . . . . . 87 mltoHNO (conc) . . . . . . . . . . 40 mlHF (48%) . . . . . . . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutionheated to 60 to 82 C (160 to180 F) until desired contrast isobtained. Rinse and dry.

Ratio HNO : HF varies.3

2

3

2

3O O O

O

Austeniticstainless steelsand nickel-base alloys

A. (NH ) SO . . . . . . . . . . 15 gH O . . . . . . . . . . . . . . 75 ml

B. FeCl . . . . . . . . . . . . . 250 gHCl (conc) . . . . . . . . . 100 ml

C. HNO (conc) . . . . . . . . 30 ml

Combine A and B, then add C.Immerse specimen in solution atroom temperature until desiredcontrast is obtained. Rinse anddry.

Lepito's etch I; mix fresh; grainstructure.

4 2 4

2

2

3

Austeniticstainless steelsand high-temperaturealloys

HCl (conc) . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 mlH O (30%) . . . . . . . . . . . 20 ml

Mix HCl and water, then heat,immerse specimen, and add H Oin several parts. Do not mix. Makeeach subsequent addition afterfoaming from previous additionhas stopped.

2

2 2

2 2

Lead and leadalloys

A. H O . . . . . . . . . . . . . . 250 mlNH OH (sp gro. 90). . . 140 mlHNO (conc) . . . . . . . . 60 mlMolybdic acid (85%) . . 100 ml

B. H O . . . . . . . . . . . . . . 960 mlHNO (conc) . . . . . . . . 400 ml

C. Glacial acetic acid. . . . 100 ml

Add A to B and let precipitateredissolve. If B is added to A, aninsoluble precipitate forms. AddC to mixture of A and B afterprecipitate has redissolved. Swabsurface of the specimen withmixed solution until desiredcontrast is obtained. Rinseand dry.

2

4

3

2

3


E

Antimoniallead

A. Glacial acetic acid. . . . 30 mlHNO (conc) . . . . . . . . 40 mlH O . . . . . . . . . . . . . . 160 ml

B. Glacial acetic acid. . . . 1 mlH O . . . . . . . . . . . . . . 400 ml

Prepare surface on silk velvetwheel with Al O abrasive at 150rpm. Etch with solution A at 42 C(108 F) then repolish until bright.Re -e t ch wi th B a t roomtemperature for 1 to 2 hours.

3

2

2

2 3O

O

Table VII—continuedAlloys Composition Procedure Comments

Table VIII—Macroetchants for Magnesium and Magnesium AlloysAlloys Composition Procedure Comments

ZK60A Glacial acetic acid . . . . . . 5 mlH O . . . . . . . . . . . . . . . . 95 ml

Immerse specimen in solution atroom temperature for 1/2 to 8minutes until desired contrast isobtained. Desmut with 50% HF(48%) in water. Rinse in rapidlyflowing water and dry.

Flow lines in forgings.2

AZ61AAZ80A

Glacial acetic acid . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 90 ml

As above, but for 1/2 to 5minutes.

Grain size, surface castingdefects.2

AZ31BAZ61AAZ80A

Glacial acetic acid . . . . . . 20 mlNaNO . . . . . . . . . . . . . . 5 gH O . . . . . . . . . . . . . . . . 80 ml


Flow pattern in forgings. Surfacecasting defects. Glycolic acid maybe substituted for acetic acid.

3

2

AZ31B HNO (conc) . . . . . . . . . . 10 mlH O . . . . . . . . . . . . . . . . 90 ml


Flow pattern in forgings. Internaldefects in cast slabs and ingots.

3

2

AZ31B Na Cr O . . . . . . . . . . . 180 gHNO (conc) . . . . . . . . . 180 mlH O to make . . . . . . . . . 1000 ml

As above, but rinse in hot water. General etch for defects in sandand die castings.

2 2 7

3

2

AZ31B CrO . . . . . . . . . . . . . . 280 gHNO (conc) . . . . . . . . . 25 mlHF (48%) . . . . . . . . . . . 10 mlH O to make . . . . . . . . . 1000 ml

Germination on sand castsurfaces. Surface defects of diecastings.

3

3

2

As above, but rinse in hot water.

AZ61AAZ80A

6% picric acid in alcohol . 100 mlH O . . . . . . . . . . . . . . . 10 mlGlacial acetic acid . . . . . 5 ml

As above for 1/2 to 3 minutes, ormay be swabbed.

Grain size and flow patterns ofboth cast and wrought forms.Requires fine finish (600 grit).

2

Antimoniallead

A. HNO (conc) . . . . . . . . 80 mlH O . . . . . . . . . . . . . . 220 ml

B. (NH ) MoO . . . . . . . . 45 gH O

Mix equal quantities of A and Bimmediately before use. Immersespecimen in solution at roomtemperature unti l desiredcontrast is obtained. Rinse anddry.

Grain structure.3

2

4 2 4

2

Antimoniallead

(NH ) MoO . . . . . . . . . . 10 gCitric acid . . . . . . . . . . . . 25 gH O . . . . . . . . . . . . . . . . 100 ml

Immerse specimen in solution atroom temperature until desiredcontrast is obtained, then rinseand dry.

Bright etch, grain structure,defects.

4 2 4

2

Antimoniallead

Acetic acid . . . . . . . . . . . 75 mlH O . . . . . . . . . . . . . . . 25 ml

Mix with strongest H O availableto minimize water content.Immerse dry specimen in solutionat room temperature until desiredcontrast is obtained, then rinseand dry.

Chemical polish-etch.2 2

2 2


ZK60A 6% picric acid in alcohol. . 50 mlH O . . . . . . . . . . . . . . . . 20 mlGlacial acetic acid . . . . . . 20 ml

Flow pattern and grain size ofhomogeneous alloy. Increasewater to increase strain contrast.Specimen should be finished on600-grit paper.

2


AZ21AZ31

6% picric acid in alcohol. . 70 mlH O . . . . . . . . . . . . . . . . 10 mlGlacial acetic acid . . . . . . 10 ml

Grain size. Specimen should befinished on 600-grit paper.2


Ag HNO . . . . . . . . . . . . . . . 10 mlMethyl alcohol . . . . . . . . 90 ml

Room temperature; few minutes. Grain contrast.3

Alloys containingCr, Fe, andother elements

Sat soln of CuSO in H O . 50 mlHCl. . . . . . . . . . . . . . . . . 50 ml

Swab etchant. Modified Marble's reagent.4 2

Table VIII—continuedAlloys Composition Procedure Comments

Table IX—Macroetchants for Nickel and Nickel AlloysAlloys Composition Procedure Comments

Table X—Macroetchants for Noble MetalsMetal Composition Condition of Use Comments

Ni CuSO . . . . . . . . . . . . . . 10 gHCl . . . . . . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solution atroom temperature until desiredcontrast is obtained. Rinse anddry.

Marble's reagent for grainstructure.

4

2

AuPt alloysPd alloys

HCl . . . . . . . . . . . . . . . . 66 mlHNO . . . . . . . . . . . . . . . 34 ml

Hot; few minutes. Grain contrast.3

Ru and alloysOs and alloysRh and alloys

Lactic acid. . . . . . . . . . . . 50 mlHNO . . . . . . . . . . . . . . . 20 mlHF . . . . . . . . . . . . . . . . . 30 ml

Room temperature; few minutes. Grain contrast.3

HCl . . . . . . . . . . . . . . . . 30 mlHNO . . . . . . . . . . . . . . . 15 mlHF . . . . . . . . . . . . . . . . . 30 ml

Room temperature; few minutes. Grain contrast.Ru and alloysOs and alloysRh and alloys

3

Pt and alloys Sat soln of NaCl in H O . . 80 mlHCl . . . . . . . . . . . . . . . . 20 ml

Electrolytic, 6V; few minutes. Grain contrast.2

Low-Ni alloys HNO . . . . . . . . . . . . . . . 20 mlH O . . . . . . . . . . . . . . . . 10 mlCuSO . . . . . . . . . . . . . . 10 g

Immerse specimen in solution atroom temperature for 20 to 30minutes; rinse and dry.

Grain structure.3

2

4

High-Ni alloys HNO (conc) . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Porosity and flowlines.3

2

Immerse specimen in solution atroom temperature for 20 to 30minutes; rinse and dry.


HNO . . . . . . . . . . . . . . . 50 mlAcetic acid . . . . . . . . . . . 50 ml

Immerse specimen in hotsolution. Rinse in hot water anddry.

Grain structure.3


Acetic acid . . . . . . . . . . . 50 mlHNO . . . . . . . . . . . . . . . 50 ml

Swab. Lepito's reagent II; etchant fornickel welds.3


HCl . . . . . . . . . . . . . . . . 100 mlH O . . . . . . . . . . . . . . . . 100 mlH O (30%) . . . . . . . . . . . 40 ml

See Table V. See Table V.2

2 2

ZK60A 4% picric acid in alcohol. . 100 mlH PO . . . . . . . . . . . . . . . 7 ml

Immerse specimen in solution atroom temperature repeatedlyuntil desired stain is obtained,rinse and dry.

Segregation of intermetalliccompounds and associatedcracks. Specimen should befinished on 600-grit paper.

2 4


Table XI—Macroetchants for Refractory MetalsMetal Composition Temperature Time Comments

Mo, W, V, Cb, and Ta HCl (conc) . . . . . . 30 mlHNO (conc) . . . . . 15 mlHF (48%) . . . . . . . 30 ml

Room temperature. 5 to 20 minutes.3

Mo, W, and V HF (48%) . . . . . . . 15 mlHNO (conc) . . . . . 35 mlH O . . . . . . . . . . . 75 ml


2

Too fast for Mo; 1 to 5seconds.

E

Iodide Ti H O (30%) . . . . . . . . . . . 60 mlH O . . . . . . . . . . . . . . . . 30 mlHF (48%) . . . . . . . . . . . . 10 ml

Swab with solution at roomtemperature unti l desiredcontrast is obtained. Rinse in coldwater and dry.

As above.2 2

2

Table XIII—Macroetchants for Titanium, Zirconium, Hafnium, and their AlloysAlloys Composition Procedure Comments

Table XIV—Macroetchants for Zinc and Zinc AlloysAlloys Composition Procedure Comments

Table XII—Macroetchants for Tin and Tin AlloysComposition Procedure Comments

Table XI—continuedMetal Composition Temperature Time Comments

Ti Alloys HCl (conc) . . . . . . . . . . . 20 mlHF (48%) . . . . . . . . . . . . 40 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse specimen in solutionheated to 49 to 66 C (120 to150 F) for 20 to 30 minutes.Rinse. If smut forms immerse in30% H SO for 3 minutes. Rinseand dry.

Desmut Ti, -13V-11 Cr-3Al alloys.Others do not normally needdesmutting.2

2 4

O O

O

Cu-free zincalloys

HCl (conc) . . . . . . . . . . . 50 mlH O . . . . . . . . . . . . . . . . 50 ml

Immerse in solution for about 15seconds. Remove smut by wipingunder running water. Repeat untildesired contrast is obtained, thendry.

Grain structure.2

Zn alloyscontaining Cu

CrO . . . . . . . . . . . . . . . 20 gNa SO . . . . . . . . . . . . . 1.5 g

orNa SO •10H O . . . . . . . 3.4 gH O . . . . . . . . . . . . . . . . 100 ml

Immerse in solution until goodcontrast is obtained. Rinse inrunning water and dry. Repeat ifnecessary.

Grain structure.3

2 4

2 4 2

2

7Al-4Mo alloys HNO (conc) . . . . . . . . . . 42 mlHF (48%) . . . . . . . . . . . . 8 mlH O . . . . . . . . . . . . . . . . 50 ml

As above.3

2

Zr, Hf, and lowalloys

H O (30%) . . . . . . . . . . . 45 mlHNO (conc) . . . . . . . . . . 45 mlHF (48%) . . . . . . . . . . . . 10 ml

Swab specimen with solution atroom temperature. Rinse 10seconds after yellow fumes form,then dry.

Should be used under a hood;chemical polishing solution.

2 2

3

Zr, Hf, high alloys H O . . . . . . . . . . . . . . . . 45 mlHNO . . . . . . . . . . . . . . . 45 mlHF . . . . . . . . . . . . . . . . . 10 ml

As above. As above.2

3

1.5Sn-0.15Fe-0.10Cr, and Hf

H O . . . . . . . . . . . . . . . . 70 mlHNO (conc) . . . . . . . . . . 30 mlHF (48%) . . . . . . . . . . . . 5 ml

As above. As above.2

3

Saturated ammonium polysulfide Immerse specimen in full strength solution atroom temperature for 20 to 30 minutes. Donot swab surface during etching.

Grain structure.

W, V, Cb, and Ta HF (48%) . . . . . . . 10 mlHNO (conc) . . . . . 30 mlLactic acid (85%) . . 50 ml


Cr H SO . . . . . . . . . 10 mlH O . . . . . . . . . . . 90 ml

Boiling. 2 to 5 minutes.2 4

2


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