Metallographic Techniques in Failure Analysis...Metallographic Techniques in Failure Analysis / 3...

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Metallographic Techniquesin Failure AnalysisGeorge F. Vander Voort, Buehler Ltd.

Fig. 1 Illustration of a cleavage fracture in a quenched and tempered low-carbon steel examined using three direct methods and three replication methods. (a) LM cross section(nickel plated). Etched with Vilella’s reagent. (b) LM fractrograph (direct). (c) SEM fractograph (direct). (d) LM replica. (e) SEM replica. (f) TEM replica

METALLOGRAPHIC EXAMINATION isone of the most important procedures used bymetallurgists in failure analysis. Development ofpowerful electron metallographic instruments,such as the scanning electron microscope, hasnot diminished the importance of light micros-copy. Basically, the light microscope is used toassess the nature of the microstructure and itsinfluence on the failure mechanism. The purposein using the light microscope may be twofold.One purpose may be to determine the relation-ship between the microstructure and the crackpath (in failures involving fracture) and/or thenature of corrosion or wear damage. The second

purpose is to determine whether processing orservice conditions have produced undesirablemicrostructural conditions that have contributedto the failure, such as abnormalities due to ma-terial quality, fabrication, heat treatment, andservice conditions. Examples are given in thisarticle to demonstrate such analytical work.

Conducting a materials failure analysis, acommon activity for many metallurgists, re-quires a carefully planned series of steps (Ref 1,2) designed to arrive at the cause of the problem.Proper implementation of light microscopy is ofcritical importance in failure analysis, and thisarticle focuses on the use of metallographic tech-

niques and examinations using the light micro-scope (LM) in failure analysis. Metallographicexamination typically should follow nondestruc-tive and macroscopic examination proceduresand should precede use of techniques of electronmicroscopy.

Examination of fractured components shouldbegin with the low-power stereomicroscope.Hand-held magnifying lenses are still widelyused to study fractures but mainly in the field.While the light microscope has limited value fordirect observation of fracture surfaces (more lim-ited for metals than nonmetals), a great deal canbe learned by indirect examination, that is, by


2 / Tools and Techniques in Failure Analysis

Fig. 3 Light micrographs of section profiles of (a) a nickel-plated ductile fracture and (b) a nickel-plated brittle fracture.Both are carbon steels etched with 2% nital.

Fig. 2 Light microscope fractographs taken with (a) bright-field and (b) dark-field illumination compared to (c) a SEM secondary-electron image fractograph of the same area. Sampleis an Fe-Al-Cr alloy.

examination of the fracture profile and secondarycracking.

Detailed observation of the fracture surface isbest accomplished by use of the scanning elec-tron microscope (SEM) or by examination ofreplicas with the transmission electron micro-scope (TEM). However, lack of access to a SEMor TEM should not be viewed as a crippling ob-stacle to performing failure analysis, becausesuch work was done successfully prior to the de-velopment of these instruments. In many studies,such equipment is not needed, while in othercases, they are very important tools. In mostcases, a more thorough job can be accomplishedusing such tools.

The LM, on the other hand, is a virtually in-dispensable tool for the failure analyst. The SEMand TEM find application in microscopy whenthe required magnification/resolution exceed thecapabilities of the LM (about 1000�), but thefirst tool of choice is the LM. Hence, it is usedfor fine-structure examination and identification.Thus, TEM and LM are complementary tools.Microstructural examination can be performedwith the SEM over the same magnification rangeas the LM, but examination with the latter ismore efficient. Contrast mechanisms for viewingmicrostructures are different for LM and SEM.Many microstructures, for example, temperedmartensite, exhibit poor contrast in the SEM andare best viewed by light microscopy. Whenatomic number contrast or topographic contrastis strong, the SEM provides good structural im-ages, particularly above 500� (Ref 3). Again,because of the limitations and advantages ofeach instrument, they are complementary ratherthan competitive tools. All studies of microstruc-tures and fractures should begin at the lowestmagnification level, the unaided human eye, andprogress upward, first using the stereomicro-scope for fractures and the LM for fracture pathand microstructural studies, before using elec-tron metallographic equipment.

Examination of Fractures

Microfractography is a relatively new field; itsroots can be traced to the light optical fracto-

graphs published by Zapffe and coworkers (Ref4) beginning in the early 1940s, although a fewstudies of historical value predated their efforts.Zapffe’s work, however, was almost exclusivelyconfined to observation of cleavage facets onrather brittle, coarse-grained specimens. Thetechnique, basically an interesting academic ex-ercise, did stimulate interest in fracture exami-nation as part of failure analysis. However, thedepth-of-field limitation of the light microscopehas restricted its use for such work. Aside fromthe published light optical fractographs made byZapffe (see Ref 5 for a review of many of these),very few optical fractographs of metallic mate-rials have been published by others. Microfrac-tography gained momentum with the develop-ment of TEM replication methods and becamecommonplace after the commercial introductionof the SEM in approximately 1965.

A flat, brittle fracture can be examined withthe light microscope by orienting the fractureperpendicularly to the optical axis. It is best tostart with a low-power objective; long-workingdistance types are preferred. Focusing revealsthe limitations of the method, because only partof the fracture is in focus at any setting. Thus,photographs reveal only a portion of the fracturein focus, depending on the coarseness and ori-entation of the fracture facets. Figure 1 shows anexample of a brittle fracture in a low-carbon steelexamined in this manner. Figure 1 also shows aLM image of the fracture profile, a LM image ofa replica of the fracture, SEM images of the frac-

ture and a replica of the fracture, and a TEMreplica of the fracture. Although Zapffe usedbright-field illumination for this work, dark-fieldillumination often produces superior results. Fig-ure 2 illustrates the use of bright-field and dark-field illumination for viewing a brittle fracture inan Fe-Cr-Al alloy, plus a SEM fractograph of thesame area. Dark-field illumination is better atcollecting the light scattered from the fracturefeatures; glare is reduced, and image contrast isimproved.

Examination of fracture replicas with the lightmicroscope (Ref 5–7) can extend the use of themethod only to a limited extent, because the rep-lica collapses slightly, producing less depth offield. Also, with a replica, the risk of damagingthe objective is eliminated.

Considerable information concerning the frac-ture mode and the relationship of the microstruc-ture to the fracture path can be obtained by LMexamination of the profile of partially fractured(Ref 8–10) or completely fractured (Ref 11–15)polished metallographic specimens. Such ex-aminations have been conducted for many years,long before the development of electron metal-lographic techniques, and continue to be usedbecause of the value of the method. If the frac-ture has progressed to complete rupture, so thatonly one side of the fracture is to be examined,it may be best to nickel plate the fracture to en-hance edge retention. This is not required if thecrack has not separated the component into twopieces, or if a secondary crack is to be examined.


Metallographic Techniques in Failure Analysis / 3

Fig. 5 Light micrographs of a partially broken and a completely broken specimen of sensitized (649 �C, or 1200 �F,for 4h) AISI 304 austenitic stainless steel, and a SEM fractograph of the broken Charpy V-notch specimen

Figure 3(b) shows a nickel-plated brittle im-pact fracture of a low-carbon steel with a ferrite-pearlite microstructure. The crack consists of nu-merous connected straight-line segments in theferrite phase. Several subsurface cleavage cracksare also present. In comparison, a ductile impactfracture in a quenched and tempered carbon platesteel is shown in Fig. 3(a). Note that the fracturesurface exhibits a much rougher appearance dueto the linking up of the microvoids. Below thefracture surface, spherical ruptures are frequentlyseen, which are also indicative of a ductile frac-ture mechanism.

Fatigue fractures can also be examined usingfracture profiles, as illustrated in Fig. 4. Thisshows a low-cycle fatigue crack that has notpropagated to final rupture. Note that the crack

exhibits some minor branching and does not tra-verse the ferrite grains in a straight-line fashion,as with cleavage (Fig. 3b). Also, there are nospherical cavities near the crack, as observedwith ductile fractures (Fig. 3a). The crack showsno preference for either the ferrite or pearlite dur-ing its growth, and the macroscopic appearanceof the crack is rather flat, which is typical offatigue cracks.

Figure 5 shows partially broken and com-pletely broken sensitized (649 �C, or 1200 �F, for4h) impact specimens of American Iron andSteel Institute (AISI) 304 stainless steel. Scan-ning electron microscope examination of thefracture face reveals extensive microvoid coales-cence, that is, ductile rupture, although the im-pact strength (at �196 �C, or �320 �F) was only40% of that of a nonsensitized sample. The par-tially broken sample reveals a fracture path thatoften follows the grain boundaries. Rupture cav-ities are observed behind and ahead of the crack;most are associated with coarse carbides. Thefracture profile of the completely broken speci-men reveals microvoid coalescence.

As a comparison to Fig. 5, Fig. 6 shows asimilar fracture made in AISI 312 stainless steelweld metal that was aged at 816 �C (1500 �F) totransform the delta ferrite to sigma phase. In this

case, the impact strength at room temperaturewas only 7% of that of an as-welded sample con-taining austenite and delta ferrite. Note that thefracture path is microscopically flatter and con-sists of numerous connected short straight-linesegments. The SEM fractograph shows numer-ous small, flat fracture regions indicative of theembrittlement due to sigma. The high-magnifi-cation LM profile view of the completely frac-tured surface reveals the presence of extensivesigma along the crack path.

Examination of the crack path using cross sec-tions is also very useful for study of fracturesdue to environmental problems. Figure 7 showsa stress-corrosion crack in a partially brokensample of solution-annealed AISI 304 stainlesssteel tested in boiling (151 �C, or 304 �F) mag-nesium chloride. The crack path is predomi-nantly intergranular, but considerable transgran-ular fracture is also present. The SEMfractograph of the specimen clearly reveals theintergranular nature of the crack.

Fracture profile examination is also very use-ful in the study of certain types of failures dueto liquid metal embrittlement (LME). Figure 8shows the microstructure adjacent to a LMEcrack in a eutectoid steel where liquid copper haspenetrated the grain boundaries at 1100 �C (2012

Fig. 4 Light micrograph of the path of a fatigue crackthrough a low-carbon steel specimen. Etched

with 2% nital



Fig. 6 Light micrographs of a partially broken and a completely broken specimen of AISI 312 stainless steel weld metalheat treated to transform the delta ferrite to sigma, and a SEM fractograph of the broken Charpy V-notch spec-

imen

�F) while the sample was austenitic and under anapplied tensile load. Light microscope exami-nation reveals a discontinuous film of copper inthe prior-austenite grain boundaries and an in-tergranular fracture path. Scanning electron mi-croscope examination of the fracture also revealsthe intergranular nature of the crack path.

Surface detail can also be studied by LM usingtaper sections (Ref 16–18). This method hasbeen used to study wear phenomena, surfacecoatings, fatigue damage, and other fine surfacedetail. In this method, the surface is sectioned ata slight angle to the surface. Polishing on thisplane produces a magnified view of the structurein the vertical direction. The degree of magnifi-cation is defined by the cosecant of the section-ing angle; an angle of 5� 43� produces a tenfoldmagnification.

Considerable progress has been made in ap-plying the principles of quantitative metallogra-phy to the study of fractures (Ref 14, 15, 19–23).Much of this work has used measurements madeon polished sections taken parallel to the crack-growth direction. This work provides new in-sight into fracture processes and should be usefulin failure analysis, although its application todate has been limited to research studies.

MetallographicSpecimen Preparation

Because the metallographer cannot predict inadvance what the microstructural examinationwill reveal, specimen preparation must be per-fect; otherwise, critical information can easily belost. This basic truth has been proven over andover, yet is violated regularly. The preparationprocedure and the prepared specimen must pro-vide the following:

● Deformation induced by sectioning, grinding,and polishing must be removed or be shallowenough to be removed by the etchant.

● Coarse grinding scratches must be removed;even very fine polishing scratches may not betolerable in examining failed parts.

● Pullout, pitting, cracking of hard particles,smear, and other preparation artifacts must beavoided.

● Relief (i.e., excessive surface-height varia-tions between structural features of differenthardness) must be minimized; otherwise, por-tions of the image are out of focus at highmagnifications. Excessive relief invalidatesimage analysis measurements and is undesir-able for wavelength-dispersive chemicalanalysis.

● The surface must be flat, particularly at edges(if they are of interest), or they cannot be ex-amined. Edge preservation is of critical im-portance in failure studies, because many fail-ures start at external surfaces.

● Coated or plated surfaces must be kept flat ifthey are to be examined, analyzed, measured,or photographed.

● An etchant should be used that reveals all ofthe structure at first. Later, it may be usefulto use a selective etchant that reveals only thephase or constituent of interest, or at least pro-duces strong contrast or color differences be-tween two or more phases present, to improvethe precision of microstructural measure-ments or to better reveal the relative presenceof undesirable constituents or phases.

If these characteristics are met, then the truestructure is revealed and can be interpreted, mea-sured, analyzed, and recorded. The preparationmethod should be as simple as possible, shouldyield consistent, high-quality results in a mini-mum of time and cost, and must be reproducible.

Preparation of metallographic specimens (Ref24) generally requires five major operations: sec-tioning, mounting (optional), grinding, polish-ing, and etching (optional).

Sectioning

It is certainly not uncommon in failure anal-ysis to encounter large specimens. Indeed, the

initial sectioning operation may be quite a chal-lenge. Bulk samples for subsequent laboratorysectioning may be removed from larger piecesusing methods such as core drilling, band- orhacksawing, flame cutting, or similar methods.Flame or torch cutting may be the only recoursein the field. If this is done, the torch-cut areamust be well away from the area to be examined,because the heat from this operation severely al-ters the original microstructure for some distancefrom the cut. Subsequent cutting can be per-formed with laboratory devices that are muchless damaging to the structure. Laboratory abra-sive-wheel cutting is recommended to establishthe desired plane of polish.

The most commonly used sectioning devicein the metallographic laboratory is the abrasivecutoff machine. All abrasive-wheel sectioningshould be performed wet. An ample flow of cool-ant, with an additive for corrosion protection andlubrication, should be directed uniformly into thecut. Wet cutting produces a smooth surface finishand, most importantly, guards against excessivesurface damage caused by overheating. Figures9(a) and (b) show the surface of quenched and



Fig. 8 Light micrograph of (a) a partially broken eutectoid carbon steel specimen embrittled by liquid copper at 1100�C (2012 �F) (arrows point to grain-boundary copper penetration), and (b) SEM fractograph of the completely

broken specimen

tempered A2 tool steel (59 HRC) cut withoutusing coolant. The cut surface was nickel platedfor edge preservation. Figure 9(a) shows a light-etching surface zone extending to a depth of ap-proximately 0.22 mm (0.009 in.), with a hard-ness of approximately 62.5 HRC. Beneath thelight-etching surface zone is a region that wassofter (53 to 56 HRC) and etches darker. Theunaffected matrix is beneath this zone, off theedge of the micrograph. Figure 9(b) shows theextreme surface at high magnification. Incipientmelting can be observed to a depth of approxi-mately 10 lm. The light-etching zone containsuntempered martensite, because the temperaturein this region was high enough to reaustenitizethe structure. The dark-etching zone beneath itsaw temperatures below the lower critical butgreater than the original tempering temperature.Abrasive wheels should be selected according tothe manufacturer’s recommendations.

Wheels consist of abrasive particles, chieflyalumina or silicon carbide (SiC), and filler in abinder material that may be a resin, rubber, or amixture of resin and rubber. Alumina is the pre-ferred abrasive for ferrous alloys, and SiC is thepreferred abrasive for nonferrous metals andminerals. Wheels have different bond strengthsand are recommended based on the suitability oftheir bond strength and abrasive type for the ma-terial to be sectioned. In general, as the hardnessof a material increases, abrasives become dullmore quickly, and the binder must be adjustedto release the abrasives when they become dull,so that fresh abrasive particles are available tomaintain cutting speed and efficiency. Conse-quently, these wheels are called “consumable”wheels, because they wear away with use. If theydo not wear at the proper rate, dull abrasives rubagainst the region being cut, generating heat andaltering the existing true microstructure. If thisheat becomes excessive, it can lead to grain orparticle coarsening, softening or phase transfor-mations, and, in extreme cases, burning or melt-ing. Different materials have different sensitivi-ties to this problem, but the need to balance thewheel break-down rate with the hardness of thepiece being sectioned produces the various rec-ommendations listed for cutting different mate-rials and metals at different hardnesses, such assteels.

Precision saws are commonly used in metal-lographic preparation to section materials thatare small, delicate, friable, extremely hard, orwhere the cut must be made as close as possibleto a feature of interest, or where the cut widthand material loss must be minimal. As the nameimplies, this type of saw is designed to makevery precise cuts. They are smaller in size thanthe usual laboratory abrasive cutoff saw and usemuch smaller blades, typically from 8 to 20 mm(0.3 to 0.8 in.) in diameter. These blades can beof the nonconsumable type, made of copper-basealloys with diamond or cubic boron nitride abra-sive bonded to the periphery of the blade, or theycan be consumable blades using alumina or SiCabrasives with a rubber-based bond. Blades forthe precision saws are much thinner than the

abrasive wheels used in an abrasive cutter, andthe load applied during cutting is much less.Consequently, less heat is generated during cut-ting, and damage depths are reduced. Whilepieces with a small section size that would nor-mally be sectioned with an abrasive cutter canbe cut with a precision saw, the cutting time isappreciably greater, but the depth of damage ismuch less. Precision saws are widely used forsectioning sintered carbides, ceramic materials,thermally sprayed coatings, printed circuitboards, electronic components, bone, teeth, andso on.

Mounting

The primary purpose of mounting metallo-graphic specimens is for convenience in han-dling specimens of difficult shapes or sizes dur-ing the subsequent steps of metallographicpreparation and examination. A secondary pur-pose is to protect and preserve extreme edges or

surface defects during metallographic prepara-tion. The method of mounting should in no waybe injurious to the microstructure of the speci-men. Pressure and heat are the most likelysources of injurious effects.

The most common mounting method uses adevice, called a mounting press, to provide therequired pressure and heat to encapsulate thespecimen with a thermosetting or thermoplasticmounting material. Common thermosetting res-ins include phenolic (often called Bakelite[Georgia-Pacific Corp.]), diallyl phthalate, andepoxy, while methyl methacrylate is the mostcommonly used thermoplastic mounting resin.Both thermosetting and thermoplastic materialsrequire heat and pressure during the molding cy-cle, but, after curing, mounts made of thermo-plastic resins must be cooled under pressure toat least 70 �C (158 �F), while mounts made ofthermosetting materials may be ejected from themold at the maximum molding temperature.However, cooling thermosetting resins under

Fig. 7 Light micrograph of a cross section of (a) a partially broken specimen and (b) a SEM fractograph of a completelybroken specimen of solution-annealed AISI 304 stainless steel after stress-corrosion crack testing in boiling (151

�C, or 304 �F) magnesium chloride



Fig. 10 Light micrographs of the surface of a carburized 8620 alloy steel specimen mounted in phenolic resin. Notethe shrinkage gap (see arrows in a) that has reduced the edge flatness. In (b), taken at 1000�, decarburization

at the surface has caused ferrite and pearlite to form, and this area is slightly out of focus. Specimen etched with nital

pressure to near-ambient temperature beforeejection significantly reduces shrinkage gap for-mation. Never rapidly cool a thermosetting resinmount with water after hot ejection from themolding temperature. This causes the metal topull away from the resin, producing shrinkagegaps that promote poor edge retention (Fig. 10a,b) because of the different rates of thermal con-traction.

Thermoset epoxy provides the best edge re-tention (Fig. 11a) of these resins (compare withFig. 11(b) for a phenolic mount and Fig. 11(c)for methyl methacrylate) and is virtually unaf-fected by hot or boiling etchants, while phenolicresins are badly damaged. The thermoplastic res-ins, such as methyl methacrylate, produce atransparent mount, which is helpful when tryingto grind to a specific feature, but provide pooredge retention (Fig. 11c). Electroless nickel plat-ing is an effective method for improving edgeretention, particularly for steels. However, if thearea to be studied is white or with low contrast,it may be difficult to determine where the nickelplating ends and the surface begins, as shown inFig. 11(d) Figure 12 shows an example of ion-nitrided hot work die steel with a brittle white-etching iron nitride surface layer that is quitevisible when mounted in epoxy resin but wouldprobably be very hard to detect if the surface hadbeen plated with nickel.

Shrinkage gaps between specimen and mountare a prime cause of loss of edge retention, asdiscussed subsequently. Besides this, abrasivescan become lodged in the gap and fall out, caus-ing contamination problems in a subsequentstep. Further, liquids can seep out of the gaps,despite best efforts to dry the specimen carefully,and obscure the microstructural details at theedge, or, worse yet, drip onto the objective (inan inverted microscope), causing loss of imageclarity or even damage. Figure 13 shows a largeshrinkage gap between a phenolic mount and apiece of 6061-T6 aluminum etched with diluteaqueous hydrofluoric acid. Nomarski differentialinterference contrast (DIC) reveals the curvatureat the edge of the specimen and water stains (ar-rows) along the edge of the specimen. Figure 14shows a high-speed steel specimen in a phenolicmount, where a large shrinkage gap is present,and the etchant, Vilella’s reagent, has seeped outand now obscures the edge detail (arrows).

An advantage of compression mounting isproduction of a mount of a predicable, conve-nient size and shape. Further, considerable in-formation can be engraved on the backside—thisis always more difficult with unmounted speci-mens. Manual (hand) polishing is simplified, be-cause the specimens are easy to hold. Also, plac-ing a number of mounted specimens in a holderfor semi- or fully-automated grinding and pol-ishing is easier with standard mounts than forunmounted specimens. Mounted specimens areeasier on the grinding/polishing surfaces thanunmounted specimens.

Cold-mounting materials require neither pres-sure nor external heat and are recommended formounting specimens that are sensitive to heat

and/or pressure. Acrylic resins are a widely usedcastable resin, due to their low cost and shortcuring time, but they are generally unsatisfactoryfor failure studies, because shrinkage is a prob-lem with acrylics. Epoxy resins, although moreexpensive than acrylics, are commonly used infailure studies, because epoxy physically adheresto specimens and can be drawn into cracks andpores, particularly if a vacuum impregnationchamber is employed and a low viscosity epoxyis used. Epoxies are very suitable for mountingfragile or friable specimens and corrosion or ox-idation specimens. Dyes or fluorescent agentsmay be added to epoxies for the study of porousspecimens such as thermal spray coated speci-mens. Epoxy resins are much more useful in fail-ure analysis work than acrylic resins.

Most epoxies are cured at room temperature,and curing times can vary from 2 to 20 h. Somecan be cured at slightly elevated temperatures inless time, as long as the higher temperature doesnot adversely affect the specimen. Acrylics dogenerate considerable heat during curing, andthis can be strongly influenced by the molding

technique used. Castable epoxy resins generatemuch less heat during curing, but this can varysubstantially. The amount of heat generated in-creases as the epoxy volume increases and as thecuring time decreases.

Edge Preservation

Edge preservation is the classic metallo-graphic problem in failure analysis work, andmany “tricks” have been promoted (most per-taining to mounting but some to grinding andpolishing) to enhance edge flatness. These meth-ods include the use of backup material in themount, the application of coatings to the surfacesbefore mounting, or the addition of a filler ma-terial to the mounting resin. Plating of a com-patible metal on the surface to be protected (elec-troless nickel has been widely used) is generallyconsidered to be the most effective procedure.However, image contrast at an interface betweena specimen and the electroless nickel may be in-adequate for certain evaluations. Figures 11(a)and (d) show the surface of a specimen of 1215

Fig. 9 Light micrographs of the surface (plated with nickel) of quenched and tempered A2 tool steel cut with an abrasivewheel without using coolant showing (a) a reaustenitized zone (light-etching area) and a back-tempered heat-

affected zone (dark-etching area), and (b) details of incipient melting at the surface (arrows). Specimen etched with nital



Fig. 12 Light micrograph of an ion-nitrided H13 toolsteel specimen mounted in epoxy thermoset-

ting resin. The arrows point to a white-etching iron nitridelayer at the surface that probably would not have beenobserved if the specimen was nickel plated for edge pro-tection. Specimen etched with nital

free-machining steel that was salt bath nitrided.One specimen was plated with electroless nickel;both were mounted in epoxy resin. It is hard totell where the nitrided layer stops for the platedspecimen (Fig. 11d), which exhibits poor imagecontrast between the nickel and the nitrided sur-face. This is not a problem for the nonplatedspecimen (Fig. 11a).

Introduction of new technology has greatly re-duced edge preservation problems. Mountingpresses that cool the specimen to near-ambienttemperature under pressure produce muchtighter mounts. Gaps that form between speci-men and resin are a major contributor to edgerounding, as shown in Fig. 10. Staining frombleed-out at shrinkage gaps obscures edge detail,(Fig. 14). Use of semiautomatic and automaticgrinding/polishing equipment, rather than man-ual (hand) preparation, increases surface flatnessand edge retention. To achieve the best results,particularly with a 200 mm (8 in.) diameterplaten and a 125 mm (5 in.) diameter holder, theposition of the specimen holder relative to theplaten should be adjusted so that the outer edgeof the specimen holder rotates out over the edgeof the surface on the platen during grinding andpolishing. This procedure can be used effectivelywith larger-diameter wheels, if the specimenholder diameter is large, relative to the platen.The use of “hard,” woven or nonwoven, naplesssurfaces for polishing with diamond abrasives(rather than softer cloths, such as canvas, bil-

liard, and felt) maintains flatness. Rigid grindingdiscs (RGDs) yield surfaces with exceptionalflatness. Final polishing with low-nap cloths forshort times introduces very little rounding, com-pared to use of higher-nap, softer cloths.

These procedures produce better edge reten-tion with all thermosetting and thermoplasticmounting materials. Nevertheless, there are stilldifferences among the polymeric materials usedfor mounting. Thermosetting resins provide bet-ter edge retention than thermoplastic resins. Ofthe thermosetting resins, diallyl phthalate pro-vides little improvement over the much-less-ex-pensive phenolic compounds. The best resultsare obtained with an epoxy-based thermosettingresin that contains a filler material. For compar-ison, Fig. 11 shows micrographs of a salt bathnitrided 1215 steel specimen mounted in a phe-nolic resin (Fig. 11b) and in methyl methacrylate(Fig. 11c) at 1000�. These specimens were pre-pared in the same specimen holder as thoseshown in Fig. 11(a) and (d), but neither displaysacceptable edge retention at 1000�.

In the 1970s, very fine alumina spheres weremixed with liquid epoxy in an effort to improveedge retention. This is not a satisfactory proce-dure, because the particles are extremely hard(�2000 HV) and their grinding/polishing char-acteristics are incompatible with softer metalsplaced inside the mount. As a result, this productis no longer promoted for improving the edgeretention of metallic specimens. Recently, a soft

ceramic shot (�775 HV) was introduced that hasgrinding/polishing characteristics compatiblewith metallic specimens placed in the mount.Figure 15 shows an example of improving edgeretention of annealed hot work die steel usingsoft ceramic shot in an epoxy mount.

Following are general guidelines for obtainingthe best possible edge retention. All of these fac-tors contribute to the overall success, althoughsome are more critical than others:

● Properly mounted specimens yield betteredge retention than unmounted specimens,because rounding is difficult, if not impossi-ble, to prevent at a free edge. Hot compres-sion mounts yield better edge preservationthan castable resins.

● Electrolytic or electroless plating of the sur-face of interest provides excellent edge reten-tion. If the compression mount is cooled tooquickly after polymerization, the plating maybe pulled away from the specimen, leaving agap. When this happens, the plating is inef-fective for edge retention.

● Thermoplastic compression mounting mate-rials are less effective than thermosetting res-ins. The best thermosetting resin for edge re-tention is an epoxy-based resin containing ahard filler material.

● Do not hot eject a thermosetting resin afterpolymerization and cool it quickly to ambient(e.g., by cooling it in water), because a gapforms between specimen and mount due tothe differences in thermal contraction rates.Fully automated mounting presses cool themounted specimen to near-ambient tempera-ture under pressure, and this greatly mini-mizes gap formation due to shrinkage.

● Automated grinding/polishing equipmentproduces flatter specimens than manual(hand) preparation.

● Use the central force mode (defined later inthis article) with an automated grinder/pol-isher, because this method provides betterflatness than individual pressure mode (de-fined later in this article).

Fig. 11 Light micrographs of specimens of 1215 carbon steel that were salt bath nitrided and mounted in differentresins. (a) Thermosetting epoxy resin. (b) Phenolic thermosetting resin. (c) Methyl methacrylate thermoplastic

resin. (d) Electroless nickel plated and mounted in epoxy resin (resin not in the field of view). All four specimens wereprepared in the same holder and were etched with nital. The arrows point to the nitrided surface layer.



Fig. 16 Light micrograph showing cutting damage (ar-rows at left) and a burr at the corner of a spec-

imen of commercial-purity titanium (ASTM F67, grade 2)etched with modified Weck’s reagent and viewed with po-larized light plus sensitive tint. The arrow along the topedge points to a surface layer containing mechanical twins.

Fig. 15 Good edge retention obtained in a cast epoxymount containing soft ceramic shot filler. (Note

the round particles in the epoxy at the top.) The specimenis annealed H13 hot work die steel, and it was etched withpicral.

● Orient the position of the smaller-diameterspecimen holder so that, as it rotates, its pe-riphery slightly overlaps the periphery of thelarger-diameter platen.

● Use pressure-sensitive-adhesive (PSA)-backed SiC grinding paper (when SiC is used)rather than water on the platen and a periph-eral holddown ring, and PSA-backed polish-ing cloths rather than stretched cloths.

● Metal-bonded or resin-bonded grinding discsproduce excellent flat surfaces for a wide va-riety of materials.

● Use hard, napless surfaces for rough polish-ing (until the final polishing step(s)) and finepolishing. Use a napless or a low- to medium-

nap cloth, depending on the material beingprepared, for the final step(s), and keep itbrief.

● Rigid grinding disks produce excellent flat-ness and edge retention and should be usedwhenever possible.

Grinding

Grinding should commence with the finest gritsize that establishes an initially flat surface andremoves the effects of sectioning within a fewminutes. An abrasive grit size of 180 or 240 inthe American National Standards Institute/

Coated Abrasive Manufacturers’ Institute(ANSI/CAMI) grading system (P180 or P280 inthe FEPA, or Federation Europeenne des Fabri-cants de Produits Abrasifs system) is coarseenough to use on specimen surfaces sectionedby an abrasive cutoff wheel. Hacksawed, band-sawed, or other rough surfaces usually requireabrasive grit sizes in the range of 120- to 180-grit (P120 to P180). Grinding must remove thedamage created by sectioning (Fig. 16). If theinitial grinding step does not remove this layer,the plane of polish may be within the zone ofsurface damage from cutting, and the true struc-ture is not observed. The abrasive used for eachsucceeding grinding operation should be one ortwo grit sizes smaller than that used in the pre-ceding step. A satisfactory grinding sequencemight involve SiC papers with grit sizes of 240-, 320-, 400-, and 600-grit (P280, P400, P800,and P1200, respectively). This sequence is usedin the “traditional” preparation approach.

As with abrasive-wheel sectioning, all grind-ing steps should be performed wet using water,provided that water has no adverse effects on anyconstituents of the microstructure. If water can-not be used during grinding, then some othernon-aqueous coolant must be used, for example,kerosene or mineral spirits. Wet grinding mini-mizes specimen heating, prevents the abrasivefrom becoming loaded with metal removed fromthe specimen being prepared, and minimizes air-borne metal-particle contamination and healthproblems.

Each grinding step, while producing damageitself, must remove the damage from the previ-ous step. The depth of damage decreases withthe abrasive size but so does the metal removalrate. For a given abrasive size, the depth of dam-age introduced is greater for soft materials thanfor hard materials. Grinding abrasive can be-come entrapped, or embedded, in the surface ofspecimens. This is especially true for soft, low-melting-point alloys ground using SiC paper.Embedding is more common with the finer-grit-

Fig. 14 Light micrograph showing stain (arrows point-ing up) from the etchant (Vilella’s reagent) that

seeped from the shrinkage gap (wide arrows pointingdown) between the phenolic resin mount and the specimenof M2 high-speed steel

Fig. 13 Light micrograph showing a very large shrinkage gap between the phenolic resin mount (PM) and a specimenof 6061-T6 aluminum etched with aqueous 0.5% hydrofluoric acid. Note the metal flow at the specimen edge

(revealed using Nomarski DIC illumination) and the water stains (arrows on the aluminum specimen).



Fig. 19 Light micrograph showing pitting (arrows) afterpreparation of cold-drawn brass (Cu-20%Zn).

Not etched

Fig. 18 Light micrograph showing residual damage (ar-rows) from preparation that was not removed

by the procedure when this specimen of commercial-puritytitanium was prepared. The specimen was etched withKroll’s reagent and photographed with Nomarski DIC illu-mination.

Fig. 17 Light micrograph showing a SiC grinding-abra-sive particle (arrow) lodged in a weldment in

6061-T6 aluminum etched with aqueous 0.5% hydro-fluoric acid

Fig. 21 Light micrograph illustrating staining (arrow)on the surface of a prepared specimen of Ti-

6%Al-2%Sn-4%Zr-2%Mo. The specimen was not etched.

Fig. 20 Light micrograph illustrating “comet tails” em-anating from hard nitrides on the surface of a

prepared specimen of H13 tool steel. The specimen is unet-ched and viewed with Nomarski DIC.

size papers. Figure 17 illustrates embedding ofSiC grinding paper abrasive in soft metals. Theelectron microprobe was used to study embed-ding of abrasives during grinding, and coating ofthe paper with candlewax or soap greatly re-duced embedding (Ref 25).

For automated preparation using a multiple-specimen holder, the initial step is called planargrinding. This step must remove the damagefrom sectioning while establishing a commonplane for all of the specimens in the holder, sothat each specimen is affected equally in subse-quent steps. Silicon carbide and alumina abra-sive papers are commonly used for the planargrinding step and are very effective. Besidesthese papers, there are a number of other optionsavailable. One option is to planar grind the spec-imens with a conventional alumina grindingstone. This requires a special-purpose machine,because the stone must rotate at a high speed,�1500 rpm, to cut effectively. The stone mustbe dressed regularly with a diamond tool tomaintain flatness, and embedding of aluminaabrasive in specimens can be a problem. How-ever, the approach provides high removal rates.

Polishing

Polishing is the final stage in producing a de-formation-free surface that is flat, scratch-free,and mirrorlike in appearance. Such a surface isnecessary for subsequent metallographic inter-pretation, both qualitative and quantitative. Thepolishing technique used should not introduceextraneous structures such as disturbed metal(Fig. 18), pitting (Fig. 19), dragging out of in-clusion, “comet tailing” (Fig. 20), staining (Fig.21), relief (height differences between differentconstituents, or between holes and constituents)(Fig. 22), or embedding (Fig. 23). Polishing usu-ally is conducted in several stages. Traditionally,rough polishing is conducted with 9, 6, or 3 lmdiamond abrasives charged onto napless or low-nap cloths. For hard materials, such as through-hardened steels, ceramics, and cemented car-bides, two rough-polishing steps may berequired. The initial rough-polishing step may befollowed by polishing with 1 lm diamond on anapless, low-nap, or medium-nap cloth. A com-patible lubricant should be used sparingly to pre-vent overheating or deformation of the surface.Intermediate polishing should be performed

thoroughly, so that final polishing may be ofminimal duration.

Manual polishing, or hand polishing, is usu-ally conducted using a rotating wheel, where theoperator rotates the specimen in a circular pathcounter to the wheel rotation direction. To obtainthe best possible surfaces, it is necessary to useanother step, typically with a 0.05 lm aluminaor colloidal silica abrasive. This step can be per-formed on a wide variety of cloths. In the tra-ditional approach, a medium-nap synthetic suedecloth is used. While this is still a useful cloth formany materials, increasing use is being made ofsynthetic polyurethane pads. This type of clothis recommended when edge retention must bemaximized.

The requirements of a good polishing clothinclude the ability to hold the abrasive media,long life, absence of any foreign material thatmay cause scratches, and absence of any pro-cessing chemical (such as dye or sizing) that mayreact with the specimen. Many cloths of differentfabrics, weaves, or naps are available for metal-lographic polishing. Napless or low-nap clothsare recommended for rough polishing with dia-mond abrasive compounds. Napless, low-, me-dium-, and, occasionally, high-nap cloths areused for final polishing. This step should be briefto minimize relief.

Mechanical polishing can be automated to ahigh degree using a wide variety of devices rang-ing from relatively simple systems to rather so-phisticated, minicomputer, or microprocessor-controlled devices. Units also vary in capacityfrom a single specimen to a half-dozen or moreat a time. These systems can be used for allgrinding and polishing steps. These devices en-able the operator to prepare a large number ofspecimens per day, with a higher degree of qual-ity than by hand polishing and at reduced con-sumable costs. Automatic polishing devices pro-duce the best surface flatness and edge retention.There are two automated approaches for holdingspecimens. Central force uses a specimen holder,with each specimen held in place rigidly. Theholder is pressed downward against the prepa-ration surface, with the force applied to the entire



Fig. 23 Light micrograph showing 6 lm diamond (ar-rows) abrasive embedded in the surface of a

partly prepared specimen of leadFig. 22 Light micrographs depicting (a) excessive and (b) low relief around voids in a braze between an austenitic

stainless steel and Monel. The specimen was etched with glyceregia.

holder. Central force yields the best edge reten-tion and specimen flatness. If the results afteretching are inadequate, the specimens must beplaced back in the holder, and the entire prepa-ration sequence must be repeated. Instead of do-ing this, most metallographers repeat the finalstep manually and then re-etch the specimen. In-dividual force machines have a holder that doesnot hold the specimen rigidly. Specimens areplaced inside a hole cut into a holder, and a pis-ton comes down and presses the specimenagainst the working surface. Thus, the specimenscan be removed and examined easily during thepreparation cycle without losing planarity. Thisprovides convenience if a step must be repeated,but the method is limited to mounted specimens,usually round mounts, and edge retention is notas good as with a central force holder.

Polishing usually involves the use of one ormore of the following abrasives: diamond, alu-mina, and amorphous silicon dioxide in colloidalsuspension. For certain materials, cerium oxide,chromium oxide, magnesium oxide, or iron ox-ide may be used, although these are used infre-quently. With the exception of diamond, theseabrasives are normally suspended in distilledwater, but if the metal to be polished is not com-patible with water, other suspensions, such asethylene glycol, alcohol, kerosene, or glycerol,may be required. The diamond abrasive shouldbe extended only with the carrier recommendedby the manufacturer. Most diamond pastes andsuspensions are water-based products, and theseare suitable for most materials. However, oil-based diamond suspensions are needed to pre-pare materials sensitive to water.

Over the past forty years, a general proce-dure has been developed that is quite success-ful for preparing most metals and alloys. Thismethod is based on grinding with SiC water-proof papers through a series of grits, thenrough polishing with one or more diamondabrasive sizes, followed by fine polishing withone or more alumina suspensions of differentparticle size. This procedure is called the “tra-ditional” method and is described in Table 1,

listing equivalent ANSI/CAMI and FEPA gritsizes for the SiC paper.

This procedure is used for manual or auto-mated preparation, although manual control ofthe force applied to a specimen would not bevery consistent. Complementary motion meansthat the specimen holder is rotated in the samedirection as the platen and does not apply tomanual preparation, because this cannot be done.In manual preparation, the specimen is held stillin grinding, aside from moving between the edgeand the center. In manual polishing, the speci-men is rotated clockwise, against the counter-clockwise wheel rotation direction. Some ma-chines can be set so that the specimen holderrotates in the direction opposite to that of theplaten, called “contra.” This provides a more ag-gressive action but was not adopted when thetraditional approach was automated. The tradi-tional method is not rigid, because other polish-ing cloths may be substituted, and one or moreof the polishing steps might be omitted. Timesand pressures could be varied, as well, to suit theneeds of the work or the material being prepared.This is the “art” of metallography.

New concepts and new preparation materialshave been introduced that enable metallogra-phers to shorten the process while producing bet-ter, more consistent results. Much of this efforthas centered on reducing or eliminating the useof SiC paper in the grinding steps. In all cases,an initial grinding step must be used, but thereis a wide range of materials that can be choseninstead of SiC paper. There is nothing wrongwith the use of SiC for the first step, except thatit has a short life. If an automated device is usedthat holds a number of specimens rigidly (centralforce), then the first step must remove the sec-tioning damage on each specimen and bring allof the specimens in the holder to a commonplane. This first step is often called planar grind-ing. Silicon carbide paper can be used for thisstep, although more than one sheet may beneeded. Alternatively, the metallographer coulduse alumina paper, an alumina stone on a dedi-cated high-speed grinder, metal- or resin-bonded

diamond discs, stainless steel mesh cloth (dia-mond is applied during use), RGDs (diamond isapplied during use), or lapping platens of severaltypes (diamond is applied and becomes embed-ded in the surface during use).

In contemporary preparation methods, one ormore steps using diamond abrasives on naplesssurfaces usually follow planar grinding. Pres-sure-sensitive-adhesive-backed silk, nylon, orpolyester cloths are widely used. These givegood cutting rates, maintain flatness, and mini-mize relief. Silk cloths provide the best flatnessand excellent surface finishes relative to the di-amond size used. Thicker hard, woven cloths aremore aggressive, give nearly as good a surfacefinish, similar excellent flatness, and longer lifethan silk cloths. Synthetic chemotextile padsgive excellent flatness and are more aggressivethan silk. They are excellent for retaining sec-ond-phase particles and inclusions. Diamondsuspensions are very popular with automatedpolishers, because they can be added easily dur-ing polishing, although it is still best to chargethe cloth initially with diamond paste of the samesize to get polishing started quickly.

Final polishing could be performed with avery fine diamond size, such as 0.1 lm diamond,depending on the material, the metallographer’sneeds, and personal preferences. Otherwise, finalpolishing is performed with colloidal silica orwith alumina slurries using napless or low- tomedium-nap cloths. For some materials, such astitanium and zirconium alloys, an attack polish-ing solution is added to the abrasive slurry toenhance deformation and scratch removal andimprove polarized light response. Contra rota-tion (head moves in the direction opposite to theplaten) is preferred, because the slurry stays onthe cloth better, although this does not work ifthe head rotates at a high revolutions per minute.Examples of generic preparation practices formany metals and alloys are found in Tables 2to 4.

The starting SiC abrasive size is chosen basedon the degree of surface roughness and depth ofcutting damage and the hardness of the material.Never start with an abrasive size coarser than



Table 4 Four-step contemporary practice for nonferrous metals using a rigid grinding disc

Surface Abrasive/sizeLoad, N

(lbf)Speed,

rpm/direction Time min

Waterproof PSA paper 240/P280- or 320/P400-grit SiC, water cooled 22 (5) 240–300Comp

Until plane

Rigid grinding disk 6 lm polycrystalline diamond suspension 22 (5) 120–150Comp

5

Synthetic woven cloth 3 lm polycrystalline diamond suspension 22 (5) 120–150Comp

4

Synthetic short nap cloth �0.05 lm colloidal silica or sol-gel alumina suspensions 22 (5) 120–150Contra

2

Note: Comp, complementary (platen and specimen holder both rotate in the same direction). Contra, platen and specimen holder rotate in oppositedirections

Table 3 Four-step contemporary practice for steels using a rigid grinding disc

Surface Abrasive/sizeLoad, N

(lbf)Speed,

rpm/direction Time, min

Waterproof PSA paper 120/P120-, 180/P180-, or 240/P280-grit SiC, water cooled 27 (6) 240–300Comp.

Until plane

Rigid grinding disk 9 lm polycrystalline diamond suspension 27 (6) 120–150Comp.

5

Synthetic woven cloth 3 lm polycrystalline diamond suspension 27 (6) 120–150Comp.

4


2

Note: Comp., Complementary (platen and specimen holder both rotate in the same direction). Contra, platen and specimen holder rotate in oppositedirections

Table 2 Generic four-step contemporary practice for many metals and alloys

Surface Abrasive/sizeLoad N

(lbf)Speed,

rpm/direction Time, min

Waterproof PSA paper 120/P120-, 180/P180, or 240/P280-grit SiC, water cooled 27 (6) 240–300Comp.

Until plane

Silk cloth 9 lm polycrystalline diamond suspension 27 (6) 120–150Comp.

5

Synthetic woven cloth 3 lm polycrystalline diamond suspension 27 (6) 120–150Comp.

4


2

Note: Comp., Complementary (platen and speciman holder both rotate in same direction). Contra, platen and specimen holder rotate in oppositedirections

Table 1 The traditional method for preparing most metals and alloys

Surface Abrasive/size Load, N (lbf) Speed, rpm/direction Time, min

Waterproof PSA paper 120/P120-grit SiC, water cooled 27 (6) 240–300Comp

Until plane


1–2


1–2


1–2


1–2

Canvas 6 lm diamond paste with lubricant 27 (6) 120–150Comp

2

Billiard or felt cloths 1 lm diamond paste with lubricant 27 (6) 120–150Comp

2

Synthetic suede pad Aqueous 0.3 lm �-alumina slurry 27 (6) 120–150Comp

2

Synthetic suede pad Aqueous 0.05 lm c-alumina slurry 27 (6) 120–150Comp

2

Note: Comp, complementary (platen and specimen holder both rotate in the same direction)

necessary to remove the cutting damage andachieve planar conditions in a reasonable time.A 1 lm diamond step can be added for moredifficult-to-prepare materials using a naplesscloth and a similar approach as the third step, buta 3 min polish.

A similar scheme can be developed usingRGDs. These discs are generally restricted tomaterials above a certain hardness level, such as175 HV, although some softer materials can beprepared using them. The disc can also be usedfor the planar grinding step. An example of sucha practice, applicable to nearly all steels (resultsare marginal for solution-annealed austeniticstainless steels), is given in Table 3.

The planar grinding step could also be per-formed using a 45 lm metal-bonded or a 30 lmresin-bonded diamond disc or with a RGD and15 or 30 lm diamond, depending on the mate-rial. Rigid grinding discs contain no abrasive;they must be charged during use, and suspen-sions are the easiest way to do this. Polycrystal-line diamond suspensions are favored over mon-ocrystalline synthetic diamond suspensions formost metals and alloys due to their higher cuttingrate. Again, a 1 lm diamond step can be addedfor difficult materials or to ensure generation ofthe required degree of perfection in the surfacefinish.

Rigid grinding discs designed for soft metalsand alloys are used in a similar manner. Thesediscs are quite versatile and can be used to pre-pare harder materials as well, although their wearrate is greater when used to prepare very hardmaterials. A generic five-step practice is givenin Table 4 for soft metals and alloys.

The planar grinding step can be performedwith the 30 lm resin-bonded diamond disc orwith a second RGD and 15 or 30 lm diamond,depending on the metal or alloy. For some verydifficult metals and alloys, a 1 lm diamond stepon a synthetic woven cloth (similar to step 3 butfor 3 min) could be added, and/or a brief vibra-tory polish (use the same cloths and abrasives asfor step 4) may be needed to produce perfectpublication-quality images.

Electrolytic Polishing

Electrolytic polishing can be used to preparespecimens with deformation-free surfaces. Thetechnique offers reproducibility and speed. Inmost cases, the published instructions for elec-trolytes tell the user to grind the surface to a 600-grit (P1200) finish and then electropolish for ap-proximately 1 to 2 min. However, the depth ofdamage after a 600-grit (P1200) finish may beseveral micrometers, but most electropolishingsolutions remove only about 1 lm/min. In thiscase, the deformation is not completely re-moved. In general, electropolished surfaces tendto be wavy rather than flat, and focusing may bedifficult at high magnifications. Further, electro-polishing tends to round edges associated withexternal surfaces, cracks, or pores. In two-phasealloys, one phase polishes at a different rate than

another, leading to excessive relief. In somecases, one phase may be attacked preferentially,and inclusions are usually attacked. Conse-quently, electrolytic polishing is not recom-

mended for failure analysis or image analysiswork, except possibly as a very brief step at theend of a mechanical polishing cycle to removewhatever minor damage may persist.



Fig. 25 Light micrographs of two cross-sectional views of a seam found on a closed-die forged pitman arm showingdecarburization and internal oxidation. Etched with 2% nital

Fig. 24 (a) Macrograph of fracture, (b) SEM fractograph, and (c) light micrograph showing shrinkage cavities in an unusual tensile fracture from a carbon steel casting. Themicrostructure was revealed using nital.

Examination of Microstructures

The second main use of light microscopy isto determine the microstructure of the materialin question to evaluate its influence on the fail-ure. Such examination is first performed at theorigin of the failure to detect any anomaliesthat may have arisen from inadequate materialquality, fabrication or heat treatment deficien-cies, or alterations due to service conditions. Infailures that do not involve fracture, for ex-ample, certain types of wear or corrosion fail-ures, the relationship of the microstructure tothe observed damage is assessed at the damagesites (Ref 26, 27). Examples of such work aregiven in the following to illustrate the value oflight microscopy.

Figure 24(a) shows a micrograph of an un-usual tensile test fracture that was obtained dur-ing the evaluation of a carbon steel (0.25% C,0.63% Mn, 0.27% Si) casting. Instead of theusual cup-and-cone tensile fracture, the surfacewas at an angle, approximately 45�, to the tensileaxis and was rather rough. Scanning electron mi-croscope examination (Fig. 24b) of the fracturerevealed numerous voids typical of shrinkagecavities. Light microscope examination (Fig.24c) also reveals these cavities. Neither SEM orLM examination by itself was satisfactory fordetermining why the fracture was unusual, buttogether they provide a more complete picture.The shrinkage cavities caused failing yieldstrength, elongation, and reduction of area re-sults.

Another example of a material quality prob-lem is shown in Fig. 25(a) and (b). These micro-graphs show a seam that was the cause of rejec-tion for a forged pitman arm. In this case, theseam is not perpendicular to the surface becauseof the metal flow during forging.

Another common material problem is thepresence of decarburization that may be presenton as-rolled stock or may form during heat treat-ment or, in some cases, during service. Light mi-croscopy is the generally accepted method fordetecting decarburization and measuring its ex-tent (Ref 28). Figures 26(a), (b), and (c) showexamples of decarburized AISI 5160 spring steelin the as-rolled condition (Fig. 26a) and afterheat treatment (Fig. 26b, c). Decarburization ofhardened coil springs is not desired, because itreduces the fatigue life of the spring.

A less common example of decarburization isshown in Fig. 27, which shows the surface of adecarburized solution-annealed austenitic man-ganese steel. At locations where the carbon con-tent is below approximately 0.50%, epsilon mar-tensite is observed (Ref 29, 30). This structuredoes not possess the remarkable work-hardeningcapacity typical of such alloys.

Figure 28 provides another example of inade-quate material quality. This shows a defect ob-served on the polished inside diameter of anAISI 420 stainless steel mold. Sectioning of themold at the defect and light microscopy exami-nation revealed a large silicate inclusion thatcaused the defect.

Numerous failures have been traced to prob-lems occurring during heat treatment. The fol-lowing examples show how light microscopywas employed to analyze such failures.

Figure 29 shows the operating surface of anAISI L6 tool steel punch that exhibited poor ser-vice life. Examination of the microstructure re-vealed that it was under-austenitized in heattreatment. The hardness was several points HRCbelow the expected value.

Figure 30 shows the microstructure of a rollmade from AISI 01 tool steel that cracked duringquenching. The quench cracks were located ad-



Fig. 27 Light micrograph showing epsilon martensiteat the surface of a decarburized (less than 0.5%

C) austenitic manganese steel specimen. Etched with 2%nital/20% sodium metabisulfite

Fig. 26 Light micrographs of decarburization observed on cross sections of as-rolled and heat treated AISI 5160H alloy steel spring. (a) Nickel plating on top of scale on an as-rolled specimen. (b) Partial decarburization at the surface of a hardened specimen. (c) Free ferrite and partial decarburization at the surface of a hardened specimen. Etched

with 2% nital

Fig. 29 (a) The working face of an AISI L6 punch that failed after limited service, because (b) the punch was under-austenitized. Specimen etched with nital

Fig. 28 (a) AISI 420 stainless steel mold containing a defect (arrow) observed after polishing the inside diametersurface. (b) Microscopic examination revealed a large silicate inclusion (unetched).

jacent to deep stamp marks, and the microstruc-ture contained considerable retained austenite.No residual carbide, normally present in thisgrade when properly heat treated, was present.Consequently, an excessively high austenitizingtemperature had been employed. The hardnesswas 56/57 HRC, which was increased to 62/64HRC after the specimen was refrigerated in liq-uid nitrogen.

Figure 31 shows two views of the microstruc-ture of a jewelry-striking die made from AISI S7tool steel that cracked soon after being placed inservice. The views show a coarse zone at thesurface, consisting of coarse plate martensite andretained austenite. The carbon content at the sur-face was 0.79%, while the interior was 0.53%.The die had been lightly carburized to a depth ofapproximately 0.5 mm (0.020 in.) due to im-proper furnace atmosphere control.

Figure 32 shows the microstructure of acracked carburized tread of a track wheel madefrom AISI 1035 carbon steel. The 60 cm (24in.) diameter track wheel was carburized at ahigher temperature than usual, and the diffu-sion cycle after carburizing had been omitted.Microstructural examination revealed thatcracking followed the grain-boundary carbide

network present due to the improper carburiz-ing practice.

Figure 33 shows the microstructure of an AISID2 tool steel powder metallurgy die that was de-formed at one end after heat treatment. Light mi-

croscopy revealed that the steel was locallymelted in this region, apparently due to flameimpingement during austenization.

As a final example of a heat-treatment-relatedfailure, Fig. 34 shows the microstructure of an



Fig. 30 Light micrograph of overaustenitized AISI 01tool steel containing coarse plate martensite

and substantial unstable retained austenite. Specimenetched with nital

Fig. 33 Light micrograph of a melted region found onan AISI D2 powder metallurgy die after heat

treatment. Specimen etched with Marble’s reagent

Fig. 32 Light micrographs of a carburized AISI 1035 track wheel that cracked due to the presence of an extensivegrain-boundary carbide film. Specimen etched with nital

Fig. 31 Light micrographs of an AISI S7 tool steel jewelry-striking die that failed due to the presence of a carbon-enriched surface layer that contained coarse plate martensite and unstable retained austenite. Specimen

etched with nital

Fig. 34 Light micrograph of a grossly overaustenitizedAISI D2 draw die insert. Specimen etched with

Marble’s reagent

AISI D2 draw die insert that exhibited gallingand chipping after limited service. Examinationof the microstructure revealed that the austeni-tizing temperature was well above the recom-mended 1010 �C (1850 �F) temperature, highenough to cause liquidation at the grain bound-aries. Note the nearly complete grain-boundarynetwork of skeletal carbides, similar in appear-ance to an as-cast condition.

Failures may also arise during fabrication pro-cesses. The next several examples illustrate suchproblems and the use of light microscopy. Awide variety of problems can occur; these ex-amples illustrate only a few of many such prob-lems.

Figure 35 shows planar and through-thicknessviews of cracks observed in several reausteniti-zed zones in a forging steel specimen. An elec-tric pencil had been used to identify the com-ponent. The cracked regions were present on theface placed against the grounding plate. Appar-ently, arcing occurred along the edges touching

the grounding plate, producing enough heat tolocally reaustenitize the steel. On cooling, thehardenability was sufficient to form as-quenchedmartensite in these regions. The transformation-related expansion caused cracking in these reaus-tenitized spots.

Figure 36 shows an example of central burst-ing (“chevron cracking”) in extruded, as-rolledAISI 4615 alloy steel. Although optimal extru-sion parameters can generally prevent such fail-ures, the ductility of the material is also an im-portant variable. Because this alloy hassubstantial hardenability, it is difficult to preventformation of bainite and martensite in small, as-rolled section sizes. In this case, the microstruc-ture consisted of ferrite, bainite, and martensite(the arrows point to microcracks present in mar-tensite patches).

Final grinding of hardened tool steel com-ponents is an important processing step thatmust be carefully controlled. Improper grindingpractices can produce a fine network of surface

cracking and a characteristic scorch pattern eas-ily revealed by macroetching (Ref 31, 32). Ex-amination of the microstructure at the cracks re-veals their shallow nature and a back-temperedcondition at the surface, as shown in Fig. 37. Insome cases, a shallow, reaustenitized light-etch-ing layer of as-quenched martensite is found at



Fig. 36 (a) Central bursting during extruding of AISI 4615 alloy steel specimen was promoted by (b) the presence ofbainite and martensite (arrows point to microcracks in the martensite) in the as-rolled stock. Specimen etched

with 4% picral, followed by 2% nital

Fig. 35 Light micrographs showing (a) planar and (b) through-thickness views of cracking that occurred at reausten-itized spots due to arcing during electric pencil coding. Specimen etched with nital

the surface (Ref 31) above the back-temperedzone. In many cases of abusive grinding, mi-crostructural examination reveals that the diewas not tempered. As-quenched tool steels arevery difficult to grind without causing suchcracking. In the example shown, however, theAISI 01 tool steel die was properly tempered,and one must conclude that the grinding opera-tion was at fault.

Electrical discharge machining (EDM) iswidely used to produce cavities in tool steels.Because it is a spark-erosion process that gen-erates considerable temperature and localizedmelting, it must be rigorously controlled. AfterEDM, the cavity surface generally is stoned toremove melted surface layers, and the part istempered. However, numerous failures havebeen observed in EDM-processed components(Ref 31, 32). Figure 38 shows a classic exampleof the microstructure of a failed part that wasmachined by this technique but not properlyposttreated. This was a plastic mold made fromAISI S7 tool steel, where pitting was observedon the cavity after polishing. The microstructureat the pit shows a large remelted surface layer(note dendrites). Beneath this layer is a white-etching zone of as-quenched martensite, typicalof such failures. Beneath this is a back-tem-pered zone, where the temperature was belowthe upper critical temperature for the steel. Inmany such EDM-related failures, the as-castlayer is not present, but the as-quenched regionis always observed.

Failures also occur due to service conditions,and the next several examples illustrate some ofthe many problems that can occur. These ex-amples are provided to illustrate the value oflight microscopy in such studies.

Figure 39 shows the microstructure of an as-cast 25%Cr-12%Ni heat-resisting alloy used asa hook for holding a basket of parts during aus-tenitizing and water quenching. The alloy con-tained delta ferrite that transformed to sigma,producing a discontinuous network that causedthe hook to crack. Electrolytic etching was usedto reveal only the sigma phase.

Figure 40 shows surface damage due to wearon a 4485 alloy steel medart roll. The specimenwas plated with electroless nickel for edge re-tention. Some of the nickel can be observed inthe crack. Note that the surface layer has beenreaustenitized due to service-generated heat.Oxidation at the surface and flow in the layercan also be observed.

It is not unusual to observe as-quenched mar-tensitic layers produced on the surface of steelssubjected to heavy wear conditions. Figure 41shows such a layer on the surface of a scrapchopper knife made from a proprietary wear-resistant tool steel. Cracking and spalling maybe produced at such layers because of their brit-tle nature. Such a condition is often observedon steel railroad rails where cracking and spall-ing has occurred. Figure 42 shows an exampleof surface cracking near a spall on a 136 lb/ydrail. Microstructural examination revealed threeregions at the surface. The outer zone is featu-

reless, typical of heavily deformed as-quenchedmartensite. The hardness was approximately 60HRC. Beneath this is a zone containing thesame white-etching martensite plus a networkof ferrite. Beneath this zone is the normal pear-

litic structure of an as-rolled rail steel. Temper-ing of the sample produced a tempered marten-site structure with spheroidized carbides in theouter zones containing the white-etching con-stituent.



Fig. 38 (a) Pitting on this mold cavity (arrow) was observed after polishing of the AISI S7 plastic mold and was causedby the use of improper post-EDM procedures. (b) The classic appearance of such failures. There is a large,

remelted surface layer above a reaustenitized, untempered zone. Specimen etched with nital

Fig. 37 (a) Abusive grinding caused this 50 mm (2 in.) diameter AISI 01 tool steel die to crack (left, after dye-penetrantinspection). (b) Typical appearance of the cracks (etchant has bled out of the crack, producing a stain around

it). Specimen etched with nital

Fig. 41 Light micrograph of the surface of a badly wornsteel chopper knife made from an air-harden-

able tool steel showing a reaustenitized surface zone anda back-tempered region below it. The large angular parti-cles are nitrides. Specimen etched with nital

Fig. 40 Light micrograph showing wear damage at thesurface of a 4485 alloy steel medart roll. The

surface was nickel plated for edge retention and etchedwith nital.

Fig. 39 Light micrograph showing sigma phase re-vealed by selective etching with 10N KOH

(electrolytic). The brittle sigma phase caused extensivecracking in a 25%Cr-12%Ni cast heat treatment baskethook.

Field Metallography

Those who regularly perform failure analysisstudies occasionally encounter situations wherethe specimen must be examined in the field. Por-table equipment (Ref 33, 34) is available for ei-ther mechanical or electrolytic polishing. Porta-ble microscopes can be used to view themicrostructures and take micrographs.

In some cases, it may be difficult to examinethe polished area with the microscope, and themetallographer must resort to replication tech-niques. Replicas can be examined with the LM

(Ref 35–38) as well as with the TEM or SEM.Quite good results can be obtained, althoughetching must be somewhat heavier than usual,and certain microstructures, for example, mar-tensite, are difficult to examine with optical rep-licas. Figure 43 shows the microstructure of areinforcing bar using replicating tape. Thisshows a ferrite-pearlite microstructure where thelamellae are resolvable. A few inclusions arealso evident, but one cannot identify them con-clusively without seeing their natural color con-trast.

Another example of an optical replica isshown in Fig. 44, along with an actual micro-

graph of the same area. The specimen is froma nitrided AISI 4150 chuck jaw that broke pre-maturely in service due to the presence of aheavy white-etching nitride layer at the surface.The replica clearly shows the surface white



Fig. 43 Light micrograph of a ferrite-pearlite micro-structure from a carbon steel reinforcing rod

revealed using replicating tape. Specimen etched with pi-cral

Fig. 42 Light micrograph of a white-etching surfacelayer formed on a rail head due to frictional

heat. This specimen was taken adjacent to a spalled area.Specimen etched with picral

Fig. 44 Light micrographs comparing images made with (a) a replica, using DIC illumination, and (b) a direct micro-graph, using bright-field illumination, of a heavily nitrided AISI 4150 chuck jaw etched with nital. Note that

the replica does not reveal the crack and is a mirror image of the bright-field micrograph.

layer and nitride in the prior-austenite grainboundaries near the surface. Examination of thereplica with DIC shows that the grain-boundarynitride film stands above the matrix; hence, it isnot a grain-boundary ferrite film. Comparisonof the actual micrograph with the replica showsthat the replica only reveals the crack where itis open. In the portion where oxide is in thecrack, the replica did not reveal the true natureof the crack. Hence, interpretation of structuresusing replicas can be more difficult and less re-liable than direct observation. Nevertheless, it isa very useful technique when no other meansof examination are possible.

REFERENCES

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