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Metallography and Microstructures of Cast Iron

Metallography and Microstructures of Cast Iron Janina M. Radzikowska, The Foundry Research Institute, Krako´w, Poland CAST IRON is an iron-carbon cast alloy with

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Text of Metallography and Microstructures of Cast Iron

Metallography andMicrostructures of Cast IronJanina M. Radzikowska, The Foundry Research Institute, Krako w, PolandCAST IRON is an iron-carbon cast alloy withother elements that is made by remelting pigiron, scrap, and other additions. For differentia-tion from steel and cast steel, cast iron is definedas a cast alloy with a carbon content (min )that ensures the solidification of the final phasewith a eutectic transformation. Depending onchemical specifications, cast irons can be non-alloyed or alloyed. Table 1 lists the range ofcompositions for nonalloyed cast irons (Ref 1).The range of alloyed irons is much wider, andthey contain either higher amounts of commoncomponents, such as silicon and manganese, orspecial additions, such as nickel, chromium, alu-minum, molybdenum, tungsten, copper, vana-dium, titanium, plus graphite is a characteristic constituent ofnonalloyed and low-alloyed cast irons. Precipi-tation of graphite directly from the liquid occurswhen solidification takes place in the range be-tween the temperatures of stable transformation(Tst) and metastable transformation (Tmst), whichare, respectively, 1153 C (2107 F) and 1147 C(2097 F), according to the iron-carbon this case, the permissible undercooling degreeisDTmax Tst Tmst. In the case of a higherundercooling degree, that is, in the temperaturesbelowTmst, primary solidification and eutecticsolidification can both take place completely orpartially in the metastable system, with precipi-tation of primary cementite or can also take place in the rangeof critical temperatures during solid-state trans-formations. The equilibrium of phases Fec Fig. 1Spheroidal graphite in as-cast ductile iron ( ) close to the edge of the specimen,which was 30 mm ( in.) in diameter. The specimen wasembedded. As-polished. 100 Table 1 Range of chemical compositions for typical nonalloyed and low-alloyed castironsType of ironComposition, %CSiMn PSGray (FG) graphite (CG) (SG) (TG) , flake graphite; SG, spheroidal graphite; TG, tempered graphite. Source: Ref 1Fe Fe3C occurs only at the temperature 723 2 C (1333 4 F), while equilibrium ofphases Fec Fe Cgroccurs at the tempera-ture 738 3 C (1360 5 F). So, in the rangeof temperatures 738 to 723 C (1360 to 1333 F),the austenite can decompose only into a mixtureof ferrite with graphite instead of with cementite(Ref 2).The previous considerations regard only pureiron-carbon alloys. In cast iron, which is a mul-ticomponent alloy, these temperatures can bechanged by different factors: chemical compo-sition, ability of cast iron for nucleation, andcooling rate. Silicon and phosphorus bothstrongly affect the carbon content of the dependence was defined as a carbon equiv-alent (Ce) value that is the total carbon contentplus one-third the sum of the silicon and phos-phorus content (Ref 2). Cast iron, with a com-position equivalent of approximately , solid-ifies as a eutectic. If the Ceis , it ishypereutectic; if it is , cast iron is hypoeu-tectic (Ref 3).Eutectic cells are the elementary units forgraphite nucleation. The cells solidify from theseparate nuclei, which are basically graphite butalso nonmetallic inclusions such as oxides andsulfides as well as defects and material discon-tinuities. Cell size depends on the nucleation ratein the cast iron. When the cooling rate and thedegree of undercooling increase, the number ofeutectic cells also increases, and their micro-structure changes, promoting radial-sphericalshape (Ref 2).Preparation for MicroexaminationPreparation of cast iron specimens for micro-structural examination is difficult due to the needto properly retain the very soft graphite phase,when present, that is embedded in a harder ma-trix. Also, in the case of gray irons with a softferritic matrix, grinding scratches can be difficultto remove in the polishing process. When shrink-age cavities are present, which is common, thecavities must not be enlarged or smeared of graphite in cast iron is a commonpolishing problem that has received considerable566 / Metallography and Microstructures of Ferrous AlloysFig. 3Flake graphite in as-cast gray iron ( ) close to the edge of the unembedded specimen,which was 30 mm ( in.) in diameter. Fig. 2Same as-cast ductile iron as in Fig. 1, but thespecimen was not embedded. The arrows showthe pulled-out graphite. As-polished. 100 Fig. 4Same as in Fig. 3 but close to the center of thespecimen. As-polished. 100 attention. Coarse grinding is a critical stage, so,if the soft graphite is lost during coarse grinding,it cannot be recovered in subsequent steps andwill be seen as an open or collapsed cavity. Sil-icon carbide (SiC) grinding papers are preferredto emery, because SiC cuts efficiently, while em-ery paper does not, and SiC produces less dam-age. Fresh paper should always be used; nevergrind with worn paper. White iron, by contrast,contains extremely hard iron carbides that resistabrasion and tend to remain in relief above thesofter matrix after polishing (Ref 4).Quality-control studies, based on image anal-ysis measurements of the amount of phases andthe graphite shape and size, also need perfectlyprepared specimens with fully retained graphitephase and with microstructural constituents cor-rectly revealed by metallographicspecimen preparation process for microstructuralinvestigations of cast iron specimens usuallyconsists of five stages: sampling, cold or hotmounting, grinding, polishing, and etching witha suitable etchant to reveal the stage presents particular problems in thecase of cast iron. Of course, the graphite phaseis studied after polishing and before the first step selecting the testlocation or locations to be evaluated metallo-graphically. Usually, cast iron castings have aconsiderable variation in microstructure betweensurface and core. Selection of the test location isvery important to obtain representative resultsfrom the microstructural examination. Samplescan be obtained by cutting them out from eithera large or small casting or from standard testbars, such as microslugs, ears, or keel bars; how-ever, the microstructure of these pieces may notbe representative for the actual casting due tosubstantial differences in the solidification saws, such as large, abrasive cutoffsaws, band saws, or power hacksaws, can beused for dividing medium-sized casting intosmaller samples. In the case of very large cast-ings, flame cutting may be used. Next, the piecesFig. 5Temper graphite in malleable iron ( ) after grinding on P1000 SiCwaterproof paper. The casting was annealed at 950 C(1740 F), held 10 h, furnace cooled to 720 C (1330 F),held 16 h, and air cooled. The arrows show the pulled-outgraphite. As-polished. 400 Fig. 6Same as in Fig. 5 but after polishing with 9lmdiamond suspension. The arrows show thepulled-out graphite. As-polished. 400 Metallography and Microstructures of Cast Iron / 567can be reduced to the desired size for metallo-graphic specimens by using a laboratory abra-sive cutoff saw or a band saw. If the casting wassectioned by flame cutting, the specimen mustbe removed well away from the heat-affectedzone. The pieces cut out for metallographic ex-amination may be ground prior to mounting (thismay be done to round off sharp cut edges or toreduce the roughness of band-saw-cut surfaces)and subsequent preparation. Overheating isavoided by proper selection of the speed of cut-off saws, the use of the correct wheel, and ade-quate water cooling. Overheating during grind-ing is avoided by using fresh abrasive paper andproper cooling. When metallographic specimensare cut out from the standard cast bars, they aresometimes prepared using standard machineshop equipment, such as turning in a lathe ormilling. These devices can deform the testpiecesurfaces to a considerable depth, so care must beexercised to remove any damage from theseoperations before starting specimen can be mounted in apolymeric material using either cold or hotmounting procedures. The mounting resin ischosen depending on the cast iron hardness (softor hard) and the need to enhance edge of an incorrect resin, or ignoring the mount-ing process, can make it very difficult to obtainproperly polished graphite in the area close tothe specimen edge. Figures 1 and 2 show themicrostructure of spheroidal graphite in ductileiron close to the edge of the specimens, whichwere cut off from a 30 mm ( in.) diameter barand polished with and without embedding in apolymer resin, respectively. In the specimen pre-pared without embedding in a resin, the graphitewas pulled out, while in the specimen that wasembedded in a resin and prepared, the graphitenodules were perfectly retained. Figures 3 and 4show that the uniform grinding of nonmountedspecimens is more difficult, and the flake graph-ite in gray iron close to the edge of such a spec-imen is not polished perfectly, in comparison towell-polished graphite in the mounted and ensure propergraphite retention, the use of an automatedgrinding-polishing machine is recommendedover manual preparation. The automated equip-ment makes it possible, in comparison to manualspecimen preparation, to properly control theorientation of the specimen surface relative tothe grinding or polishing surface, to maintainconstantly the desired load on the specimens, touniformly rotate the specimens relative to thework surface, and to control the time for eachpreparation step. Proper control of these factorsinfluences graphite retention, although other fac-tors are also good, general principle is to minimize thenumber of grinding and polishing stages. Also,the load on each specimen, or on all specimensin the holder, must be chosen to obtain a cor-rectly polished surface in the shortest possibletime. This precludes the risk of pulling out thegraphite phase and ensures that the graphite pre-cipitates will be perfectly flat with sharp recommended procedure for automatedpreparation of the specimens of nonalloyed andlow-alloyed cast iron with graphite specimens isto grind with a high-quality, waterproof 220- or240-grit (or equivalent) SiC paper until plane,with a load of 100 N for six specimens mountedin the sample holder, with central loading. Pol-Fig. 8Same as in Fig. 7 but after final polishing withthe 1lm diamond suspension applied on anapped cloth. The arrows show the 400 Fig. 9White high-chromium iron ( ). Eutecticand secondary carbides in the matrix. Specimen was pre-pared correctly. The casting was austenitized at 1000 C(1830 F), held 1 h, furnace cooled to 400 C (750 F) for2 h, taken to salt bath at 400 C (750 F), held for 4 h, andair cooled. Etched with glyceregia. 500 Fig. 10White high-chromium iron ( ). Eutectic and secondary carbides in the was prepared incorrectly. The casting was aus-tenitized at 1000 C (1830 F), held 1 h, furnace cooled to700 C (1290 F) for 2 h, taken to salt bath at 700 C (1290 F), held 4 h, and air cooled. Etched with glyceregia. 500 Fig. 7Same as in Fig. 6 but after final polishing withthe 1lm diamond paste applied on a naplesscloth. Graphite is free of any visible pullouts. 568 / Metallography and Microstructures of Ferrous Alloysishing is carried out in four steps with a differentgrain size diamond paste:StepDiamond pastegrain size,lmLoad,NDurationRecommendedpolishin g cloth19120(a)5 min(d)Napless woven23120(a)3 min(d)Napless woven31120(a)2 min(d)Napless woven4(b)25(c) 45 60 s(e) Napless syntheticpolyurethane(a) Load per six specimens. (b) Aqueous alumina suspension.(c) Load per single specimen (switching to individual force to makespecimen cleaning easier). (d) Comp direction. (e) ContraFigure 5 shows temper graphite in malleableiron after the last step of grinding, which wascarried out in three steps using, consecutively,SiC grit papers P220, P500, and P1000. Figure6 shows the same specimen after grinding onP220 SiC paper and then polishing with 9lmdiamond paste according to the procedure givenpreviously. In both cases, there is some pulled-out graphite after these steps. Each specimen wasprepared further. The specimen ground withthree SiC steps was polished with 3lm diamondsuspension on a napless cloth and then with 1lm diamond suspension on a napped cloth. Thepulled-out graphite was still visible. However,the specimen ground with P220 SiC and pol-ished with 9lm diamond paste, when finishedwith the recommended practice given previ-ously, was free of any visible pullouts, as shownin Fig. 7. By using a napped cloth and an aque-ous 1lm diamond suspension for the final dia-mond polishing step, it was impossible to obtainperfectly retained graphite, as shown in Fig. cloths should not be used with diamondabrasive, either in paste, suspension, or aerosolform. Graphite retention appears to be slightlybetter using diamond paste and the preferred lu-bricant than with an aqueous suspension, al-though more work needs to be conducted to de-termine if this difference is important. Finalpolishing with an alumina suspension, such asMasterprep alumina (Buehler, Ltd.), makes thegraphite boundaries sharper by removing thematrix, which was smeared over the edge of thegraphite during grinding and was not removedby the diamond polishing chromium iron is much harder, and adifferent preparation procedure must be grinding process is carried out in three steps,and polishing is carried out in three steps, al-though only two polishing steps are needed formost routine work. Grinding of the specimens,mounted in the sample holder, used central load-ing (150 N/six specimens), with high-qualitySiC waterproof paper (water cooled) with thefollowing grit sizes:Table 2 EtchantsNo. of etchantName of etchantCompositionCommentsRef1Nital96 98 mL ethanol2 4 mL nitric acid (HNO3)Most common etchant for iron, carbon, alloyed steels, and cast iron. Revealsalpha grain boundaries and constituents. The 2 or 4% solution is commonlyused. Use by immersion of sample for up to 60 , p 6482Picral4 g picric acid ((NO2)3C6H2OH)100 mL ethanolRecommended for structures consisting of ferrite and carbides. Does not revealferrite grain boundaries and martensite as-quenched. Addition of 1% zephiran chloride improves etch rate and , p 6483Glyceregia (modified)3 parts glycerine2 parts hydrochloric acid (HCl)1 part nitric acid (HNO3)For austenitic stainless steels and cast irons. Reveals grain structure; outlinessigma and carbides. Mix fresh; do not store. Use by swabbing. Heat up to 50 C (120 F) when etching time at 20 C (70 F) does not bring , p 6344Alkaline sodium picrate(ASP)2 g picric acid ((NO2)3C6H2OH))25 g sodium hydroxide (NaOH)100 mL distilled waterImmerse sample in solution at 60 70 C (140 160 F) for 1 3 min. Colorscementite (Fe3C) dark brown to black, depending on etching , p 6465Klemm I50 mL sat. aq. sodium thiosulfate(Na2S2O3 5H2O)1 g potassium metabisulfite (K2S2O5)Immerse sample for 40 100 s. Reveals phosphorus segregation (white); colorsferrite blue or red; martensite brown; cementite and austenite are unaffected4, p 6426Beraha CdS240 g aq. sodium thiosulfate(Na2S2O3 5H2O)30 g citric acid (C6H8O7 H2O)20 25 g cadmium chloride(CdCl2 )100 mL distilled waterTint etch for iron, steel, cast irons, and ferritic and martensitic stainless in order shown. Allow each to dissolve before adding next. Allow toage 24 h at 20 C (70 F) in a dark bottle. Before use, filter 100 mL of solutionto remove precipitates. Preetch with a general-purpose reagent. Etch 20 90 s;good for 4 h. For steels and cast irons, after 20 40 s only ferrite is colored, redor violet. Longer times color all constituents: ferrite is colored yellow or lightblue; phosphide, brown; carbide, violet or blue. For stainless steels, immersesample 60 90 s; carbides are colored red or violet-blue; matrix, yellow; colorsof ferrite vary. Sulfides red-brown after 90 s4, p 6447. . .28 g sodium hydroxide (NaOH)4 g picric acid ((NO2)3C6H2OH))1 g potassium metabisulfite (K2S2O5)(a)100 mL distilled waterImmerse sample in hot solution (close to boiling temperature) for 30 60 min. Thisreagent reveals silicon segregation in ductile iron. The colors of microstructurechange themselves from green through red, yellow, blue, and dark brown tolight brown as the silicon content is reduced from the graphite nodule to cellboundaries. The regions with lowest silicon content at the cell boundariesremain colorless. Before etching, ferritization of the specimen is recommendedto enhance the visibility of the reagent10 g potassium ferricyanide (K3Fe(CN)6)10 g potassium hydroxide (KOH) orsodium hydroxide (NaOH)100 mL distilled waterUse fresh, cold or hot. Cold, at 20 C (70 F) for up to min, tints chromiumcarbides; Fe3C unattacked or barely attacked. Hot, at 50 C (120 F) for 3 min,tints iron phosphide. The higher temperature or etching time also tintscementite into yellow , p 6469Beraha reagent with selenicacid2 mL hydrochloric acid (HCl) mL selenic acid (H2SeO4)100 mL ethanolFor differentiation of the constituents in steadite in cast iron, immerse sample for7 10 min; iron phosphide colored blue or green, cementite colored red, andferrite is bright (unaffected). Preetching with nital is , p 64310Beraha-Martensite (B-M)2 g ammonium bifluoride (NH4F HF)2 g potassium metabisulfite (K2S2O5)100 mL stock solution: 1:5, HCl todistilled waterImmerse sample for 1 3 s. Coarse martensite is blue or yellow; fine martensiteand bainite are brown. Use fresh reagent, and wet sample with tap water ,p261110% sodium metabisulfite(SMB)(b)10 g sodium pyrosulfite (Na2S2O5)100 mL distilled waterTints as-quenched martensite into brown; bainite into blue; carbides, phosphides,and residual austenite, unaffected. Immerse sample in etchant solutionapproximately 20 s. Preetching sample with nital is , p 64212Lichtenegger and Bloech I(LB I)20 g ammonium bifluoride (NH4F HF) g potassium metabisulfite (K2S2O5)100 mL hot distilled waterDissolve in given order. In austenitic Cr-Ni alloys, it tints austenite and revealsdendritic segregation. Ferrite and carbides remain unaffected. Wet-etch for 1 5min immediately after ,p5113. . .50% aq. hydrochloric acid (HCl)Immerse sample for 30 90 min. Every 15 20 min, wash sample with distilledwater, quickly etch in hydrofluoric acid (conc.), and wash in tap water. Whenthe etching process is finished, immerse sample in 5% aq. KOH or NaOH for10 20 min, wash with distilled water in an ultrasonic washer, then in ethanol,and dry with blowing hot ,p97(a) Potassium metabisulfite and potassium pyrosulfite are both synonomous with K2S2O5. (b) Sodium metabisulfite and sodium pyrosulfite are synonomous with and Microstructures of Cast Iron / 569 First step:P220 grit until plane. Second step:P500 grit for 3 min. Third step:P1000 grit for 3 minPolishing is done in three steps, with differentgrain size diamond in paste for the first two steps: First step:3lm diamond, 120 N load/sixspecimens for 3 min with a napless cloth. Second step:1lm diamond, 100 N load/sixspecimens for 3 min with a napless clothThe last polishing step is carried out with a col-loidal silica suspension on a napless syntheticpolyurethane pad but in a single specimenholder with an individual load of 30 N for 9 shows the microstructure of a heattreated chromium iron after this preparation. Thecarbides are perfectly flat, with very sharp edgesand boundaries etched uniformly. Figure 10shows the primary eutectic carbides in the mi-crostructure of a high-chromium iron. They ap-pear to be sticking out from the matrix, and theirboundaries are not outlined uniformly. This re-sult occurs if the load is too low or the final pol-ishing time on the silica suspension is too problems will result in too much removalof the softer matrix that was surrounding the pri-mary grinding, the paper must be moistenedwith flowing tap water, and the specimens shouldbe washed with water after each step. Also, dur-ing the first planning step, the sheet of papershould be changed every 1 min. Used grit paperis not effective and will introduce heat and dam-age, impairing specimen flatness. During polish-ing with diamond paste from a tube, the cloth ismoistened with the recommended lubricant forthe paste. If a water-based diamond suspensionis applied on the cloth, the use of an additionallubricant is not speed of the grinding-polishing head was150 rpm, and it was constant. The speed of theplaten during grinding was always 300 rpm, andduring polishing was always 150 rpm. After eachgrinding step, the specimens were washed withrunning tap water and dried with compressed air,while after each polishing step, they werewashed with alcohol and dried with hot air froma hair 11Ductile iron ( ) after annealing. Ferrite andapproximately 5% pearlite. Etched with 2% nital. 100 .Courtesy of Vander Voort, Buehler 12As-cast gray iron ( ). Pearlite. Etched with4% nital. Arrows show the white areas with weakly etchedor nonetched pearlite. 500 Fig. 13Same as in Fig. 12 but after etching with 4%picral. Pearlite was etched uniformly. 500 Fig. 14As-cast high-chromium white iron ( ). Eutectic chromium carbides typeM7C3in austenitic matrix. Etched with glyceregia. 500 Fig. 15Same as in Fig. 14 but after etching with 4%nital. 500 570 / Metallography and Microstructures of Ferrous AlloysMicroexamination MethodsChemical examination of theiron microstructure with a light optical micro-scope is always the first step for phase identifi-cation and morphology. One should always be-gin microstructural investigations by examiningthe as-polished specimen before etching. This isa necessity, of course, for cast iron specimens, ifone is to properly examine the graphite see the microstruc-tural details, specimens must be etched. Etchingmethods based on chemical corrosive processeshave been used by metallographers for manyyears to reveal structures for of nonalloyed and low-alloyedirons containing ferrite, pearlite, the phosphoruseutectic (steadite), cementite, martensite, andbainite can be etched successfully with nital atroom temperature to reveal all of these micro-structural constituents. Usually, this isa2to4%alcohol solution of nitric acid (HNO3) (Table 2,etch No. 1). Figure 11 shows a nearly ferriticannealed ductile iron with uniformly etchedgrain boundaries of ferrite and a small amountof pearlite. Nital is very sensitive to the crystal-lographic orientation of pearlite grains, so, in thecase of a fully pearlitic structure, it is recom-mended to use picral, which is an alcohol solu-tion of 4% picric acid (Table 2, etch No. 2). Fig-ures 12 and 13 show the differences in revealingthe microstructure of pearlite with nital or does not etch the ferrite grain boundaries,or as-quenched martensite, but it etches the pear-litic structure more uniformly, while nital leaveswhite, unetched areas, especially in the casewhere pearlite is very the austempering heat treatment is veryshort, the microstructure of austempered ductileiron (ADI) consists of martensite and a smallamount of acicular ferrite. After etching in 4%nital, martensite as well as acicular ferrite areboth etched intensively, which makes it very dif-Fig. 16As-cast ductile iron ( ). C,cementite; L, ledeburite; F, ferrite; and P, pearlite. Etchedwith 4% nital. 650 (microscopic magnification 500 )Fig. 17Same as in Fig. 16 but after etching with hotalkaline sodium picrate. C, eutectic cementite;L, ledeburite; F, ferrite; and P, pearlite with slightly etchedcementite. 650 (microscopic magnification 500 )Fig. 18As-cast gray iron ( ). E, phosphorousternary eutectic. Etched with 4% nital. 100 Fig. 19Same as in Fig. 18 but after etching withKlemmI reagent. E, phosphorous ternary Fig. 20Austempered ductile iron ( ). CB, cell bound-aries; H, ferritic halo around the graphite nodules. Etchedwith Klemm I reagent. 200 Metallography and Microstructures of Cast Iron / 571ficult to distinctly see the needles of acicular fer-rite. Picral reveals this phase very well; marten-site is barely etched due to the very shortaustempering heat treatment of the specimen,which was 2 min, because the martensite was as-quenched. The needles of acicular ferrite aredark and very sharp (Fig. 32, 33). In this case,picral is very convenient for estimating theamount and morphology of the acicular ferrite inthe ADI microstructure. Picral is safer to bestored in the lab than nital, which can be an ex-plosive mixture under certain conditions when itis stored in a tightly closed (Table 2, etch No. 3), which is amixture of glycerine, hydrochloric acid (HCl),and nitric acid (HNO3), is recommended for re-vealing the microstructure of high-chromiumand chromium-nickel-molybdenum irons. Fig-ure 14 shows the microstructure of a high-chro-mium cast iron after etching with glyceregia (seealso Fig. 43, 49, and 96). Nital can be also usedfor revealing the carbide morphology in the mi-crostructure of chromium or chromium-nickelirons when the carbon and chromium contentpromotes solidification of eutectic the microstructure of a high-chromiumwhite iron contains columnar primary carbides,glyceregia is 15 shows the microstructure of thesame cast iron as Fig. 14 but after etching with4% nital (see also Fig. 40, 41). Both etchantsreveal carbide boundaries sharply and Color the black-and-white etchants are inadequate for positive iden-tification of the iron microstructures, other pro-cedures must be used, such as selective coloretching. The reagents referred to as tint etchantsare usually acidic solutions with either water oralcohol as a solvent. They are chemically bal-anced to deposit a thin (40 to 500 nm), trans-parent film of oxide, sulfide, complex molyb-date, elemental selenium, or chromate on thespecimen surface. Coloration is developed by in-terference between light rays reflected at the in-Fig. 21Same as in Fig. 20 but after etching with Ber-aha s CdS reagent. H, ferritic halo; CB, cellboundaries. 250 Fig. 22Nodular iron ( ). Sili-con microsegregation was revealed. The casting was an-nealed. Etched with hot aqueous solution of sodiumhydroxide, picric acid, and potassium pyrosulfite (Table 2,etchant No. 7). 500 Fig. 23As-cast gray iron ( ). Ternary phos-phorus eutectic. Etched with 4% nital. 1300 (micro-scopic magnification 1000 )Fig. 24Same as in Fig. 23 but after etching with hotalkaline sodium picrate. C, cementite; F, ferrite(unaffected); IP, iron phosphide ferrite; and TiN,titaniumnitride. 1300 (microscopic magnification 1000 )Fig. 25Same as in Fig. 23 but after etching withKlemmI reagent. F, ferrite; C, cementite; and C IP,cementite iron phosphide inside the precipitate of phos-phorous eutectic. 1300 (microscopic magnification1000 )572 / Metallography and Microstructures of Ferrous Alloysner and outer film surfaces. Crystallographic ori-entation, local chemical composition, andetching time affect film thickness and controlcolor production. The use of selective etchantsis invaluable for quantitative metallography, afield of growing importance (Ref 5).When ferritic-pearlitic microstructures of castiron contain large amounts of cementite and led-eburite, the differentiation of the white structuralconstituents is difficult after etching a specimenwith nital. In such cases, hot alkaline sodium pic-rate (ASP) is recommended (Table 2, etch ), which reveals cementite, tinted a browncolor, while ferrite remains unaffected. This etchis a mixture of sodium hydroxide (NaOH), picricacid, and distilled water. Figure 16 shows themicrostructure of a thin-walled, chilled ductileiron casting after etching with nital, while shows the microstructure of the same speci-men after etching with ASP; the brown-coloredcementite and ledeburite are clearly visible in thepearlitic-ferritic matrix (the cementite in pearlitewas also slightly tinted). It allows one to esti-mate the amount of cementite that should be re-moved from the casting microstructure in the an-nealing of silicon and phosphorus in ironis very strong and can be revealed with selectivecolor etching methods. Klemm s I reagent (Table2, etch No. 5), which tints ferrite while austeniteand carbides remain colorless, consists ofpotassium metabisulfite (K2S2O5) and a cold-sat-urated water solution of sodium thiosulfate(Na2S2O3 5H2O) and is one of the etchants thatcan be used to reveal phosphorus and silicon , the distribution of the phosphorus eu-tectic, which solidifies in gray iron on the cellboundaries, is revealed by etching up to 4 minin 4% nital. Figure 18 shows the microstructureof as-cast gray iron with the ternary phosphorouseutectic. The cell boundaries, filled with steadite,Fig. 28Same as in Fig. 25 but after etching with Ber-aha s reagent with selenic acid. IP, iron phos-phide; C, cementite; and F, ferrite. The dark points in pearl-ite, which look like artifacts, can be iron phosphideprecipitates or fine, nonmetallic (micro-scopic magnification 1000 )Fig. 29As-cast gray iron ( ). E, binary phosphorous eutectic; F,ferrite at the graphite precipitate; and P, pearlite. Etchedwith 4% nital. 500 Fig. 26Same as in Fig. 23 but after etching with hotMurakami s reagent. IP, iron phosphide; C F,cementite ferrite inside the precipitate of phosphorouseutectic. 1300 (microscopic magnification 1000 )Fig. 30Same as in Fig. 29 but after etching with hotalkaline sodium picrate and 4% nital. Pearliticmatrix is revealed; phosphorous eutectic is Fig. 27Same as in Fig. 26 but after overetching withhot Murakami s reagent. IP, iron phosphide; C,cementite; and F, ferrite. 1300 (microscopic magnifica-tion 1000 )Metallography and Microstructures of Cast Iron / 573are white, while their interiors are almost blackdue to overetching the pearlitic-ferritic 19 shows the same microstructure afteretching with Klemm s I reagent. The microre-gions inside the eutectic cells with a lower phos-phorus content are tinted a blue color, while theareas with the ternary eutectic, situated at theboundaries of the eutectic cells, are almost col-orless. In both cases, the cementite and ironphosphide in steadite are not etched, and the net-work is clearly 20 shows silicon segregation in an ADImicrostructure after etching with Klemm s I re-agent. The regions outlining the cell boundaries,low in silicon, are tinted a blue color, while thevery thin halos around the graphite nodules,where the silicon content is highest, remain col-orless. Acicular ferrite is orange, and austenite isnot tinted. Figure 21 shows the same microstruc-ture after etching with Beraha s CdS reagent (Ta-ble 2, etch No. 6), an aqueous solution of sodiumthiosulfate(Na2S2O3 5H2O),citricacid(C6H8O7 H2O),andcadmiumchloride(CdCl2 ). The silicon segregation is re-vealed the same way as after using Klemm s reveal silicon segregation in nonalloyedductile cast iron inside eutectic cells, the hotaqueous solution of sodium hydroxide, picricacid, and potassium pyrosulfite (K2S2O5) can beused (Table 2, etch No. 7). Figure 22 shows thedifferent colors of the microstructure, whichchange from green through red, yellow, blue, anddark brown to light brown as the silicon contentis changing from the graphite nodule to the cellboundaries. The regions with the lowest siliconcontent at the cell boundaries remain etching, ferritization of the specimen wascarried out to enhance the visibility of the mi-crostructural revealing and differentiation of all con-stituents in steadite is invaluable for the deter-mination of the type of eutectic as well as theFig. 33Same as in Fig. 32 but after etching with 4%picral. AF, acicular ferrite; PM, plate marten-site. 1000 Fig. 32Austempered ductile iron ( ). Martensiteand acicular ferrite. The casting was austempered at 900 C (1650 F), held 2 h, taken to salt bath at 360 C (680 F),held 2 min, and air cooled. Etched with 4% nital. 1000 Fig. 31Same as in Fig. 29 but after etching with hotMurakami reagent. Only brown-tinted ironphosphide was revealed. 500 Fig. 34Same as in Fig. 32 but after etching with Ber-aha-Martensite reagent. PM, blue-yellow platemartensite; FM, brown fine martensite; and AF A, darkneedles of acicular ferrite surrounded with colorless aus-tenite. 1000 Fig. 35Same as in Fig. 32 but after etching with 10%sodium metabisulfite. PM, plate martensite;FM, fine martensite; and AF A, acicular ferrite and aus-tenite. 1000 574 / Metallography and Microstructures of Ferrous Alloysamount of each constituent. In the case of theternary phosphorous eutectic, which consists offerrite, cementite, and iron phosphide (Fe3P), ni-tal does not help in the identification of the con-stituents, nor does it provide enough informationabout distribution of the eutectic 23 shows the microstructure of the ter-nary phosphorous eutectic in gray iron after etch-ing in 4% nital. The white areas that surroundthe ternary eutectic and are also visible insidethe eutectic can be either ferrite or 24 shows the microstructure of the samespecimen after selective color etching with hotASP (Table 2, etchant No. 4). Cementite in theeutectic is tinted brown and blue colors (also inpearlite), while ferrite and iron phosphide are nottinted. To reveal the ferrite, the same specimenwas etched with Klemm s I reagent. Figure 25shows the eutectic regions with precipitates ofbrown ferrite (also in pearlite), while cementiteand iron phosphide are not s reagent (Table 2, etch No. 8),used at 50 C (120 F) for 3 min and containingpotassium hydroxide (KOH), potassium ferri-cyanide (K3Fe(CN)6), and distilled water, can beused for revealing and estimating the amount ofiron phosphide in steadite. Figure 26 shows thisconstituent of the ternary phosphorous eutecticmicrostructure, which was tinted a light-browncolor, while cementite and ferrite remained col-orless. The microstructure of the same specimenafter overetching (5 min) with the same reagentis shown in Fig. 27. This time, cementite wasalso revealed and was tinted a yellow color,while ferrite remained white. The color of theiron phosphide changed to a dark-brown andgray-blue color. Nevertheless, extending theetching time beyond 3 min is not recommended,because this can falsify the true results of themicrostructural differentiation of all constituents in theternary phosphorus eutectic can be obtained withBeraha s reagent (Table 2, etch No. 9), a mixtureof hydrochloric acid (HCl), selenic acidFig. 36Austempered ductile iron ( ). AF, acicular ferrite; A,austenite; and M, martensite. The casting was austemperedat 900 C (1650 F), held 2 h, taken to salt bath at 360 C(680 F), held 30 min, and air cooled. Etched with 4% Fig. 37Same as in Fig. 36 but after etching with Ber-aha-Martensite reagent. AF, acicular ferrite; A,austenite; and M, martensite. 1000 Fig. 38Same as in Fig. 36 but after etching with 10%sodium metabisulfite. AF, acicular ferrite; A,austenite; and M, martensite. 1000 Fig. 39White alloyed cast iron ( ). Etched with Ber-aha-Martensite. PM, plate martensite; FM, fine martensite;EC, eutectic carbides type M7C3; SC, secondary carbides;and MS, manganese sulfite. The casting was heat treated:austenitized at 750 C (1380 F), held 2 h, and air cooled;tempered at 250 C (480 F), held 4 h, and air (microscopic magnification 1000 )Fig. 40Same white iron as in Fig. 39 but after etchingwith 4% nital. M, martensite; EC, eutectic car-bides; and SC, secondary carbides. 1300 (microscopicmagnification 1000 )Metallography and Microstructures of Cast Iron / 575(H2SeO4), and ethanol. According to Beraha,this etchant tints iron phosphide a dark-bluecolor, cementite a violet or dark red, and ferriteremains white. Figure 28 shows the microstruc-ture of the ternary eutectic, with cementite tinteda light-pink color, while the rest of the constit-uents were colored 29 shows the microstructure of thepseudobinary phosphorous eutectic, which con-sists of iron phosphide and ferrite, after etchingwith 4% nital. The same specimen was etchedwith hot ASP (Table 2, etchant No. 4). This didnot tint the iron phosphide or the ferrite. Becausecementite is not present in the eutectic, the onlyetching was of cementite in the pearlite, whichshowed up very lightly. To reveal the microstruc-ture of the eutectic, the specimen was nextetched with 4% nital. Figure 30 shows the resultsafter using hot ASP and then nital. Hot Murak-ami s reagent perfectly tinted the iron phosphidein the binary phosphorous eutectic a browncolor, while pearlite was colorless, as shown inFig. (B-M) (Table 2, etch ) and aqueous 10% sodium metabisulfite(SMB) (Table 2, etch No. 11) reagents for selec-tive color etching are very useful in cases wheremicrostructural details are very fine and barelyvisible after etching with nital. They reveal allof the constituents, tinting them to expected col-ors that are useful for verifying that the heattreatment process was carried out B-M etchant is a mixture of stock solution(1:5, HCl to water), potassium metabisulfite(K2S2O5),andammoniumacidflu oride(NH4F HF). According to Ref 6, B-M tints mar-tensite a blue color and bainite a brown color,while the residual austenite and carbides B-M etchant can be used for identificationof the constituents after heat treatment of castiron by tinting phases to different colors. It alsoFig. 42Same as in Fig. 41 but after etching with Lich-tenegger and Bloech I. Austenite is darkbrown,and dendritic segregation is visible around unaffected car-bides. 1000 Fig. 43As-cast high-chromium white iron ( ). PC, primary carbides; EC, eutectic carbides,both M7C3type. Etched with glyceregia. 500 Fig. 41Same white iron as in Fig. 39 and 40 but as-cast. Eutectic carbides in austenitic with glyceregia. 500 Fig. 44Same as in Fig. 43 but after etching with Mu-rakami s reagent (at room temperature). PC, or-ange primary carbides; EC, orange and gray eutectic car-bides. 400 Fig. 45Same as in Fig. 43 but as-polished and exam-ined in differential interference contrast. Pri-mary and eutectic carbides are sticking up from the softeraustenitic matrix. 400 576 / Metallography and Microstructures of Ferrous Alloysimproves microstructural contrast, enhancingvisibility and permitting estimation of evensmall amounts of the residual austenite (althoughx-ray diffraction results are always more than10% greater than by light microscopy) and finecarbides. Figure 32 shows the black-white mi-crostructure of ADI after etching with 4% nital,while Fig. 33 shows the same microstructure af-ter etching with 4% picral. Figures 34 and 35show the microstructure of the same specimenafter color etching, respectively, with B-M andwith aqueous 10% SMB etchants. The SMB tintsmartensite a brown color and bainite a blue color,while austenite and carbides are colorless. Bothetching time in B-M and the different crystallo-graphic orientations affected the color of thecoarse, high-carbon martensitic plates, whichvary from blue to yellow. The brown areas in themicrostructure (Fig. 34) are the patches of finemartensite. This color differentiation of micro-structure occurs due to the change in size of theplate martensite as transformation , this is not the only factor, becausesome of the larger plates are also brown. Thereis only a very small amount of austenite, whichsurrounds the acicular ferrite at the graphite nod-ules and in the matrix. The SMB etchant is evenmore useful than the B-M etchant in the casewhere the dominating phase in the ADI micro-structure is martensite, and acicular ferrite isweakly visible. In ADI, the martensite of bothtypes is tinted a brown color, the acicular ferriteis colored the same as bainite, that is, blue color,while austenite is colorless, which was shown inFig. 35 (see the section Ductile Iron in thisarticle).Figure 36 shows the microstructure of ADIafter etching with 4% nital, while Fig. 37 and 38show the same microstructure after etching withB-M and 10% SMB, respectively. Nital etchedthe acicular ferrite, while the austenite is areas that were darkened may be marten-site, but there is no clear distinction betweenmartensite and acicular ferrite with nital. Selec-Fig. 46Same white iron as in Fig. 39 but after slightetching with 4% nital and examined in bright-field illumination. EC, eutectic carbides type M7C3;SC,secondary carbides; and M, martensite. 1000 Fig. 47Same as in Fig. 46 but examined in dark-fieldillumination. EC, eutectic carbides; SC, sec-ondary carbides; and M, martensite. 1000 Fig. 48Same as Fig. 46 but examined in differentialinterference contrast. EC, eutectic carbides;SC, secondary carbides; and M, martensite. 1000 Fig. 49Same white iron as in Fig. 14 and 15 but cast-ing was heat treated at 1000 C (1830 F), held1 h, furnace cooled to 400 C (750 F) for 2 h, taken to saltbath at 400 C (750 F), held 4 h, and air cooled. Examinedin bright-field illumination. EC, eutectic carbides typeM7C3; SC, secondary carbides. Etched with Fig. 50Same as in Fig. 49 but examined in dark-fieldillumination. EC, eutectic carbides; SC, sec-ondary carbides. 1000 Metallography and Microstructures of Cast Iron / 577tive color etching with the two previously men-tioned reagents clearly revealed small patches ofmartensite, which were blue after etching withB-M and brown after etching with 10% SMB;acicular ferrite was colored blue (darker withB-M than with SMB), and austenite same results were achieved with the useof B-M reagent to reveal the microstructure ofalloyed cast iron after heat treatment. The mi-crostructure, which is shown in Fig. 39, consistsof brown patches of fine martensite (which mayhave transformed from austenite during or aftertempering), while the blue needles are high-car-bon plate martensite. Figure 40 shows the mi-crostructure of the same specimen after etchingwith 4% nital; in this case, the recognition ofmartensite is not 41 and 42 show the microstructure ofthe same iron but in the as-cast condition afteretching with glyceregia and with Lichteneggerand Bloech I (LBI), respectively (Table 2, etch-ant No. 3 and 12). The LBI is an aqueous solu-tion of ammonium bifluoride (NH4F HF) andpotassium metabisulfite (K2S2O5) (Table 2, etchNo. 12). In chromium-nickel alloys, LB I tintsaustenite, while carbides and ferrite (if present)remain unaffected (white). Glyceregia outlinesonly the eutectic carbides, while the LB I etchantalso reveals microsegregation. The microstruc-ture shown in Fig. 42 consists of austenite, tintedbrown and blue color, and white (noncolored)carbides. The blue austenitic areas surroundingFig. 51Graphite nodule examined in bright-field illu-mination. As-polished. 1000 Fig. 52Same as in Fig. 51, but graphite nodule wasexamined in crossed polarized light. 1000 Fig. 53Flake graphite in as-cast gray iron examined incrossed polarized light. As-polished. 200 Fig. 54Same as in Fig. 33 but microstructure was ex-amined in crossed polarized light. Acicular fer-rite is shining brightly; plate martensite is Fig. 55Flake graphite in as-cast gray iron examinedwith SEM. Sample was deeply etched with50% HCl. 500 Fig. 56Nodular graphite in as-cast ductile iron exam-ined with SEM. Sample was deeply etchedwith50% HCl. 1000 578 / Metallography and Microstructures of Ferrous Alloysthe eutectic carbides indicate regions with alower concentration of carbide-forming ele-ments, such as carbon and chromium, and per-haps a higher concentration of elements that arenot present in the carbides, such as silicon 43 shows the microstructure of a high-chromium cast iron after etching with glycere-gia, revealing columnar primary carbides and eu-tectic carbides, both (FeCr)7C3type, uniformlydistributed in the matrix. Figure 44 shows themicrostructure of the same specimen after etch-ing with the standard Murakami s etchant atroom temperature. The carbides are tinted an or-ange color, while the matrix is not colored. X-ray diffraction determined that the matrix wasaustenitic-ferritic; the matrix was not colored us-ing the LB I tint etchant due to missing nickelin that basic etchants mentioned previously,which are suitable for revealing microstructuresas well as for phase identification in irons, arelisted in Table Illumination and DifferentialInterference illumination(DFI) technique for microstructural examinationproduces a very strong image contrast that makesit possible to see features in the microstructurethat are invisible or weakly visible in bright-fieldillumination (BFI) when the surface of the spec-imen is normal to the optical axis of the micro-scope and white light is used. This image con-trast is a result of reversing the image, which isbright in BFI, into a dark one when DFI is 3 Constituents commonly found incast iron microstructures, and their generaleffect on physical propertiesConstituentCharacteristics and effectsGraphite(hexagonalcrystalstructur e)Free carbon; soft; improvesmachinability and dampingproperties; reduces shrinkage andmay reduce strength severely,depending on shapeAustenite (c-iron)Face-centered cubic crystal character of the primary phase,which solidifies from theoversaturated liquid alloy indendrite form, is maintained untilroom temperature. Austenite ismetastable or stable equilibriumphase (depending upon alloycomposition). Usually transformsinto other phases. Seen only incertain alloys. Soft and ductile,approximately 200 HBFerrite ( -iron)Body-centered cubic crystal with elements in solidsolution, which is a stableequilibrium, low-temperaturephase. Soft, 80 90 HB; contributesductility but little strengthCementite (Fe3C)Complex orthorhombic crystalstructure. Hard, intermetallic phase,800 1400 HV depending uponsubstitution of elements for Fe;imparts wear resistance; reducesmachinabilityPearliteA metastable lamellar aggregate offerrite and cementite due toeutectoidal transformation ofaustenite above the bainite strength withoutbrittleness; has good machinability,approximately 230 HBMartensiteGeneric term for microstructures thatform by diffusionlesstransformation, where the parentand product phases have a specificcrystallographic relationship. Hardmetastable phaseSteadite(phosphorouseutectic)A pseudobinary or ternary eutectic offerrite and iron phosphide orferrite, iron phosphide, andcementite, respectively. It can formin gray iron or in mottled iron witha phosphorous content and brittle; solidifies from theliquid on cooling at the cellboundaries as a last constituent ofthe microstructureLedeburiteMassive eutectic phase composed ofcementite and austenite; austenitetransforms to cementite andpearlite on cooling. Produces highhardness and wear resistance;virtually unmachinableSource: Ref 3, 10Fig. 57Compacted graphite examined was deeply etched with 50% HCl. 1500 Fig. 58Hypoeutectic as-cast gray iron ( ). Flakegraphite type A. As-polished. 100 Fig. 59Hypoeutectic as-cast gray iron ( ). Flakegraphite type D. As-polished. 100 Fig. 60Hypereutectic as-cast gray iron ( ). Flake graphite type A. As-polished. 100 Metallography and Microstructures of Cast Iron / 579Features that are perpendicular to the optic axisin BFI appear white, while in DFI they are , features that are at an angle to the sur-face, such as grain boundaries and phase bound-aries, appear black in BFI but are white (self-luminous in appearance) in crossed polarized light is used alongwith a double-quartz prism (Wollaston prism)placed between the objective and the vertical il-luminator, two light beams are produced that ex-hibit coherent interference in the image leads to two slightly displaced (laterally)images differing in phase (k/2), which produceshigher contrast for nonplanar detail. This iscalled differential interference contrast (DIC),and the most common system is the one devel-oped by Nomarski. The image reveals topo-graphic detail somewhat similar to that producedby oblique illumination but without the loss ofresolution. Images can be viewed with naturalcolors similar to those observed in bright field,or artificial coloring can be introduced by addinga sensitive tint plate (Ref 4).The microstructure of Fe-Cr-Mn cast irons canbe examined in DIC after polishing. Figure 45shows the microstructure of the high-chromiumcast iron (shown previously in Fig. 43 and 44)but as-polished (unetched) and examined in chromium carbides, which are much harderthan the matrix, stand above it in relief and canbe seen very DFI and DIC methods also can be usedfor revealing the details of microstructures in al-loyed irons, for example, a chromium-nickeliron after heat treatment, which were barely visi-ble after etching with nital. Figures 46 to 48show the microstructure of the Fe-Cr-Ni alloyediron after heat treatment and after etching with4% nital, but for a shorter time than usual. Thethree micrographs show the results for the samefield after examination with BFI, DFI, and DIC,respectively. The BFI image reveals the marten-site poorly, and the secondary carbides are barelyvisible. However, both the DFI and DIC imagesreveal the martensite, although not strongly,Fig. 61Hypereutectic as-cast gray iron ( ). Flakegraphite type E. As-polished. 100 . Courtesy of Van-der Voort, Buehler 63Hypereutectic as-cast gray iron ( ). Pointlike flake graphite type D. Fig. 64Hypereutectic as-cast gray iron ( ).Flakegraphite type C (kish graphite). As-polished. 100 Fig. 65As-cast gray iron. Flake graphite type V (star-like graphite). As-polished. 100 Fig. 62Hypereutectic as-cast gray iron ( ). Flakegraphite type B. As-polished. 90 (microscopic examina-tion 100 )580 / Metallography and Microstructures of Ferrous Alloyswhile the secondary carbides are very distinct(see also Fig. 39, 40).The microstructure of an Fe-C-Cr cast iron af-ter heat treatment, revealed with glyceregia andexamined with BFI and DFI, is shown in Fig. 49and 50, respectively. The details of the micro-structure were revealed much more strongly withDFI than with Light light isobtained by placing a polarizer (usually a Polar-oid filter, Polaroid Corp.) in front of the con-denser lens of the microscope and placing an an-alyzer (another Polaroid filter) before theeyepiece. The polarizer produces plane-polar-ized light that strikes the surface and is reflectedthrough the analyzer to the eyepieces. If an an-isotropic metal is examined with the analyzer set90 to the polarizer, the grain structure will bevisible. However, viewing of an isotropic metal(cubic metals) under such conditions will pro-duce dark, extinguished conditions (completedarkness is not possible using Polaroid filters).Polarized light is particularly useful in metallog-raphy for revealing grain structures and twinningin anisotropic metals and alloys as well as foridentifying anisotropic phases and method also improves the contrast of mi-crostructures, providing more structural details(Ref. 4).Figure 51 shows the microstructure of agraphite nodule in BFI, while Fig. 52 shows thesame nodule in crossed polarized light. Polarizedlight reveals much better than BFI the radialstructure of the graphite nodule that grows fromthe central nucleus. Also, the cross, which ischaracteristic of the perfectly formed graphitenodule, can be made visible only with the use ofthis technique. Figure 53 shows the anisotropiclayered structure of graphite flakes, which alsorespond to polarized light. In both cases, the col-ors of graphite vary due to the anisotropy ofgraphite. Figure 54 shows the microstructure ofADI, consisting of martensite and a smallFig. 66As-cast ductile iron ( ). Nodular graphite size is 20lm. As-polished. 100 Fig. 67As-cast ductile iron ( ). Nodular graphite size is 40lm. Fig. 68As-cast ductile iron. Nodular graphite size is100lm. As-polished. 100 Fig. 69As-cast ductile iron. Imperfectly formed graph-ite nodules. As-polished. 100 Fig. 70As-cast ductile iron. Exploded 100 Metallography and Microstructures of Cast Iron / 581amount of acicular ferrite. The specimen wasetched with 4% picral to reveal the acicular fer-rite in the background of the almost invisiblemartensite (see Fig. 33). When examination wascarried out with crossed polarized light plus asensitive tint filter, shown in Fig. 54, the ferriticneedles can be seen by shining with a whitecolor. Also, many of the coarsest martensiticplates (body-centered tetragonal crystal struc-ture) with the proper lattice orientation were visi-ble in polarized light but barely visible in 3 lists the characteristics of typical con-stituents of cast iron microstructures and theireffect on mechanical properties (Ref 3, 10).Microstructure of GraphiteThe eutectic graphite cell has a continuousgraphite skeleton, but on the metallographiccross section, the three-dimensional nature is notobvious. Figures 55 to 57 show scanning elec-tron microscopy (SEM) micrographs of flake,nodular, and compacted graphite, to flake graphite, where the flakes so-lidify as an aggregate and each such agglomer-ation is one eutectic cell, both spheroidal andcompacted graphite solidify as separate precipi-tates. For SEM examination, the specimens weredeeply etched (Table 2, etch No. 13).Characteristic properties of graphite precipi-tations are shape, size, and distribution. There isa correlation between different graphite mor-phologies, the chemical composition, and thecooling Graphite in Gray (lamel-lar) graphite is characteristic of gray iron, andcomponents such as aluminum, carbon, and sil-icon promote its formation. When the coolingrate increases, the flakes get finer, and their dis-tribution tends to be interdendritic. Figure 58shows a hypoeutectic gray iron with a uniformdistribution of randomly oriented graphite flakeswith a maximum length of 320lm (type A flakegraphite in the ASTM A 247 classifications).Figure 59 shows a fine, type D flake graphitewith a maximum length of 40lm, also in a hy-poeutectic alloy, but it solidified at a higher cool-ing rate, which promoted the interdendritic dis-tribution. Type A graphite in a hypereutecticgray iron is shown in Fig. 60. It has the mostdesired shape and distribution, and it is verycharacteristic of gray iron casts with high mach-inability. The maximum length of graphite flakesis 160lm. Figures 61 and 62 show type E andtype B graphite, respectively, which occur in hy-pereutectic gray iron at high cooling rates. Notein Fig. 62 that each rosette group of graphiterepresents one eutectic cell. That type of graphiteis characteristic of thin-walled 71Austempered ductile iron ( ). Chunky (CH) and spiky(SP) types of graphite. As-polished. 100 Fig. 72As-cast iron with compacted graphite ( ). Graphite size is 80 to 160lm. As pol-ished. 100 Fig. 73Malleable iron ( ). Temper graphite type III with maxi-mum size of 80lm. As-polished. 100 Fig. 74The diagram of correlation between a type of matrix in nonalloyed cast irons and silicon and phosphoruscontent as well as the thickness,R, of the casting wall, which relates to the cooling C (Si logR)is a coefficient of graphitization, and CE C 1 3Si 1 3P is the coefficient of saturation (carbon equivalent). Region Iis white cast iron (pearlite, cementite, no graphite); Region IIa is mottled cast iron (pearlite, graphite, cementite); RegionIIb is ferritic-pearlitic cast iron; Region III is ferritic cast iron. Source: Ref 13582 / Metallography and Microstructures of Ferrous AlloysA high degree of undercooling of hypereutec-tic gray iron can promote the solidification ofvery fine, pointlike type D graphite with an in-terdendritic distribution, as shown in Fig. 63. Inthe other direction, undertreatment of the graph-itizing inoculants, such as ferrosilicon, producesother flake graphite types in gray iron. For ex-ample, Fig. 64 shows a hypereutectic gray ironwith graphite type C, where very coarse, needle-like flakes (kish graphite) form before the eutec-tic, which is very fine. Kish graphite, which isshown in Fig. 64, can be changed into a starlikegraphite, shown in Fig. 65, under higher coolingrates, which is referred to as type V (plate I ofASTM A 247). The carbide-forming alloy ele-ments, for example, chromium, manganese, andvanadium, and the low-melting-point metals, forexample, bismuth, lead, and sulfur, also affectgraphite Graphite in Ductile ad-dition of magnesium in the inoculation processdesulfurizes the iron and makes graphite precip-itate as nodules rather than flakes. Moreover, me-chanical properties are greatly improved overgray iron; hence, nodular iron is widely knownas ductile iron. Nodule size and shape perfectioncan vary, depending on composition and coolingrate. Figure 66 shows fine nodules with a max-imum diameter of 20lm in a chill-cast thin sec-tion, while Fig. 67 and 68 show coarser nodules,with maximum diameters of 40 and 100lm, re-spectively. Note that the number of nodules perunit area is different and changes from approxi-mately 350 to 125 to 22/mm2, respectively, forFig. 66 to factors can cause weak 69 shows an irregular graphite shape dueto poor inoculation or excessive fading of inoc-ulant. Exploded graphite, shown in Fig. 70, mayoccur due to excessive rare earth additions. Nor-mally, it is found in thick-section castings or athigher-carbon equivalents (Ref 11). Figure 71shows chunky and spiky types of graphite. Thefirst one is caused by high-purity charge mate-Fig. 75As-cast gray iron ( ). Pearlitic with 4% nital. 100 Fig. 76Same as in Fig. 75. Fine pearlite. 500 Fig. 77As-cast gray iron. Pearlitic-ferritic matrix withphosphorous eutectic (E). Etched with 4% Fig. 78Same as in Fig. 77. E, ternary phosphorous eu-tectic; P, pearlite; and F, ferrite. 500 Fig. 79As-cast gray iron ( ). Ferri-tic-pearlitic matrix. Etched with 4% nital. 100 Metallography and Microstructures of Cast Iron / 583rials and excess rare earth additions in high-car-bon-equivalent compositions, while the secondone is caused by small amounts of tramp ele-ments, for example, lead, bismuth, tin, and tita-nium, in the absence of cerium (Ref 12).Compacted that producecompacted graphite cast iron include the treat-ment of molten iron with rare earth inoculants,adding a lower amount of magnesium than isrequired to obtain nodular graphite, or addingelements such as titanium and aluminum thatmake it difficult to spheroidize graphite. Thistype of iron is a more recent development, madein an effort to improve the mechanical propertiesof flake gray iron. Figure 72 shows an examplewhere the longest compacted graphite precipi-tations are in the 80 to 160lm Graphite in Malleable graphite is formed by annealing white castiron castings to convert carbon in the form ofcementite to graphite, called temper-carbon nod-ules. Figure 73 shows the type III graphite pre-cipitation with 80lm maximum of MatrixThe matrix of gray, nodular, compacted, andmalleable cast irons can be pearlitic, pearlitic-ferritic, ferritic-pearlitic, or ferritic. The samematrix constituents can be present in white castiron, but cementite precipitates from the melt,rather than graphite, due to crystallization in ametastable matrix microstructure depends on chem-ical composition as well as on the temperatureof the eutectoidal transformation. Figure 74shows the diagram of the correlation between thetype of matrix in nonalloyed cast irons and thesilicon and phosphorus content as well as thethickness,R, of the casting wall, which relatesto the cooling C (Si logR) givesthe coefficient of graphitization, and CE C 1 3Si 1 3P is the coefficient of saturation (car-bon equivalent). A low value ofKgpromotes so-lidification of white cast iron, with cementite andpearlite as the microstructure, regardless of thetotal carbon content, C. When theKgcoefficientand the silicon content increase, the microstruc-Fig. 80As-cast gray iron ( ). Ferritic matrix. Etched in 4% nital. 100 Fig. 81The Fe-C-P equilibrium diagram. Thex-axis isthe carbon content, and they-axis is the phos-phorus content. Source: Ref 13Fig. 82As-cast ductile iron ( ). Pearlite matrix with ferritic halos aroundgraph-ite nodules. Etched with 4% nital. 100 Fig. 83As-cast ductile iron. Ferritic-pearlitic with 4% nital. 100 Fig. 84Ductile iron. Ferritic matrix. The casting wasannealed at 900 C (1650 F), held 2 h, quickfurnace cooled to 730 C (1345 F), slow furnace cooled to600 C (1110 F), and air cooled. Etched with 4% 584 / Metallography and Microstructures of Ferrous Alloysture of the cast iron matrix tends to be pearliticthrough pearlitic-ferritic to ferritic, and it de-pends on the CEvalue (Ref 13).In pearlitic-ferritic cast irons, the regions withferrite always occur within the eutectic cells andin the neighborhood of graphite precipitates dueto microsegregation. The microregions of solid-ification, like the axis of dendrites and the inte-riors of eutectic cells, contain more silicon,which promotes ferrite formation. Slow cooling,as well as higher silicon contents, usually pro-duces ferrite, while a very fast cooling rate canproduce free cementite. Ferritic microstructuresalso can be obtained by annealing of pearliticcast irons or in thick-walled castings (Ref 13).Gray ironis classified according to minimumtensile strength of the test bars. The matrix ispredominantly pearlitic (Fig. 75, 76) but also canbe pearlitic-ferritic (Fig. 77, 78), ferritic-pearlitic(Fig. 79), or ferritic (Fig. 80).A common characteristic constituent of grayiron microstructures is the phosphorus eutecticknown as steadite. Figure 81 shows the Fe-C-Pequilibrium diagram, while Table 4 shows thetransformations that occur in this system duringsolidification as well as in the solid state. Thecharacteristic property of this system is a largearea of the ternary phosphorous eutectic due tothe strong tendency for phosphorus to type of eutectic appears in the microstruc-ture of cast irons already at P (Ref 13).The form of the phosphorus eutectic dependson the chemical composition of the gray iron. Inirons with an average tendency to graphitizationand a phosphorus content of , the fine-grain ternary eutectic solidifiesfrom the liquid and consists of ferrite Fe3P Fe3C. In gray iron with a strong tendency forcementite solidification and with carbide-form-ing elements, the ternary eutectic may also con-tain large columnar precipitates of the amount of strong graphitizingelements, such as silicon, promotes solidificationof the binary phosphorous eutectic, ferrite Fe3P Cgr, instead of the ternary one. The bi-Table 4 Transformations in the range of solidification and in the solid state according tothe Fe-C-P diagram (Fig. 81)Symbol on theFe-C-P diagramTransformation(a)Type of transformationTemperatureComposition of phases(b)TJL c Fe3PPeritectic1005 C (1840 F)L C, P C, Pc C, PEL c Fe3P Fe3CEutectic950 C (1740 F)L C, Pc C, PMNc Fe3P Fe3CPeritectoidal745 C (1375 F)c C, P C, P(a) L, melt (liquid metal); , ferrite ( -iron);c, austenite (c-iron); Fe3P, iron phosphide; Fe3C, cementite. (b) Maximum solubility of chemicalcomponents in the particular phases. Source: Ref 13Fig. 85As-cast ductile iron ( ). C, cementite; L, ledeburite; and P, pearliteandferrite around graphite nodules. Etched with 4% (microscopic magnification is 100 )Fig. 86Same as in Fig. 85. C, cementite; L, ledeburite;F, ferrite; and P, pearlite. 500 Fig. 87As-cast austenitic ductile iron ( ). Austenite and eutectic carbides typeM7C3. Etched with glyceregia. 500 Fig. 88Austempered ductile iron (the same ductileiron as in Fig. 32). Acicular ferrite and austen-ite. The casting was austempered: 900 C (1650 F), held 2h, taken to salt bath at 360 C (680 F), held 180 min, andair cooled. Etched with 4% nital. 500 Metallography and Microstructures of Cast Iron / 585nary euetectic is called a pseudobinary eutectic,because carbon is removed from the eutectic dur-ing diffusion in the solid state (Ref 13).Ductile ironmicrostructures normally con-sist of pearlite or pearlite and ferrite with graph-ite nodules surrounded with ferrite, which looklike a white halo, although the more commonname of this structure is the bull s-eye 82 and 83 show this type of microstruc-ture, while Fig. 84 shows a fully ferritic-matrixmicrostructure that was achieved after annealinga chilled casting with a microstructure contain-ing cementite, ferrite, and pearlite (see ). In some cases, usually at a high coolingrate, cementite occurs as a separate constituentin the precipitates of cementite are situated inthe exterior regions of the eutectic cells or in theinterdendritic spaces of the transformed austen-ite due to the microsegregation of carbide-form-ing elements, such as manganese, chromium, orvanadium. Cementite appears very frequently inchill castings or in thin-walled sand castings (Ref13). Figures 85 and 86 show the microstructureof pearlitic ductile iron with cementite and led-eburite. In this case, cementite was a desired con-stituent of the microstructure to improve thewear resistance of cast iron. It was achieved byfeeding the melted metal with an iron-magne-sium foundry alloy, without inoculation. Thegraphite nodules do not have a perfect 87 shows the microstructure of a heat-and wear-resistant ductile iron containing chro-mium and nickel, which consists of chromiumcarbides in an austenitic austempering heat treatment is used toachieve better mechanical properties in ductileiron. The casting is heated to a temperature rangeof 840 to 950 C (1545 to 1740 F) and held atthis temperature until the entire matrix is trans-formed to austenite saturated with carbon. Thecasting is then quenched rapidly to austemperingtemperature, between 230 and 400 C (445 and750 F), and held at this temperature for the re-quired time (Ref 14). When this cycle is properlyperformed, the final structure of the matrix,which is called ausferrite, consists of acicularferrite and austenite, and the iron is called aus-tempered ductile iron (ADI). The morphologyand amount of ferrite depends on the time andtemperature of the austempering process. Fig-ures 88 and 89 show an ADI microstructure afteraustempering heat treatment, when the casting inboth cases was held in the furnace for 180 min,and the annealing temperatures were use of a higher temperature changed themorphology of the ferrite and increased theamount of austenite. Figure 90 shows the ADImicrostructure after 2 min austempering heattreatment, when the transformation of austeniteto acicular ferrite was just started, so martensiteis the dominant phase, and there is only a smallamount of acicular ferrite, mostly surroundingthe graphite nodules (see also Fig. 31 to 37).Compacted graphite-ironmatrix micro-structures consist of ferrite and pearlite; theamount of each constituent depends on the cool-ing rate and the chemical composition (elementsthat promote either ferrite or pearlite solidifica-Fig. 89Austempered ductile iron (the same composi-tion as in Fig. 32). Acicular ferrite and austen-ite. The casting was austempered: 900 C (1650 F), held 2h, taken to salt bath at 380 C (715 F), held 180 min, andair cooled. Etched with 4% nital. 500 Fig. 90Austempered ductile iron (the same composi-tion as in Fig. 32). Martensite and smallamountof acicular ferrite (AF). The casting was austempered: 900 C (1650 F), held 2 h, taken to salt bath at 300 C (570 F),held 2 min, and air cooled. Etched with 4% nital. 1000 Fig. 91As-cast iron with compacted graphite ( ). Ferritic-pearlitic matrix. Etched with 4% Fig. 92Malleable iron ( ). The casting was annealed: at 950 C(1740 F), held 10 h, furnace cooled to 720 C (1330 F),held 16 h, and air cooled. Pearlitic-ferritic matrix. Etchedwith 4% nital. 125 (microscopic magnification 100 )586 / Metallography and Microstructures of Ferrous Alloystion). Figure 91 shows a ferritic-pearlitic micro-structure with a small amount of nodular ironmatrix microstructures aremostly ferritic, ferritic-pearlitic, or pearlitic, de-pending on the cast-making process used. Thetype of matrix is a result of transformation in thesolid state during the annealing of white castiron. Figures 92 and 93 show the microstructureof the pearlitic-ferritic and ferritic malleableiron, respectively. The annealing process was thesame one in both cases, but the final microstruc-tures were achieved by the use of different foun-dry processes. In the case of the pearlitic-ferriticmicrostructure, the inoculation and deoxidationprocesses were not carried out. When these twoprocesses were used, that is, deoxidation withaluminum and inoculation with iron-silicon andpure bismuth, the result was a ferritic microstruc-ture after white iron microstructuresarecreated when the castings solidify in the meta-stable system and the structure is free of graph-ite. The matrix may consist of cementite andpearlite, as shown in Fig. 94, or, after heat treat-ment, of cementite and martensite, as shown inFig. 95. This type of white iron belongs to theclass of abrasion-resistant cast irons and usuallyis produced as chill white ironis a type ofhighly wear-resistant iron with a microstructurethat consists of primary and eutectic carbides, forexample, (FeCr)3C, (FeCr)7C3, and (FeCr)23C6,depending on the chemical composition andcooling rate during solidification. The primarycarbides have a typical shape, which in cross sec-tion is similar to the letter L, and have thechemical formula M3C at higher cooling rate andlow chromium content. When the cooling rateslows down and the chromium content increasesto, for example, 30% and higher, the primarychromium carbides have a hexagonal shape witha characteristic hole in the center. The eutecticcarbides are always type M7C3, independent ofchromium content and cooling rate (Ref 15).The type M23C6carbides solidify when thechromium content is approximately 50 to 60%(Ref 16). The same factors that affect the type ofcarbides also influence the type of matrix, whichcan be austenitic but also ferritic or the cooling rate is respectively slow, sec-ondary precipitations can occur around the aus-tenitic dendrites (Ref 17).It has been proven that the annealing heattreatment destabilizes the matrix, which willtransform into either bainite or martensite withsmall secondary carbide precipitates. The result-ing transformation product is a function of thecooling rate from annealing as well as the chem-ical composition; for example, when Cr/C 5,the matrix microstructure was bainitic, while ex-ceeding this value produced martensitic trans-formation (Ref 18). Figure 96 shows the micro-structure of high-chromium white cast iron afterisothermal heat treatment; the secondary, finecarbides precipitated from the austenitic matrix,which transformed into martensite (see also , 44, and 45).ACKNOWLEDGMENTSFor samples and examination results, manythanks to: Colleagues at the Foundry Research Institute,Krako w, Poland: Adam Kowalski, W. Wierz-Fig. 94As-cast white iron ( ). C, cementite; P,pearlite. Etched with 4% nital. 400 Fig. 93Malleable iron ( ). The casting was annealed as Etched with 4% nital. 125 (microscopic magni-fication 100 )Fig. 95White cast iron after heat treatment. A networkof massive cementite and tempered with 4% picral. 140 . Courtesy of VanderVoort, Buehler 96White high-chromium cast iron (see Fig. 43).PC, primary carbides; EC, eutectic carbides inmartensitic matrix with fine, globular secondary casting was heat treated at 1000 C (1830 F), held 1h, furnace cooled to 550 C (1020 F), held 4 h in a 400 C(750 F) salt bath, and air cooled. Etched with Metallography and Microstructures of Cast Iron / 587chowski, J. Turzyn ski, W. Madej, ownia, A. Pytel, and J. Bryniarska M. Sorochtej of The Krako w Technical Uni-versityAlso, many thanks to M. Warmuzek at theFoundry Research Institute for making the SEMmicrographs of different types of thanks to Vander Voort fromBuehler Ltd. for his advice and help in preparingthis Guide to Engineered Materials, PropertyComparison Tables,Adv. Mater. Process.,Vol 159 (No. 12), Dec 2001, p 462. Cz. Podrzucki and Cz. Kalata,Metalurgia iodlewnictwo z eliwa (Metallurgy and Foun-drying of Cast Iron),S la sk-Katowice, Po-land, 1971, p 32, 54 59, 703. W. Sakwa,Z eliwo (Cast Iron),S la sk-Katowice, Poland, 1974, p 534. Vander Voort,Metallography: Princi-ples and Practice,McGraw-Hill Book Co.,1984; Reprinted by ASM International,1999, p 129, 294 303, 632, 634, 642 644,646, Vander Voort, Phase Identification bySelective Etching,Applied Metallography, Vander Voort, Ed., Van Nostrand Rein-hold Co., 1986, p 1 Zhou, Dalian, W. Schmitz, and S. Engler,UnterschungderGefu gebildungvonGu eisen mit Kugelgraphit bei langsamerErstarrung,Giessereiforschung,V ol 39 ( ), 1987, p 55 Skoc ovsky ,Colour Contrast in Metallo-graphic Microscopy,Z ilina (Slovak Repub-lic), 1993, p Weck and E. Leistner,Metallographic In-structions for Color Etching by Immersion,Part II: Beraha Color Etchants and TheirDifferent Variations,Deutscher Verlag fu rSchwei technik, GmbH, Du sseldorf, 1983, Liu and Loper, Observation on theGraphite Morphology in Cast Iron, ,Vol 88, 1980, p Nelson, Cast Irons,Metallography andMicrostructures,Vo l 9 ,ASM Handbook,American Society for Metals, 1985, p 24211. Poor Nodularity in Ductile Iron, TechnicalInformation 25, Revision No. 1, ElkemASA, Silicon Division, 200112. Graphite Structures in Cast Irons, poster,Elkem Se kowski, J. Piaskowski, and Z. Wojtow-icz, Atlas struktur znormalizowanych sto-po w odlewniczych(Atlas of the StandardMicrostructures of Foundry Alloys),WNTWarszawa, Poland, 1972, p 54 55, 74 Kovacs, On the Terminology andStructure of ADI,Trans. AFS,Vol 102,1994, p 417 Ohide and G. Ogira, Solidification ofHigh Chromium Alloyed Cast Irons, ,Vol 76 (No. 1), 1983, p S ittner, Prima rni kristalizace chromov-ych litin, C ast II (Primary Solidification ofthe Chromium Cast Irons, Part II),Sle va r-enstv ,Vol 33 (No. 9), 1985, p Dupin and Schissler, Etude structuralde l e tat brut de fontes blanches a` 17% Cr,Hommes et Fonderie,No. 151, 1985, p Tong, T. Suzuki, and T. Umeda, TheInfluence of Chemical Composition and De-stabilization on Transformation Character-istics of High Chromium Cast Irons,Imono(J. Jpn. Foundrymen s Soc.),Vol 62 (No. 5),1990, p 344ASM International is the society for materials engineers and scientists, a worldwide network dedicated to advancing industry, technology, and applications of metals and materials. ASM International, Materials Park, Ohio, USA This publication is copyright ASM International . All rights reserved. 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