Metallography and Microstructures of Cast Iron

Metallography andMicrostructures of Cast IronJanina M. Radzikowska, The Foundry Research Institute, Kraków, PolandCAST IRON is an iron-carbon cast alloy withother elements that is made by remelting pigiron, scrap, and other additions. For differentiationfrom steel and cast steel, cast iron is definedas a cast alloy with a carbon content (min 2.03%)that ensures the solidification of the final phasewith a eutectic transformation. Depending onchemical specifications, cast irons can be nonalloyedor 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, aluminum,molybdenum, tungsten, copper, vanadium,titanium, plus others.Free graphite is a characteristic constituent ofnonalloyed and low-alloyed cast irons. Precipitationof graphite directly from the liquid occurswhen solidification takes place in the range betweenthe temperatures of stable transformation(T st ) and metastable transformation (T mst ), whichare, respectively, 1153 C (2107 F) and 1147 C(2097 F), according to the iron-carbon diagram.In this case, the permissible undercooling degreeis DT max T st T mst . In the case of a higherundercooling degree, that is, in the temperaturesbelow T mst , primary solidification and eutecticsolidification can both take place completely orpartially in the metastable system, with precipitationof primary cementite or ledeburite.Graphitization can also take place in the rangeof critical temperatures during solid-state transformations.The equilibrium of phases Fe c Fe Fe 3 C occurs only at the temperature 723 2 C (1333 4 F), while equilibrium ofphases Fe c Fe C gr occurs at the temperature738 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 multicomponentalloy, these temperatures can bechanged by different factors: chemical composition,ability of cast iron for nucleation, andcooling rate. Silicon and phosphorus bothstrongly affect the carbon content of the eutectic.That dependence was defined as a carbon equivalent(C e ) value that is the total carbon contentplus one-third the sum of the silicon and phosphoruscontent (Ref 2). Cast iron, with a compositionequivalent of approximately 4.3, solidifiesas a eutectic. If the C e is 4.3, it ishypereutectic; if it is 4.3, cast iron is hypoeutectic(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 discontinuities.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 microstructurechanges, promoting radial-sphericalshape (Ref 2).Preparation for MicroexaminationPreparation of cast iron specimens for microstructuralexamination is difficult due to the needto properly retain the very soft graphite phase,when present, that is embedded in a harder matrix.Also, in the case of gray irons with a softferritic matrix, grinding scratches can be difficultto remove in the polishing process. When shrinkagecavities are present, which is common, thecavities must not be enlarged or smeared over.Retention of graphite in cast iron is a commonpolishing problem that has received considerableTable 1ironsType of ironRange of chemical compositions for typical nonalloyed and low-alloyed castComposition, %C Si Mn P SGray (FG) 2.5–4.0 1.0–3.0 0.2–1.0 0.002–1.0 0.02–0.025Compacted graphite (CG) 2.5–4.0 1.0–3.0 0.2–1.0 0.01–0.1 0.01–0.03Ductile (SG) 3.0–4.0 1.8–2.8 0.1–1.0 0.01–0.1 0.01–0.03White 1.8–3.6 0.5–1.9 0.25–0.8 0.06–0.2 0.06–0.2Malleable (TG) 2.2–2.9 0.9–1.9 0.15–1.2 0.02–0.2 0.02–0.2FG, flake graphite; SG, spheroidal graphite; TG, tempered graphite. Source: Ref 1Fig. 1 Spheroidal graphite in as-cast ductile iron (Fe-3.7%C-2.4%Si-0.59%Mn-0.025%P-0.01%S-0.095%Mo-1.4%Cu) close to the edge of the specimen,which was 30 mm (1.2 in.) in diameter. The specimen wasembedded. As-polished. 100

Metallography and Microstructures of Cast Iron / 567can be reduced to the desired size for metallographicspecimens by using a laboratory abrasivecutoff saw or a band saw. If the casting wasFig. 7 Same as in Fig. 6 but after final polishing withthe 1 lm diamond paste applied on a naplesscloth. Graphite is free of any visible pullouts. As-polished.400sectioned by flame cutting, the specimen mustbe removed well away from the heat-affectedzone. The pieces cut out for metallographic examinationmay 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 cutoffsaws, the use of the correct wheel, and adequatewater cooling. Overheating during grindingis 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 preparation.Mounting. Specimens 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 retention.Use of an incorrect resin, or ignoring the mountingprocess, 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 (1.2 in.) diameter barand polished with and without embedding in apolymer resin, respectively. In the specimen preparedwithout 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 graphitein gray iron close to the edge of such a specimenis not polished perfectly, in comparison towell-polished graphite in the mounted specimen.Grinding and Polishing. To ensure propergraphite retention, the use of an automatedgrinding-polishing machine is recommendedover manual preparation. The automated equipmentmakes 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 factorsare also important.A 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 correctlypolished surface in the shortest possibletime. This precludes the risk of pulling out thegraphite phase and ensures that the graphite precipitateswill be perfectly flat with sharp boundaries.The 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. 8 Same as in Fig. 7 but after final polishing withthe 1 lm diamond suspension applied on anapped cloth. The arrows show the pulled-out graphite.Aspolished.400Fig. 9 White high-chromium iron (Fe-3.2%C-4.65%Cr-2.9%Mn-0.51%Si-0.050%P-0.024%S). Eutecticand secondary carbides in the matrix. Specimen was preparedcorrectly. 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. 500Fig. 10 White high-chromium iron (Fe-3.16%C-8.86%Cr-0.50%Si-3.04%Mn-0.051%P-0.018%S). Eutectic and secondary carbides in the matrix.Specimen was prepared incorrectly. The casting was austenitizedat 1000 C (1830 F), held 1 h, furnace cooled to700 C (1290 F) for 2 h, taken to salt bath at 700 C (1290F), held 4 h, and air cooled. Etched with glyceregia. 500

Metallography 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:Fig. 11 Ductile iron (Fe-3.8%C-2.4%Si-0.28%Mn-1.0%Ni-0.05%Mg) after annealing. Ferrite andapproximately 5% pearlite. Etched with 2% nital. 100.Courtesy of G.F. Vander Voort, Buehler Ltd.● First step: 3 lm 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 colloidalsilica suspension on a napless syntheticFig. 13 Same as in Fig. 12 but after etching with 4%picral. Pearlite was etched uniformly. 500polyurethane pad but in a single specimenholder with an individual load of 30 N for 1.5min.Figure 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 microstructureof a high-chromium iron. They appearto be sticking out from the matrix, and theirboundaries are not outlined uniformly. This resultoccurs if the load is too low or the final polishingtime on the silica suspension is too long.Both problems will result in too much removalof the softer matrix that was surrounding the primarycarbides.During grinding, the paper must be moistenedwith flowing tap water, and the specimens shouldbe washed with water after each step. Also, duringthe first planning step, the sheet of papershould be changed every 1 min. Used grit paperis not effective and will introduce heat and damage,impairing specimen flatness. During polishingwith 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 required.The 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 dryer.Fig. 12 As-cast gray iron (Fe-2.8%C-0.8%Si-0.4%Mn-0.1%S-0.35%P-0.3%Cr). Pearlite. Etched with4% nital. Arrows show the white areas with weakly etchedor nonetched pearlite. 500Fig. 14 As-cast high-chromium white iron (Fe-1.57%C-18.64%Cr-2.86%Mn-0.53%Si-0.036%P-0.013%S). Eutectic chromium carbides typeM 7C 3 in austenitic matrix. Etched with glyceregia. 500Fig. 15 Same as in Fig. 14 but after etching with 4%nital. 500

570 / Metallography and Microstructures of Ferrous AlloysMicroexamination MethodsFig. 16 As-cast ductile iron (Fe-3.07%C-0.06%Mn-2.89%Si-0.006%P-0.015%S-0.029%Mg). C,cementite; L, ledeburite; F, ferrite; and P, pearlite. Etchedwith 4% nital. 650 (microscopic magnification 500)Chemical Etching. The examination of theiron microstructure with a light optical microscopeis always the first step for phase identificationand morphology. One should always beginmicrostructural investigations by examiningthe as-polished specimen before etching. This isa necessity, of course, for cast iron specimens, ifone is to properly examine the graphite phase.Fig. 18 As-cast gray iron (Fe-3.24%C-2.32%Si-0.54%Mn-0.71%P-0.1%S). E, phosphorousternary eutectic. Etched with 4% nital. 100Standard Etchants. To see the microstructuraldetails, specimens must be etched. Etchingmethods based on chemical corrosive processeshave been used by metallographers for manyyears to reveal structures for black-and-whiteimaging.Specimens 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 microstructuralconstituents. Usually, this is a2to4%alcohol solution of nitric acid (HNO 3 ) (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 crystallographicorientation of pearlite grains, so, in thecase of a fully pearlitic structure, it is recommendedto use picral, which is an alcohol solutionof 4% picric acid (Table 2, etch No. 2). Figures12 and 13 show the differences in revealingthe microstructure of pearlite with nital or picral.Picral does not etch the ferrite grain boundaries,or as-quenched martensite, but it etches the pearliticstructure more uniformly, while nital leaveswhite, unetched areas, especially in the casewhere pearlite is very fine.When 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. 17 Same 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. 19 Same as in Fig. 18 but after etching with KlemmI reagent. E, phosphorous ternary eutectic.100Fig. 20 Austempered ductile iron (Fe-3.6%C-2.5%Si-0.06%P-1.5%Ni-0.7%Cu). CB, cell boundaries;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 ferrite.Picral reveals this phase very well; martensiteis barely etched due to the very shortaustempering heat treatment of the specimen,Fig. 21 Same as in Fig. 20 but after etching with Beraha’sCdS reagent. H, ferritic halo; CB, cellboundaries. 250which was 2 min, because the martensite was asquenched.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 beFig. 23 As-cast gray iron (Fe-3.33%C-1.64%Si-0.31%Mn-1.37%P-0.107%S). Ternary phosphoruseutectic. Etched with 4% nital. 1300 (microscopicmagnification 1000)stored in the lab than nital, which can be an explosivemixture under certain conditions when itis stored in a tightly closed bottle.Glyceregia (Table 2, etch No. 3), which is amixture of glycerine, hydrochloric acid (HCl),and nitric acid (HNO 3 ), is recommended for revealingthe microstructure of high-chromiumand chromium-nickel-molybdenum irons. Figure14 shows the microstructure of a high-chromiumcast iron after etching with glyceregia (seealso Fig. 43, 49, and 96). Nital can be also usedfor revealing the carbide morphology in the microstructureof chromium or chromium-nickelirons when the carbon and chromium contentpromotes solidification of eutectic carbides.When the microstructure of a high-chromiumwhite iron contains columnar primary carbides,glyceregia is recommended.Figure 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 uniformly.Selective Color Etching. If the black-andwhiteetchants are inadequate for positive identificationof the iron microstructures, other proceduresmust 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 balancedto deposit a thin (40 to 500 nm), transparentfilm of oxide, sulfide, complex molybdate,elemental selenium, or chromate on thespecimen surface. Coloration is developed by interferencebetween light rays reflected at the in-Fig. 22 Nodular iron (Fe-3.9%C-2.9%Si-0.32%Mn-0.06%P-0.037%Mg-1.5%Ni-0.57%Cu). Siliconmicrosegregation was revealed. The casting was annealed.Etched with hot aqueous solution of sodiumhydroxide, picric acid, and potassium pyrosulfite (Table 2,etchant No. 7). 500Fig. 24 Same 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. 25 Same as in Fig. 23 but after etching with KlemmI reagent. F, ferrite; C, cementite; and C IP,cementite iron phosphide inside the precipitate of phosphorouseutectic. 1300 (microscopic magnification1000)

572 / Metallography and Microstructures of Ferrous Alloysner and outer film surfaces. Crystallographic orientation,local chemical composition, andetching time affect film thickness and controlcolor production. The use of selective etchantsFig. 26 Same 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. 28 Same as in Fig. 25 but after etching with Beraha’sreagent with selenic acid. IP, iron phosphide;C, cementite; and F, ferrite. The dark points in pearlite,which look like artifacts, can be iron phosphideprecipitates or fine, nonmetallic inclusions. 1300 (microscopicmagnification 1000)is invaluable for quantitative metallography, afield of growing importance (Ref 5).When ferritic-pearlitic microstructures of castiron contain large amounts of cementite and ledeburite,the differentiation of the white structuralconstituents is difficult after etching a specimenwith nital. In such cases, hot alkaline sodium picrate(ASP) is recommended (Table 2, etch No.4), 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 Fig.17 shows the microstructure of the same specimenafter 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 estimatethe amount of cementite that should be removedfrom the casting microstructure in the annealingprocess.Segregation 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 (K 2 S 2 O 5 ) and a cold-saturatedwater solution of sodium thiosulfate(Na 2 S 2 O 3 •5H 2 O) and is one of the etchants thatcan be used to reveal phosphorus and silicon segregation.Usually, the distribution of the phosphorus eutectic,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. 27 Same as in Fig. 26 but after overetching withhot Murakami’s reagent. IP, iron phosphide; C,cementite; and F, ferrite. 1300 (microscopic magnification1000)Fig. 29 As-cast gray iron (Fe-3.62%C-2.03%Si-1.13%P-0.61%Mn-0.137%S-0.113%Cr-0.478%Ni-0.004%Al). E, binary phosphorous eutectic; F,ferrite at the graphite precipitate; and P, pearlite. Etchedwith 4% nital. 500Fig. 30 Same as in Fig. 29 but after etching with hotalkaline sodium picrate and 4% nital. Pearliticmatrix is revealed; phosphorous eutectic is unaffected.500

Metallography and Microstructures of Cast Iron / 573are white, while their interiors are almost blackdue to overetching the pearlitic-ferritic matrix.Figure 19 shows the same microstructure afteretching with Klemm’s I reagent. The microregionsinside the eutectic cells with a lower phosphoruscontent are tinted a blue color, while theFig. 31 Same as in Fig. 29 but after etching with hotMurakami reagent. Only brown-tinted ironphosphide was revealed. 500areas with the ternary eutectic, situated at theboundaries of the eutectic cells, are almost colorless.In both cases, the cementite and ironFig. 33 Same as in Fig. 32 but after etching with 4%picral. AF, acicular ferrite; PM, plate martensite.1000phosphide in steadite are not etched, and the networkis clearly visible.Figure 20 shows silicon segregation in an ADImicrostructure after etching with Klemm’s I reagent.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 colorless.Acicular ferrite is orange, and austenite isnot tinted. Figure 21 shows the same microstructureafter etching with Beraha’s CdS reagent (Table2, etch No. 6), an aqueous solution of sodiumthiosulfate (Na 2 S 2 O 3 •5H 2 O), citric acid(C 6 H 8 O 7 •H 2 O), and cadmium chloride(CdCl 2 •2.5H 2 O). The silicon segregation is revealedthe same way as after using Klemm’s Ireagent.To reveal silicon segregation in nonalloyedductile cast iron inside eutectic cells, the hotaqueous solution of sodium hydroxide, picricacid, and potassium pyrosulfite (K 2 S 2 O 5 ) 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 colorless.Before etching, ferritization of the specimen wascarried out to enhance the visibility of the microstructuralcolors.The revealing and differentiation of all constituentsin steadite is invaluable for the determinationof the type of eutectic as well as theFig. 32 Austempered ductile iron (Fe-3.6%C-2.5%Si-0.056%P-0.052%Mg-0.7%Cu). Martensiteand acicular ferrite. The casting was austempered at 900C (1650 F), held 2 h, taken to salt bath at 360 C (680 F),held 2 min, and air cooled. Etched with 4% nital. 1000Fig. 34 Same as in Fig. 32 but after etching with Beraha-Martensitereagent. PM, blue-yellow platemartensite; FM, brown fine martensite; and AF A, darkneedles of acicular ferrite surrounded with colorless austenite.1000Fig. 35 Same as in Fig. 32 but after etching with 10%sodium metabisulfite. PM, plate martensite;FM, fine martensite; and AF A, acicular ferrite and austenite.1000

574 / Metallography and Microstructures of Ferrous AlloysFig. 36 Austempered ductile iron (Fe-3.6%C-2.5%Si-0.052%Mg-0.7%Cu). 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% nital.1000Fig. 38 Same as in Fig. 36 but after etching with 10%sodium metabisulfite. AF, acicular ferrite; A,austenite; and M, martensite. 1000amount of each constituent. In the case of theternary phosphorous eutectic, which consists offerrite, cementite, and iron phosphide (Fe 3 P), nitaldoes not help in the identification of the constituents,nor does it provide enough informationabout distribution of the eutectic constituents.Figure 23 shows the microstructure of the ternaryphosphorous eutectic in gray iron after etchingin 4% nital. The white areas that surroundthe ternary eutectic and are also visible insidethe eutectic can be either ferrite or cementite.Figure 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 tinted.Murakami’s reagent (Table 2, etch No. 8),used at 50 C (120 F) for 3 min and containingpotassium hydroxide (KOH), potassium ferricyanide(K 3 Fe(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 colorless.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 examination.Good 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. 37 Same as in Fig. 36 but after etching with Beraha-Martensitereagent. AF, acicular ferrite; A,austenite; and M, martensite. 1000Fig. 39 White alloyed cast iron (Fe-3.4%C-0.92%Mn-1.89%Si-9.52%Cr-6.27%Ni). Etched with Beraha-Martensite.PM, plate martensite; FM, fine martensite;EC, eutectic carbides type M 7C 3; 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 cooled.1300 (microscopic magnification 1000)Fig. 40 Same white iron as in Fig. 39 but after etchingwith 4% nital. M, martensite; EC, eutectic carbides;and SC, secondary carbides. 1300 (microscopicmagnification 1000)

Metallography and Microstructures of Cast Iron / 575Fig. 41 Same white iron as in Fig. 39 and 40 but ascast.Eutectic carbides in austenitic matrix.Etched with glyceregia. 500(H 2 SeO 4 ), 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 microstructureof the ternary eutectic, with cementite tinteda light-pink color, while the rest of the constituentswere colored properly.Fig. 43 As-cast high-chromium white iron (Fe-4.52%C-0.4%Si-2.86%Mn-35.0%Cr-0.06%P-0.012%S). PC, primary carbides; EC, eutectic carbides,both M 7C 3 type. Etched with glyceregia. 500Figure 29 shows the microstructure of thepseudobinary phosphorous eutectic, which consistsof 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 microstructureof the eutectic, the specimen was nextetched with 4% nital. Figure 30 shows the resultsafter using hot ASP and then nital. Hot Murakami’sreagent perfectly tinted the iron phosphidein the binary phosphorous eutectic a browncolor, while pearlite was colorless, as shown inFig. 31.Beraha-Martensite (B-M) (Table 2, etch No.10) and aqueous 10% sodium metabisulfite(SMB) (Table 2, etch No. 11) reagents for selectivecolor 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 colorsthat are useful for verifying that the heattreatment process was carried out correctly.The B-M etchant is a mixture of stock solution(1:5, HCl to water), potassium metabisulfite(K 2 S 2 O 5 ), and ammonium acid fluoride(NH 4 F•HF). According to Ref 6, B-M tints martensitea blue color and bainite a brown color,while the residual austenite and carbides remainunaffected.The B-M etchant can be used for identificationof the constituents after heat treatment of castiron by tinting phases to different colors. It alsoFig. 42 Same as in Fig. 41 but after etching with Lichteneggerand Bloech I. Austenite is dark brown,and dendritic segregation is visible around unaffected carbides.1000Fig. 44 Same as in Fig. 43 but after etching with Murakami’sreagent (at room temperature). PC, orangeprimary carbides; EC, orange and gray eutectic carbides.400Fig. 45 Same as in Fig. 43 but as-polished and examinedin differential interference contrast. Primaryand eutectic carbides are sticking up from the softeraustenitic matrix. 400

576 / Metallography and Microstructures of Ferrous AlloysFig. 46 Same white iron as in Fig. 39 but after slightetching with 4% nital and examined in brightfieldillumination. EC, eutectic carbides type M 7C 3;SC,secondary carbides; and M, martensite. 1000improves 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 microstructureof ADI after etching with 4% nital,while Fig. 33 shows the same microstructure afteretching with 4% picral. Figures 34 and 35Fig. 48 Same as Fig. 46 but examined in differentialinterference contrast. EC, eutectic carbides;SC, secondary carbides; and M, martensite. 1000show 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 crystallographicorientations 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 microstructureoccurs due to the change in size of theplate martensite as transformation progresses.However, 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 nodulesand in the matrix. The SMB etchant is evenmore useful than the B-M etchant in the casewhere the dominating phase in the ADI microstructureis 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 white.Some areas that were darkened may be martensite,but there is no clear distinction betweenmartensite and acicular ferrite with nital. Selec-Fig. 47 Same as in Fig. 46 but examined in dark-fieldillumination. EC, eutectic carbides; SC, secondarycarbides; and M, martensite. 1000Fig. 49 Same white iron as in Fig. 14 and 15 but castingwas 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 typeM 7C 3; SC, secondary carbides. Etched with glyceregia.1000Fig. 50 Same as in Fig. 49 but examined in dark-fieldillumination. EC, eutectic carbides; SC, secondarycarbides. 1000

Metallography and Microstructures of Cast Iron / 577tive color etching with the two previously mentionedreagents 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 remainedcolorless.The same results were achieved with the useof B-M reagent to reveal the microstructure ofalloyed cast iron after heat treatment. The microstructure,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-carbonplate martensite. Figure 40 shows the microstructureof the same specimen after etchingwith 4% nital; in this case, the recognition ofmartensite is not straightforward.Figures 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, etchantNo. 3 and 12). The LBI is an aqueous solutionof ammonium bifluoride (NH 4 F•HF) andpotassium metabisulfite (K 2 S 2 O 5 ) (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 microstructureshown in Fig. 42 consists of austenite, tintedbrown and blue color, and white (noncolored)carbides. The blue austenitic areas surroundingFig. 51 Graphite nodule examined in bright-field illumination.As-polished. 1000Fig. 53 Flake graphite in as-cast gray iron examined incrossed polarized light. As-polished. 200Fig. 55 Flake graphite in as-cast gray iron examinedwith SEM. Sample was deeply etched with50% HCl. 500Fig. 52 Same as in Fig. 51, but graphite nodule wasexamined in crossed polarized light. 1000Fig. 54 Same as in Fig. 33 but microstructure was examinedin crossed polarized light. Acicular ferriteis shining brightly; plate martensite is slightly gray-blue.1000Fig. 56 Nodular graphite in as-cast ductile iron examinedwith SEM. Sample was deeply etched with50% HCl. 1000

578 / Metallography and Microstructures of Ferrous Alloysthe eutectic carbides indicate regions with alower concentration of carbide-forming elements,such as carbon and chromium, and perhapsa higher concentration of elements that arenot present in the carbides, such as silicon andnickel.Figure 43 shows the microstructure of a highchromiumcast iron after etching with glyceregia,revealing columnar primary carbides and eutecticcarbides, both (FeCr) 7 C 3 type, uniformlydistributed in the matrix. Figure 44 shows themicrostructure of the same specimen after etchingwith the standard Murakami’s etchant atroom temperature. The carbides are tinted an orangecolor, while the matrix is not colored. X-ray diffraction determined that the matrix wasaustenitic-ferritic; the matrix was not colored usingthe LB I tint etchant due to missing nickelin that iron.The basic etchants mentioned previously,which are suitable for revealing microstructuresas well as for phase identification in irons, arelisted in Table 2.Dark-Field Illumination and DifferentialInterference Contrast. Dark-field 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 specimenis normal to the optical axis of the microscopeand white light is used. This image contrastis a result of reversing the image, which isbright in BFI, into a dark one when DFI is used.Table 3 Constituents commonly found incast iron microstructures, and their generaleffect on physical propertiesFig. 57 Compacted graphite examined with SEM. Samplewas deeply etched with 50% HCl. 1500Fig. 58 Hypoeutectic as-cast gray iron (Fe-2.8%C-1.85%Si-0.5%Mn-0.04%P-0.025%S).graphite type A. As-polished. 100FlakeFig. 59 Hypoeutectic as-cast gray iron (Fe-2.1%C-2.8%Si-0.38%Mn-0.06%P-0.03%S).graphite type D. As-polished. 100FlakeFig. 60 Hypereutectic as-cast gray iron (Fe-3.5%C-2.95%Si-0.4%Mn-0.08%P-0.02%S-0.13%Ni-0.15%Cu). Flake graphite type A. As-polished. 100ConstituentGraphite(hexagonalcrystalstructure)Austenite (c-iron)Ferrite (-iron)Cementite (Fe 3C)PearliteMartensiteSteadite(phosphorouseutectic)LedeburiteSource: Ref 3, 10Characteristics and effectsFree carbon; soft; improvesmachinability and dampingproperties; reduces shrinkage andmay reduce strength severely,depending on shapeFace-centered cubic crystal structure.The 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 HBBody-centered cubic crystal structure.Iron with elements in solidsolution, which is a stableequilibrium, low-temperaturephase. Soft, 80–90 HB; contributesductility but little strengthComplex orthorhombic crystalstructure. Hard, intermetallic phase,800–1400 HV depending uponsubstitution of elements for Fe;imparts wear resistance; reducesmachinabilityA metastable lamellar aggregate offerrite and cementite due toeutectoidal transformation ofaustenite above the bainite region.Contributes strength withoutbrittleness; has good machinability,approximately 230 HBGeneric term for microstructures thatform by diffusionlesstransformation, where the parentand product phases have a specificcrystallographic relationship. Hardmetastable phaseA 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 0.06%.Hard and brittle; solidifies from theliquid on cooling at the cellboundaries as a last constituent ofthe microstructureMassive eutectic phase composed ofcementite and austenite; austenitetransforms to cementite andpearlite on cooling. Produces highhardness and wear resistance;virtually unmachinable

Metallography and Microstructures of Cast Iron / 579Features that are perpendicular to the optic axisin BFI appear white, while in DFI they are black.Likewise, features that are at an angle to the surface,such as grain boundaries and phase boundaries,appear black in BFI but are white (selfluminousin appearance) in DFI.Fig. 61 Hypereutectic as-cast gray iron (Fe-2.18%C-2.49%Si-0.7%Mn-0.06%P-0.05%S). Flakegraphite type E. As-polished. 100. Courtesy of G.F. VanderVoort, Buehler Ltd.Fig. 63 Hypereutectic as-cast gray iron (Fe-3.4%C-3.4%Si-0.07%Mn-0.04%P-0.03%Cr-0.47%Cu). Pointlike flake graphite type D. As-polished.100When crossed polarized light is used alongwith a double-quartz prism (Wollaston prism)placed between the objective and the vertical illuminator,two light beams are produced that exhibitcoherent interference in the image plane.This 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 developedby Nomarski. The image reveals topographicdetail 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 DIC.The chromium carbides, which are much harderthan the matrix, stand above it in relief and canbe seen very well.The DFI and DIC methods also can be usedfor revealing the details of microstructures in alloyedirons, for example, a chromium-nickeliron after heat treatment, which were barely visibleafter 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 martensitepoorly, and the secondary carbides are barelyvisible. However, both the DFI and DIC imagesreveal the martensite, although not strongly,Fig. 62 Hypereutectic as-cast gray iron (Fe-3.3%C-2.75%Si-0.88%Mn-0.42%P-0.086%S). Flakegraphite type B. As-polished. 90 (microscopic examination100)Fig. 64 Hypereutectic as-cast gray iron (Fe-4.3%C-1.5%Si-0.5%Mn-0.12%P-0.08%S). Flakegraphite type C (kish graphite). As-polished. 100Fig. 65 As-cast gray iron. Flake graphite type V (starlikegraphite). As-polished. 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 afterheat treatment, revealed with glyceregia andFig. 66 As-cast ductile iron (Fe-3.45%C-2.25%Si-0.30%Mn-0.04%P-0.01%S-0.8%Ni-0.07%Mg-0.55%Cu). Nodular graphite size is 20 lm. Aspolished.100examined with BFI and DFI, is shown in Fig. 49and 50, respectively. The details of the microstructurewere revealed much more strongly withDFI than with BFI.Fig. 68 As-cast ductile iron. Nodular graphite size is100 lm. As-polished. 100Polarized Light Response. Polarized light isobtained by placing a polarizer (usually a Polaroidfilter, Polaroid Corp.) in front of the condenserlens of the microscope and placing an analyzer(another Polaroid filter) before theeyepiece. The polarizer produces plane-polarizedlight that strikes the surface and is reflectedthrough the analyzer to the eyepieces. If an anisotropicmetal 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 producedark, extinguished conditions (completedarkness is not possible using Polaroid filters).Polarized light is particularly useful in metallographyfor revealing grain structures and twinningin anisotropic metals and alloys as well as foridentifying anisotropic phases and inclusions.This method also improves the contrast of microstructures,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 colorsof graphite vary due to the anisotropy ofgraphite. Figure 54 shows the microstructure ofADI, consisting of martensite and a smallFig. 67 As-cast ductile iron (Fe-3.6%C-2.9%Si-0.14%Mn-0.04%P-0.02%S-0.16%Ni-0.06%Mg). Nodular graphite size is 40 lm. As-polished.100Fig. 69 As-cast ductile iron. Imperfectly formed graphitenodules. As-polished. 100Fig. 70 As-cast ductile iron. Exploded graphite. As-polished.100

NÚMERO ESPECIAL DEL XII SÍNODO GENERALLa Gruta del Getsemaní en el jardín de los Santos Juan y Pablomultirracial y multirreligioso. El último CapítuloGeneral trató proféticamente el tema de la globalización.El Documento Capitular afirma:“‘Solidaridad’ es la palabra escogida para describiruna nueva manera de estar juntos comopasionistas en misión por la vida del mundo. Las‘nuevas’ realidades requieren ‘nuevas’ respuestasen la fe. La solidaridad exige de cada uno unaconversión profunda de la mente y del corazón. Setrata de un crecimiento en la comprensión de quela vida es un don para compartir” (DC 4.6).Ha llegado ya el momento de crear “una nuevamanera de estar juntos”, de “dar respuestas nuevasa realidades nuevas” no sólo en el ámbito comunitarioo provincial, sino en toda la Congregación.Reestructurar para revitalizar, reestructurarpara permitir un mejor flujo de vida de una partede la Congregación hacia la otra “en un sólo cuerpoy un sólo espíritu”. Ha llegado el tiempo deabrirse al don de la vida para que todos en laCongregación tengamos la posibilidad de unanueva vida. Donándonos permanecemos; preservándonosy rechazando la posibilidad de abrirnosnos engañamos, al creer que permanecemos ysobrevivimos, cerrando de esta manera el horizontede futuro: “quien quiera salvar la propiavida, la perderá; pero quien la pierda por mí ypor el evangelio, la salvará” (Mc 8, 35).Es tiempo de pensar más como Congregaciónque como Provincia, recuperando la frescuraevangélica y la capacidad de diálogo entre todaslas partes de la Congregación, con intercambio dedones entre las diversas culturas y naciones.Donde hay una auténtica y sincera comunicación,allí se realiza la verdadera comunión. Es necesarioentrar en la “cultura del otro” para comprendersus ideas, compartir sus emociones, compartir sussueños. Uno de estos sueños consiste en que laCongregación se transforme de tal manera queparezca una sola provincia y en cuanto tal viva ysea enviada a todas las razas del mundo paraanunciar la Buena Noticia. Jesús nos quiere multiculturalesy multiétnicos: “Id y haced discípulosa todas las naciones” (Mt 28, 19).Pero, más allá de preguntarse el por qué deuna Reestructuración y de llegar a la convicciónde que ya es inevitable realizarla, es necesariopreguntarse y discernir cuál Reestructuración senecesita hoy para revitalizar la Congregación ypara ser, por tanto, eficaces en la Misión. ¿Quétipo de Congregación queremos para el mundode hoy con su secularización, su violencia y suterrorismo, su agresividad en el ámbito global yfamiliar, difundida incluso en las pequeñas cosascotidianas? Mucho nace del olvido de la Pasiónde Jesús y de los grandes “valores” humanos ycristianos y de la incapacidad de amar y de reconciliarse.¿Qué Congregación fundaría hoy San Pablode la Cruz para los males de nuestro tiempo ypara la vitalidad misma de la Congregación? Y,por ende, nosotros, ¿qué tipo de Congregaciónpodemos establecer como hipótesis para nuestrosdías? ¿Y dentro de diez años? ¿Cuál vidndomly oriented graphite flakeswith a maximum length of 320 lm (type A flakegraphite in the ASTM A 247 classifications).Figure 59 shows a fine, type D flake graphitewith a maximum length of 40 lm, also in a hypoeutecticalloy, but it solidified at a higher coolingrate, which promoted the interdendritic distribution.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 machinability.The maximum length of graphite flakesis 160 lm. Figures 61 and 62 show type E andtype B graphite, respectively, which occur in hypereutecticgray 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 castings.Fig. 72 As-cast iron with compacted graphite (Fe-3.7%C-2.3%Si-0.21%Mn-0.03%P-0.01%S-0.82%Ni-0.02%Mg). Graphite size is 80 to 160 lm. As polished.100Fig. 74 The 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 rate. K g C (Si logR)is a coefficient of graphitization, and C E 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 13

582 / Metallography and Microstructures of Ferrous AlloysFig. 75 As-cast gray iron (Fe-2.8%C-1.85%Si-1.05%Mn-0.04%P-0.025%S). Pearlitic matrix.Etched with 4% nital. 100Fig. 77 As-cast gray iron. Pearlitic-ferritic matrix withphosphorous eutectic (E). Etched with 4% nital.100A high degree of undercooling of hypereutecticgray iron can promote the solidification ofvery fine, pointlike type D graphite with an interdendriticdistribution, as shown in Fig. 63. Inthe other direction, undertreatment of the graphitizinginoculants, such as ferrosilicon, producesother flake graphite types in gray iron. For example,Fig. 64 shows a hypereutectic gray ironwith graphite type C, where very coarse, needlelikeflakes (kish graphite) form before the eutectic,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 elements,for example, chromium, manganese, andvanadium, and the low-melting-point metals, forexample, bismuth, lead, and sulfur, also affectgraphite morphology.Nodular Graphite in Ductile Iron. The additionof magnesium in the inoculation processdesulfurizes the iron and makes graphite precipitateas nodules rather than flakes. Moreover, mechanicalproperties 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 maximumdiameter of 20 lm in a chill-cast thin section,while Fig. 67 and 68 show coarser nodules,with maximum diameters of 40 and 100 lm, respectively.Note that the number of nodules perunit area is different and changes from approximately350 to 125 to 22/mm 2 , respectively, forFig. 66 to 68.Certain factors can cause weak nodularity.Figure 69 shows an irregular graphite shape dueto poor inoculation or excessive fading of inoculant.Exploded graphite, shown in Fig. 70, mayoccur due to excessive rare earth additions. Normally,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. 76 Same as in Fig. 75. Fine pearlite. 500Fig. 78 Same as in Fig. 77. E, ternary phosphorous eutectic;P, pearlite; and F, ferrite. 500Fig. 79 As-cast gray iron (Fe-3.4%C-3.4%Si-0.07%Mn-0.04%P-0.03%Cr-0.47%Cu). Ferritic-pearliticmatrix. Etched with 4% nital. 100

Metallography and Microstructures of Cast Iron / 583rials and excess rare earth additions in high-carbon-equivalentcompositions, while the secondone is caused by small amounts of tramp elements,for example, lead, bismuth, tin, and titanium,in the absence of cerium (Ref 12).Compacted Graphite. Methods that producecompacted graphite cast iron include the treatmentof 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 exampleFig. 82 As-cast ductile iron (Fe-3.7%C-1.25%Si-0.03%Mn-0.02%P-0.02%S-0.24%Ni-0.06%Mg). Pearlite matrix with ferritic halos around graphitenodules. Etched with 4% nital. 100where the longest compacted graphite precipitationsare in the 80 to 160 lm range.Temper Graphite in Malleable Iron. Tempergraphite is formed by annealing white castiron castings to convert carbon in the form ofcementite to graphite, called temper-carbon nodules.Figure 73 shows the type III graphite precipitationwith 80 lm maximum size.Microstructure of MatrixThe matrix of gray, nodular, compacted, andmalleable cast irons can be pearlitic, pearliticferritic,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 system.The matrix microstructure depends on chemicalcomposition 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 rate. K g C (Si logR) givesthe coefficient of graphitization, and C E C 1⁄3Si 1 ⁄3P is the coefficient of saturation (carbonequivalent). A low value of K g promotes solidificationof white cast iron, with cementite andpearlite as the microstructure, regardless of thetotal carbon content, C. When the K g coefficientand the silicon content increase, the microstruc-Fig. 80 As-cast gray iron (Fe-3.4%C-3.2%Si-0.09%Mn-0.04%P-0.01%S-0.86%Cu-0.01%Mg). Ferritic matrix. Etched in 4% nital. 100Fig. 81 The Fe-C-P equilibrium diagram. The x-axis isthe carbon content, and the y-axis is the phosphoruscontent. Source: Ref 13Fig. 83 As-cast ductile iron. Ferritic-pearlitic matrix.Etched with 4% nital. 100Fig. 84 Ductile 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% nital.100

Metallography and Microstructures of Cast Iron / 585nary euetectic is called a pseudobinary eutectic,because carbon is removed from the eutectic duringdiffusion in the solid state (Ref 13).Fig. 89 Austempered ductile iron (the same compositionas in Fig. 32). Acicular ferrite and austenite.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. 500Ductile iron microstructures normally consistof pearlite or pearlite and ferrite with graphitenodules surrounded with ferrite, which looklike a white halo, although the more commonname of this structure is the bull’s-eye structure.Figures 82 and 83 show this type of microstructure,while Fig. 84 shows a fully ferritic-matrixmicrostructure that was achieved after annealinga chilled casting with a microstructure containingcementite, ferrite, and pearlite (see alsoFig.11). In some cases, usually at a high coolingrate, cementite occurs as a separate constituentin the matrix.The precipitates of cementite are situated inthe exterior regions of the eutectic cells or in theinterdendritic spaces of the transformed austenitedue to the microsegregation of carbide-formingelements, 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 ledeburite.In this case, cementite was a desired constituentof the microstructure to improve thewear resistance of cast iron. It was achieved byfeeding the melted metal with an iron-magnesiumfoundry alloy, without inoculation. Thegraphite nodules do not have a perfect shape.Figure 87 shows the microstructure of a heatandwear-resistant ductile iron containing chromiumand nickel, which consists of chromiumcarbides in an austenitic matrix.The 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 transformedto 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 requiredtime (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 austemperedductile iron (ADI). The morphologyand amount of ferrite depends on the time andtemperature of the austempering process. Figures88 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 different.The 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-iron matrix microstructuresconsist of ferrite and pearlite; theamount of each constituent depends on the coolingrate and the chemical composition (elementsthat promote either ferrite or pearlite solidifica-Fig. 90 Austempered ductile iron (the same compositionas in Fig. 32). Martensite and small amountof acicular ferrite (AF). The casting was austempered: 900C (1650 F), held 2 h, taken to salt bath at 300 C (570 F),held 2 min, and air cooled. Etched with 4% nital. 1000Fig. 91 As-cast iron with compacted graphite (Fe-2.8%C-1.9%Si-0.55%Mn-0.04%P-0.2%S-0.018%Mg). Ferritic-pearlitic matrix. Etched with 4% nital.100Fig. 92 Malleable iron (Fe-2.95%C-1.2%Si-0.53%Mn-0.06%P-0.21%S-0.08%Cr-0.10%Cu-0.07%Ni-0.01%Al). 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 microstructurewith a small amount of nodular graphite.Fig. 93 Malleable iron (Fe-2.9%C-1.5%Si-0.53%Mn-0.06%P-0.22%S-0.09%Cr-0.10%Cu-0.08%Ni-0.02%Al). The casting was annealed as in Fig. 92.Ferrite. Etched with 4% nital. 125 (microscopic magnification100)Malleable iron matrix microstructures aremostly ferritic, ferritic-pearlitic, or pearlitic, dependingon 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 microstructureFig. 95 White cast iron after heat treatment. A networkof massive cementite and tempered martensite.Etched with 4% picral. 140. Courtesy of G.F. VanderVoort, Buehler Ltd.of the pearlitic-ferritic and ferritic malleableiron, respectively. The annealing process was thesame one in both cases, but the final microstructureswere achieved by the use of different foundryprocesses. 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 microstructureafter annealing.Unalloyed white iron microstructures arecreated when the castings solidify in the metastablesystem and the structure is free of graphite.The matrix may consist of cementite andpearlite, as shown in Fig. 94, or, after heat treatment,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 castings.High-chromium white iron is a type ofhighly wear-resistant iron with a microstructurethat consists of primary and eutectic carbides, forexample, (FeCr) 3 C, (FeCr) 7 C 3 , and (FeCr) 23 C 6 ,depending on the chemical composition andcooling rate during solidification. The primarycarbides have a typical shape, which in cross sectionis similar to the letter “L,” and have thechemical formula M 3 C 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 M 7 C 3 , independent ofchromium content and cooling rate (Ref 15).The type M 23 C 6 carbides 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 pearlitic.When the cooling rate is respectively slow, secondaryprecipitations can occur around the austeniticdendrites (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 resultingtransformation product is a function of thecooling rate from annealing as well as the chemicalcomposition; for example, when Cr/C 5,the matrix microstructure was bainitic, while exceedingthis value produced martensitic transformation(Ref 18). Figure 96 shows the microstructureof high-chromium white cast iron afterisothermal heat treatment; the secondary, finecarbides precipitated from the austenitic matrix,which transformed into martensite (see also Fig.43, 44, and 45).Fig. 94 As-cast white iron (Fe-3.0%C-2.7%Si-0.45%Mn-0.07%P-0.025%S). C, cementite; P,pearlite. Etched with 4% nital. 400Fig. 96 White high-chromium cast iron (see Fig. 43).PC, primary carbides; EC, eutectic carbides inmartensitic matrix with fine, globular secondary carbides.The 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 glyceregia.1000ACKNOWLEDGMENTSFor samples and examination results, manythanks to:●Colleagues at the Foundry Research Institute,Kraków, Poland: Adam Kowalski, W. Wierz-

Metallography and Microstructures of Cast Iron / 587chowski, J. Turzyński, W. Madej, K.Głownia, A. Pytel, and J. Bryniarska● M. Sorochtej of The Kraków Technical UniversityAlso, many thanks to M. Warmuzek at theFoundry Research Institute for making the SEMmicrographs of different types of graphite.Special thanks to G.F. Vander Voort fromBuehler Ltd. for his advice and help in preparingthis article.REFERENCES1. Guide to Engineered Materials, PropertyComparison Tables, Adv. Mater. Process.,Vol 159 (No. 12), Dec 2001, p 462. Cz. Podrzucki and Cz. Kalata, Metalurgia iodlewnictwo żeliwa (Metallurgy and Foundryingof Cast Iron), Śląsk-Katowice, Poland,1971, p 32, 54–59, 703. W. Sakwa, Żeliwo (Cast Iron), Śląsk-Katowice, Poland, 1974, p 534. G.F. Vander Voort, Metallography: Principlesand Practice, McGraw-Hill Book Co.,1984; Reprinted by ASM International,1999, p 129, 294–303, 632, 634, 642–644,646, 6485. G.F. Vander Voort, Phase Identification bySelective Etching, Applied Metallography,G.F. Vander Voort, Ed., Van Nostrand ReinholdCo., 1986, p 1–196. J. Zhou, Dalian, W. Schmitz, and S. Engler,Unterschung der Gefügebildung vonGußeisen mit Kugelgraphit bei langsamerErstarrung, Giessereiforschung, Vol 39 (No.2), 1987, p 55–707. P. Skočovský, Colour Contrast in MetallographicMicroscopy, Žilina (Slovak Republic),1993, p 268. E. Weck and E. Leistner, Metallographic Instructionsfor Color Etching by Immersion,Part II: Beraha Color Etchants and TheirDifferent Variations, Deutscher Verlag fürSchweißtechnik, GmbH, Düsseldorf, 1983,p519. P.C. Liu and C.R. Loper, Observation on theGraphite Morphology in Cast Iron, Trans.AFS, Vol 88, 1980, p 9710. J.A. Nelson, Cast Irons, Metallography andMicrostructures, Vol 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 ASA13. K. Sękowski, J. Piaskowski, and Z. Wojtowicz,Atlas struktur znormalizowanych stopówodlewniczych (Atlas of the StandardMicrostructures of Foundry Alloys), WNTWarszawa, Poland, 1972, p 54–55, 74–7714. B.V. Kovacs, On the Terminology andStructure of ADI, Trans. AFS, Vol 102,1994, p 417–42015. T. Ohide and G. Ogira, Solidification ofHigh Chromium Alloyed Cast Irons, Br.Foundryman, Vol 76 (No. 1), 1983, p 716. M. Šittner, Primárni kristalizace chromovychlitin, Čast II (Primary Solidification ofthe Chromium Cast Irons, Part II), Slévárenství,Vol 33 (No. 9), 1985, p 36317. P. Dupin and J.M. Schissler, Etude structuralde l’état brut de fontes blanches à 17% Cr,Hommes et Fonderie, No. 151, 1985, p 1918. C.P. Tong, T. Suzuki, and T. Umeda, TheInfluence of Chemical Composition and Destabilizationon Transformation Characteristicsof High Chromium Cast Irons, Imono(J. Jpn. Foundrymen’s Soc.), Vol 62 (No. 5),1990, p 344

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