Metallography and Microstructures of Cast Iron

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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)
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