Metallography and Microstructures of Cast Iron

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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
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Metallography and Microstructures of Cast Iron

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