Metallography and Microstructures of Cast Iron

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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
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Metallography and Microstructures of Cast Iron

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