Metallography and Microstructures of Cast Iron

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