Metallography and Microstructures of Cast Iron

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)