CAST IRON, like the term steel, identifies a large family of ferrous alloys. Cast irons primarily are iron alloys that contain more than 2% carbon and from 1 to 3% silicon. Wide variations in properties can be achieved by varying the balance between carbon and silicon, by alloying with various metallic elements, and by varying melting, casting, and heat treating practices.
The five types of commercial cast iron are gray, ductile, malleable, compacted graphite, and white iron. With the exception of a white cast iron, all cast irons have in common a microstructure that consists of graphite phase in a matrix that may be ferritic, pearlitic, bainitic, tempered martensitic, or combinations thereof. The four types of graphitic cast irons are roughly classified according to the morphology of the graphite phase. Gray iron has flake-shaped graphite, ductile iron has nodular or spherically shaped graphite, compacted graphite iron (also called vermicular graphite iron) is intermediate between these two, and malleable iron has irregularly shaped globular or "popcorn-shaped" graphite that is formed during tempering of white cast iron. Table 1 shows the correspondence between commercial and microstructural classification, as well as final processing stage in obtaining common cast irons.
Table 1 Classification of cast irons by commercial designation, microstructure, and fracture
Commercial designation
Carbon-rich phase
Matrix(a)
Fracture
Final structure after
Gray iron
Lamellar graphite
P
Gray
Solidification
Ductile iron
Spheroidal graphite
F, P, A
Silver-gray
Solidification or heat treatment
Compacted graphite iron
Compacted (vermicular) graphite
F, P
White iron
Fe3C
P, M
White
Solidification and heat treatment(b)
Mottled iron
Lamellar Gr + Fe3C
Mottled
Malleable iron
Temper graphite
Heat treatment
Austempered ductile iron
At
(a)
F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite).
(b)
White irons are not usually heat treated, except for stress relief and to continue austenite transformation.
White cast irons, so named because of the characteristically white fracture surfaces, do not have any graphite in the microstructures. Instead the carbon is present in the form of carbides, chiefly of the types Fe3C and Cr7C3. Often, complex carbides are also present, such as (Fe,Cr)3C from additions of 3 to 5% Ni and 1.5 to 2.5% Cr, (Cr,Fe)7C3 from additions of 11 to 35% Cr, or those containing other carbide-forming elements.
Cast irons may also be classified as either unalloyed cast irons or alloy cast irons. Unalloyed cast irons are essentially iron-carbon-silicon alloys containing small amounts of manganese, phosphorus, and sulfur. The range of composition for typical unalloyed cast irons is given in Table 2. Figure 1 shows the range of carbon and silicon for common cast irons as compared with steel.
Table 2 Range of compositions for typical unalloyed common cast irons
Type of iron
Composition, %
C
Si
Mn
S
2.5-4.0
1.0-3.0
0.2-1.0
0.002-1.0
0.02-0.25
Compacted graphite
0.01-0.1
0.01-0.03
Ductile
3.0-4.0
1.8-2.8
0.1-1.0
1.8-3.6
0.5-1.9
0.25-0.8
0.06-0.2
Malleable
2.2-2.9
0.9-1.9
0.15-1.2
0.02-0.2
Fig. 1 Approximate ranges of carbon and silicon for steel and various cast irons
A section through the ternary Fe-Fe3C-Si diagram at 2% Si (which approximates the silicon contents of many cast irons) provides a convenient reference for discussing the metallurgy of cast iron. The diagram in Fig. 2 resembles the binary Fe-Fe3C diagram but exhibits important differences characteristic of ternary systems. Eutectic and eutectoid temperatures change from single values in the Fe-Fe3C system to temperature ranges in the Fe-Fe3C-Si system; the eutectic and eutectoid points shift to lower carbon contents.
Fig. 2 Section through the Fe-Fe3C-Si ternary equilibrium diagram at 2% Si
Figure 2 represents the metastable equilibrium between iron and iron carbide (cementite), a metastable system. The silicon that is present remains in solid solution in the iron, in both ferrite and austenite, and so does not affect the composition of the carbide phase but only the conditions and the kinetics of the carbide formation on cooling. The designations , , and Fe...
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