Introduction
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 |
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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 |
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Fig. 1 Approximate ranges of carbon and silicon for steel and various cast irons |
The Iron-Iron Carbide-Silicon System
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.
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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 Fe3C, therefore, are used in the ternary system to identify the same phases that occur in the Fe-Fe3C binary system. Some of the silicon may precipitate along with the carbide, but it cannot be distinguished as a different phase. The solidification of certain compositions does not occur in the metastable system, but rather in the stable system, where the products are iron and graphite rather than iron and carbide. These compositions encompass the gray, ductile, and compacted graphite cast irons.
If the section through the ternary diagram at 2% Si is to be used in tracing the phase changes that occur, its use can be justified only on the assumption that silicon concentration remains at 2% in all parts of the alloy under all conditions. This obviously is not strictly true, but there is little evidence that silicon segregates to any marked degree in cast iron.
Carbon Equivalence
Both carbon and silicon influence the nature of iron castings, so it is necessary to develop an approximation of their impact on solidification. This has been accomplished through development of the concept of carbon equivalence, CE. Using this approach, carbon equivalence is calculated as:
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or more precisely, taking phosphorus into consideration:
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Comparison of CE with the eutectic composition in the Fe-C system (4.3% C) will indicate whether a cast iron will behave as a hypoeutectic or hypereutectic alloy during solidification. When CE is near the eutectic value, the liquid state persists to a relatively low temperature and solidification takes place over a small temperature range. The latter characteristic can be important in promoting uniformity of properties within a given casting.
In hypereutectic irons (CE greater than about 4.3%), there is a tendency for kish graphite--proeutectic graphite that forms and floats free in the molten iron--to precipitate on solidification under normal cooling conditions. In hypoeutectic irons, the lower the CE, the greater the tendency for white or mottled iron to form on solidification.
Characteristics of Cast Irons
White Cast Iron. White iron is formed when the carbon in solution in the molten iron does not form graphite on solidification but remains combined with the iron, often in the form of massive carbides. White irons are hard and brittle and produce white, crystalline fracture surfaces.
White cast irons have high compressive strength and good retention of strength and hardness at elevated temperature, but they are most often used for their excellent resistance to wear and abrasion. The massive carbides in the microstructure are chiefly responsible for these properties.
Gray Cast Iron. When the composition of the iron and the cooling rate at solidification are suitable, a substantial portion of the carbon content separates out of the liquid to form flakes of graphite. When a piece of the solidified alloy is broken, the fracture path follows the graphite flakes, and the fracture surfaces appear gray because of the predominance of exposed graphite.
Gray cast iron has several unique properties that are derived from the existence of flake graphite in the microstructure. Gray iron can be machined easily at hardnesses conducive to good wear resistance. It resists galling under boundary-lubrication conditions (conditions wherein the flow of lubricant is insufficient to maintain a full fluid film). It has outstanding properties for applications involving vibrational damping or moderate thermal shock.
Ductile Cast Iron. Ductile iron, which is also known as nodular iron or spheroidal graphite cast iron, is very similar to gray iron in composition, but during casting of ductile iron the graphite is caused to nucleate as spherical particles, or spherulites, rather than as flakes. This is accomplished through the addition of a very small but definite amount of magnesium and/or cerium to the molten iron in a process step called nodulizing.
Ductile iron is produced from the same types of raw material as gray iron, but usually requires slightly higher purity, especially in regard to sulfur. Casting properties of ductile iron, such as fluidity, are comparable to those of gray iron.
The chief advantage of ductile iron over gray iron is its combination of high strength and ductility--up to 18% minimum elongation for ferritic ductile iron with a tensile strength of 415 MPa (60 ksi) as opposed to only about 0.6% elongation for a gray iron of comparable strength. Martensitic ductile irons with tensile strengths of about 830 MPa (120 ksi) exhibit at least 2% elongation, and the newer austempered ductile irons exhibit in excess of 5% elongation at even higher tensile strengths (1000 MPa, or 145 ksi).
For most applications, some deviation from true spherical shape can be tolerated without unacceptable loss of properties. However, the quasi-flake and crab form (see Table 3) are unacceptable for most applications.
Table 3 Summary and description of ASTM and equivalent ISO classification of graphite shapes in cast iron |
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Compacted graphite (CG) cast iron, also known as vermicular graphite cast iron, is characterized by graphite that is interconnected within eutectic cells as is the flake graphite in gray iron. Compared with the graphite in gray iron, however, the graphite in CG iron is coarser and more rounded, similar in metallographic appearance to ASTM type IV, quasi-flake graphite (Table 3). The structure can be considered intermediate between those of gray iron and ductile iron. Individual properties can also be considered intermediate between those of gray and ductile irons, but the unique combinations of properties obtainable in CG irons make them superior to either gray or ductile iron in applications such as disc-brake rotors and diesel-engine heads.
Compacted graphite cast iron can be obtained by very carefully controlling the amount of magnesium added as an inoculant in a process very similar to the process used to make ductile iron. Unfortunately, either undertreatment, resulting in a gray iron structure, or overtreatment, resulting in a ductile iron structure, can occur if the ideal quantity of magnesium is missed by as little as 50 ppm. Current commercial production of CG iron is accomplished by inoculation with magnesium to give a residual content of 50 to 600 ppm in the presence of 0.15 to 0.5% titanium and 10 to 150 ppm of rare earths, such as cerium. In effect, the process is one in which the nodulizing reaction due to the addition of magnesium is poisoned by the presence of a controlled amount of titanium, and in which cerium is added to eliminate a need to control sulfur at a low concentration.
Malleable cast iron encompasses yet another form of graphite called temper carbon. This form of graphite is produced by the heat treatment of white cast iron, which does not contain graphite, but does contain a high percentage of cementite. When a white cast iron is heated for an extended period of time (about 60 h) at a temperature of 960 °C (1760 °F), the cementite decomposes into austenite and graphite. By slow cooling from 960 °C (1760 °F), the austenite transforms into ferrite or pearlite, depending on the cooling rate and the diffusion rate of carbon. The ductility and toughness of malleable iron falls between that of ductile cast iron and gray cast iron. Because white iron can only be produced in cast sections up to about 100 mm (4 in.) thick, malleable iron is limited in section size. In recent years, malleable irons have been replaced by the more economically processed ductile irons for many applications.
Selected References
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Casting, Vol 15, ASM Handbook, ASM International, 1988 |
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J.R. Davis, Ed., ASM Specialty Handbook: Cast Irons, ASM International, 1996 |
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R. Elliot, Cast Iron Technology, Butterworths, 1988 |
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H. Fredriksson and M Hillert, Ed., The Physical Metallurgy of Cast Iron, Proc. Materials Research Society, North Holland, 1985 |
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B. Lux et al., Ed., The Metallurgy of Cast Iron, Georgi Publishing, 1975 |
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I. Minkoff, The Physical Metallurgy of Cast Iron, John Wiley & Sons, 1983 |
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Properties and Selections: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990, p 3-104 |
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C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, 1981 |
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