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Machine Design

Cast Iron

Cast irons include many metals having a wide variety of properties. Although cast iron is often considered a simple metal to produce and to specify, the metallurgy of cast iron is more complex than that of steel and most other metals.

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Steels and cast irons are both primarily iron with carbon as the main alloying element. Steels contain less than 2 and usually less than 1% carbon; all cast irons contain more than 2% carbon. Two percent is about the maximum carbon content at which iron can solidify as a single-phase alloy with all the carbon solution in austenite. Thus, the cast irons, by definition, solidify as heterogeneous alloys and always have more than one constituent in their microstructure. In addition to carbon, cast irons must also contain silicon, usually from 1 to 3%; thus, they are actually iron-carbon-silicon alloys.

High-carbon content and silicon in cast irons give them excellent castability. Their melting temperatures are appreciably lower than those of steel. Molten iron is more fluid than molten steel and less reactive with molding materials. Formation of lower density graphite during solidification makes production of complex shapes possible. Cast irons, however, do not have sufficient ductility to be rolled or forged.

Iron's carbon content is the key to its distinctive properties. The precipitation of carbon (as graphite) during solidification counteracts the normal shrinkage of the solidifying metal, producing sound sections. Graphite also provides excellent machinability (even at wear-resisting hardness levels), damps vibration, and aids lubrication on wearing surfaces (even under borderline lubrication conditions). When most of the carbon remains combined with the iron (as in white iron), the presence of hard iron carbides provides good abrasion resistance.

In some cases, iron microstructure may be all ferrite -- the same constituent that makes low-carbon steels soft and easily machined. But the ferrite of iron is different because it contains sufficient dissolved silicon to eliminate the characteristic gummy nature of low-carbon steel. Thus, cast irons containing ferrite do not require sulfur or lead additions in order to be free machining.

Because a casting's size and shape control its solidification rate and strength, design of the casting and the casting process involved must be considered in selecting the type of iron to be specified. While most other metals are specified by a standard chemical analysis, a single analysis of cast iron can produce several entirely different types of iron, depending upon foundry practice and shape and size of the casting, all of which influence cooling rate. Thus, iron is usually specified by mechanical properties. For applications involving high temperatures or requiring specific corrosion resistance, however, some analysis requirements may also be specified.

Patternmaking is no longer a necessary step in manufacturing cast-iron parts. Many gray, ductile, and alloy-iron components can be machined directly from bar that is continuously cast to near-net shape. Not only does this "parts-without-patterns" method save the time and expense of patternmaking, continuous-cast iron also provides a uniformly dense, fine-grained structure, essentially free from porosity, sand, or other inclusions. Keys to the uniform microstructure of the metal are the ferrostatic pressure and the temperature-controlled solidification that are unique to the process.

For each basic type of cast iron, there are a number of grades with widely differing mechanical properties. These variations are caused by differences in the microstructure of the metal that surrounds the graphite (or iron carbides). Two different structures can exist in the same casting. The microstructure of cast iron can be controlled by heat treatment, but once graphite is formed, it remains.

Pearlitic cast-iron grades consist of alternating layers of soft ferrite and hard iron carbide. This laminated structure -- called pearlite -- is strong and wear resistant, but still quite machinable. As laminations become finer, hardness and strength of the iron increase. Lamination size can be controlled by heat treatment or cooling rate.

Cast irons that are flame hardened, induction hardened, or furnace heated and subsequently oil quenched contain a martensite structure. When tempered, this structure provides machinability with maximum strength and good wear resistance.

Specification methods: ASTM specifications for iron castings are based on a different method than that of the SAE. ASTM specifications designate the properties of the metal to be obtained in an appropriately sized but separately cast test bar, which is poured under the same conditions as are the castings. SAE specifications, on the other hand, require that the microstructure of the casting be appropriate for the specified grade of metal and that the hardness of each casting at a designated location be within the specified range.

Commercially, the ASTM specification is more commonly used for general engineering applications where the strength of the iron necessary in the part has been established. SAE specifications are usually used for large quantities of smaller cast components such as those used in automobiles, and in agricultural and refrigeration equipment. In these cases, the suitability of a particular grade of iron is established, not only on design considerations, but also on actual proof in operation; the purpose of the specification is to ensure a consistent product comparable to those found, by experience, to be satisfactory.

Gray iron: This is a supersaturated solution of carbon in an iron matrix. The excess carbon precipitates out in the form of graphite flakes. Gray iron is specified by a two-digit designation; Class 20, for example, specifies a minimum tensile strength of 20,000 psi. In addition, gray iron is specified by the cross section and minimum strength of a special test bar. Usually, the test-bar cross section matches or is related to a particularly critical section of the casting. This second specification is necessary because the strength of gray iron is highly sensitive to cross section (the smaller the cross section, the faster the cooling rate and the higher the strength).

Impact strength of gray iron is lower than that of most other cast ferrous metals. In addition, gray iron does not have a distinct yield point (as defined by classical formulas) and should not be used when permanent, plastic deformation is preferred to fracture. Another important characteristic of gray iron -- particularly for precision machinery -- is its ability to damp vibration. Damping capacity is determined principally by the amount and type of graphite flakes. As graphite decreases, damping capacity also decreases.

Gray iron's high compressive strength -- three to five times tensile strength -- can be used to advantage in certain situations. For example, placing ribs on the compression side of a plate instead of the tension side produces a stronger, lighter component.

Gray irons have excellent wear resistance. Even the softer grades perform well under certain borderline lubrication conditions (as in the upper cylinder walls of internal-combustion engines, for example).

To increase the hardness of gray iron for abrasive-wear applications, alloying elements can be added, special foundry techniques can be used, or the iron can be heat treated. Gray iron can be hardened by flame or induction methods, or the foundry can use a chill in the mold to produce hardened, "white-iron" surfaces.

Typical applications of gray iron include automotive engine blocks, gears, flywheels, brake discs and drums, and machine bases. Gray iron serves well in machinery applications because of its good fatigue resistance.

Ductile iron: Ductile, or nodular, iron contains trace amounts of magnesium which, by reacting with the sulfur and oxygen in the molten iron, precipitates out carbon in the form of small spheres. These spheres improve the stiffness, strength, and shock resistance of ductile iron over gray iron. Different grades are produced by controlling the matrix structure around the graphite, either as-cast or by subsequent heat treatment.

A three-part designation system is used to specify ductile iron. The designation of a typical alloy, 60-40-18, for example, specifies a minimum tensile strength of 60,000 psi, a minimum yield strength of 40,000 psi, and 18% elongation in 2 in.

Ductile iron is used in applications such as crankshafts because of its good machinability, fatigue strength, and high modulus of elasticity; in heavy-duty gears because of its high yield strength and wear resistance; and in automobile door hinges because of its ductility. Because it contains magnesium as an additional alloying element, ductile iron is stronger and more shock resistant than gray iron. But although ductile iron also has a higher modulus of elasticity, its damping capacity and thermal conductivity are lower than those of gray iron.

By weight, ductile iron castings are more expensive than gray iron. Because they offer higher strength and provide better impact resistance, however, overall part costs may be about the same.

Although it is not a new treatment for ductile iron, austempering has become increasingly known to the engineering community in the past five to 10 years. Austempering does not produce the same type of structure as it does in steel because of the high carbon and silicon content of iron. The matrix structure of austempered ductile iron (ADI) sets it apart from other cast irons, making it truly a separate class of engineering materials.

In terms of properties, the ADI matrix almost doubles the strength of conventional ductile iron while retaining its excellent toughness. Like ductile iron, ADI is not a single material; rather, it is a family of materials having various combinations of strength, toughness, and wear resistance. Unfortunately, the absence of a standard specification for the materials has restricted its widespread acceptance and use. To help eliminate this problem, the Ductile Iron Society has proposed property specifications for four grades of austempered ductile iron.

Most current applications for ADI are in transportation equipment -- automobiles, trucks, and railroad and military vehicles. The same improved performance and cost savings are expected to make these materials attractive in equipment for other industries such as mining, earthmoving, agriculture, construction, and machine tools.

White iron: White iron is produced by "chilling" selected areas of a casting in the mold, which prevents graphitic carbon from precipitating out. Both gray and ductile iron can be chilled to produce a surface of white iron, consisting of iron carbide, or cementite, which is hard and brittle. In castings that are white iron throughout, however, the composition of iron is selected according to part size to ensure that the volume of metal involved can solidify rapidly enough to produce the white-iron structure.

The principal disadvantage of white iron is its brittleness. This can be reduced somewhat by reducing the carbon content or by thoroughly stress relieving the casting to spheroidize the carbides in the matrix. However, these measures increase cost and reduce hardness.

Chills produce castings with white-iron working surfaces and cores that are a tougher and more easily machinable gray or ductile iron. During chilling, that portion of the casting that is to resist wear is cooled by a metal or graphite heat sink (chill) in the mold. When the molten iron contacts the chill, it solidifies so rapidly that the iron and carbon cannot become dissociated.

Chilling should not be confused with heat-treat hardening, which involves an entirely different metallurgical mechanism. White iron, so called because of its very white fracture, can be formed only during solidification. It will not soften except by extended annealing, and it retains its hardness even above 1,000°F.

White irons are used primarily for applications requiring wear and abrasion resistance such as mill liners and shot-blasting nozzles. Other uses include railroad brake shoes, rolling-mill rolls, clay-mixing and brickmaking equipment, and crushers and pulverizers. Generally, plain (unalloyed) white iron costs less than other cast irons.

Compacted graphite iron: Until recently, compacted graphite iron (CGI), also known as vermicular iron, has been primarily a laboratory curiosity. Long known as an intermediate between gray and ductile iron, it possesses many of the favorable properties of each. However, because of process-control difficulties and the necessity of keeping alloy additions within very tight limits, CGI has been extremely difficult to produce successfully on a commercial scale. For example, if the magnesium addition varied by as little as 0.005%, results would be unsatisfactory.

Processing problems have been solved by the joint development efforts of the Foote Mineral Co. and the British Cast Iron Research Association. An alloy-addition package provides the essential alloying ingredients -- magnesium, titanium, and rare earths -- in exactly the right proportions.

Strength of CGI parts approaches that of ductile cast iron. CGI also offers high thermal conductivity, and its damping capacity is almost as good as that of gray iron; fatigue resistance and ductility are similar to those properties in ductile iron. Machinability is superior to that of ductile iron, and casting yields are high because shrinkage and feeding characteristics are more like gray iron.

The combination of high strength and high thermal conductivity suggests the use of CGI in engine blocks, brake drums, and exhaust manifolds of vehicles. CGI gear plates have replaced aluminum in high-pressure gear pumps because of the iron's ability to maintain dimensional stability at pressures above 1,500 psi.

Malleable iron: Malleable iron is white iron that has been converted by a two-stage heat treatment to a condition having most of its carbon content in the form of irregularly shaped nodules of graphite, called temper carbon. Resulting properties are opposite from those of the white iron from which it is derived. Rather than being hard and brittle, it is malleable and easily machined. Malleable-iron castings generally cost slightly less than ductile-iron castings.

The three basic types of malleable iron are ferritic, pearlitic, and martensitic. Ferritic grades are more machinable and ductile, whereas the pearlitic grades are stronger and harder. Generally, the martensitic grades are grouped with the pearlitic materials; they might be thought of as extensions (at the higher strength end of the range) of pearlitic malleable iron.

In sharp contrast to ferritic malleable iron, whose microstructure is free from combined carbon, pearlitic malleable iron contains from 0.3 to 0.9% carbon in the combined form. Since this constituent can be transformed readily into the hardest form of combined carbon by a simple heating and quenching treatment, pearlitic malleable-iron castings can be selectively hardened. Depth of hardening is controlled by the rate of heat input, time at temperature, and quenching rate. Heat treating can produce surface hardness to about Rockwell C 60.

Carbon in malleable irons helps retain and store lubricants. In extreme-wear service, the pearlitic malleable-iron surface wears away in harmless, micron-size particles, which are less damaging than other types of iron particles. The porous malleable-iron surface traps abrasive debris that accumulates between bearing surfaces. Gall streaks can form on malleable iron but galling does not usually progress.

Malleable-iron castings are often used for heavy-duty bearing surfaces in automobiles, trucks, railroad rolling stock, and farm and construction machinery. Pearlitic grades are highly wear resistant, with hardnesses ranging from 152 to over 300 Bhn. Applications are limited, however, to relatively thin-sectioned castings because of the high shrinkage rate and the need for rapid cooling to produce white iron.

High-alloy irons: High-alloy irons are ductile, gray, or white irons that contain 3 to more than 30% alloy content. Properties by specialized foundries, are significantly different from those of unalloyed irons. These irons are usually specified by chemical composition as well as by various mechanical properties.

White high-alloy irons containing nickel and chromium develop a microstructure with a martensite matrix around primary chromium carbides. This structure provides a high hardness with extreme wear and abrasion resistance. High-chromium irons (typically, about 16%), combine wear and oxidation resistance with toughness. Irons containing from 14 to 24% nickel are austenitic; they provide excellent corrosion resistance for nonmagnetic applications. The 35%-nickel irons have an extremely low coefficient of thermal expansion and are also nonmagnetic and corrosion resistant.

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