Page 328 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 12.4 Economics of Casting 30

e. Compacted-graphite iron. First produced commercially in 1976, compacted-
graphite iron (CGI) has properties that are between those of gray and ductile
irons. Gray iron has good damping and thermal conductivity, but low ductili-
ty, whereas ductile iron has poor damping and thermal conductivity, but high
tensile strength and fatigue resistance. Compacted-graphite iron has damping
and thermal properties similar to gray iron and strength and stiffness compa-
rable to those of ductile iron. Because of its strength, parts made of CGI can be
smaller and thus lighter. It is easy to cast and has consistent properties
throughout the casting, and its machinability is better than that of ductile iron
(which is an important consideration, since compacted-graphite iron is used
for automotive engine blocks and cylinder heads). New casting techniques are
being developed to further improve the machinability of CGI.

Cast Steels. Because of the high temperatures required to melt steels (up to about
1650°C, casting them requires considerable experience. The high temperatures in-
volved present difficulties in the selection of mold materials, particularly in view of
the high reactivity of steels with oxygen during the melting and pouring of the metal.
Steel castings possess properties that are more uniform (isotropic) than those made
by mechanical working processes (Part III). Cast steels can be welded; however,
welding alters the cast microstructure in the heat-affected zone (see Fig. 30.17), thus
influencing the strength, ductility, and toughness of the base metal. Subsequent heat
treatment must be performed to restore the mechanical properties of the casting.
Cast weldments have gained importance for assembling large machines and struc-
tures in which complex configurations or the size of the casting may prevent casting
of the part economically in one location. Cast steels have important applications in
mining, chemical plants, oil fields, heavy construction, and equipment for railroads.

Cast Stainless Steels. Casting of stainless steels involves considerations similar to
those for steels. Stainless steels generally have long freezing ranges and high melting
temperatures. They can develop several structures, depending on their composition
and processing parameters. Cast stainless steels are available in various composi-
tions, and they can be heat treated and welded. Cast stainless-steel products have
high heat and corrosion resistance, especially in the chemical and food industries.
Nickel-based casting alloys are used for severely corrosive environments and for
very high temperature service.

l2.4 Economics of Casting

As is the case with all manufacturing processes, the cost of each cast part (unit cost)
depends on several factors, including materials, equipment, and labor. Of the vari-
ous casting processes discussed in Chapter 1 1, some require more labor than others,
some require expensive dies and machinery, and some require a great deal of time to
produce the castings (Table 12.6). Each of these individual factors thus affects the
overall cost of a casting operation to varying degrees. As described in greater detail
in Section 40.9, the cost of a product includes the costs of materials, labor, tooling,
and equipment. Preparations for casting a product include the production of molds
and dies that require raw materials, time, and effort-all of which also influence
product cost.
As can be seen in Table 12.6, relatively little cost is involved in molds for sand
casting. On the other hand, molds for various processes and die-casting dies require