Page 80 - Shigley's Mechanical Engineering Design

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Materials 55
to provide a very hard surface within a particular area of the casting, while at the same
time retaining the more desirable gray structure within the remaining portion. This pro-
duces a relatively tough casting with a wear-resistant area.
Malleable Cast Iron
If white cast iron within a certain composition range is annealed, a product called
malleable cast iron is formed. The annealing process frees the carbon so that it is pres-
ent as graphite, just as in gray cast iron but in a different form. In gray cast iron the
graphite is present in a thin flake form, while in malleable cast iron it has a nodular
form and is known as temper carbon. A good grade of malleable cast iron may have
a tensile strength of over 350 MPa (50 kpsi), with an elongation of as much as 18 per-
cent. The percentage elongation of a gray cast iron, on the other hand, is seldom over
1 percent. Because of the time required for annealing (up to 6 days for large and
heavy castings), malleable iron is necessarily somewhat more expensive than gray
cast iron.

Alloy Cast Irons
Nickel, chromium, and molybdenum are the most common alloying elements used in
cast iron. Nickel is a general-purpose alloying element, usually added in amounts up to
5 percent. Nickel increases the strength and density, improves the wearing qualities, and
raises the machinability. If the nickel content is raised to 10 to 18 percent, an austenitic
structure with valuable heat- and corrosion-resistant properties results. Chromium
increases the hardness and wear resistance and, when used with a chill, increases the
tendency to form white iron. When chromium and nickel are both added, the hardness
and strength are improved without a reduction in the machinability rating. Molybdenum
added in quantities up to 1.25 percent increases the stiffness, hardness, tensile strength,
and impact resistance. It is a widely used alloying element.

Cast Steels
The advantage of the casting process is that parts having complex shapes can be man-
ufactured at costs less than fabrication by other means, such as welding. Thus the
choice of steel castings is logical when the part is complex and when it must also have
a high strength. The higher melting temperatures for steels do aggravate the casting
problems and require closer attention to such details as core design, section thicknesses,
fillets, and the progress of cooling. The same alloying elements used for the wrought
steels can be used for cast steels to improve the strength and other mechanical proper-
ties. Cast-steel parts can also be heat-treated to alter the mechanical properties, and,
unlike the cast irons, they can be welded.

2–18 Nonferrous Metals
Aluminum
The outstanding characteristics of aluminum and its alloys are their strength-weight
ratio, their resistance to corrosion, and their high thermal and electrical conductivity.
3
3
The density of aluminum is about 2770 kg/m (0.10 lbf/in ), compared with 7750 kg/m 3
3
(0.28 lbf/in ) for steel. Pure aluminum has a tensile strength of about 90 MPa (13 kpsi),
but this can be improved considerably by cold working and also by alloying with other
materials. The modulus of elasticity of aluminum, as well as of its alloys, is 71.7 GPa
(10.4 Mpsi), which means that it has about one-third the stiffness of steel.