Page 327 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

0 Chapter 12 Metal Casting: Design, Materials, and Economics

Zinc-based Alloys. A low-melting-point alloy group, zinc-based alloys have good
corrosion resistance, good fluidity, and sufficient strength for structural applications.
These alloys commonly are used in die casting, particularly for parts with thin walls
and intricate shapes.

Tin-based Alloys. Although low in strength, these alloys have good corrosion
resistance and typically are used for bearing surfaces.
Lead-based Alloys. These alloys have applications similar to tin-based alloys, but
the toxicity of lead is a major drawback to their wider application.

High-temperature Alloys. High-temperature alloys have a wide range of proper-
ties and typically require temperatures of up to 1650°C for casting titanium and su-
peralloys, and even higher for refractory alloys (Mo, Nb, W and Ta). Special
techniques are used to cast these alloys for nozzles and various jet- and rocket-engine
components. Some high-temperature alloys are more suitable and economical for
casting than for shaping by other manufacturing methods, such as forging.

l2.3.2 Ferrous Casting Alloys
Commonly cast ferrous alloys are as follows:

Cast Irons. Cast irons represent the largest quantity of all metals cast, and they
can be cast easily into intricate shapes. They generally possess several desirable
properties, such as wear resistance, high hardness, and good machinability. The
term cast iron refers to a family of alloys, and as described in Section 4.6, they
are classified as gray cast iron (gray iron), ductile (nodular or spheroidal) iron, white
cast iron, malleable iron, and compacted-graphite iron. Their general properties and
typical applications are given in Tables 12.3 and 12.4.

a. Gray cast iron. Castings of gray cast iron have relatively few shrinkage cavi-
ties and low porosity. Various forms of gray cast iron are ferritic, pearlitic, and
martensitic. Because of differences in their structures, each type has different
properties. The mechanical properties for several classes of gray cast iron are
given in Table 12.4. Typical uses of gray cast iron are in engine blocks, electric-
motor housings, pipes, and wear surfaces for machines. Also, its high damping
capacity has made gray iron a common material for machine-tool bases. Gray
cast irons are specified by a two-digit ASTM designation. For example, class
20 specifies that the material must have a minimum tensile strength of 140
MPa.
b. Ductile (nodular) iron. Typically used for machine parts, housings, gears, pipe,
rolls for rolling mills, and automotive crankshafts, ductile irons are specified by
a set of two-digit numbers. For example, class or grade 80-55-06 indicates that
the material has a minimum tensile strength of 80 ksi (550 MPa), a minimum
yield strength of 55 ksi (380 MPa), and 6% elongation in 2 in. (50 mm).
c. White cast iron. Because of its extreme hardness and wear resistance, white
cast iron is used mainly for rolls for rolling mills, railroad-car brake shoes, and
liners in machinery for processing abrasive materials.
d. Malleable iron. The principal use of malleable iron is for railroad equipment
and various types of hardware, fittings, and components for electrical applica-
tions. Malleable irons are specified by a five-digit designation. For example,
35 018 indicates that the yield strength of the material is 35 ksi (240 MPa) and
its elongation is 18% in 2 in. (50 mm).