Page 79 - Shigley's Mechanical Engineering Design

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54 Mechanical Engineering Design
2–17 Casting Materials

Gray Cast Iron
Of all the cast materials, gray cast iron is the most widely used. This is because it has
a very low cost, is easily cast in large quantities, and is easy to machine. The principal
objections to the use of gray cast iron are that it is brittle and that it is weak in tension.
In addition to a high carbon content (over 1.7 percent and usually greater than 2 percent),
cast iron also has a high silicon content, with low percentages of sulfur, manganese,
and phosphorus. The resultant alloy is composed of pearlite, ferrite, and graphite, and
under certain conditions the pearlite may decompose into graphite and ferrite. The
resulting product then contains all ferrite and graphite. The graphite, in the form of
thin flakes distributed evenly throughout the structure, darkens it; hence, the name gray
cast iron.
Gray cast iron is not readily welded, because it may crack, but this tendency may
be reduced if the part is carefully preheated. Although the castings are generally used in
the as-cast condition, a mild anneal reduces cooling stresses and improves the machin-
ability. The tensile strength of gray cast iron varies from 100 to 400 MPa (15 to 60 kpsi),
and the compressive strengths are 3 to 4 times the tensile strengths. The modulus of
elasticity varies widely, with values extending all the way from 75 to 150 GPa (11 to
22 Mpsi).

Ductile and Nodular Cast Iron
Because of the lengthy heat treatment required to produce malleable cast iron, engineers
have long desired a cast iron that would combine the ductile properties of malleable
iron with the ease of casting and machining of gray iron and at the same time would
possess these properties in the as-cast conditions. A process for producing such a material
using magnesium-containing material seems to fulfill these requirements.
Ductile cast iron, or nodular cast iron, as it is sometimes called, is essentially the
same as malleable cast iron, because both contain graphite in the form of spheroids.
However, ductile cast iron in the as-cast condition exhibits properties very close to
those of malleable iron, and if a simple 1-h anneal is given and is followed by a slow
cool, it exhibits even more ductility than the malleable product. Ductile iron is made by
adding MgFeSi to the melt; since magnesium boils at this temperature, it is necessary
to alloy it with other elements before it is introduced.
Ductile iron has a high modulus of elasticity (172 GPa or 25 Mpsi) as compared
with gray cast iron, and it is elastic in the sense that a portion of the stress-strain
curve is a straight line. Gray cast iron, on the other hand, does not obey Hooke’s law,
because the modulus of elasticity steadily decreases with increase in stress. Like
gray cast iron, however, nodular iron has a compressive strength that is higher than
the tensile strength, although the difference is not as great. In 40 years it has become
extensively used.

White Cast Iron
If all the carbon in cast iron is in the form of cementite and pearlite, with no graphite
present, the resulting structure is white and is known as white cast iron. This may be
produced in two ways. The composition may be adjusted by keeping the carbon and
silicon content low, or the gray-cast-iron composition may be cast against chills in order
to promote rapid cooling. By either method, a casting with large amounts of cementite
is produced, and as a result the product is very brittle and hard to machine but also very
resistant to wear. A chill is usually used in the production of gray-iron castings in order