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gray irons are equally strong, or equally
hard. As in steel, tensile strength and hardness are closely related. In gray
irons, tensile strength ranges from
about 14 MPa (20,000 psi) to more than 35 MPa (50,000 psi). The hardness of
the strongest grades is double that
of the weakest grades. All gray irons have high compressive strength three
to four times
their tensile strength. While
all gray cast irons contain free carbon (graphite) in flake form, they also contain
combined carbon (iron carbide)
in almost every case. This combined carbon is often present in pearlite grains,
such as found in most carbon
steels. It may also be found as cementite or martensite. The composition of the
cast iron, the rate at which it
cooled after casting, and heat treatment after casting all have a bearing on the
structure. Small amounts of alloying
elements are used in the strongest gray irons; they tend to prevent the formation
of pearlite. While the hardness
and strength of steel almost always increase as carbon content rises, in the case
of gray cast iron the strongest,
hardest grades have less carbon than some of the lower-strength, less expensive
grades. Gray iron
is usually cast in sand molds, and allowed to cool normally in the mold. Heat
treatment after casting is not
always necessary, but is frequently employed, either to increase or to decrease
hardness. Almost all gasoline and
diesel engine blocks are gray iron castings. Whenever industry desires an intricate
form which can be machined
to close tolerances, and must withstand abrasive wear, gray iron gets consideration.
Only when it is essential
that the finished item have some ductility and good shock resistance is some other
material such as nodular
cast iron or cast steel, both more expensive likely to be substituted.
White iron, mentioned above, is about
the same as gray iron in composition, but has been cooled rapidly so that
graphite does not have time to form,
and all the carbon winds up in the combined form, as pearlite, cementite, or
martensite. Many white iron castings
are subsequently converted to malleable iron, which we shall take up next.
However, some gray iron castings are
made with white iron wearing surfaces, since white iron is much harder than
gray iron, although extremely brittle.
This is accomplished by inserting metal or graphite chill blocks at appropriate
places in the mold. The molten metal
that solidifies against those chill blocks cools so rapidly that white iron
surfaces are created. Plowshares, railroad
car wheels, and various types of dies are often made with such chilled
white iron surfaces.