producing malleable cast iron. White cast iron derived
of the material, rather than being properties
its name from the bright silvery appearance it has
themselves.
when fractured. White cast iron is hard, brittle, wear
resistant, and unmachinable, largely because the
TYPES OF METAL
carbon it contains is present as cementite.
Gray cast iron always contains iron, carbon, and
Metals are divided into two major fields: ferrous
silicon, and generally contains more carbon and
metals and nonferrous metals. Ferrous metals are
silicon than white cast iron. Carbon is always present
those that are composed primarily of iron. Nonferrous
in the form of graphite flakes. In addition, gray cast
metals are those that are composed primarily of some
iron often contains appreciable amounts of nickel and
element or elements other than iron. Nonferrous
other alloying elements. Gray cast iron is of three
metals or alloys sometimes contain a small amount of
varieties: common, high strength, and alloy. All three
iron as an alloying element or impurity.
are machinable and have good damping capacity
(ability to absorb and dampen vibrations). The
FERROUS METALS
common gray cast irons are quite soft; the others are
somewhat stronger, particularly the alloy gray cast
Ferrous metals include all forms of iron and steel.
iron. All three are brittle because the carbon they
Ferrous metals are widely used in the construction of
contain is largely present in graphite flakes, which act
ships.
as severe stress raisers. Whether a cast iron is gray or
white depends upon the cooling rate and carbon,
Iron
silicon, and nickel content.
Commercially pure iron is known as INGOT
Malleable cast iron is made by heating white
IRON. This iron is 99.85 percent iron; carbon,
castings to 1700F for about 50 hours, followed by
manganese, phosphorous, sulfur, and a trace of silicon
slow cooling to room temperature. The castings are
make up the remainder. The chemical composition of
packed in a neutral slag or scale during heating and
cooling. During the heating period, the cementite in
of the lowest carbon steel.
the structure tends to decompose into ferrite plus
temper carbon. Malleable cast iron is strong,
WROUGHT IRON was used extensively for
machinable, and ductile. The mechanical properties of
construction and even for machinery before steels
malleable cast iron compare favorably with those of
came into use. Wrought iron is a mixture of very pure
low-carbon steels. Malleable cast iron is a great deal
iron and silica slag. The slag gives wrought iron some
more ductile than either white or gray cast iron.
of its desirable properties--corrosion resistance,
weldability, and ductility, among others. Wrought
Nodular cast iron is produced in the same way as
iron is still used for some piping systems on auxiliary
gray cast iron, but with much closer control of
ships.
The molten iron is inoculated with an alloy that will
CAST IRON is produced by resmelting a charge
of pig iron and scrap iron and removing some of the
produce spherical graphite rather than flake graphite.
Nodular cast iron possesses the good machinability,
fluxing agents. There are many grades of cast iron,
damping capacity, and castability of gray cast iron. Its
strength is comparable to alloy gray cast irons and cast
rated as to strength and hardness. The four major kinds
carbon steel. The ductility of nodular cast iron is about
of cast iron are white cast iron, gray cast iron,
malleable cast iron, and nodular cast iron. With the
half that of cast steel, far better than gray cast iron.
exception of similarity between nodular and malleable
cast iron, there are considerable differences in the
Steel
properties of the various kinds of cast iron. The form
in which the carbon exists (graphite or cementite) and
the mode of its distribution are chiefly responsible for
Steels and other metals are classified on the basis
the differences in properties.
of the method of manufacture, method of shaping,
White cast iron essentially consists of an alloy of
method of heat treatment, properties, intended use,
iron, carbon, and silicon. It is known chiefly for its
and chemical composition. In addition, certain steels
good wear resistance and as the starting point for
and other metals are often referred to by trade names.
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