Ferrous Metals
susceptible to corrosive agents by either coating or
alloying them with other metals that are corrosion
Iron ore, the basis of all ferrous metals, is
resistant.
converted to metal (pig iron) in a blast furnace.
Alloying elements can be added later to the pig iron to
Heat Resistance
obtain a wide variety of metals with different
characteristics. The characteristics of metal can be
Heat resistance is the property of a steel or alloy
further changed and improved by heat treatment and
that permits it to retain its properties at high tempera-
by hot or cold working.
tures. Examples are tungsten steel, which can cut
other metals even when red hot, and chromium
PIG IRON.--The product of the blast furnace is
molybdenum steel, which is used for piping and
called pig iron. Pig iron is composed of approxi-
valves in high temperature, high-pressure steam
mately 93 percent iron, 3 to 5 percent carbon, and
systems.
varying amounts of impurities. It is seldom used
directly as an industrial manufacturing material, but it
is the basic ingredient in cast iron, wrought iron, and
Weldability
steel.
Weldability refers to the relative ease with which
CAST IRON.--Cast iron is produced by
a metal can be welded. Weldability depends on many
resmelting a charge of pig iron and scrap iron in a
different factors. The basic factor is the chemical
furnace and removing some of the impurities from the
composition or the elements that were added during
molten metal by using various fluxing agents. There
the metal's manufacture. A steel with a low carbon
are many grades of cast iron, based on strength and
content will be much easier to weld than one with a
hardness. The quality depends upon the extent of
high carbon content. You can weld a low alloy steel
refining, the amount of scrap iron used, and the
that has low hardenability easier than one with a high
method of casting and cooling the molten metal when
hardenability. You also must consider the welding
it is drawn from the furnace. The higher the
procedure, such as gas or arc welding. Charts provide
proportion of scrap iron, the lower the grade of cast
guidelines concerning the weldabilty of a metal and
iron. Cast iron has some degree of corrosion
the recommended welding procedure. Always make
resistance and great compressive strength, but it is
weldability an integral part of planning a job that
brittle and has a comparatively low tensile strength. It
requires welding.
has limited use in marine service.
WROUGHT IRON.--Wrought iron is a highly
Machinability
refined pure iron that contains uniformly distributed
particles of slag. It is considerably softer than cast
iron and has a fibrous internal structure. Like cast
Machinability is the relative ease with which a
metal can be machined. Several factors affect the
iron, it is fairly resistant to corrosion and fatigue.
machinability of metal. They are different alloying
Wrought iron, is used extensively for low-pressure
elements, the method used by the manufacturer to
pipes, rivets, and nails.
form the metal bar (physical condition), any heat
PLAIN-CARBON STEEL.--Pig iron is
treatment that has changed the hardness, whether you
converted into steel by a process that separates and
use a high-speed steel or carbide cutting tool, and
removes impurities from the molten iron by use of
whether you use a cutting fluid. We'll discuss some
various catalytic agents and extremely high
of these factors later in this chapter.
temperatures. During the refining process, practically
all the carbon originally present in the pig iron is
TYPES OF METALS
burned out. In the final stages when higher carbon
alloys are needed, measured amounts of carbon are
added to the relatively pure liquid iron to produce
Metals are divided into two general types--
carbon steel of a desired grade. The amount of carbon
ferrous and nonferrous. Ferrous metals are those
added controls the mechanical properties of the
whose major element is iron. Iron is the basis for all
finished steel. After the steel has been drawn from
steels. Nonferrous metals are those whose major
the furnace and allowed to solidify, it may be sent
element is not iron, but they may contain a small
either to the stockpile or to shaping mills for rolling
amount of iron as an impurity.
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