Figure 15-5.--Typical structure of low-carbon steel.
The structure of an alloy is described as being a
MECHANICAL MIXTURE when two or more
structural forms are mixed together but are still
separately distinguishable. A mechanical mixture of an
alloy is comparable-though on a smaller scale-to the
mixture of sand and gravel in concrete.
One of the most important mechanical mixtures that
occurs in many steels is known as PEARLITE. Pearlite,
so called because it has a pearly luster when seen under
a microscope, is an intimate mechanical mixture of
Figure 15-6.--Controlled atmosphere electric furnace used for
ferrite and cementite in alternate plates or layers. Ferrite
heat-treating.
is a solid solution, and cementite or iron carbide is an
intermetallic compound; in pearlite, the two are closely
HEAT-TREATING EQUIPMENT
mixed to form a characteristically layered structure.
The equipment required for heat-treating consists
When the entire structure of the alloy is in the form of
of furnaces or other heating devices, quenching baths
or other cooling devices, temperature controls and
indicators, and other controls and indicators required
is called the EUTECTOID or the EUTECTOID
for the proper operation of the equipment or for the
STRUCTURE.
control of the process. In addition, heat-treating equip-
The internal structure of an alloy may show various
ment includes tongs and other devices for handling and
holding the work while it is being heated or cooled.
metallic compounds, and mechanical mixtures. Many
Because heat-treating equipment varies so widely,
it is not practical to describe or to give operating
instructions for different types in this course. The
alloy. Figure 15-5 shows, very much enlarged, a typical
equipment described here is typical of heat-treating
combination that occurs when plain-carbon steel
equipment you may be required to use, but it does not
containing less than 0.83 percent carbon is heated to a
include all possible types of equipment. If you are
certain temperature and then cooled slowly. This
required to heat-treat metals, find out all you can about
combination consists of the solid solution ferrite and the
the equipment that is available before you begin to use
mechanical mixture pearlite, each in crystal form,
it. The instructions furnished by the manufacturer are
distributed throughout the alloy. The relative
usually your best-and sometimes only-source of
proportions of ferrite and pearlite in this combination
authoritative information on a specific item of heat-
treating equipment.
depend largely upon the carbon content of the alloy.
15-6