Figure 15-7.--Exploded view of electric furnace used for heat-treating.
avoided, however, a neutral atmosphere rather than a
HEATING EQUIPMENT
reducing or oxidizing atmosphere is used.
Equipment designed for the heating of metals
A NEUTRAL ATMOSPHERE is neither oxidizing
includes electric furnaces, fuel-fired furnaces, bath
nor reducing because it contains no oxygen (oxidizing
furnaces, and devices for the measurement and control
agent) and no carbon monoxide (reducing agent). In
of temperature. Improvised heating devices may
reality, other factors often tend to produce either
include oxyacetylene torches, Hauck burners, forges,
oxidation or decarburization; therefore, an atmosphere
and temporary ovens constructed of firebrick and sheet
that is described as neutral may not always be totally
asbestos. We will discuss electric furnaces since they
neutral.
are what most Machinery Repairmen will be using.
The furnace atmosphere may be the result of the
The gases that circulate through the furnace heating
deliberate introduction of a gas or a mixture of gases
chamber and surround the metal as it is being heated
into the heating chamber. When gases are deliberately
make up the FURNACE ATMOSPHERE. By con-
introduced into the heating chamber for the purpose of
trolling the composition of the furnace atmosphere, you
controlling the atmosphere, the furnace is said to have
can produce the type of atmosphere best suited for the
a CONTROLLED or PROTECTIVE ATMOSPHERE.
heating of a particular type of metal. Furnace
atmospheres are generally classified as being oxidizing,
Electric Furnaces
reducing, or neutral.
An OXIDIZING ATMOSPHERE exists when
Electric furnaces with a controlled atmosphere are
frequently used for heat-treating on repair ships and
excess air is circulated through the furnace. Some
tenders. Quite often two such units are used on the same
electric furnaces are designed to operate with an
ship. One is a relatively low-temperature furnace used
oxidizing atmosphere at all times. An oxidizing
atmosphere is indicated by bright, clean walls and a
for preheating or tempering, and the other is a higher
temperature furnace used for hardening. Both types are
clear, transparent atmosphere in the furnace. Metals
equipped with control devices for regulating
heated in an oxidizing atmosphere occasionally develop
films of metal oxides (scale) on their surfaces. In some
temperature. The high-temperature furnace may also be
equipped with rheostats used to increase the rate of
cases, this is undesirable; but in other cases, it is desir-
heating. A typical electric furnace for shipboard use is
able because it tends to prevent further decarburization
of the metal. Decarburization is a term used to describe
the loss of carbon from the surface of ferrous metals
The outer casing of the furnace is usually made of
with consequent softening of the material.
A REDUCING ATMOSPHERE tends to remove
sheet steel. Just inside the casing is a layer of insulating
oxygen from the surface of the metal. A reducing
material, such as mica, spun glass, or asbestos. Inside
atmosphere is often desirable in heat treatment since it
this insulating material is a lining of refractory material,
tends to prevent oxide formation and other surface
such as firebrick and insulating brick. The refractory
lining insulates the furnace, helps maintain the required
15-7