Heat treating and inspection of metals

TM 1-423
HEAT TREATING AND INSPECTION OF METALS 3-4
individual case; however, the general construction of the furnaces is
very similar.
(1) Furnaces may be heated by means of oil, gas, or electricity. In
the oil- and gas-fired types, the furnace proper or fire box is inclosed
in a casing of steel plates, electrically welded together and mounted on
a steel frame. The lining of the furnace is constructed of fire brick,
insulated by two or more inches of magnesia. The magnesia permits
expansion of the brickwork without danger of damage to the steel
casing. The heating chamber is made of semirefractory brick which
allows for quick heating. The hearth plates are usually constructed
of a heat-resisting alloy with suitable flanges for holding the work in
place. Furnaces are designed to maintain a uniform temperature
throughout all parts of the heating chamber.
(2) The electrically heated furnaces are used extensively and have
the advantage of being quiet, clean, and constant in operation. The
heating element of an electric furnace may be either of the metal or
carbon resistor type. Metal resistors may be used where temperatures
do not exceed 2,0000 F., while carbon resistors are recommended for
higher temperatures. Electrically heated furnaces are illustrated in
figures 1, 2, and 3. The furnaces illustrated in figures 1 and 2 are con-
ventional production types, whereas the furnace shown in figure 3 is
usually used in laboratories for experimental purposes.
b. A salt bath is often used for small parts that have been finish-
machined and for rivets requiring heat treatment. Parts and assem-
blies too large to be heated in the conventional furnace may also be
treated in this manner. In the salt bath, the parts are heated by sub-
mersion in molten salt, which is kept at the required temperatures by
means of electrical resistors. Figure 4 is a schematic drawing of a
furnace used for this purpose which is simply a large crucible sur-
rounded by fire brick. Parts heated in the salt bath are free from
scaling although extreme care must be used to remove all traces of the
salt after treatment.
4. Quenching tanks and liquids.--a. Suitable tanks must be
provided for quenching baths. The size of tanks should be sufficiently
large to allow the liquids to remain approximately at room tempera-
ture. Circulating pumps and coolers may be used for maintaining
approximately constant temperatures where a large amount of quench-
ing is done. The location of these tanks is very important due to the
fact that insufficiently rapid transfer from the furnace to the quench-
ing medium may destroy the effects of the heat treatment in many in-
stances. A washing tank and provisions for scrubbing must also be
provided for parts that have been heat-treated in salt brine.

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United States. War Dept. Heat treating and inspection of metals. Washington, D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc96657/. Accessed November 28, 2014.