tight to prevent mixing of the mediums. Each
compartment is equipped with a drain plug, a screen in
the bottom to catch scale and other foreign matter, and
a mesh basket to hold the parts being quenched. The
mesh basket and the wire screen are suspended in the
tank and held in position by clips that fit over the rim of
the tank A portable electric pump may be attached to
the rim of the tank to circulate the liquid.
Stationary quenching tanks are usually designed to
contain only one liquid. In a stationary quenching tank,
the mesh basket that holds the work is usually raised
and lowered by air pressure and is controlled by a three-
way air valve. The basket can usually be positioned at
any level and can be raised above the level of the liquid
so the parts can be drained after they have been cooled.
Figure 15-10.--Portable quenching tank for use in heat-treating.
Stationary quenching tanks usually have built-in
electric pumps to circulate the liquid. WATER is often
used as a quenching medium for plain-carbon steels and
cooling is much faster than when the cooling medium
for aluminum and other nonferrous metals. The water
is not in motion. The volume of the cooling medium is
must be kept clean by frequent changing. The
also important. As the metal cools, the cooling medium
absorbs heat. If the volume is insufficient, the cooling
temperature most often used for water quenching is
about 65F. Normally, the volume of water in the tank
medium will become too hot to cool the work at the
required rate. In regular heat-treating shops where the
should prevent a temperature rise of more than 20F.
cooling mediums must be used continuously,
When very heavy pieces are being quenched, the
mechanical cooling systems are used to maintain the
temperature rise may exceed 20F, but it should always
cooling medium at the correct temperature.
be kept as low as possible.
Liquids, gases, and solids are all used as cooling
BRINE is used for many quenching operations. At
mediums for heat-treating operations. Table 15-3 shows
any given temperature, brine cannot hold as much
dissolved air as fresh water can hold. With brine,
the relative cooling rates of some commonly used
liquids and gases. Solid materials such as lime, sand,
therefore, there are fewer air bubbles or gas pockets on
ashes, and cast-iron chips are sometimes used when the
the surface of the work Brine wets the surface more
rate of cooling must be slower than that produced by
thoroughly and cools the work more rapidly and more
liquids or gases.
uniformly than plain water.
Liquid quenching is accomplished either by
Brine solutions usually contain from 7 to 10 percent
S T I L L - B A T H QUENCHING or by FLUSH
salt by weight or 3/4 of a pound of salt for each gallon
QUENCHING. In still-bath quenching the metal is
of water. The correct temperature for a brine quench
cooled in a tank of still liquid. The only movement of
ranges from 65 to 100F. Plain-carbon steels and low-
the liquid is that caused by the movement of the hot
alloy steels are often quenched in brine. High-carbon
metal. Flush quenching is used for parts that have
steels and all alloy steels that are uneven in cross section
recesses or cavities that would not be properly quenched
must be quenched very carefully if brine is the cooling
by the still-bath method. In flush quenching, the liquid
medium. Brine cools the material so rapidly that great
is sprayed under pressure onto the surface of the piece
internal stresses may develop and crack the work Brine
and into every cavity or recess. This procedure is often
is not used as a quenching medium for nonferrous
used to minimize distortion by providing a relatively
metals because of its high corrosive effect on these
uniform quench to all parts of the piece.
metals.
Portable quenching tanks of the type shown in
OIL is used to quench high-speed steels and
oil-hardened steels. It is also the preferred quenching
not have permanent, built-in equipment. Portable
medium for almost all other steels except where the
quenching tanks may have one compartment or several.
necessary hardness cannot be obtained by such a
When more than one quenching medium is to be used,
relatively slow quench. Although nonferrous metals are
the seal between the compartments must be absolutely
not normally quenched in oil, they may be in special
15-11