Salt Bath Heat-Treating Equipment,Salt Bath Heat-Treating Equipment,Salt Bath Heat-Treating EquipmentHEAT TREATERS can no longer use heat-treating methods that are only uniform, quick, efficient, and economical. They
must also be environmentally responsible. Salt bath heat-treating methods are uniform, quick, efficient, economical, and
environmentally responsible.
Salt baths are used in a wide variety of commercial heat-treating operations including neutral hardening, liquid
carburizing, liquid nitriding, austempering, martempering, and tempering applications. Salt bath equipment is well
adapted to heat treatment of ferrous and nonferrous alloys.
Parts that are heated in molten salt baths are heated by conduction; the molten salt bath provides a ready source of heat as
required. Although materials being heated come in contact with heat through their surfaces, the core of a part rises in
temperature at approximately the same rate as its surface. Heat is quickly drawn to the core from the surface, and salt
baths provide heat at an equal rate over the total part.
Neither convection nor radiation heating methods are able to maintain the rate of heating required to reach equilibrium
with the rate of heat absorption. The ability of a molten salt bath to supply heat at a rapid rate accounts for the uniform,
high quality of parts heat treated in salt baths. Heat-treating times are also shortened; for example, a 25 mm (1 in.) diam
bar can be heated to temperature equilibrium in 4 min in a salt bath, whereas 20 to 30 min would be required to obtain the
same properties in either convection or radiation furnaces.
Salt baths are very efficient methods of heat treating; about 93 to 97% of the electric power consumed with a covered salt
bath operation goes directly into heating of the parts. In atmosphere furnaces, 60% of the energy goes for heating, and the
remaining 40% is released up the furnace stack as waste. Steels that are heat treated in molten salts typically are processed
in ceramic-lined furnaces with submerged or immersed electrodes containing chloride-based salts.
Applications
Applications of the various furnace designs and auxiliary equipment to specific heat-treating processes, such as
austempering and martempering, are described in this and other articles in this Volume. Basic advantages of salt bath
treatment include surface protection and control of distortion.
Surface Protection. Parts immersed in a molten salt bath develop a thin cocoon of solidified salt, which can be easily
washed from the surface after treatment. This surface protection afforded by salt baths can eliminate the formation of
damaging oxide scales. Moreover, because salt baths do not contain the oxygen, carbon dioxide, and water vapor levels
found in most non-vacuum (or atmosphere) furnaces, immersed parts are protected further from scale formation.
Decarburization of steel parts from contact with oxygen and carbon dioxide are also eliminated by the use of molten salts.
Vacuum furnaces provide similar advantages in surface protection.
Control of Distortion. Salt baths offer a way to minimize the bad effects of nonuniform heating, lack of support, and
poor quenching that may cause size and shape distortion. Unlike parts in an atmosphere or vacuum furnace, parts
immersed in molten salts are supported by the density of the medium. Due to its buoyancy, sagging or bending of the
parts is minimized in a molten salt bath.
Heating in molten salts is also very uniform. The temperature uniformity in a molten salt bath averages ±3 °C (±5 °F)
throughout the bath, depending on furnace design. The layer of solidified salt around a part can also protect the part from
rapid initial heating and the resulting thermal shock. As the cocoon of salt melts, the part is gradually and uniformly
heated, minimizing distortion and preventing cracking.
Selecting a Salt for a Given Application. Information concerning the various salts suitable for heat-treating
furnaces is available from many sources, such as the many competent salt companies. Also, military specification MIL-
10699 describes the salts in detail. When selecting a salt for a given application, the following must be considered.
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