Liquid baths are often used for heating steel in the heat - treating processes. They are usually cylindrical and consist of a sheet steel casing lined with insulating bricks faced with refractory material. This forms the heating chamber in which is suspended a steel pot. The materials which serve as the heating media are usually salt, lead, and oil. Heating may be by gas, oil or electricity. In the case of gas, low-pressure forced- drought fan is coupled with the gas supply and provides a high flame temperature. In the electrically - heated types, there are externally-heated types, and immersed electrode type.
The externally - heated bath furnaces are used for quenching and tempering up to about 700*C. The electrical resistance elements are located in the walls of furnace and heat the outside wall of the metal pot by radiation. A refractory lining is made to withstand the high temperature and prevent rapid loss of heat.
The immersed electrodes salt furnaces are used for preheating, Carburizing and hardening, and the working temperature is between 700*C and 1000*C. In this furnace, electrodes , usually two in number, are immersed in the liquid bath, and a low voltage alternating current is passed between them, since direct current tends to electrolytically decompose the liquid salt.
SALT BATH :
Salt Bath are filled with a salt or mixture of different salts, which melt when heated , and so form a liquid heating medium. The workpiece is inserted into the liquid and the heat of the liquid raises the temperature to the required point. The heating of large number of small parts are done conveniently when they are strung on wires or placed in wire baskets. The salts used must be such that it will not attack the steel , i.e., will not alter its composition, will not produce oxidation or decarburization, corrosion or erosion ; and it will not attack the point in which it is held.
Common salts are sodium and potassium chlorides, nitrates, and cyanides. They are mixed in various proportions and with other salts to obtain different melting points for services in various ranges from 180*C to 1350*C and for various purposes.
LEAD BATH :
In the Lead Bath, the pot which is made of cast steel or some heat - resisting alloy, is filled with lead. The lead used in the bath is ordinary commercial pig lead. A covering of charcoal is placed on top of the bath to decrease the oxidation of the lead. Owing to the high specific gravity of molten lead, steel will float in it, and , therefore, must be held below the surface.
With its low melting point, 327*C, it is soon raised to temperature, and can be effectively employed for temperature up to 1285*C, i.e., for most hardening and tempering operations. Lead bath is particularly favorable for heating in the hardening of files, broaches, reamers, drills, axes, and many other tools. The advantages of the lead bath for heating steel preparatory to hardening or for tempering are :
(1) That uniform temperature can be obtained through out the work.
(2) that the steel will not oxidized during heating period.
(3) that the rate of heating is rapid in the lead bath.
The disadvantage in this bath is that the work must be perfectly dry before immersion in the bath, or steam may be formed, leading to dangerous explosions.
OIL BATH :
Oils are used only in quenching and tempering steel parts at temperatures up to 260*C. Mineral oil having a high flash point is satisfactory for this purpose and heated in a metal container. The equipment is similar to salt bath furnace.
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