1.4) According to working environment:
According to working environment, furnaces are classified in to two.
a) Air convection furnace and
b) Controlled atmosphere furnace.
a) Air convection furnace:
Most of the heat-treating furnaces are of this type.
b) Controlled atmosphere furnace:
Controlled atmosphere furnaces are used under specific conditions and for only limited metals and alloys. A controlled atmosphere can be defined as a furnace atmosphere that protects the metal from oxidation and maintain the desired properties at the surface of metal during heat treatment. Broadly speaking such an atmosphere is either protective or chemically active. The composition and distribution of atmosphere are deliberately controlled to produce specific surface characteristics. Bright annealing, normalizing and atmospheric annealing of black heart cast iron are examples where a protective atmosphere is used. Carburizing, carbonitriding, and nitriding are examples of chemically active atmosphere.
· Commercially available atmospheres are
· Town gas or other fuel gases
· Ammonia
· Charcoal
· Liquid organic mixture and
· Vacuum.
Bath furnaces:
Different types of bath furnaces are salt bath, oil bath and lead bath furnaces. It consists of a cylindrical outer case with refractory lining. A ceramic pot is suspended from top maintaining an empty space with the heating chamber. The ceramic pot carries oil, lead or salt. The furnace can be oil fire, gas fired or electrically heated. Two electrodes are suspended from top. Oil bath is used for temperatures up to 260°C. Lead bath is used up to 327°C and slat bath for temperatures ranging between 160 and 1330°C.
Salt bath furnace:
It consists of a steel or ceramic container. This contains the molten salt in which the work pieces are immersed. This can be used over a wide range of temperatures (160-130°C). Commonly used salts are Nitrates, Chlorides, Carbonates, Cyanide and Caustic soda. The mode of heat transfer is convection through the molten bath. Since the heat capacity of the molten bath is high and the heat transfer is by convection the work pieces are heated up quickly compared with the conventional furnaces. This is about five times faster than other furnaces. They can be heated by fuel or electrically. They can be classified into externally heated; immersion heated, and immersed electrode type. They are use for processes like cyaniding, liquid carburizing, austempering, martempering, hardening and tempering etc.
Salt bath furnaces possess certain advantages. All work pieces are at uniform temperature and have identical surroundings. Such a condition results in better surface conditions and consistent and reproducible results. Since the work is in direct contact with the bath there is no oxidation or decarburization. Selective heat treatment can be performed by immersing
only the desired portion o the work. Time requirement is lesser. Objects with variable section thickness can be treated along with simpler pieces. Heavy and light objects can be treated in the same bath.
COMMENTS