Heat Treatment Furnace Types
Furnaces used for heat treatment can be split into two broad categories, batch furnaces and continuous furnaces.
Batch furnaces are usually manually loaded and unloaded where continuous furnaces have an automatic conveying system to provide a constant load into the furnace chamber.
Batch systems usually consist of an insulated chamber with a steel shell, a heating system, and an access door to the chamber.
Many basic box type furnaces have been upgraded to a semi-continuous batch furnace with the addition of integrated quench tanks and slow-cool chambers. These upgraded furnaces are a very commonly used piece of equipment for heat-treating.
Also known as a “bogie hearth”, the car furnace is an extremely large batch furnace. The floor is constructed as an insulated movable car that is moved in and out of the furnace for loading and unloading. The car is usually sealed using sand seals or solid seals when in position. Due to the difficulty in getting a sufficient seal, car furnaces are usually used for non-atmosphere processes.
Fluidised bed heat treatment line
Similar in type to the car furnace, except that the car and hearth are rolled into position beneath the furnace and raised by means of a motor driven mechanism. Elevator furnaces can handle large heavy loads and often eliminate the need for any external cranes and transfer mechanisms.
Bell furnaces have removable covers called “bells”, which are lowered over the load and hearth by crane. An inner bell is placed over the hearth and sealed to supply a protective atmosphere. An outer bell is the lowered to provide the heat supply.
Furnaces which are constructed in a pit and extend to floor level or slightly above are called pit furnaces. Workpieces can be suspended from fixtures, held in baskets or placed on bases in the furnace. Pit furnaces are suited to heating long tubes, shafts and rods by holding them in a vertical position. This manner of loading provides minimal distortion.
Salt Bath Furnaces
Salt baths are used in a wide variety of heat treatment processes including neutral hardening, liquid carburising, liquid nitriding, austempering, martempering and tempering.
Parts are loaded into a pot of molten salt where they are heated by conduction, giving a very readily available source of heat. The core temperature of a part rises in temperature at approximately the same rate as its surface in a salt bath.
Salt baths utilize a variety of salts for heat treatment with cyanide salts being the most extensively used. Concerns about associated occupation health and safety, and expensive waste management and disposal due to their environmental effects has made the use of salt baths less attractive in recent years. Consequently, many salt baths are being replaced by more environmentally friendly fluidised bed furnaces.
Fluidised Bed Furnaces
A fluidised bed consists of a cylindrical retort made from high temperature alloy, filled with sand-like aluminium oxide particulate. Gas (air or nitrogen) is bubbled through the oxide and the sand moves in such a way that is exhibits fluid like behaviour, hence the term fluidised. The solid-solid contact of the oxide gives very high thermal conductivity and excellent temperature uniformity throughout the furnace, comparable to those seen in a salt bath.
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