Editing Heat treating (section)

== Types of furnaces ==
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, whereas continuous furnaces have an automatic conveying system to provide a constant load into the furnace chamber.<ref name=":0">ASM International Handbook Committee. (1991). ''ASM Handbook'', Volume 04 - Heat Treating. ASM International.</ref>

=== Batch furnaces ===
Batch systems usually consist of an insulated chamber with a steel shell, a [[heating system]], and an access door to the chamber.<ref name=":0" />

=== Box-type furnace ===
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.<ref name=":0" />

=== Car-type furnace ===
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. {{citation needed|date=January 2021|reason=Need a reference for paragraph}}

=== Elevator-type furnace ===
[[File:Fluidised Beds.jpg|thumb|Fluidised bed heat treatment line|542x542px]]

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 [[crane (machine)|crane]]s and transfer mechanisms.<ref name=":0" />

=== Bell-type furnace ===
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 lowered to provide the heat supply.<ref name=":0" />

=== Pit furnaces ===
Furnaces that 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.<ref name=":0" />

=== Salt bath furnaces ===
Salt baths are used in a wide variety of heat treatment processes including neutral hardening, liquid carburising, [[Nitriding#Salt bath nitriding|liquid nitriding]], [[austempering]], [[martempering]] and [[tempering (metallurgy)|tempering]].

Parts are loaded into a pot of [[molten salt]] where they are heated by [[conduction (heat)|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.<ref name=":0" />

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 have made the use of salt baths less attractive in recent years. Consequently, many salt baths are being replaced by more environmentally friendly fluidized bed furnaces.<ref>{{Cite web|title = Made in the Midlands {{!}} Fluidised beds: A Green Alternative to Salt Baths|url = http://claytonholdings.madeinthemidlands.com/news/fluidised-beds-a-green-alternative-to-salt-baths/813|website = claytonholdings.madeinthemidlands.com|access-date = 2015-06-02|url-status = live|archive-url = https://web.archive.org/web/20160207021500/http://claytonholdings.madeinthemidlands.com/news/fluidised-beds-a-green-alternative-to-salt-baths/813|archive-date = 2016-02-07}}</ref>

=== Fluidised bed furnaces ===
A [[fluidised bed]] consists of a cylindrical [[retort]] made from high-temperature alloy, filled with sand-like aluminum oxide particulate. Gas (air or nitrogen) is bubbled through the oxide and the sand moves in such a way that it exhibits fluid-like behavior, hence the term ''fluidized''. 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.<ref name=":0" />