Heat treatment furnace may be defined as a refractory-lined chamber in which the metal part to be heat-treated, is heated to the required temperature. Usually a heat-treatment furnace is a box-like structure i.e., a steel shell with an access door, a refractory lining, temperature controls and indicators.
Furnaces used for heat treatment are classified as:
(a) Hearth furnace
(i) Stationary hearth furnace
(ii) Movable hearth furnace
(b) Bath furnace
(i) Salt bath
(ii) Lead bath
(iii) Oil bath
Stationary Hearth Furnaces
These are of two types:
(a) Direct Fuel Fired Furnace. It burns the fuel in the space occupied by the charge. It is of low cost and is suitable for all ordinary temperature ranges. This furnace is used for rough heating, such as in forging, and also used for heat treatment, particularly at lower temperature. All the forging jobs can be heat treated in this type of furnace.
(b) Indirect Fuel Fired Furnace/Muffle Furnace. Indirect fuel fired furnace has a heating chamber and a muffle that separates the combustion space from the work space. Since the products of combustion do not enter the work chamber, the furnace atmosphere can be controlled. It has an upper temperature limit of about 1100ºC. Reduced scaling and contamination from the fuels are the advantages of this type of furnace. These types of furnaces are used for heat treatment of small size components.
Movable Hearth Furnaces
These are of two types:
(a) Car Bottom Furnace. A car bottom furnace has a moveable hearth like a flat car that can be rolled in and out of the furnace for charging and unloading the components. Commonly, the metal charge is placed on heat-resistance alloy or refractory base and spaces are provided for circulation of hot air. Car bottom furnace is used for large and heavy parts (e.g. big casting and forgings).
(b) Rotary Hearth Furnace. Rotary furnace is used for continuous production. It has a round shell and horizontal hearth that rotates slowly. Material is charged through a door and taken out from the same or another door (Refer Fig17.9). It is used for hardening, tempering and carburizing. Hammers and chisels can be heat treated in this type of furnace. The temperature range of this furnace is 100ºC to 1200ºC.
These are further classified as:
(a) Salt Bath. A molten salt bath furnace consist of a pot i.e. a ceramic or metal container filled with molten salt in which small jobs are immersed for either heating or cooling within the temperature range of 150°c- 1315°c. Molten bath contains one or more salts such as Nitrates, Carbonates, Chlorides Cyanides and Caustic soda. In this furnace the heat is developed either by oil, gas or electricity.
(b) Lead Bath. Molten lead is used in lead bath. It is used for tempering steel parts. The parts are preheated then immersed in this bath, which is already heated to a tempering temperature. This bath is used for temperature range from 327º C to 1285º C. Heat treatment of Files, Reamers and Drills is carried out in this furnace.
(c) Oil Bath. Mineral oils are used for oil baths. Light oil baths are used for temperature up to 230°C. Heavy oil baths are used for temperature range from 340°C– 370°C.