A pit furnace is constructed by digging a pit and building a furnace structure within it. The furnace body is made of steel plates lined with ceramic fiber insulation. It has a lid with a central fan for circulating gases. Pit furnaces can be used to melt metals by placing crucibles containing metal inside. Coke fuel is burned to heat the furnace to the desired temperature for processes like carburizing, hardening, annealing, and nitriding steel parts. Pit furnaces provide flexibility, economic operation, and precision for heat treating large loads.

1. PIT FURNACE

2. Pit Furnace:
What is pit furnace?
 A furnace made in pit for melting metal for taking casting process is called
a pit furnace
 It is a furnace for which first a pit had to be made first according to the
height of furnace.
 After this, the furnace is fitted in pit so that blower should b fitted properly
below the furnace.

3. Construction of Pit Furnace:
 The furnace body is made of heavy steel plate reinforced with steel profiles and profile
bent steel sheet. The heavy-duty upper flange is reinforced with internal braces.
 The furnace lining is mainly ceramic fiber.
 The furnace hot face is made up of vacuum-formed fiber into which metallic supports for
the elements are fixed .
 The lid is formed as a concave end cover. Between the end cover and the upper part the
lid is insulated with ceramic fiber of the same thickness as the rest of the furnace .

4. Construction (cont.):
 The circulation fan is mounted centrally in the lid. The fan is a water-cooled
high temperature fan with a cast fan wheel mounted on a cone
 In the lid there are also connection points for oxygen measuring cell,
thermocouple, process gas supply and exhaust pipe. There is also an extra
connection point.
 The retort is formed as a cylinder with a heavy-duty flange on top. On the
flange there is a circular cooling tube housing two seals to tighten against
the furnace lid.

5. Construction (cont.):
 The furnace bottom functions as a support for the furnace charge as well as
a seal for the atmosphere, which is built up in the furnace during the
process.
 The retort is provided with an inner corrugated guide cylinder for the
circulation of gas.
 The furnace is equipped with a safety waste gas burner system for
combustible gases.
 The gases making up the atmosphere in the furnace can be chosen for the
desired process, we want. Such as LPG for melting metal.

6. Construction Involves :
 3/16" thick steel plate furnace shell
 1/2" thick steel top plate
 Energy-saving lightweight ceramic fiber insulation reduces operating costs
 Insulating refractory floor to support heavy loads
 Excellent temperature uniformity throughout work space
 Fast heat-up and cool-down

7. Schematic Diagram Of Pit Furnace:

8. Working Of Pit Furnace:
 To prepare the furnace for melting, a deep bed of coke is kindled and
allowed to burn until a state of good combustion is attained.
 Some of the coke is removed to make place for the crucible .The crucible
is then lowered into furnace .
 The coke is replaced and additional coke is put to surround the crucible
on all sides.
 Metal is then charged in the crucible and the furnace lid is replaced to
give natural draft.

9. Working (cont.):
 The draft of the air through the furnace may be a natural draft for low
melting temperature metals but a force draft with the help of a blower to
accelerate the melting process in case of higher melting temperature
metals and alloys.
 .When the metal melts and reaches the desired temperature the crucible is
removed from the furnace with special long handle tongs.
 The molten metal is then put into casting molds for the casting of metal.

10. Key Features That Must Be Observed:
 Maximum temperature: up to 1000 °C
 High temperature uniformity
 Highly efficient atmosphere circulation system
 Highly efficient system for load cooling
 Use of the most precise and modern instrumentation currently available

11. Types:
Type of pit furnace design:
 pit furnaces with closed bottom
 pit furnaces- with opened bottom and fixed hearth
Type of pit furnace heating system:
 Electrically heated furnaces
 Gas fired furnaces

12. Technological Application of Pit
Furnace:
 Gas carburizing
 Hardening
 Bright Annealing
 Normalizing
 Carbon Nitriding

13. Gas Carburizing:
 Gas Carburizing Process is a surface chemistry process, which improves
the hardness of a component by diffusing carbon into the surface layer to
improve wear and fatigue resistance. The work pieces are pre-heated and
then held for a period of time at an elevated temperature, typically
between 820 and 940°C.
 During the thermal cycle the components are subject to an enriched
carbon atmosphere such that nascent species of carbon can diffuse into
the surface layers of the component.

14. Hardening:
 Hardening is a metallurgical and metal working process used to increase
the hardness of a metal.
 A harder metal will have a higher resistance to plastic deformation than a
less hard metal

15. Bright Annealing:
 What is Annealing?
Heat (metal or glass) and allow it to cool slowly, in order to remove
internal stresses and toughen it.
 An annealing process that is carried out in a controlled atmosphere
furnace or vacuum in order that oxidation is reduced to a minimum and
the surface remains relatively bright.

16. Normalizing:
 Normalization is a type of annealing process used to relieve stress in
harden able steels after cold work and to improve ductility and
toughness properties
 Normalizing Heat Treatment process is heating a steel above the critical
temperature, holding for a period of time long enough for transformation to
occur, and air cooling.

17. Carbon-nitriding:
 Carbonitriding is a metallurgical surface modification technique that is
used to increase the surface hardness of a metal, thereby reducing wear.
During the process, atoms of carbon and nitrogen diffuse interstitially into
the metal, increasing the hardness and modulus near the surface.
 Carbonitriding is often applied to inexpensive, easily machined low
carbon steel to impart the surface properties of more expensive and
difficult to work grades of steel.

18. Main advantages of pit furnace:
 Working flexibility.
 Economic effectiveness, due to the relatively small requirements for media
needed for furnace operation.
 Easy service and maintenance.
 Very high repeatability of processes and very precise heat treatment of
various parts.
 High accuracy of process control (temperature and carbon potential).
 Safe operation.
 Ability to treat large parts.

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