PIT FURNACE

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.

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 .

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.

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.

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

Schematic Diagram Of Pit Furnace:

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.

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.

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

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

Technological Application of Pit

Furnace:

Gas carburizing

Hardening

Bright Annealing

Normalizing

Carbon Nitriding

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.

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

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.

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.

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.

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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