The crucible furnace
Crucible furnace
The crucible way is the oldest one used for making high carbon steel and
special steels. High carbon steel is made by melting scrap steel in a
crucible about 20 inches high and one foot in diameter, made of graphite
which can stand great heat. The percentage of carbon the scrap steel
contains must be known. The amount of carbon wanted is then placed on top of
the steel. A cover is placed over the top and a number of these crucibles
are placed in a hot furnace. The melted iron mixes with the carbon, thus
making steel containing any percentage of carbon necessary. The melted steel
is then poured into forms. Special steels are made the same way, but some
additional materials are also put in the crucible. Crucible steel is a
higher grade, stronger and more expensive steel than either the Bessemer or
open-hearth ones. Crucible steel is used for pens, knives and some machine
parts where very high hardness and strength are needed.
CRUCIBLE PROCESS
High grade tool steels and some alloy steels are still made by the crucible
process, although the electric furnace is now capable of making steel equal
in quality to crucible steel. In the crucible process, wrought iron, or good
scrap, together with a small amount of high purity pig-iron, ferro-manganese,
the necessary alloying metals, and slagging materials are placed in a day or
clay-graphite crucible, covered with an old crucible bottom and melted in a
gas-or-coke-fired furnace. After the charge is entirely molten, with
sufficient time allowed for the gases and impurities to rise to the surface,
the crucible is withdrawn, the slag removed with a cold iron bar, and the
resulting fifty or one hundred pounds of steel poured into a small ingot
which is subsequently forged to the desired shape. The crucible process
differs from other steel-making in that little or no refining is included;
the purity of the metal depends almost entirely upon the purity of the
materials charged. The chief advantage of the process is that it removes
most of the impurities, including oxygen and entangled particles.
The Puddling Furnace
Puddling furnace is the simplest type of furnace. It is used to make wrought,
iron. It has the fire at one end and is separated from the hearth by a wall.
When the flame passes over the hearth its heat is thrown back from the roof
of the furnace on to the metal, which is placed on the hearth. Through, an
opening in the side wall the puddler stirs the metal with a long rake called
a rabble. In this way impurities and carbon are burned away. And the metal
is little by little made into a paste that looks like boiled rice. From time
to time flux-is thrown in to collect ash and impurities. The puddler
collects the granules into a ball. He rolls it back and forth over the
hearth. And finally he takes it out at exactly the right moment. This ball
is then passed through squeezers; or it is worked under the hammer into bars.
In 1783 Peter Onion of Wales, patented a puddling furnace. A few years later
Henry Cort greatly improved it. The puddling furnace was of the type we call
the reverberatory furnace. It is a furnace in which the flame passes over
the charge but does not come in contact with it. The charge in the
reverberatory furnace is placed on a hearth. The heat comes from the fire at
one end of the furnace. The flame is directed against the arched roof over
the hearth. When the flame passes along the arch its heat is thrown downward.
It melts out the iron as it falls on it and burns away the impurities. Along
the sides of the furnace are openings through which the molten metal is
stirred with rabbles. The finished iron is removed in balls weighing about
one hundred pounds apiece.