The introduction of mechanically-operated bellows and taller shaft furnaces
allowed higher temperatures to be achieved. These were called charcoal blast
furnaces, but they did require the use of an excessive amount of fuel to reach
temperatures up to 1200°C. The walls of the furnace were well lagged to keep
the temperatures up, while the extra height allowed the iron ore to be in contact
with the carbon monoxide gas for a longer period of time. At these higher tem-
peratures, the solid iron slowly dissolved the charcoal, which lowered the
melting point of the iron. When the carbon content becomes approximately
4.5%, the metal becomes liquid at the maximum furnace temperature and
forms a layer beneath the now molten slag. This molten metal has the com-
position of cast iron. The slag can be separated now from the cast iron by
draining the top liquid layer of slag away. The slag produced by this blast fur-
nace has much lower iron content and is a more efficient method for producing
ferrous metals. The molten cast iron is run off from the furnace into blocks
called pigs or into the desired shape clay moulds. The first blast furnace built
of this type in England was at Sussex in 1496, and the earliest artefact of cast
iron has been dated as 1509.
Wrought iron could be produced from cast iron by blowing air over the
surface of molten cast iron. This caused the carbon to oxidise to form carbon
dioxide gas and this was continued until all of the carbon had been removed.
The melting point of steels is only a few degrees lower than that of pure iron
and it was impossible to produce these temperatures until the middle of the
eighteenth century. Previous to this, it was found that wrought iron, reheated in
a charcoal fire for a sufficiently long time could be made far harder than the
original wrought iron. The reason was simple the carbon had diffused into the
iron to form steel. Eventually, the cementation process was developed where
wrought iron was packed with charcoal in stone boxes and heated to 900°C for
one week. Later developments included the re-casting of cementation steel to
remove the slag. This produced superior quality steel and became known as
crucible cast steel and improved the fame of Sheffield watchmakers. Methods to
produce a large tonnage of steel began in 1856 with the introduction of the
Bessemer process and the Siemens–Martin (open hearth) in 1865. In the former
process, air (oxygen) is blown through a charge of molten “pig iron” (cast
iron) contained in a pear-shaped converter. The air converts the carbon to a gas
as well as most of the other impurities in the cast iron. The reaction rates are so
high that the temperature rises above the melting point of steel. The molten
steel was subsequently cast into moulds. The process of converting cast iron to
steel took only approximately 30 min, but control of the quality of the steel was
difficult. The principle of the Siemens–Martin process was similar but the time
taken to produce steel was 8–10 h. The quality of the steel was easier to control
by this method. Both processes flourished for over 100 years until 1952 with the
development of the oxygen lance process.
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