Full resolution (JPEG) - On this page / på denna sida - Second part - IX. Mining Industry and Metal Production - 2. The Iron and Steel Industry. By the late Prof. J. G. Wiborgh - Ingot iron (Steel)
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738- ix. mining industry and metal production op sweden.
Care is taken, to be sure, not to blow soft iron from perfectly white pig
iron and hard steel from perfectly grey pig iron, for in the former case it is
known that the product will be too cold, in the latter rendered too impure br
silicon and manganese. In the first place, one is dependent upon the running
of the blast furnace, which, when its product is intended for a large quantity of
steel of the same composition must be extremely even and constantly produce a
pig iron with the percentage of silicon and manganese which is best suited for
blowing of steel of this composition.
By carefully assorting the charcoal and by drying it if necessary, by keeping
the slag at a fixed percentage of silica, and regulating the temperature by
means of good hot blast stoves, it is possible to get a uniform pig iron for a
long time, but many times disturbances also occur in the running of the blast
furnace, the pig iron turning out too white or too grey for the purpose intended,
and then it is best to make a steel from this pig iron with another percentage
of carbon, for which the pig iron is more suitable.
This circumstance is the greatest disadvantage of the Swedish Bessemer process.
In order as much as possible to avoid impurities in the steel and »pipe»
in the ingot, the pig iron must be such that the temperature in the converter
does not become unnecessarily high, but then it also happens, as Caspersson has
shown, that the gases form belt-blowholes, which is a fortunate circumstance.
During the blowing process itself, there are also certain signs which
characterize a low or a high temperature of the iron in the converter, as the length of
time from the beginning of the blow until the boiling begins (the term boiling
is applied to that stage of the process when the carbon as carbonic oxide with
a violent bubbling leaves the liquid mass of iron), the nature of the flame issuing
from the Bessemer furnace, the appearance of the slag, etc., etc.
There are also means for increasing or diminishing the temperature of
the metal during the course of the process; in the former case ferro-silicoi
is added, in the latter case cold iron scrap. It is best, however, from the
beginning, to have a pig iron which will give the required temperature to
the steel.
The Swedish Bessemer process has the great advantage of making an addition
of ferro-manganese unnecessary at the close of the blow, to take away the red
shortness from the steel on account of oxidized iron, which is due partly to the
high percentage of manganese in the pig iron, and partly to the blowing being
interrupted while the carbon is still high and the iron consequently cannot
contain oxidized iron. Only in case soft iron is to be produced, an addition of
ferro-manganese is made.
In order to get ingots entirely free from blowholes, however, there is
sometimes added, when the blow is finished, either ferro-silicon-manganese or
aluminium, the former in the converter, the latter while the iron is poured
into the moulds. The amount of aluminium to be added depends upon the
percentage of carbon in the steel; for hard steel only O’oos % is used, but
for soft steel up to 0-02 % and more. A larger addition of aluminium makes
the steel less fluid and increases the number of blowholes instead of
diminishing them.
The converter. The Bessemer furnaces or converters at present in use are
of the same form as those first used at Sandviken in 1863, only the size of the
converter and the mechanical appliances for tipping it having been altered. The
furnaces made of iron plate lined with silica bricks or with gannister, is suspended
on two trunnions, one of which is hollow and connected with the blast-pipe,
which is then continued by another pipe to a box under the bottom of the
converter, whence the blast enters through a large number of tuyeres to the furnace
and passes through the liquid iron.
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