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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wrought iron.
737
hardly be carried out without a considerably higher temperature of the iron
than the melting temperature. With a larger percentage of silicon in the
iron, and the consequent high temperature, the difficulty to get a good product
in the Swedish Bessemer process is also increased (the process being interrupted
when the desired percentage of carbon has been reached), for the refining
consists in the oxidation of silicon, manganese, and carbon in the iron by means
of oxygen, but the affinity to oxygen of these substances varies with the
temperature, so that at a temperature of about the melting point of pig iron, these
substances are oxidized in the order in which they have been mentioned above,
but if the temperature is extremely high, their oxidation and removal from the
pig iron may even take place in an entirely opposite order, and consequently
great care must be taken that the process be carried out at the right
temperature, unless the blow either will run too cold, so that the iron will have a
tendency to solidify in the converter or ladle, or become too hot, in which case
the steel is deteriorated by too high a percentage of silicon and manganese.
But this is not all; the blowholes which the metal always contains to a higher
or lower degree after solidifying and which are caused by gases escaping during
the cooling, are also as to quantity and distribution throughout the ingot
dependent upon the temperature.
Mr. C. A. Caspersson has made numerous observations at the Bessemer
works of Vestanfors in regard to this matter, and has tried to determine certain
temperatures during the blow, which he has denoted by the figures 0, 1, 2, 3,
and 4. With the lowest degree of temperature, which he has denoted with 0,
the metal has a large number of blowholes distributed everywhere throughout
the ingot; at the next degree of temperature, No. 1, the number of blowholes
diminishes, and they are then situated in the form of a belt in the ingot, the
outer and inner parts of which on the contrary are free from blowholes. If the
temperature be still higher, degree No. 2, the number of blowholes diminishes still
more, but the blowholes formed are situated near the surface of the ingot, and
furthermore the ingot will have a tendency to get hollow in the middle at the
top, which, however, is not caused by the gases but by the shrinkage of the
metal during cooling, and which is called »pipe*. If the temperature be increased
still more (No. 3 and 4), the formation of blowholes ceases, but the »pipe»
grows still larger. The blowholes near the surface of the ingot and the
»pipe», are most injurious, for their inner walls are oxidized by the air at the
heating of the ingot, and afterwards they cannot be welded together but cause
cracks, which have an injurious effect on the tensile strength of the steel, but
this is not the case with the blowholes in the belt. For this reason temperature
No. 1 is the only one to be used. Furthermore, these temperatures thus denoted
by figures are not constant but vary with the carbon (iron with increased
percentage of carbon becoming more fusible), so that a temperature which for
soft iron is denoted, e. g., by 0, and thus is totally unfit, is on the other hand
suitable for steel, etc.
The Caspersson method of determining the right temperature at which the
steel should be tapped, is frequently spoken of as being too complicated and
difficult to apply. This, however, is not the case, for these different degrees of
temperature which Caspersson has denoted by figures, depend in the first instance
on the percentage of silicon in the pig iron, which is easily determined by its
appearance.
The generator of heat is the silicon (and manganese) in the pig iron, but
the amount of silicon can well be estimated from the amount of graphite in
the pig iron sample cast in the test mould, and the carbon in the product
must, to a certain degree, conform to this, if the steel is to be of a first class
quality.
Swtden. 47
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