Methods of Making Steel
There are three methods of making steel from pig iron. These are:
Bessemer, Open Hearth, and Cementation.
The Bessemer process, named from Henry Bessemer who patented it
in 1855, is as follows: First the pig iron is poured into a pear-shaped
vessel called a converter. This vessel is made of steel plates riveted
together to form a shell which is lined with ground quartz or silica.
The vessel is turned down to a horizontal position and 8 to 15 tons
of molten iron are poured into the smaller end. The great pot is
then turned up again and a blast of air is blown through it. The
carbon in the iron soon bursts into flame and in a few minutes the
carbon is burned out and most of the other impurities removed. The
flame then dies down, fresh carbon is added in the form of '* spiegel-eisen,"
which supplies exactly the amount required, and the process is finished.
Open Hearth Process
In the open hearth process the pig iron and scrap are melted in
a dish-shaped chamber, or hearth, containing about 50 tons. At each
end of the chamber are openings which admit the fuel gas and air,
and at the rear a tapping hole to let out the steel when the process
of conversion is completed.
The cementation process is the one used for making fine tool steel
and cutlery. The cementation furnace consists of two converting
pots or chambers from 8 to
15 feet long, 3 or 4 feet wide, and 3 feet deep. These are placed
side by side with the fire beneath them. The bars of white cast
iron or of wrought iron are placed in the pots and completely surrounded
by carbon in the form of charcoal. The pots are closed so that they
are air-tight and the temperature of the furnace kept at 10000 C
or over for three or four weeks.
By this process the carbon has been absorbed by the iron, but it
is not evenly distributed through the bar. The center of the bar
may not be changed at all. In that case, the bars must be cut up
and melted, or else reheated, hammered, and rolled.
When cast iron is put into the cementation furnace the processes
of purifying and carburizing, that is, combining with carbon, are
combined. For producing fine tool steel, wrought iron is used.
The electric furnace is coming into use in making wrought iron
and steel, as there are no impurities to be absorbed from the fuel
and therefore the process is simpler and the product better. The
greater cost has thus far prevented a more general change. Steel
for tools or cutlery must be annealed, hardened, and tempered before
it is ready for use.