The Electric Process
: STEEL MAKING: The Working Of Steel
The fourth method of manufacturing steel is by the electric furnace.
These furnaces are of various sizes and designs; their size may be
sufficient for only 100 lb. of metal--on the other hand electric
furnaces for making armor-plate steel will hold 40 tons of steel.
Designs vary widely according to the electrical principles used.
A popular furnace is the 6-ton Heroult furnace illustrated in Fig. 5.
It is
a dished bottom and mounted so it can be tilted forward slightly
and completely drained. This kettle is lined with special fire
brick which will withstand most intense heat and resist the cutting
action of hot metal and slag. For a roof, a low dome of fire brick
is provided. The shell and lining is pierced in front for a pouring
spout, and on either side by doors, through which the raw material
is charged.
Two or three carbon electrodes--18-in. cylinders of specially
prepared coke or graphite--extend through holes in the roof. Electrical
connections are made to the upper ends, and a very high current
sent through them. This causes tremendous arcs to form between
the lower ends of the electrodes and the metal below, and these
electric arcs are the only source of heat in this style of furnace.
Electric furnaces can be used to do the same work as is done in
crucible furnaces--that is to say, merely melt a charge of carefully
selected pure raw materials. On the other hand it can be used to
produce very high-grade steel from cheap and impure metal, when
it acts more like an open-hearth furnace. It can push the refining
even further than the latter furnace does, for two reasons: first
the bath is not swept continuously by a flaming mass of gases;
second, the temperature can be run up higher, enabling the operator
to make up slags which are difficult to melt but very useful to
remove small traces of impurities from the metal.
Electric furnaces are widely used, not only in the iron industry,
but in brass, copper and aluminum works. It is a useful melter of
cold metal for making castings. It can be used to convert iron
into steel or vice versa. Its most useful sphere, however, is as a
refiner of metal, wherein it takes either cold steel or molten steel
from open hearth or bessemer furnaces, and gives it the finishing
touches.
As an illustration of the furnace reactions that take place the
following schedule is given, showing the various stages in the
making of a heat of electric steel. The steel to be made was a
high-carbon chrome steel used for balls for ball bearings:
6-TON HEROULT FURNACE
11:50 A.M.--Material charged:
Boiler plate 5,980 lb.
Stampings 5,991 lb.
-----------
11,971 lb.
Limestone 700 lb.
12:29 P.M.--Completed charging (current switched on).
3:20 P.M.--Charge melted down.
Preliminary analysis under black slag.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.06 0.014 0.032 0.009 0.08
Note the practical elimination of phosphorus.
3:40 P.M.--The oxidizing (black) slag is now poured and skimmed off as
clean as possible to prevent rephosphorizing and to permit of adding
carburizing materials. For this purpose carbon is added in the form
of powdered coke, ground electrodes or other forms of pure carbon.
The deoxidizing slag is now formed by additions of lime, coke and
fluorspar (and for some analyses ferrosilicon). The slag changes
from black to white as the metallic oxides are reduced by these
deoxidizing additions and the reduced metals return to the bath.
A good finishing slag is creamy white, porous and viscous. After
the slag becomes white, some time is necessary for the absorption
of the sulphur in the bath by the slag.
The white slag disintegrates to a powder when exposed to the atmosphere
and has a pronounced odor of acetylene when wet.
Further additions of recarburizing material are added as needed to
meet the analysis. The further reactions are shown by the following:
3:40 P.M.--Recarburizing material added:
130 lb. ground electrodes.
25 lb. ferromanganese.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.76 0.011 0.030 0.008 0.26
To form white slag there was added:
225 lb. lime.
75 lb. powdered coke.
55 lb. fluorspar.
4:50 P.M.--
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.75 0.014 0.012 0.008 0.28
Note reduction of the sulphur content.
During the white-slag period the following alloying additions were
made:
500 lb. pig iron.
80 lb. ferrosilicon.
9 lb. ferromanganese.
146 lb. 6 per cent carbon ferrochrome.
The furnace was rotated forward to an inclined position and the
charge poured into the ladle, from which in turn it was poured
into molds.
5:40 P.M.--Heat poured.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese Chromium
0.97 0.25 0.014 0.013 0.33 0.70
Ingot weight poured 94.0 per cent
Scull 2.7 per cent
Loss 3.3 per cent
Total current consumption for the heat, 4,700 kW.-hr. or 710 kw.-hr.
per ton.
Electric steel, in fact, all fine steel, should be cast in big-end-up
molds with refractory hot tops to prevent any possibility of pipage
in the body of the ingot. In the further processing of the ingot,
whether in the rolling mill or forge, special precautions should
be taken in the heating, in the reduction of the metal and in the
cooling.
No attempt is made to compare the relative merits of open hearth
and electric steel; results in service, day in and day out, have,
however, thoroughly established the desirability of electric steel.
Ten years of experience indicate that electric steel is equal to
crucible steel and superior to open hearth.
The rare purity of the heat derived from the electric are, combined
with definite control of the slag in a neutral atmosphere, explains
in part the superiority of electric steel. Commenting on this recently
Dr. H. M. Howe stated that in the open hearth process you have such
atmosphere and slag conditions as you can get, and in the electric
you have such atmosphere and slag conditions as you desire.
Another type of electric furnace is shown in Figs. 7 and 8. This
is the Ludlum furnace, the illustrations showing a 10-ton size.
Figure 7 shows it in normal, or melting position, while in Fig.
8 it is tilted for pouring. In melting, the electrodes first rest
on the charge of material in the furnace. After the current is
turned on they eat their way through, nearly to the bottom. By
this time there is a pool of molten metal beneath the electrode
and the charge is melted from the bottom up so that the roof is
not exposed to the high temperature radiating from the open arc.
The electrodes in this furnace are of graphite, 9 in. in diameter
and the current consumed is about 500 kw.-hr. per ton.
One of the things which sometimes confuse regarding the contents
of steel is the fact that the percentage of carbon and the other
alloys are usually designated in different ways. Carbon is usually
designated by points and the other alloys by percentages. The
point is one ten-thousandth while 1 per cent is one one-hundredth
of the whole. In other words, one hundred point carbon is steel
containing 1 per cent carbon. Twenty point carbon, such as is used
for carbonizing purposes is 0.20 per cent. Tool steel varies from
one hundred to one hundred and fifty points carbon, or from 1.00
to 1.50 per cent.
Nickel, chromium, etc., are always given in per cent, as a 3.5
per cent nickel, which means exactly what it says--3-1/2 parts in
100. Bearing this difference in mind all confusion will be avoided.
More
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