The Electric Process





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 seen to be a squat kettle, made of heavy sheet steel, with

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.





The Effect Of Tempering On Water-quenched Gages The Forging Of Steel facebooktwittergoogle_plusredditpinterestlinkedinmail

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