THE OPEN-HEARTH PROCESS
Bessemer’s was a wonderful process, but the time seemed to be ripe for
great development along metallurgical lines, and the method of
converting pig iron into steel which he devised soon had a competitor
which was destined eventually to take the lead in steel production. Many
years passed before the tonnage turned out annually by Bessemer’s
process was equaled by that of the new, but as shown in the last chapter
the Siemens-Martin or open-hearth process in 1907 produced the greater
tonnage. It has since retained its lead and probably will continue to do
so.
As far back as 1845 John Marshall Heath took out a patent for a process
for making steel patterned after the old puddling process. In a way he
may therefore be said to have devised or forecast the open-hearth
process, but because of the great obstacles that had to be surmounted in
getting a furnace that would fulfill the requirements he was unable to
carry out his scheme. You will remember that in the puddling furnace the
purified metal became pasty because of its high melting point. Because
of the great heat required it was not until the invention of the
regenerative system by C. W. Siemens in 1860 that the open-hearth
process was possible. Siemen’s furnace was the first one that could keep
the iron molten. It was in Birmingham, England, that the first
successful open-hearth furnace was used.
[Illustration:
EARLY TYPE OF GAS PRODUCER, REGENERATORS, AND OPEN-HEARTH FURNACE.
COURSE TAKEN BY AIR, GAS, AND PRODUCTS OF COMBUSTION ARE PLAINLY
SHOWN, AS ARE THE VALVES THAT REVERSE DIRECTION OF FLOW
]
While not as speedy nor as prolific a producer as the Bessemer process
and far less spectacular, the open-hearth has several advantages.
The acid Bessemer was always handicapped because pig iron with less than
0.1 per cent of phosphorus was necessary. The majority of ores carry
more than this amount. The basic Bessemer requires pig iron containing
not less than 2 per cent of phosphorus. The vast quantities of material
which contain percentages of phosphorus between these limits are useless
as far as the Bessemer process is concerned.
To be successfully used the pig iron must be further limited as to
composition. It must have sufficient silicon, manganese, and carbon to
give the heat required for Bessemerizing, as the burning of these
metalloids has to be depended upon for the conversion to steel and to
give proper fluidity to the finished alloy.
Then, too, the large amount of air forced through to a certain extent
“over-oxidizes” the bath and some of the gases are mechanically retained
by the steel no matter how complete the deoxidation. There also is loss
of metal due to unavoidable “spitting,” for the rapid streams of air
mechanically carry some metal and slag with the flame out of the vessel.
On the other hand, for the open-hearth process can be used pig iron of
widely varied character and composition and, further, large percentages
of low-priced steel scrap can be utilized in the charges; as no air is
blown through the metal and little comes in contact with it, the
conversion takes place quietly and smoothly and with much less loss by
oxidation, the yield of steel usually being from 90 to 97 per cent of
the metal charged as against 83 to 87 per cent which is the yield by the
Bessemer process; besides giving less over-oxidation and gases in the
metal, the slowness of the conversion is an advantage, as control is
very easy, and, when desired, samples for test may be taken. From his
tests the melter can be quite certain when he taps out the steel that it
is of the composition desired.
[Illustration:
SECTION THROUGH TYPICAL STATIONARY OPEN-HEARTH FURNACE, SHOWING
CONSTRUCTION OF FURNACE, LINING, BATH AND AIR AND GAS PORTS
]
The melting in an open-hearth furnace is done largely by indirect or
radiated heat, and it is not intended that the flame shall impinge too
directly upon the surface of the bath.
[Illustration:
BOXES OF STEEL SCRAP AND ELECTRIC CHARGING MACHINE IN FRONT OF
CHARGING DOORS AT REAR OF FURNACES
]
Except during the melting down of the pig iron and other materials
charged in the furnace, the flame and air take little part in the actual
elimination of the metalloids. Their main function is to furnish the
heat necessary. Being used so indirectly—mostly by radiation from the
roof and walls—very great heat must be used and much would be wasted if
special precautions were not taken to save it. The bath must be kept hot
enough to remain molten after purification of the metal, which we were
unable to accomplish in the wrought iron puddling furnace.
Under each end of the rectangular furnace are two chambers built up with
checker-work of fire brick. These sets are in duplicate and each has one
chamber for air and one for gas.
[Illustration:
CHARGING MACHINE WITH BOX OF SCRAP HALF WAY INTO FURNACE
]
Thus an open-hearth furnace will be seen to occupy a sort of hollow
square, the furnace proper forming one side, the regenerative chambers
two sides, with the chimney and flues the remaining side. “Reversing”
valves force the incoming gas and air to travel each through its
respective hot regenerating chamber up through the ports and into the
furnace where they unite and burn with a very hot flame. The hot gases
leave through similar ports in the other end of the furnace and on their
way to the chimney heat the checker-work in the regenerative chamber.
Every fifteen or twenty minutes the valves are reversed and the
direction of flow is changed. In this way the incoming gas and air are
preheated and in the furnace burn with a very much hotter flame than
would cold gas with cold air. No blast is required, the draft caused by
the chimney being sufficient.
For protection of the roof from the great heat developed and the metal
of the bath from too great oxidation, the air ports usually are located
above the gas ports. The streams of air, while protecting the roof from
the flame, at the same time are prevented from directly impinging upon
and too strongly oxidizing the metal of the bath.
The diagrammatic sketches given show roughly a furnace, regenerative
chambers, ports, etc.
[Illustration:
CHARGING “HOT” METAL
]
The original intention was to melt pig iron and reduce it; i. e., burn
out the silicon, manganese, and carbon by action of the flame and
addition of iron ore. This was the process worked out by Siemens in
England. In France, P. and E. Martin altered the method by diluting
molten pig iron in the Siemens furnace by melting and dissolving in it
steel scrap. It was soon found that a combination of the two methods was
better than either one alone and the open-hearth process acquired its
name—the Siemens-Martin—in this way.
In the United States about 20,000,000 tons of steel are made annually by
the basic open-hearth process while only 1,100,000 tons are produced by
the acid open-hearth process.
The two processes are practically the same except that by the basic
process the phosphorus as well as the silicon, manganese, and carbon are
reduced or eliminated. In order to take out the phosphorus, additions of
lime (i. e., calcium oxide or calcium carbonate) are made just as
occurred with the basic Bessemer process.
Should we use lime in a furnace having an acid lining, much of the lime,
which is a “base,” would react with the “acid” (silica) bricks of the
lining, and, becoming neutralized, would not do its work. So, as in the
basic Bessemer process, we here have to use either “basic” or “neutral”
lining.
The material generally used is burnt magnesium carbonate which is known
as “magnesite.” Dolomite, which is a combination of the carbonates of
calcium and magnesium, is sometimes used. Chrome bricks, the usual
neutral material, are rather too expensive for extensive use. The best
magnesite comes from Austria and is usually not very cheap. As acid
materials (those of silica or clay) are cheaper and mechanically
stronger, a compromise is ofttimes effected by using basic materials for
the furnace bottom and acid bricks for the sidewalls and roof. A few
rows of chrome bricks may be put in to form a neutral dividing line just
at and above the edge of the bath where the action of the slag is the
most severe. It also serves to keep the basic and acid materials apart
and from reacting with each other.
At the commencement of charging, limestone or sometimes burnt lime is
shoveled in upon the bottom or “hearth” of the white-hot furnace.
When cold metal is charged, the pigs of iron are conveyed into the
furnace by the melter and his helpers by means of long handled flat iron
tools called “peels.” This is followed by charging some or all of the
scrap or iron which is to be made a part of the charge.
Even in the smaller 15 or 25–ton furnaces hand charging takes a great
deal of time, sometimes as much as six or eight hours, and the labor
cost as well as the heat loss is therefore excessive. Modern machine
charging which requires not more than an hour is therefore highly
desirable.
[Illustration:
ROW OF OPEN-HEARTH FURNACES SHOWING PIT OR TAPPING SIDE
]
During the melting down of the pig iron with the scrap that has been
charged, the air and flame burn out about half of the silicon and
manganese of the metal. To remove the remainder of these and the carbon
of the charge, additions are made from time to time of sufficient ore to
keep the bath “boiling.” This phenomenon results from the giving off of
carbon monoxide gas formed from the oxygen of the iron ore and the
carbon of the metal, just as happened when the puddler in the
manufacture of wrought iron used iron ore in his furnace. The covering
of slag which forms and protects the bath from the flame undoubtedly
transfers oxygen from the furnace gases to the bath and this helps to
burn out the carbon.
The lime charged unites with the phosphorus of the iron and takes it
into the slag which covers the bath. If necessary, further additions of
lime may be made from time to time during the melting and the “working
down” (elimination of the metalloids) of the charge. As long as the slag
is kept basic it retains the phosphorus, but should it turn acid the
iron of the bath would take the phosphorus back again.
These reactions are all chemical, just as much so as are the burning of
wood and coal and the thousands of reactions which are brought about in
chemical laboratories.
Additions of ore are made from time to time and the bath rabbled.
Samples are taken now and then with a long handled iron spoon or ladle
and these are poured into molds to form small bars of steel, which,
after quenching, are broken.
[Illustration:
OPEN-HEARTH FURNACE “TAPPING”
]
The melter has become very proficient in judging the composition of the
metal of the bath from the fracture of these broken test pieces. By
means of the samples taken he watches the elimination of the metalloids.
When he thinks the reactions have progressed far enough he takes a last
sample which is rushed to the chemist who makes a hurried “control”
analysis for carbon and phosphorus, the metal being held in the furnace
meanwhile. If the results of this analysis show the bath to have the
desired composition the steel is poured. If the reactions have not been
complete, the chemist’s report shows that the carbon and perhaps the
phosphorus are still too high, in which case the charge must be still
further worked down.
Some melters are able to make fairly uniform and satisfactory steel
without a chemist, but for best results a chemical laboratory is
desirable.
When ready to tap, the big ladle is suspended from a crane under the
spout of the furnace. With a tapping bar the plug of clay is removed
from the tap hole and the molten steel gushes out into the ladle. The
slag which has covered the bath is the last to drain out. Many times
this will overflow the ladle, making a beautiful cascade as it pours
over the sides all around to the floor beneath. Especially at night is
this a glorious sight.
[Illustration:
TEEMING THE STEEL INTO INGOT MOLDS
]
Recarburization is not done to the same extent as it is in the Bessemer
process. As the open-hearth elimination of carbon is slower and under so
much better control, the furnace usually is tapped when the carbon has
been reduced to the percentage desired in the finished steel. When it is
necessary to add carbon it is done sometimes by adding pig iron to the
bath and sometimes by throwing a weighed amount of coal or coke in the
ladle as the steel is going in. Molten iron and steel have strong
appetites for carbon and dissolve it very readily. Ferro-manganese is
used to prevent red-shortness and to deoxidize the metal. This also is
usually put into the ladle as too much loss would occur were it added in
the furnace.
While the furnace is again being charged through the charging doors at
the rear, the steel is teemed through the nozzle of the big ladle into
the waiting ingot molds. These go to the stripper, to the soaking pits,
and then to the rolls of the blooming mills just as did the Bessemer
ingots.
[Illustration:
AT THE “STRIPPER”
]
In the acid-lined furnace no attempt is made to reduce the phosphorus.
It would be futile. Therefore the materials charged must be very low in
phosphorus and sulphur. No lime additions are made, the flame simply
melts down the pig iron and scrap, the iron oxide later is added from
time to time to keep up the boil until the test bars show that the
carbon as well as the silicon and manganese have been eliminated as
fully as is desired. The metal is then tapped as described above.
Three or more hours are usually required to melt down cold charges. The
elimination of the remainder of the silicon, manganese, and the carbon
requires about four or five hours more. So for each heat the open-hearth
furnace requires from eight to twelve hours, depending largely upon the
speed of charging and melting.
Of late years the difficulties attending the use of molten metal from
the blast furnace in place of cold pig iron have been largely
surmounted. The use of uniform metal from the “mixer,” which was
described in the article on the Bessemer process, has aided the
open-hearth process also. Of course, when molten metal is added none of
its silicon and manganese is reduced by the flame as occurred with the
cold metals during the melting down, so the molten metal charged is
usually low in these elements to compensate. By use of “hot” (molten)
metal the time necessary to produce a “heat” of steel is considerably
shortened.
The first and perhaps the majority of furnaces yet building are
“stationary.” Some have found it advantageous to construct furnaces that
can be tipped to pour the metal into the ladle. Such are known as
“tilting” furnaces. One furnace designer has even gone so far in a
smaller type used for steel castings as to make the furnace removable,
thus doing away with a ladle entirely. The big crane simply lifts the
whole furnace out from between the housings which contain the ports. It
is taken bodily to the molds which are poured directly.
Open-hearth furnaces have been built of larger and larger capacity. A
great many fifty-ton furnaces have been built and furnaces which produce
eighty or more tons at a heat are now not uncommon.
Furnaces of the Talbot type are built for as much as 200 and even 300
tons of metal, but from these only part of the finished steel is tapped
at a time, the remainder being left to help work down the additions of
new material which is added to replace the steel tapped out.
The rolling mill industry is so intimately connected with and dependent
upon the steel-making methods and equipment that each is designed with
reference to the other.
Bessemer steel has been largely used for the manufacture of rails, rod,
wire, pipe, merchant bar, etc., while open-hearth steel has gone into
plate, boiler tubes, structural shapes, billets for axles, etc. Recently
it is being used for rails and very many of the products which were
formerly made from Bessemer steel.
[Illustration:
LOWER HALF OF A “BATTERY” OF MODERN GAS PRODUCERS
]
It should not be inferred from this that Bessemer steel is no longer in
demand or that it is not good steel. As you will notice from the table
given in the last chapter, the production of Bessemer steel has not
declined appreciably, if at all. The fact is that open-hearth steel
production has been increasing at a great rate, while the production of
Bessemer has remained stationary. With the growing scarcity of ores
suitable for pig iron for Bessemerizing, the open-hearth process is
becoming able to compete with the Bessemer process in the matter of
cost. For some purposes the steel is considered to be a little more
desirable, but, as is the case with many good things, the pendulum
swings too far and there is no doubt that open-hearth steel is often
demanded and used for purposes for which Bessemer steel would be just as
good and perhaps better.
For many years it has been said that the Bessemer process is “doomed.”
This, of course, was because of the scarcity of low phosphorus ores.
Just how “doomed” it is, it is perhaps impossible to say. Certainly it
is still a very live process and the combining of processes, such as
“duplexing,” will probably prolong its life.
By the “duplex” process, molten blast furnace iron from the mixer is
“desiliconized” in the Bessemer converter. Before too much of the carbon
has been burned, the metal is transferred to a basic open-hearth furnace
where the remainder of the carbon and most of the phosphorus is removed.
By this method the advantages of the open-hearth and much of the speed
of the Bessemer process are combined. The output of the open-hearth
furnace is thus greatly increased.
To-day all kinds of combinations of Bessemer, open-hearth, and electric
furnace are being projected and it is difficult and likely impossible
for any one to predict the future of any of the processes.
Lest the metallurgical facts scattered through several chapters escape,
let us summarize a little. Roughly speaking, the capabilities of and
materials required for the processes are as follows—the chemical symbols
for silicon, manganese, carbon, phosphorus, and sulphur being used for
brevity:
PROCESS REFINING CAPABILITY MATERIAL REQUIRED
Crucible Process. Removes no metalloids, Very low Si. P. S. and
but simply remelts. C.
Acid Bessemer Process. Takes out Si. Mn. and Very low P. and S.
C. (under 0.1%).
Basic Bessemer Process. Takes out Si. Mn. C. P. Very high P. (2% and
and some S. over).
Acid Open-Hearth Takes out Si. Mn. C. Very low P. and S.
Process. (under 0.1% of each).
Basic Open-Hearth Takes out Si. Mn. C. P. Wider Variety.
Process. and some S.
Electric Furnace Takes out Si. Mn. C. P. Wider Variety.
Process. and S.
In further explanation of the competition in quality of Bessemer and of
open-hearth steels it should be understood that in both the acid
Bessemer and the acid open-hearth furnaces we get out in quality just
what we put in. While for some purposes phosphorus and sulphur of 0.1
per cent is allowable, for other purposes they should not be over 0.025
or 0.03 per cent. To produce steel of the latter high quality, material
containing slightly less than this of sulphur and phosphorus must be
charged, and these are usually much higher in price than are pig iron
and scrap containing greater percentages of these metalloids.
[Illustration:
CHARGING FLOOR OF THE BATTERY OF GAS PRODUCERS SHOWING ROCKING ARMS
FOR GRADUAL FEEDING OF THE COAL
]
Where materials of 2.5 to 3 per cent of phosphorus are obtainable, as,
generally speaking, they are not in this country, the basic Bessemer
should make as low phosphorus steel as does the basic open-hearth.
The great advantages of the basic open-hearth process, then, are that
for it can be utilized a much wider variety of raw materials than is
possible with the acid open-hearth or either of the Bessemer processes,
and, particularly that here, at least, the proper materials are readily
available.
The fuels used vary, of course, according to what is most available,
considering quantity, quality, and price. Natural gas has been a
favorite fuel, as also has oil. But in many localities natural gas never
was available and in others which were thus blessed, the supply has been
exhausted. By-product coke oven gas and tar are being experimented with
with some success.
Largely because of the great size of the open-hearth furnace solid fuel,
such as coal which can be used for puddling furnaces, is not adaptable.
As far back as 1839 attempts were made to gasify coal by burning it to
ash and utilizing the gaseous products for industrial purposes. These
attempts succeeded and the process has been brought to quite a high
state of development. There are to-day a large number of efficient types
of “gas producers” which furnish gas for general industrial use and it
is with this “producer gas” that a great deal of the steel nowadays is
made.
While endeavoring to leave out of these articles most of the chemistry
and as much of the technical detail as is consistent with clearness, the
chemistry of combustion and the “gas producer” is so interesting that it
will be well to explain that carbon (coal, coke, wood, etc.) can burn
either in one or two stages. Nearly every one has noticed the blue flame
with which coal burns in the parlor coal heater or in other furnaces
where little draft is used and most of us remember that the gas which is
given off from such a fire has asphyxiated many who were unfortunate
enough to be sleeping in a closed room, when through insufficient
chimney draft or a leaky stove some of the unburnt gas filled the room.
This gas, which is carbon monoxide, is labeled CO in books on chemistry.
It is the result of burning the coal with insufficient air. Chemically
it is explained by the second of the chemical “equations” which follow.
The third equation explains the second stage of the burning which would
occur were further air or oxygen admitted to the upper part of the
furnace.
_The usual one-stage combustion with plenty of air_:
1. C (carbon) + 2O (oxygen) burns to CO_{2} (carbon dioxide).
Non-poisonous.
_The two-stage combustion with insufficient air_:
2. C + O burns to CO (carbon monoxide). Poisonous.
3. CO + O burns to CO_{2}. Non-poisonous.
Carbon monoxide asphyxiates by forming a chemical compound with the
hæmoglobin of the blood, which therefore is prevented from supplying the
body with the oxygen that is required for the sustenance of life.
Carbon dioxide is no such poisonous product, as may be inferred when we
remember that it is the gas with which our carbonated waters are charged
and which is so commonly served with ice cream in ice cream soda.
Now in a gas producer, by maintaining a sufficiently thick bed of
glowing coal and admitting only such amounts of air as will produce
mainly carbon monoxide gas, a product of high burning value is obtained.
A kilogram (2.2 pounds) of carbon in burning from C to CO generates only
2450 calories or heat units, whereas its complete burning to CO_{2}
would give 8080 calories. So by conducting the carbon monoxide gas—the
product of the first stage of the combustion—through brick-lined pipes
to the furnace, and in the latter by addition of air allowing it to burn
to CO_{2}, the greater amount of heat (i. e., 8080 minus 2450 or 5630
calories) is evolved in the furnace. Of course, some of this theoretical
two-thirds which is in this way made available at the furnace is lost
because a little CO_{2} is formed, and always the nitrogen of the air
used greatly dilutes the gas. But there are gains, notably the great
heat which is carried over by the hot gas from the glowing bed of coal
and that from the water-gas which is formed from steam used in the
producer. So, all in all, the gas generated in a “battery” of gas
producers, all of which discharge into one large main or header to
maintain gas of average composition, is quite a satisfactory fuel.