Chapter XIV: Part 14

Whatever be the form into which the steel is to be rolled, it must in
general first be poured from the Bessemer converter in which it is
made into a large clay-lined ladle, and thence cast in vertical
pyramidal ingots. To bring them to a temperature suitable for rolling,
these ingots must be set in heating or soaking furnaces (S 125), and
this should be done as soon as possible after they are cast, both to
lessen the loss of their initial heat, and to make way for the next
succeeding lot of ingots, a matter of great importance, because the
charges of steel follow each other at such very brief intervals. A
pair of working converters has made 4958 charges of 10 tons each, or a
total of 50,547 tons, in one month, or at an average rate of a charge
every seven minutes and twenty-four seconds throughout every working
day. It is this extraordinary rapidity that makes the process so
economical and determines the way in which its details must be carried
out. Moreover, since the mould acts as a covering to retard the loss
of heat, it should not be removed from the ingot until just before the
latter is to be placed in its soaking furnace. These conditions are
fulfilled by the car casting system of F. W. Wood, of Sparrows Point,
Md., in which the moulds, while receiving the steel, stand on a train
of cars, which are immediately run to the side of the soaking furnace.
Here, as soon as the ingots have so far solidified that they can be
lifted without breaking, their moulds are removed and set on an
adjoining train of cars, and the ingots are charged directly into the
soaking furnace. The mould-train now carries its empty moulds to a
cooling yard, and, as soon as they are cool enough to be used again,
carries them back to the neighbourhood of the converters to receive a
new lot of steel. In this system there is for each ingot and each
mould only one handling in which it is moved as a separate unit, the
mould from one train to the other, the ingot from its train into the
furnace. In the other movements, all the moulds and ingots of a given
charge of steel are grouped as a train, which is moved as a unit by a
locomotive. The difficulty in the way of this system was that, in
pouring the steel from ladle to mould, more or less of it occasionally
spatters, and these spatterings, if they strike the rails or the
running gear of the cars, obstruct and foul them, preventing the
movement of the train, because the solidified steel is extremely
tenacious. But this cannot be tolerated, because the economy of the
process requires extreme promptness in each of its steps. On account
of this difficulty the moulds formerly stood, not on cars, but
directly on the floor of a casting pit while receiving the molten
steel. When the ingots had so far solidified that they could be
handled, the moulds were removed and set on the floor to cool, the
ingots were set on a car and carried to the soaking furnace, and the
moulds were then replaced in the casting pit. Here each mould and each
ingot was handled as a separate unit twice, instead of only once as in
the car casting system; the ingots radiated away great quantities of
heat in passing naked from the converting mill to the soaking
furnaces, and the heat which they and the moulds radiated while in the
converting mill was not only wasted, but made this mill, open-doored
as it was, so intolerably hot, that the cost of labour there was
materially increased. Mr Wood met this difficulty by the simple device
of so shaping the cars that they completely protect both their own
running gear and the track from all possible spattering, a device
which, simple as it is, has materially lessened the cost of the steel
and greatly increased the production. How great the increase has been,
from this and many other causes, is shown in Table III.

TABLE III.--_Maximum Production of Ingots by a Pair of American
Converters._

Gross Tons per Week.
1870 254
1880 3,433
1889 8,549
1899 (average for a month) 11,233
1903 15,704

Thus in thirty-three years the rate of production per pair of vessels
increased more than sixty-fold. The production of European Bessemer
works is very much less than that of American. Indeed, the whole
German production of acid Bessemer steel in 1899 was at a rate but
slightly greater than that here given for one pair of American
converters; and three pairs, if this rate were continued, would make
almost exactly as much steel as all the sixty-five active British
Bessemer converters, acid and basic together, made in 1899.

96. _Range in Size of Converters._--In the Bessemer process, and
indeed in most high-temperature processes, to operate on a large scale
has, in addition to the usual economies which it offers in other
industries, a special one, arising from the fact that from a large hot
furnace or hot mass in general a very much smaller proportion of its
heat dissipates through radiation and like causes than from a smaller
body, just as a thin red-hot wire cools in the air much faster than a
thick bar equally hot. Hence the progressive increase which has
occurred in the size of converters, until now some of them can treat a
20-ton charge, is not surprising. But, on the other hand, when only a
relatively small quantity of a special kind of steel is needed, very
much smaller charges, in some cases weighing even less than half a
ton, have been treated with technical success.

97. _The Bessemer Process for making Steel Castings._--This has been
particularly true in the manufacture of steel castings, i.e. objects
usually of more or less intricate shape, which are cast initially in
the form in which they are to be used, instead of being forged or
rolled to that form from steel cast originally in ingots. For making
castings, especially those which are so thin and intricate that, in
order that the molten steel may remain molten long enough to run into
the thin parts of the mould, it must be heated initially very far
above its melting-point, the Bessemer process has a very great
advantage in that it can develop a much higher temperature than is
attainable in either of its competitors, the crucible and the
open-hearth processes. Indeed, no limit has yet been found to the
temperature which can be reached, if matters are so arranged that not
only the carbon and silicon of the pig iron, but also a considerable
part of the metallic iron which is the iron itself, are oxidized by
the blast; or if, as in the Walrand-Legenisel modification, after the
combustion of the initial carbon and silicon of the pig iron has
already raised the charge to a very high temperature, a still further
rise of temperature is brought about by adding more silicon in the
form of ferro-silicon, and oxidizing it by further blowing. But in the
crucible and the open-hearth processes the temperature attainable is
limited by the danger of melting the furnace itself, both because some
essential parts of it, which, unfortunately, are of a destructible
shape, are placed most unfavourably in that they are surrounded by the
heat on all sides, and because the furnace is necessarily hotter than
the steel made within it. But no part of the Bessemer converter is of
a shape easily affected by the heat, no part of it is exposed to the
heat on more than one side, and the converter itself is necessarily
cooler than the metal within it, because the heat is generated within
the metal itself by the combustion of its silicon and other calorific
elements. In it the steel heats the converter, whereas in the
open-hearth and crucible processes the furnace heats the steel.

98. The _open-hearth process_ consists in making molten steel out of pig or cast iron and "scrap," i.e. waste pieces of steel and iron melted together on the "open hearth," i.e. the uncovered basin-like bottom of a reverberatory furnace, under conditions of which fig. 18 may give a general idea. The conversion of cast iron into steel, of course, consists in lessening its content of the several foreign elements, carbon, silicon, phosphorus, &c. The open-hearth process does this by two distinct steps: (1) by oxidizing and removing these elements by means of the flame of the furnace, usually aided by the oxygen of light charges of iron ore, and (2) by diluting them with scrap steel or its equivalent. The "pig and ore" or "Siemens" variety of the process works chiefly by oxidation, the "pig and scrap" or "Siemens-Martin" variety chiefly by dilution, sometimes indeed by extreme dilution, as when 10 parts of cast iron are diluted with 90 parts of scrap. Both varieties may be carried out in the basic and dephosphorizing way, i.e. in presence of a basic slag and in a basic- or neutral-lined furnace; or in the acid and undephosphorizing way, in presence of an acid, i.e. silicious slag, and in a furnace with a silicious lining.

Half Section showing condition of charge when boiling very gently.

Half Section showing condition of charge when boiling violently during
oreing.]

The charge may be melted down on the "open hearth" itself, or, as in the more advanced practice, the pig iron may be brought in the molten state from the blast furnace in which it is made. Then the furnaceman, controlling the decarburization and purification of the molten charge by his examination of test ingots taken from time to time, gradually oxidizes and so removes the foreign elements, and thus brings the metal simultaneously to approximately the composition needed and to a temperature far enough above its present melting-point to permit of its being cast into ingots or other castings. He then pours or taps the molten charge from the furnace into a large clay-lined casting ladle, giving it the final additions of manganese, usually with carbon and often with silicon, needed to give it exactly the desired composition. He then casts it into its final form through a nozzle in the bottom of the casting ladle, as in the Bessemer process.

The oxidation of the foreign elements must be very slow, lest the effervescence due to the escape of carbonic oxide from the carbon of the metal throw the charge out of the doors and ports of the furnace, which itself must be shallow in order to hold the flame down close to the charge. It is in large part because of this shallowness, which contrasts so strongly with the height and roominess of the Bessemer converter, that the process lasts hours where the Bessemer process lasts minutes, though there is the further difference that in the open-hearth process the transfer of heat from flame to charge through the intervening layer of slag is necessarily slow, whereas in the Bessemer process the heat, generated as it is in and by the metallic bath itself, raises the temperature very rapidly. The slowness of this rise of the temperature compels us to make the removal of the carbon slow for a very simple reason. That removal progressively raises the melting-point of the metal, after line Aa of Fig. 1, i.e. makes the charge more and more infusible; and this progressive rise of the melting-point of the charge must not be allowed to outrun the actual rise of temperature, or in other words the charge must always be kept molten, because once solidified it is very hard to remelt. Thus the necessary slowness of the heating up of the molten charge would compel us to make the removal of the carbon slow, even if this slowness were not already forced on us by the danger of having the charge froth so much as to run out of the furnace.

The general plan of the open-hearth process was certainly conceived by Josiah Marshall Heath in 1845, if not indeed by Reaumur in 1722, but for lack of a furnace in which a high enough temperature could be generated it could not be carried out until the development of the Siemens regenerative gas furnace about 1860. It was in large part through the efforts of Le Chatelier that this process, so long conceived, was at last, in 1864, put into actual use by the brothers Martin, of Sireuil in France.

99. _Siemens Open-Hearth Furnace._--These furnaces are usually
stationary, but in that shown in figs. 19 to 22 the working chamber or
furnace body, G of fig. 22, rotates about its own axis, rolling on the
rollers M shown in fig. 21. In this working chamber, a long
quasi-cylindrical vessel of brickwork, heated by burning within it
pre-heated gas with pre-heated air, the charge is melted and brought
to the desired composition and temperature. The working chamber indeed
is the furnace proper, in which the whole of the open-hearth process
is carried out, and the function of all the rest of the apparatus,
apart from the tilting mechanism, is simply to pre-heat the air and
gas, and to lead them to the furnace proper and thence to the chimney.
How this is done may be understood more easily if figs. 19 and 20 are
regarded for a moment as forming a single diagrammatic figure instead
of sections in different planes. The unbroken arrows show the
direction of the incoming gas and air, the broken ones the direction
of the escaping products of their combustion. The air and gas, the
latter coming from the gas producers or other source, arrive through H
and J respectively, and their path thence is determined by the
position of the reversing valves K and K'. In the position shown in
solid lines, these valves deflect the air and gas into the left-hand
pair of "regenerators" or spacious heat-transferring chambers. In
these, bricks in great numbers are piled loosely, in such a way that,
while they leave ample passage for the gas and air, yet they offer to
them a very great extent of surface, and therefore readily transfer to
them the heat which they have as readily sucked out of the escaping
products of combustion in the last preceding phase. The gas and air
thus separately pre-heated to about 1100 deg. C. (2012 deg. F.) rise
thence as two separate streams through the uptakes (fig. 22), and
first mix at the moment of entering the working chamber through the
ports L and L' (fig. 19). As they are so hot at starting, their
combustion of course yields a very much higher temperature than if
they had been cold before burning, and they form an enormous flame,
which fills the great working chamber. The products of combustion are
sucked by the pull of the chimney through the farther or right-hand
end of this chamber, out through the exit ports, as shown by the
dotted arrows, down through the right-hand pair of regenerators,
heating to perhaps 1300 deg. C. the upper part of the loosely-piled
masses of brickwork within them, and thence past the valves K and K'
to the chimney-flue O. During this phase the incoming gas and air have
been withdrawing heat from the left-hand regenerators, which have thus
been cooling down, while the escaping products of combustion have been
depositing heat in the right-hand pair of regenerators, which have
thus been heating up. After some thirty minutes this condition of
things is reversed by turning the valves K and K' 90 deg. into the
positions shown in dotted lines, when they deflect the incoming gas
and air into the right-hand regenerators, so that they may absorb in
passing the heat which has just been stored there; thence they pass up
through the right-hand uptakes and ports into the working chamber,
where as before they mix, burn and heat the charge. Thence they are
sucked out by the chimney-draught through the left-hand ports, down
through the uptakes and regenerators, here again meeting and heating
the loose mass of "regenerator" brickwork, and finally escape by the
chimney-flue O. After another thirty minutes the current is again
reversed to its initial direction, and so on. These regenerators are
the essence of the Siemens or "regenerative furnace"; they are
heat-traps, catching and storing by their enormous surface of
brickwork the heat of the escaping products of combustion, and in the
following phase restoring the heat to the entering air and gas. At any
given moment one pair of regenerators is storing heat, while the other
is restoring it.

Figs. 19 to 22.--Diagrammatic Sections of Tilting Siemens Furnace.
G, Furnace body.
H, Air supply.
J, Gas supply.
K, Air reversing valve.
K', Gas reversing valve.
L, Air port.
L', Gas port.
M, Rollers on which the furnace tilts.
N, Hydraulic cylinder for tilting the furnace.
O, Flue leading to chimney.
P, Slag pockets.
R, Charging boxes.
W, Water-cooled joints between furnace proper, G, and ports L, L'.]

The tilting working chamber is connected with the stationary ports L
and L' by means of the loose water-cooled joint W in Campbell's
system, which is here shown. The furnace, resting on the rollers M, is
tilted by the hydraulic cylinder N. The slag-pockets P (fig. 22),
below the uptakes, are provided to catch the dust carried out of the
furnace proper by the escaping products of combustion, lest it enter
and choke the regenerators. Wellman's tilting furnace rolls on a fixed
rack instead of on rollers. By his charging system a charge of as much
as fifty tons is quickly introduced. The metal is packed by unskilled
labourers in iron boxes, R (fig. 21), standing on cars in the
stock-yard. A locomotive carries a train of these cars to the track
running beside a long line of open-hearth furnaces. Here the charging
machine lifts one box at a time from its car, pushes it through the
momentarily opened furnace door, and empties the metal upon the hearth
of the furnace by inverting the box, which it then replaces on its
car.

100. The proportion of pig to scrap used depends chiefly on the
relative cost of these two materials, but sometimes in part also on
the carbon content which the resultant steel is to have. Thus part at
least of the carbon which a high-carbon steel is to contain may be
supplied by the pig iron from which it is made. The length of the
process increases with the proportion of pig used. Thus in the
Westphalian pig and scrap practice, scrap usually forms 75 or even 80%
of the charge, and pig only from 20 to 25%, indeed only enough to
supply the carbon inevitably burnt out in melting the charge and
heating it up to a proper casting temperature; and here the charge
lasts only about 6 hours. In some British and Swedish "pig and ore"
practice (S 98), on the other hand, little or no scrap is used, and
here the removal of the large quantity of carbon, silicon and
phosphorus prolongs the process to 17 hours. The common practice in
the United States is to use about equal parts of pig and scrap, and
here the usual length of a charge is about ll(1/2) hours. The pig and
ore process is held back, first by the large quantity of carbon, and
usually of silicon and phosphorus, to be removed, and second by the
necessary slowness of their removal. The gangue of the ore increases
the quantity of slag, which separates the metal from the source of its
heat, the flame, and thus delays the rise of temperature; and the
purification by "oreing," i.e. by means of the oxygen of the large
lumps of cold iron ore thrown in by hand, is extremely slow, because
the ore must be fed in very slowly lest it chill the metal both
directly and because the reaction by which it removes the carbon of
the metal, Fe2O3 + C = 2FeO + CO, itself absorbs heat. Indeed, this
local cooling aggravates the frothing. A cold lump of ore chills the
slag immediately around it, just where its oxygen, reacting on the
carbon of the metal, generates carbonic oxide; the slag becomes cool,
viscous, and hence easily made to froth, just where the froth-causing
gas is evolved.

The length of these varieties of the process just given refers to the
basic procedure. The acid process goes on much faster, because in it
the heat insulating layer of slag is much thinner. For instance it
lasts only about 8(1/2) hours when equal parts of pig and scrap are
used, instead of the 11(1/2) hours of the basic process. Thus the
actual cost of conversion by the acid process is materially less than
by the basic, but this difference is more than outweighed in most
places by the greater cost of pig and scrap free enough from
phosphorus to be used in the undephosphorizing acid process.

101. _Three special varieties of the open-hearth process_, the
Bertrand-Thiel, the Talbot and the Monell, deserve notice. Bertrand
and Thiel oxidize the carbon of molten cast iron by pouring it into a
bath of molten iron which has first been oxygenated, i.e. charged with
oxygen, and superheated, in an open-hearth furnace. The two metallic
masses coalesce, and the reaction between the oxygen of one and the
carbon of the other is therefore extremely rapid because it occurs
throughout their depth, whereas in common procedure oxidation occurs
only at the upper surface of the bath of cast iron at its contact with
the overlying slag. Moreover, since local cooling, with its consequent
viscosity and tendency to froth, are avoided, the frothing is not
excessive in spite of the rapidity of the reaction. The oxygenated
metal is prepared by melting cast iron diluted with as much scrap
steel as is available, and oxidizing it with the flame and with iron
ore as it lies in a thin molten layer on the hearth of a large
open-hearth furnace; the thinness of the layer hastens the oxidation,
and the large size of the furnace permits considerable frothing. But
the oxygenated metal might be prepared easily in a Bessemer converter.

To enlarge the scale of operations makes strongly for economy in the
open-hearth process as in other high temperature ones. Yet the use of
an open-hearth furnace of very great capacity, say of 200 tons per
charge, has the disadvantage that such very large lots of steel,
delivered at relatively long intervals, are less readily managed in
the subsequent operations of soaking and rolling down to the final
shape, than smaller lots delivered at shorter intervals. To meet this
difficulty Mr B. Talbot carries on the process as a quasi-continuous
instead of an intermittent one, operating on 100-ton or 200-ton lots
of cast iron in such a way as to draw off his steel in 20-ton lots at
relatively short intervals, charging a fresh 20-ton lot of cast iron
to replace each lot of steel thus drawn off, and thus keeping the
furnace full of metal from Monday morning till Saturday night. Besides
minor advantages, this plan has the merit of avoiding an ineffective
period which occurs in common open-hearth procedure just after the
charge of cast iron has been melted down. At this time the slag is
temporarily rich in iron oxide and silica, resulting from the
oxidation of the iron and of its silicon as the charge slowly melts
and trickles down. Such a slag not only corrodes the furnace lining,
but also impedes dephosphorization, because it is irretentive of
phosphorus. Further, the relatively low temperature impedes
decarburization. Clearly, no such period can exist in the continuous
process.

At a relatively low temperature, say 1300 deg. C., the phosphorus of
cast iron oxidizes and is removed much faster than its carbon, while
at a higher temperature, say 1500 deg. C., carbon oxidizes in
preference to phosphorus. It is well to remove this latter element
early, so that when the carbon shall have fallen to the proportion
which the steel is to contain, the steel shall already be free from
phosphorus, and so ready to cast. In common open-hearth procedure,
although the temperature is low early in the process, viz. at the end
of the melting down, dephosphorization is then impeded by the
temporary acidity of the slag, as just explained. At the Carnegie
works Mr Monell gets the two dephosphorizing conditions, low
temperature and basicity of slag, early in the process, by pouring his
molten but relatively cool cast iron upon a layer of pre-heated lime
and iron oxide on the bottom of the open-hearth furnace. The lime and
iron oxide melt, and, in passing up through the overlying metal, the
iron oxide very rapidly oxidizes its phosphorus and thus drags it into
the slag as phosphoric acid. The ebullition from the formation of
carbonic oxide puffs up the resultant phosphoric slag enough to make
most of it run out of the furnace, thus both removing the phosphorus
permanently from danger of being later deoxidized and returned to the
steel, and partly freeing the bath of metal from the heat-insulating
blanket of slag. Yet frothing is not excessive, because the slag is
not, as in common practice, locally chilled and made viscous by cold
lumps of ore.

102. In the _duplex process_ the conversion of the cast iron into
steel is begun in the Bessemer converter and finished in the
open-hearth furnace. In the most promising form of this process an
acid converter and a basic open-hearth furnace are used. In the former
the silicon and part of the carbon are moved rapidly, in the latter
the rest of the carbon and the phosphorus are removed slowly, and the
metal is brought accurately to the proper temperature and composition.
The advantage of this combination is that, by simplifying the
conditions with which the composition of the pig iron has to comply,
it makes the management of the blast furnace easier, and thus lessens
the danger of making "misfit" pig iron, i.e. that which, because it is
not accurately suited to the process for which it is intended, offers
us the dilemma of using it in that process at poor advantage or of
putting it to some other use, a step which often implies serious loss.

For the acid Bessemer process the sulphur-content must be small and
the silicon-content should be constant; for the basic open-hearth
process the content of both silicon and sulphur should be small, a
thing difficult to bring about, because in the blast furnace most of
the conditions which make for small sulphur-content make also for
large silicon-content. In the acid Bessemer process the reason why the
sulphur-content must be small is that the process removes no sulphur;
and the reason why the silicon-content should be constant is that,
because silicon is here the chief source of heat, variations in its
content cause corresponding variations in the temperature, a most
harmful thing because it is essential to the good quality of the steel
that it shall be finished and cast at the proper temperature. It is
true that the use of the "mixer" (S 77) lessens these variations, and
that there are convenient ways of mitigating their effects.
Nevertheless, their harm is not completely done away with. But if the
conversion is only begun in the converter and finished on the
open-hearth, then there is no need of regulating the temperature in
the converter closely, and variations in the silicon-content of the
pig iron thus become almost harmless in this respect. In the basic
open-hearth process, on the other hand, silicon is harmful because the
silica which results from its oxidation not only corrodes the lining
of the furnace but interferes with the removal of the phosphorus, an
essential part of the process. The sulphur-content should be small,
because the removal of this element is both slow and difficult. But if
the silicon of the pig iron is removed by a preliminary treatment in
the Bessemer converter, then its presence in the pig iron is harmless
as regards the open-hearth process. Hence the blast furnace process,
thus freed from the hampering need of controlling accurately the
silicon-content, can be much more effectively guided so as to prevent
the sulphur from entering the pig iron.

Looking at the duplex process in another way, the preliminary
desilicidizing in the Bessemer converter should certainly be an
advantage; but whether it is more profitable to give this treatment in
the converter than in the mixer remains to be seen.

103. In the _cementation process_ bars of wrought iron about 1/2 in. thick are carburized and so converted into high carbon "blister steel," by heating them in contact with charcoal in a closed chamber to about 1000 deg. C. (1832 deg. F.) for from 8 to 11 days. Low-carbon steel might thus be converted into high-carbon steel, but this is not customary. The carbon dissolves in the hot but distinctly solid [gamma]-iron (compare fig. 1) as salt dissolves in water, and works its way towards the centre of the bar by diffusion. When the mass is cooled, the carbon changes over into the condition of cementite as usual, partly interstratified with ferrite in the form of pearlite, partly in the form of envelopes enclosing kernels of this pearlite (see ALLOYS, Pl. fig. 13). Where the carbon, in thus diffusing inwards, meets particles of the slag, a basic ferrous silicate which is always present in wrought iron, it forms carbonic oxide, FeO + C = Fe + CO, which puffs the pliant metal up and forms blisters. Hence the name "blister steel." It was formerly sheared to short lengths and formed into piles, which were then rolled out, perhaps to be resheared and rerolled into bars, known as "single shear" or "double shear" steel according to the number of shearings. But now the chief use for blister steel is for remelting in the crucible process, yielding a product which is asserted so positively, so universally and by such competent witnesses to be not only better but very much better than that made from any other material, that we must believe that it is so, though no clear reason can yet be given why it should be. For long all the best high-carbon steel was made by remelting this blister steel in crucibles (S 106), but in the last few years the electric processes have begun to make this steel (S 108).

104. _Case Hardening._--The many steel objects which need an extremely hard outer surface but a softer and more malleable interior may be carburized superficially by heating them in contact with charcoal or other carbonaceous matter, for instance for between 5 and 48 hours at a temperature of 800 deg. to 900 deg. C. This is known as "case hardening." After this carburizing these objects are usually hardened by quenching in cold water (see S 28).

105. _Deep Carburizing; Harvey and Krupp Processes._--Much of the heavy side armour of war-vessels (see ARMOUR-PLATE) is made of nickel steel initially containing so little carbon that it cannot be hardened, i.e. that it remains very ductile even after sudden cooling. The impact face of these plates is given the intense hardness needed by being converted into high-carbon steel, and then hardened by sudden cooling. The impact face is thus carburized to a depth of about 1(1/4) in. by being held at a temperature of 1100 deg. for about a week, pressed strongly against a bed of charcoal (Harvey process). The plate is then by Krupp's process heated so that its impact face is above while its rear is below the hardening temperature, and the whole is then cooled suddenly with sprays of cold water. Under these conditions the hardness, which is very extreme at the impact face, shades off toward the back, till at about quarter way from face to back all hardening ceases, and the rest of the plate is in a very strong, shock-resisting state. Thanks to the glass-hardness of this face, the projectile is arrested so abruptly that it is shattered, and its energy is delivered piecemeal by its fragments; but as the face is integrally united with the unhardened, ductile and slightly yielding interior and back, the plate, even if it is locally bent backwards somewhat by the blow, neither cracks nor flakes.

106. The _crucible process_ consists essentially in melting one or another variety of iron or steel in small 80-lb. charges in closed crucibles, and then casting it into ingots or other castings, though in addition the metal while melting may be carburized. Its chief, indeed almost its sole use, is for making tool steel, the best kinds of spring steel and other very excellent kinds of high-carbon and alloy steel. After the charge has been fully melted, it is held in the molten state from 30 to 60 minutes. This enables it to take up enough silicon from the walls of the crucible to prevent the evolution of gas during solidification, and the consequent formation of blowholes or internal gas bubbles. In Great Britain the charge usually consists of blister steel, and is therefore high in carbon, so that the crucible process has very little to do except to melt the charge. In the United States the charge usually consists chiefly of wrought iron, and in melting in the crucible it is carburized by mixing with it either charcoal or "washed metal," a very pure cast iron made by the Bell-Krupp process (S 107).

Compared with the Bessemer process, which converts a charge of even as
much as 20 tons of pig iron into steel in a few minutes, and the
open-hearth process which easily treats charges of 75 tons, the
crucible process is, of course, a most expensive one, with its little
80-lb. charges, melted with great consumption of fuel because the heat
is kept away from the metal by the walls of the crucible, themselves
excellent heat insulators. But it survives simply because crucible
steel is very much better than either Bessemer or open-hearth steel.
This in turn is in part because of the greater care which can be used
in making these small lots, but probably in chief part because the
crucible process excludes the atmospheric nitrogen, which injures the
metal, and because it gives a good opportunity for the suspended slag
and iron oxide to rise to the surface. Till Huntsman developed the
crucible process in 1740, the only kinds of steel of commercial
importance were blister steel made by carburizing wrought iron without
fusion, and others which like it were greatly injured by the presence
of particles of slag. Huntsman showed that the mere act of freeing
these slag-bearing steels from their slag by melting them in closed
crucibles greatly improved them. It is true that Reaumur in 1722
described his method of making molten steel in crucibles, and that the
Hindus have for centuries done this on a small scale, though they let
the molten steel resolidify in the crucible. Nevertheless, it is to
Huntsman that the world is immediately indebted for the crucible
process. He could make only high-carbon steel, because he could not
develop within his closed crucibles the temperature needed for melting
low-carbon steel. The crucible process remained the only one by which
slagless steel could be made, till Bessemer, by his astonishing
invention, discovered at once low-carbon steel and a process for
making both it and high-carbon steel extremely cheaply.

107. In the _Bell-Krupp_ or "pig-washing" process, invented independently by the famous British iron-master, Sir Lowthian Bell, and Krupp of Essen, advantage is taken of the fact that, at a relatively low temperature, probably a little above 1200 deg. C., the phosphorus and silicon of molten cast iron are quickly oxidized and removed by contact with molten iron oxide, though carbon is thus oxidized but slowly. By rapidly stirring molten iron oxide into molten pig iron in a furnace shaped like a saucer, slightly inclined and turning around its axis, at a temperature but little above the melting-point of the metal itself, the phosphorus and silicon are removed rapidly, without removing much of the carbon, and by this means an extremely pure cast iron is made. This is used in the crucible process as a convenient source of the carbon needed for high-carbon steel.

108. _Electric steel-making processes_, or more accurately processes in which electrically heated furnaces are used, have developed very rapidly. In steel-making, electric furnaces are used for two distinct purposes, first for making steel sufficiently better than Bessemer and open-hearth steels to replace these for certain important purposes, and second for replacing the very expensive crucible process for making the very best steel. The advantages of the electric furnaces for these purposes can best be understood after examining the furnaces themselves and the way in which they are used. The most important ones are either "arc" furnaces, i.e. those heated by electric arcs, or "induction" ones, i.e. those in which the metal under treatment is heated by its own resistance to a current of electricity induced in it from without. The Heroult furnace, the best known in the arc class, and the Kjellin and Roechling-Rodenhauser furnaces, the best known of the induction class, will serve as examples.

The Heroult furnace (fig. 23) is practically a large closed crucible,
ABCA, with two carbon electrodes, E and F, "in series" with the bath,
H, of molten steel. A pair of electric arcs play between these
electrodes and the molten steel, passing through the layer of slag, G,
and generating much heat. The lining of the crucible may be of either
magnesite (MgO) or chromite (FeO.Cr2O3). The whole furnace, electrodes
and all, rotates about the line KL for the purpose of pouring out the
molten slag and purified metal through the spout J at the end of the
process. This spout and the charging doors A, A are kept closed except
when in actual use for pouring or charging.

The Kjellin furnace consists essentially of an annular trough, AA
(fig. 24), which contains the molten charge. This charge is heated,
like the filaments of a common household electric lamp, by the
resistance which it offers to the passage of a current of electricity
induced in it by means of the core C and the frame EEE. The ends of
this core are connected above, below and at the right of the trough A,
by means of that frame, so that the trough and this core and frame
stand to each other in a position like that of two successive links of
a common oval-linked chain. A current of great electromotive force
(intensity or voltage) passed through the coil D, induces, by means of
the core and frame, a current of enormous quantity (volume or
amperage), but very small electromotive force, in the metal in the
trough. Thus the apparatus is analogous to the common transformers
used for inducing from currents of great electromotive force and small
quantity, which carry energy through long distances, currents of great
quantity and small electromotive force for incandescent lights and for
welding. The molten metal in the Kjellin trough forms the "secondary"
circuit. Like the Heroult furnace, the Kjellin furnace may be lined
with either magnesite or chromite, and it may be tilted for the
purpose of pouring off slag and metal.

The shape which the molten metal under treatment has in the Kjellin
furnace, a thin ring of large diameter, is evidently bad, inconvenient
for manipulation and with excessive heat-radiating surface. In the
Roechling-Rodenhauser induction furnace (fig. 25), the molten metal
lies chiefly in a large compact mass A, heated at three places on its
periphery by the current induced in it there by means of the three
coils and cores CCC. The molten metal also extends round each of these
three coils, in the narrow channels B. It is in the metal in these
channels and in that part of the main mass of metal which immediately
adjoins the coils that the current is induced by means of the coils
and cores, as in the Kjellin furnace.

When the Heroult furnace is used for completing the purification of
molten steel begun in the Bessemer or open-hearth process, and this is
its most appropriate use, the process carried out in it may be divided
into two stages, first dephosphorization, and second deoxidation and
desulphurization.

In the first stage the phosphorus is removed from the molten steel by
oxidizing it to phosphoric acid, P2O5, by means of iron oxide
contained in a molten slag very rich in lime, and hence very basic and
retentive of that phosphoric acid. This slag is formed by melting lime
and iron oxide, with a little silica sand if need be. Floating on top
of the molten metal, it rapidly oxidizes its phosphorus, and the
resultant phosphoric acid combines with the lime in the overlying slag
as phosphate of lime. When the removal of the phosphorus is
sufficiently complete, this slag is withdrawn from the furnace.

Next comes the deoxidizing and desulphurizing stage, of which the
first step is to throw some strongly deoxidizing substance, such as
coke or ferro-silicon, upon the molten metal, in order to remove thus
the chief part of the oxygen which it has taken up during the
oxidation of the phosphorus in the preceding stage. Next the metal is
covered with a very basic slag, made by melting lime with a little
silica and fluor spar. Coke now charged into this slag first
deoxidizes any iron oxide contained in either slag or metal, and next
deoxidizes part of the lime of the slag and thus forms calcium, which,
uniting with the sulphur present in the molten metal, forms calcium
sulphide, CaO + FeS + C = CaS + Fe + CO. This sulphide is nearly
insoluble in the metal, but is readily soluble in the overlying basic
slag, into which it therefore passes. The thorough removal of the
sulphur is thus brought about by the deoxidation of the calcium. It is
by forming calcium sulphide that sulphur is removed in the manufacture
of pig iron in the iron blast furnace, in the crucible of which, as in
the electric furnaces, the conditions are strongly deoxidizing. But in
the Bessemer and open-hearth processes this means of removing sulphur
cannot be used, because in each of them there is always enough oxygen
in the atmosphere to re-oxidize any calcium as fast as it is
deoxidized. Here sulphur may indeed be removed to a very important
degree in the form of manganese sulphide, which distributes itself
between metal and slag in rough accord with the laws of equilibrium.
But if we rely on this means we have difficulty in reducing the
sulphur content of the metal to 0.03% and very great difficulty in
reducing it to 0.02%, whereas with the calcium sulphide of the
electric furnaces we can readily reduce it to less than 0.01%.

When the desulphurization is sufficiently complete, the
sulphur-bearing slag is removed, the final additions needed to give
the metal exactly the composition aimed at are made, and the molten
steel is tapped out of the furnace into its moulds. If the initial
quantity of phosphorus or sulphur is large, or if the removal of these
impurities is to be made very thorough, the dephosphorizing or the
desulphurizing slagging off may be repeated. While the metal lies
tranquilly on the bottom of the furnace, any slag mechanically
suspended in it has a chance to rise to the surface and unite with the
slag layer above.

In addition to this work of purification, the furnace may be used for
melting down the initial charge of cold metal, and for beginning the
purification--in short not only for finishing but also for roughing.
But this is rarely expedient, because electricity is so expensive that
it should be used for doing only those things which cannot be
accomplished by any other and cheaper means. The melting can be done
much more cheaply in a cupola or open-hearth furnace, and the first
part of the purification much more cheaply in a Bessemer converter or
open-hearth furnace.

The normal use of the Kjellin induction furnace is to do the work
usually done in the crucible process, i.e. to melt down very pure iron
for the manufacture of the best kinds of steel, such as fine tool and
spring steel, and to bring the molten metal simultaneously to the
exact composition and temperature at which it should be cast into its
moulds. This furnace may be used also for purifying the molten metal,
but it is not so well suited as the arc furnaces for dephosphorizing.
The reason for this is that in it the slag, by means of which all the
purification must needs be done, is not heated effectively; that hence
it is not readily made thoroughly liquid; that hence the removal of
the phosphoric slag made in the early dephosphorizing stage of the
process is liable to be incomplete; and that hence, finally, the
phosphorus of any of this slag which is left in the furnace becomes
deoxidized during the second or deoxidizing stage, and is thereby
returned to befoul the underlying steel. The reason why the slag is
not heated effectively is that the heat is developed only in the layer
of metal itself, by its resistance to the induced current, and hence
the only heat which the slag receives is that supplied to its lower
surface by the metal, while its upper side is constantly radiating
heat away towards the relatively cool roof above.

The Roechling-Rodenhauser furnace is unfitted, by the vulnerability of
its interior walls, for receiving charges of cold metal to be melted
down, but it is used to good advantage for purifying molten basic
Bessemer steel sufficiently to fit it for use in the form of railway
rails.

We are now in a position to understand why electricity should be used as a source of heat in making molten steel. Electric furnaces are at an advantage over others as regards the removal of sulphur and of iron oxide from the molten steel, because their atmosphere is free from the sulphur always present in the flame of coal-fired furnaces, and almost free from oxygen, because this element is quickly absorbed by the carbon and silicon of the steel, and in the case of arc furnaces by the carbon of the electrodes themselves, and is replaced only very slowly by leakage, whereas through the Bessemer converter and the open-hearth furnace a torrent of air is always rushing. As we have seen, the removal of sulphur can be made complete only by deoxidizing calcium, and this cannot be done if much oxygen is present. Indeed, the freedom of the atmosphere of the electric furnaces from oxygen is also the reason indirectly why the molten metal can be freed from mechanically suspended slag more perfectly in them than in the Bessemer converter or the open-hearth furnace. In order that this finely divided slag shall rise to the surface and there coalesce with the overlying layer, the metal must be tranquil. But tranquillity is clearly impossible in the Bessemer converter, in which the metal can be kept hot only by being torn into a spray by the blast. It is practically unattainable in the open-hearth furnace, because here the oxygen of the furnace atmosphere indirectly oxidizes the carbon of the metal which is kept boiling by the escape of the resultant carbonic oxide. In short the electric furnaces can be used to improve the molten product of the Bessemer converter and open-hearth furnace, essentially because their atmosphere is free from sulphur and oxygen, and because they can therefore remove sulphur, iron oxide and mechanically suspended slag, more thoroughly than is possible in these older furnaces. They make a better though a dearer steel.

Further, the electric furnaces, e.g. the Kjellin, can be used to replace the crucible melting process (S 106), chiefly because their work is cheaper for two reasons. First, they treat a larger charge, a ton or more, whereas the charge of each crucible is only about 80 pounds. Second, their heat is applied far more economically, directly to the metal itself, whereas in the crucible process the heat is applied most wastefully to the outside of the non-conducting walls of a closed crucible within which the charge to be heated lies. Beyond this sulphur and phosphorus can be removed in the electric furnace, whereas in the crucible process they cannot. In short electric furnaces replace the old crucible furnace primarily because they work more cheaply, though in addition they may be made to yield a better steel than it can.

Thus we see that the purification in these electric furnaces has
nothing to do with electricity. We still use the old familiar
purifying agents, iron oxide, lime and nascent calcium. The
electricity is solely a source of heat, free from the faults of the
older sources which for certain purposes it now replaces. The electric
furnaces are likely to displace the crucible furnaces completely,
because they work both more cheaply and better. They are not likely to
displace either the open-hearth furnace or the Bessemer converter,
because their normal work is only to improve the product of these
older furnaces. Here their use is likely to be limited by its
costliness, because for the great majority of purposes the superiority
of the electrically purified steel is not worth the cost of the
electric purification.

109. _Electric Ore-smelting Processes._--Though the electric processes which have been proposed for extracting the iron from iron ore, with the purpose of displacing the iron blast furnace, have not become important enough to deserve description here, yet it should be possible to devise one which would be useful in a place (if there is one) which has an abundance of water power and iron ore and a local demand for iron, but has not coke, charcoal or bituminous coal suitable for the blast furnace. But this ancient furnace does its fourfold work of deoxidizing, melting, removing the gangue and desulphurizing, so very economically that it is not likely to be driven out in other places until the exhaustion of our coal-fields shall have gone so far as to increase the cost of coke greatly.

110. _Comparison of Steel-making Processes._--When Bessemer discovered that by simply blowing air through molten cast iron rapidly he could make low-carbon steel, which is essentially wrought iron greatly improved by being freed from its essential defect, its necessarily weakening and embrittling slag, the very expensive and exhausting puddling process seemed doomed, unable to survive the time when men should have familiarized themselves with the use of Bessemer steel, and should have developed the evident possibilities of cheapness of the Bessemer process. Nevertheless the use of wrought iron actually continued to increase. The first of the United States decennial censuses to show a decrease in the production of wrought iron was that in 1890, 35 years after the invention of the Bessemer process. It is still in great demand for certain normal purposes for which either great ease in welding or resistance to corrosion by rusting is of great importance; for purposes requiring special forms of extreme ductility which are not so confidently expected in steel; for miscellaneous needs of many users, some ignorant, some very conservative; and for remelting in the crucible process. All the best cutlery and tool steel is made either by the crucible process or in electric furnaces, and indeed all for which any considerable excellence is claimed is supposed to be so made, though often incorrectly. But the great mass of the steel of commerce is made by the Bessemer and the open-hearth processes. Open-hearth steel is generally thought to be better than Bessemer, and the acid variety of each of these two processes is thought to yield a better product than the basic variety. This may not necessarily be true, but the acid variety lends itself more readily to excellence than the basic. A very large proportion of ores cannot be made to yield cast iron either free enough from phosphorus for the acid Bessemer or the acid open-hearth process, neither of which removes that most injurious element, or rich enough in phosphorus for the basic Bessemer process, which must rely on that element as its source of heat. But cast iron for the basic open-hearth process can be made from almost any ore, because its requirements, comparative freedom from silicon and sulphur, depend on the management of the blast-furnace rather than on the composition of the ore, whereas the phosphorus-content of the cast iron depends solely on that of the ore, because nearly all the phosphorus of the ore necessarily passes into the cast iron. Thus the basic open-hearth process is the only one which can make steel from cast iron containing more than 0.10% but less than 1.80% of phosphorus.

The restriction of the basic Bessemer process to pig iron containing at least 1.80% of phosphorus has prevented it from getting a foothold in the United States; the restriction of the acid Bessemer process to pig iron very low in phosphorus, usually to that containing less than 0.10% of that element, has almost driven it out of Germany, has of late retarded, indeed almost stopped, the growth of its use in the United States, and has even caused it to be displaced at the great Duquesne works of the Carnegie Steel Company by the omnivorous basic open-hearth process, the use of which has increased very rapidly. Under most conditions the acid Bessemer process is the cheapest in cost of conversion, the basic Bessemer next, and the acid open-hearth next, though the difference between them is not great. But the crucible process is very much more expensive than any of the others.