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.

[Illustration: FIG. 19.--Section on EF through Furnace and Port Ends.]

[Illustration: FIG. 20.--Plan through Regenerators, Flues and

Reversing Valves.]

[Illustration: FIG. 21.--Section on CD through Body of Furnace.]

[Illustration: FIG. 22.--Section on AB through Uptake, Slag Pocket and

Regenerator.

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.

[Illustration: FIG. 23.--Heroult Double-arc Electric Steel Purifying

Furnace.]

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.

[Illustration: FIG. 24.--Kjellin Induction Electric Steel Melting

Furnace.]

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.

[Illustration: FIG. 25.--Plan of Roechling-Rodenhauser Induction

Electric Furnace.]

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.

Until very lately the Bessemer process, in either its acid or its

basic form, made all of the world's rail steel; but even for this work

it has now begun to be displaced by the basic open-hearth process,

partly because of the fast-increasing scarcity of ores which yield pig

iron low enough in phosphorus for the acid Bessemer process, and

partly because the increase in the speed of trains and in the loads on

the individual engine- and car-wheels has made a demand for rails of a

material better than Bessemer steel.

111. _Iron founding_, i.e. the manufacture of castings of cast iron,

consists essentially in pouring the molten cast iron into moulds, and,

as preparatory steps, melting the cast iron itself and preparing the

moulds. These are usually made of sand containing enough clay to give it

the needed coherence, but of late promising attempts have been made to

use permanent iron moulds. In a very few places the molten cast iron as

it issues from the blast furnace is cast directly in these moulds, but

in general it is allowed to solidify in pigs, and then remelted either

in cupola furnaces or in air furnaces. The cupola furnace (fig. 26) is a

shaft much like a miniature blast furnace, filled from top to bottom by

a column of lumps of coke and of iron. The blast of air forced in

through the tuyeres near the bottom of the furnace burns the coke there,

and the intense heat thus caused melts away the surrounding iron, so

that this column of coke and iron gradually descends; but it is kept at

its full height by feeding more coke and iron at its top, until all the

iron needed for the day's work has thus been charged. As the iron melts

it runs out through a tap hole and spout at the bottom of the furnace,

to be poured into the moulds by means of clay-lined ladles. The air

furnace is a reverberatory furnace like that used for puddling (fig.

14), but larger, and in it the pigs of iron, lying on the bottom or

hearth, are melted down by the flame from the coal which burns in the

firebox. The iron is then held molten till it has grown hot enough for

casting and till enough of its carbon has been burnt away to leave just

the carbon-content desired, and it is then tapped out and poured into

the moulds.

[Illustration: FIG. 26.--Cupola Furnace for Remelting Pig Iron.]

Of the two the cupola is very much the more economical of fuel, thanks

to the direct transfer of heat from the burning coke to the pig iron

with which it is in contact. But this contact both causes the iron to

absorb sulphur from the coke to its great harm, and prevents it from

having any large part of its carbon burnt away, which in many cases

would improve it very greatly by strengthening it. Thus it comes about

that the cupola, because it is so economical, is used for all but the

relatively few cases in which the strengthening of the iron by the

removal of part of its carbon and the prevention of the absorption of

sulphur are so important as to compensate for the greater cost of the

air-furnace melting.

112. _Cast iron for foundry purposes_, i.e. for making castings of cast

iron. Though, as we have seen in S 19, steel is rarely given a

carbon-content greater than 1.50% lest its brittleness should be

excessive, yet cast iron with between 3 and 4% of carbon, the usual cast

iron of the foundry, is very useful. Because of the ease and cheapness

with which, thanks to its fluidity and fusibility (fig. 1), it can be

melted and run even into narrow and intricate moulds, castings made of

it are very often more economical, i.e. they serve a given purpose more

cheaply, in the long run, than either rolled or cast steel, in spite of

their need of being so massive that the brittleness of the material

itself shall be endurable. Indeed this high carbon-content, 3 to 4%, in

practice actually leads to less brittleness than can readily be had with

somewhat less carbon, because with it much of the carbon can easily be

thrown into the relatively harmless state of graphite, whereas if the

carbon amounts to less than 3% it can be brought to this state only with

difficulty. For crushing certain kinds of rock, the hardness of which

cast iron is capable really makes it more valuable, pound for pound,

than steel.

113. _Qualities needed in Cast Iron Castings._--Different kinds of

castings need very different sets of qualities, and the composition of

the cast iron itself must vary from case to case so as to give each the

qualities needed. The iron for a statuette must first of all be very

fluid, so that it will run into every crevice in its mould, and it must

expand in solidifying, so that it shall reproduce accurately every

detail of that mould. The iron for most engineering purposes needs

chiefly to be strong and not excessively brittle. That for the

thin-walled water mains must combine strength with the fluidity needed

to enable it to run freely into its narrow moulds; that for most

machinery must be soft enough to be cut easily to an exact shape; that

for hydraulic cylinders must combine strength with density lest the

water leak through; and that for car-wheels must be intensely hard in

its wearing parts, but in its other parts it must have that

shock-resisting power which can be had only along with great softness.

Though all true cast iron is brittle, in the sense that it is not

usefully malleable, i.e. that it cannot be hammered from one shape into

another, yet its degree of brittleness differs as that of soapstone does

from that of glass, so that there are the intensely hard and brittle

cast irons, and the less brittle ones, softer and unhurt by a shock

which would shiver the former.

Of these several qualities which cast iron may have, fluidity is given

by keeping the sulphur-content low and phosphorus-content high; and this

latter element must be kept low if shock is to be resisted; but

strength, hardness, endurance of shock, density and expansion in

solidifying are controlled essentially by the distribution of the carbon

between the states of graphite and cementite, and this in turn is

controlled chiefly by the proportion of silicon, manganese and sulphur

present, and in many cases by the rate of cooling.

114. _Constitution of Cast Iron._--Cast iron naturally has a high

carbon-content, usually between 3 and 4%, because while molten it

absorbs carbon greedily from the coke with which it is in contact in

the iron blast furnace in which it is made, and in the cupola furnace

in which it is remelted for making most castings. This carbon may all

be present as graphite, as in typical grey cast iron; or all present

as cementite, Fe3C, as in typical white cast iron; or, as is far more

usual, part of it may be present as graphite and part as cementite.

Now how does it come about that the distribution of the carbon between

these very unlike states determines the strength, hardness and many

other valuable properties of the metal as a whole? The answer to this

is made easy by a careful study of the effect of this same

distribution on the constitution of the metal, because it is through

controlling this constitution that the condition of the carbon

controls these useful properties. To fix our ideas let us assume that

the iron contains 4% of carbon. If this carbon is all present as

graphite, so that in cooling the graphite-austenite diagram has been

followed strictly (S 26), the constitution is extremely simple;

clearly the mass consists first of a metallic matrix, the carbonless

iron itself with whatever silicon, manganese, phosphorus and sulphur

happen to be present, in short an impure ferrite, encased in which as

a wholly distinct foreign body is the graphite. The primary graphite

(S 26) generally forms a coarse, nearly continuous skeleton of curved

black plates, like those shown in fig. 27; the eutectic graphite is

much finer; while the pro-eutectoid and eutectoid graphite, if they

exist, are probably in very fine particles. We must grasp clearly this

conception of metallic matrix and encased graphite skeleton if we are

to understand this subject.

[Illustration: FIG. 27.--Graphite in Grey Cast Iron.]

Now this matrix itself is equivalent to a very low-carbon steel,

strictly speaking to a carbonless steel, because it consists of pure

ferrite, which is just what such a steel consists of; and the cast

iron as a whole is therefore equivalent to a matrix of very low-carbon

steel in which is encased a skeleton of graphite plates, besides some

very fine scattered particles of graphite.

Next let us imagine that, in a series of cast irons all containing 4%

of carbon, the graphite of the initial skeleton changes gradually into

cementite and thereby becomes part of the matrix, a change which of

course has two aspects, first, a gradual thinning of the graphite

skeleton and a decrease of its continuity, and second, a gradual

introduction of cementite into the originally pure ferrite matrix. By

the time that 0.4% of graphite has thus changed, and in changing has

united with 0.4 X 14 = 5.6% of the iron of the original ferrite

matrix, it will have changed this matrix from pure ferrite into a

mixture of

Cementite 0.4 + 5.6 = 6.0

Ferrite 96.0 - 5.6 = 90.4

----

96.4

The residual graphite skeleton forms 4 - 0.4 = 3.6

----

100.0

But this matrix is itself equivalent to a steel of about 040% of

carbon (more accurately 0.40 X 100 / 96.4 = 0.415%), a rail steel,

because it is of just such a mixture of ferrite and cementite in the

ratio of 90.4 : 6 or 94% and 6%, that such a rail steel consists. The

mass as a whole, then, consists of 96.4 parts of metallic matrix,

which itself is in effect a 0.415% carbon rail steel, weakened and

embrittled by having its continuity broken up by this skeleton of

graphite forming 3.6% of the whole mass by weight, or say 12% by

volume.

As, in succeeding members of this same series of cast irons, more of

the graphite of the initial skeleton changes into cementite and

thereby becomes part of the metallic matrix, so the graphite skeleton

becomes progressively thinner and more discontinuous, and the matrix

richer in cementite and hence in carbon and hence equivalent first to

higher and higher carbon steel, such as tool steel of 1% carbon, file

steel of 1.50%, wire-die steel of 2% carbon and then to white cast

iron, which consists essentially of much cementite with little

ferrite. Eventually, when the whole of the graphite of the skeleton

has changed into cementite, the mass as a whole becomes typical or

ultra white cast iron, consisting of nothing but ferrite and

cementite, distributed as follows (see fig. 2):--

Eutectoid ferrite 40.0

" cementite 6.7

----

" Interstratified as pearlite 46.7

Cementite, primary, eutectoid and pro-eutectoid 53.3

----

100.0

Total ferrite 40.0

Total cementite 60.0

----

100.0

[Illustration: FIG. 28.--Physical Properties and assumed Microscopic

Constitution of Cast Iron containing 4% of carbon, as affected by the

distribution of that carbon between the combined and graphitic

states.]

The constitution and properties of such a series of cast irons, all

containing 4% of carbon but with that carbon shifting progressively

from the state of graphite to that of cementite as we pass from

specimen to specimen, may, with the foregoing picture of a

skeleton-holding matrix clearly in our minds be traced by means of

fig. 28. The change from graphite into cementite is supposed to take

place as we pass from left to right. BC and OH give the proportion of

ferrite and cementite respectively in the matrix, DEF, KS and TU

reproduced from fig. 3 give the consequent properties of the matrix,

and GAF, RS and VU give, partly from conjecture, the properties of the

cast iron as a whole. Above the diagram are given the names of the

different classes of cast iron to which different stages in the change

from graphite to cementite correspond, and above these the names of

kinds of steel or cast iron, to which at the corresponding stages the

constitution of the matrix corresponds, while below the diagram are

given the properties of the cast iron as a whole corresponding to

these stages, and still lower the purposes for which these stages fit

the cast iron, first because of its strength and shock-resisting

power, and second because of its hardness.

115. _Influence of the Constitution of Cast Iron on its

Properties._--How should the hardness, strength and ductility, or

rather shock-resisting power, of the cast iron be affected by this

progressive change from graphite into cementite? First, the hardness

(VU) should increase progressively as the soft ferrite and graphite

are replaced by the glass-hard cementite. Second, though the

brittleness should be lessened somewhat by the decrease in the extent

to which the continuity of the strong matrix is broken up by the

graphite skeleton, yet this effect is outweighed greatly by that of

the rapid substitution in the matrix of the brittle cementite for the

very ductile copper-like ferrite, so that the brittleness increases

continuously (RS), from that of the very grey graphitic cast irons,

which, like that of soapstone, is so slight that the metal can endure

severe shock and even indentation without breaking, to that of the

pure white cast iron which is about as brittle as porcelain. Here let

us recognize that what gives this transfer of carbon from graphite

skeleton to metallic matrix such very great influence on the

properties of the metal is the fact that the transfer of each 1% of

carbon means substituting in the matrix no less than 15% of the

brittle, glass-hard cementite for the soft, very ductile ferrite.

Third, the tensile strength of steel proper, of which the matrix

consists, as we have already seen (fig. 3), increases with the

carbon-content till this reaches about 1.25%, and then in turn

decreases (fig. 28, DEF). Hence, as with the progressive transfer of

the carbon from the graphitic to the cementite state in our imaginary

series of cast irons, the combined carbon present in the matrix

increases, so does the tensile strength of the mass as a whole for two

reasons; first, because the strength of the matrix itself is

increasing (DE), and second, because the discontinuity is decreasing

with the decreasing proportion of graphite. With further transfer of

the carbon from the graphitic to the combined state, the matrix itself

grows weaker (EF); but this weakening is offset in a measure by the

continuing decrease of discontinuity due to the decreasing proportion

of graphite. The resultant of these two effects has not yet been well

established; but it is probable that the strongest cast iron has a

little more than 1% of carbon combined as cementite, so that its

matrix is nearly equivalent to the strongest of the steels. As regards

both tensile strength and ductility not only the quantity but the

distribution of the graphite is of great importance. Thus it is

extremely probable that the primary graphite, which forms large

sheets, is much more weakening and embrittling than the eutectic and

other forms, and therefore that, if either strength or ductility is

sought, the metal should be free from primary graphite, i.e. that it

should not be hyper-eutectic.

The presence of graphite has two further and very natural effects.

First, if the skeleton which it forms is continuous, then its planes

of junction with the metallic matrix offer a path of low resistance to

the passage of liquids or gases, or in short they make the metal so

porous as to unfit it for objects like the cylinders of hydraulic

presses, which ought to be gas-tight and water-tight. For such

purposes the graphite-content should be low. Second, the very genesis

of so bulky a substance as the primary and eutectic graphite while the

metal is solidifying (fig. 5) causes a sudden and permanent expansion,

which forces the metal into even the finest crevices in its mould, a

fact which is taken advantage of in making ornamental castings and

others which need great sharpness of detail, by making them rich in

graphite.

To sum this up, as graphite is replaced by carbon combined as

cementite, the hardness, brittleness and density increase, and the

expansion in solidification decreases, in both cases continuously,

while the tensile strength increases till the combined carbon-content

rises a little above 1%, and then in turn decreases. That strength is

good and brittleness bad goes without saying; but here a word is

needed about hardness. The expense of cutting castings accurately to

shape, cutting on them screw threads and what not, called "machining"

in trade parlance, is often a very large part of their total cost; and

it increases rapidly with the hardness of the metal. On the other

hand, the extreme hardness of nearly graphiteless cast iron is of

great value for objects of which the chief duty is to resist abrasion,

such as parts of crushing machinery. Hence objects which need much

machining are made rich in graphite, so that they may be cut easily,

and those of the latter class rich in cementite so that they may not

wear out.

116. _Means of controlling the Constitution of Cast Iron._--The

distribution of the carbon between these two states, so as to give the

cast iron the properties needed, is brought about chiefly by

adjusting the silicon-content, because the presence of this element

favours the formation of graphite. Beyond this, rapid cooling and the

presence of sulphur both oppose the formation of graphite, and hence

in cast iron rich in sulphur, and in thin and therefore rapidly

cooling castings, the silicon-content must be greater than in thick

ones and in those freer from sulphur. Thus thick machinery castings

usually contain between 1.50 and 2.25% of silicon, whereas thin

castings and ornamental ones which must reproduce the finest details

of the mould accurately may have as much as 3 or even 3.40% of it.

Castings which, like hydraulic press cylinders and steam radiators,

must be dense and hence must have but little graphite lest their

contents leak through their walls, should not have more than 1.75% of

silicon and may have even as little as 1% if impenetrability is so

important that softness and consequent ease of machining must be

sacrificed to it. Cast iron railroad car-wheels, the tread or rim of

which must be intensely hard so as to endure the grinding action of

the brakeshoe while their central parts must have good shock-resisting

power, are given such moderate silicon-content, preferably between

0.50 and 0.80%, as in and by itself leaves the tendencies toward

graphite-forming and toward cementite-forming nearly in balance, so

that they are easily controlled by the rate of cooling. The "tread" or

circumferential part of the mould itself is made of iron, because

this, by conducting the heat away from the casting rapidly, makes it

cool quickly, and thus causes most of the carbon here to form

cementite, and thus in turn makes the tread of the wheel intensely

hard; while those parts of the mould which come in contact with the

central parts of the wheel are made of sand, which conducts the heat

away from the molten metal so slowly that it solidifies slowly, with

the result that most of its carbon forms graphite, and here the metal

is soft and shock-resisting.

117. _Influence of Sulphur._--Sulphur has the specific harmful effects

of shifting the carbon from the state of graphite to that of

cementite, and thus of making the metal hard and brittle; of making it

thick and sluggish when molten, so that it does not run freely in the

moulds; and of making it red short, i.e. brittle at a red heat, so

that it is very liable to be torn by the aeolotachic contraction in

cooling from the molten state; and it has no good effects to offset

these. Hence the sulphur present is, except in certain rare cases,

simply that which the metallurgist has been unable to remove. The

sulphur-content should not exceed 0.12%, and it is better that it

should not exceed 0.08 % in castings which have to be soft enough to

be machined, nor 0.05% in thin castings the metal for which must be

very fluid.

118. _Influence of Manganese._--Manganese in many cases, but not in

all, opposes the formation of graphite and thus hardens the iron, and

it lessens the red shortness (S 40), which sulphur causes, by leading

to the formation of the less harmful manganese sulphide instead of the

more harmful iron sulphide. Hence the manganese-content needed

increases with the sulphur-content which has to be endured. In the

better classes of castings it is usually between 0.40 and 0.70%, and

in chilled railroad car-wheels it may well be between 0.15 and 0.30%;

but skilful founders, confronted with the task of making use of cast

iron rich in manganese, have succeeded in making good grey iron

castings with even as much as 2.20% of this element.

119. _Influence of Phosphorus._--Phosphorus has, along with its great

merit of giving fluidity, the grave defect of causing brittleness,

especially under shock. Fortunately its embrittling effect on cast

iron is very much less than on steel, so that the upper limit or

greatest tolerable proportion of phosphorus, instead of being 0.10 or

better 0.08% as in the case of rail steel, may be put at 0.50% in case

of machinery castings even if they are exposed to moderate shocks; at

1.60% for gas and water mains in spite of the gravity of the disasters

which extreme brittleness here might cause; and even higher for

castings which are not exposed to shock, and are so thin that the iron

of which they are made must needs be very fluid. The permissible

phosphorus-content is lessened by the presence of either much sulphur

or much manganese, and by rapid cooling, as for instance in case of

thin castings, because each of these three things, by leading to the

formation of the brittle cementite, in itself creates brittleness

which aggravates that caused by phosphorus.

120. _Defects in Steel Ingots._--Steel ingots and other steel castings

are subject to three kinds of defects so serious as to deserve notice

here. They are known as "piping," "blowholes" and "segregation."

121. _Piping._--In an early period of the solidification of a molten

steel ingot cast in a cold iron mould we may distinguish three parts:

(1) the outer layers, i.e. the outermost of the now solid metal; (2)

the inner layers, i.e. the remainder of the solid metal; and (3) the

molten lake, i.e. the part which still is molten. At this instant the

outer layers, because of their contact with the cold mould, are

cooling much faster than the inner ones, and hence tend to contract

faster. But this excess of their contraction is resisted by the almost

incompressible inner layers so that the outer layers are prevented

from contracting as much as they naturally would if unopposed, and

they are thereby virtually stretched. Later on the cooling of the

inner layers becomes more rapid than that of the outer ones, and on

this account their contraction tends to become greater than that of

the outer ones. Because the outer and inner layers are integrally

united, this excess of contraction of the inner layers makes them draw

outward towards and against the outer layers, and because of their

thus drawing outward the molten lake within no longer suffices to fill

completely the central space, so that its upper surface begins to

sink. This ebb continues, and, combined with the progressive narrowing

of the molten lake as more and more of it solidifies and joins the

shore layers, gives rise to the pipe, a cavity like an inverted pear,

as shown at C in fig. 29. Because this pipe is due to the difference

in the rates of contraction of interior and exterior, it may be

lessened by retarding the cooling of the mass as a whole, and it may

be prevented from stretching down deep by retarding the solidification

of the upper part of the ingot, as, for instance, by preheating the

top of the mould, or by covering the ingot with a mass of burning fuel

or of molten slag. This keeps the upper part of the mass molten, so

that it continues to flow down and feed the pipe during the early part

of its formation in the lower and quicker-cooling part of the ingot.

In making castings of steel this same difficulty arises; and much of

the steel-founder's skill consists either in preventing these pipes,

or in so placing them that they shall not occur in the finished

casting, or at least not in a harmful position. In making

armour-plates from steel ingots, as much as 40% of the metal may be

rejected as unsound from this cause. An ingot should always stand

upright while solidifying, so that the unsound region due to the pipe

may readily be cut off, leaving the rest of the ingot solid. If the

ingot lay on its side while solidifying, the pipe would occur as shown

in fig. 30, and nearly the whole of the ingot would be unsound.

[Illustration: FIG. 29.--Diagram showing how a Pipe is formed.

A, Superficial blowholes.

B, Deep-seated blowholes.

C, Pipe.]

[Illustration: FIG. 30.--Diagram showing a Pipe so formed as to render

Ingot unsound.]

122. _Blowholes._--Iron, like water and many other substances, has a

higher solvent power for gases, such as hydrogen and nitrogen, when

molten, i.e. liquid, than when frozen, i.e. solid. Hence in the act of

solidifying it expels any excess of gas which it has dissolved while

liquid, and this gas becomes entangled in the freezing mass, causing

gas bubbles or _blowholes_, as at A and B in fig. 29. Because the

volume of the pipe represents the excess of the contraction of the

inner walls and the molten lake jointly over that of the outer walls,

between the time when the lake begins to ebb and the time when even

the axial metal is too firm to be drawn further open by this

contraction, the space occupied by blowholes must, by compensating for

part of this excess, lessen the size of the pipe, so that the more

abundant and larger the blowholes are, the smaller will the pipe be.

The interior surface of a blowhole which lies near the outer crust of

the ingot, as at A in fig. 29, is liable to become oxidized by the

diffusion of the atmospheric oxygen, in which case it can hardly be

completely welded later, since welding implies actual contact of metal

with metal; it thus forms a permanent flaw. But deep-seated blowholes

like those at B are relatively harmless in low-carbon easily welding

steel, because the subsequent operation of forging or rolling usually

obliterates them by welding their sides firmly together.

Blowholes may be lessened or even wholly prevented by adding to the

molten metal shortly before it solidifies either silicon or aluminium,

or both; even as little as 0.002% of aluminium is usually sufficient.

These additions seem to act in part by deoxidizing the minute quantity

of iron oxide and carbonic oxide present, in part by increasing the

solvent power of the metal for gas, so that even after freezing it can

retain in solution the gas which it had dissolved when molten. But,

because preventing blowholes increases the volume of the pipe, it is

often better to allow them to form, but to control their position, so

that they shall be deep-seated. This is done chiefly by casting the

steel at a relatively low temperature, and by limiting the quantity of

manganese and silicon which it contains. Brinell finds that, for

certain normal conditions, if the sum of the percentage of manganese

plus 5.2 times that of the silicon equals 1.66, there will be no

blowholes; if this sum is less, blowholes will occur, and will be

injuriously near the surface unless this sum is reduced to 0.28. He

thus finds that this sum should be either as great as 1.66, so that

blowholes shall be absent; or as low as 0.28, so that they shall be

harmlessly deep-seated. These numbers must be varied with the

variations in other conditions, such as casting temperature, rapidity

of solidification, &c.

123. _Segregation._--The solidification of an ingot of steel takes

place gradually from without inwards, and each layer in solidifying

tends to expel into the still molten interior the impurities which it

contains, especially the carbon, phosphorus, and sulphur, which by

this process are in part concentrated or _segregated_ in the

last-freezing part of the ingot. This is in general around the lower

part of the pipe, so that here is a second motive for rejecting the

piped part of the ingot. While segregation injures the metal here,

often fatally, by giving it an indeterminate excess of phosphorus and

sulphur, it clearly purifies the remainder of the ingot, and on this

account it ought, under certain conditions, to be promoted rather than

restrained. The following is an extreme case:--

+------------------+---------+----------+------------+-------------+----------+

| | Carbon. | Silicon. | Manganese. | Phosphorus. | Sulphur. |

+------------------+---------+----------+------------+-------------+----------+

| Composition of | | | | | |

| the initial | | | | | |

| metal per cent | 0.24 | 0.336 | 0.97 | 0.089 | 0.074 |

| Composition of | | | | | |

| the segregate | 1.27 | 0.41 | 1.08 | 0.753 | 0.418 |

+------------------+---------+----------+------------+-------------+----------+

The surprising fact that the degree of segregation does not increase

greatly either with the slowness of solidification or with the size of

the ingot, at least between the limits of 5 in. sq. and 16 in. sq.,

has been explained by the theory that the relative quiet due to the

gentleness of the convection currents in a slowly cooling mass favours

the formation of far outshooting pine-tree crystals, and that the

tangled branches of these crystals landlock much of the littoral

molten mother metal, and thus mechanically impede that centreward

diffusion and convection of the impurities which is the essence of

segregation.

124. _Castings and Forgings._--There are two distinct ways of making the

steel objects actually used in the arts, such as rails, gear wheels,

guns, beams, &c., out of the molten steel made by the Bessemer, open

hearth, or crucible process, or in an electric furnace. The first is by

"steel founding," i.e. casting the steel as a "steel casting" in a mould

which has the exact shape of the object to be made, e.g. a gear wheel,

and letting it solidify there. The second is by casting it into a large

rough block called an "ingot," and rolling or hammering this out into

the desired shape. Though the former certainly seems the simpler way,

yet its technical difficulties are so great that it is in fact much the

more expensive, and therefore it is in general used only in making

objects of a shape hard to give by forging or rolling. These technical

difficulties are due chiefly to the very high melting point of the

metal, nearly 1500 deg. C (2732 deg. F.), and to the consequent great

contraction which it undergoes in cooling through the long range between

this temperature and that of the room. The cooling of the thinner, the

outer, and in general the more exposed parts of the casting outruns that

of the thicker and less exposed parts, with the consequence that, at any

given instant, the different parts are contracting at very different

rates, i.e. aeolotachically; and this aeolotachic contraction is very

likely to concentrate severe stress on the slowest cooling parts at the

time when they are passing from the molten to the solid state, when the

steel is mushy, with neither the fluidity of a liquid nor the strength

and ductility of a solid, and thus to tear it apart. Aeolotachic

contraction further leads to the "pipes" or contraction cavities already

described in S 121, and the procedure must be carefully planned first so

as to reduce these to a minimum, and second so as to induce them to form

either in those parts of the casting which are going to be cut off and

re-melted, or where they will do little harm. These and kindred

difficulties make each new shape or size a new problem, and in

particular they require that for each and every individual casting a new

sand or clay mould shall be made with care by a skilled workman. If a

thousand like gears are to be cast, a thousand moulds must be made up,

at least to an important extent by hand, for even machine moulding

leaves something for careful manipulation by the moulder. It is a

detail, one is tempted to say a retail, manufacture.

In strong contrast with this is the procedure in making rolled products

such as rails and plates. The steel is cast in lots, weighing in some

cases as much as 75 tons, in enduring cast iron moulds into very large

ingots, which with their initial heat are immediately rolled down by a

series of powerful roll trains into their final shape with but slight

wear and tear of the moulds and the machinery. But in addition to the

greater cost of steel founding as compared with rolling there are two

facts which limit the use of steel castings: (1) they are not so good as

rolled products, because the kneading which the metal undergoes in

rolling improves its quality, and closes up its cavities; and (2) it

would be extremely difficult and in most cases impracticable to cast the

metal directly into any of the forms in which the great bulk of the

steel of commerce is needed, such as rails, plates, beams, angles, rods,

bars, and wire, because the metal would become so cool as to solidify

before running far in such thin sections, and because even the short

pieces which could thus be made would pucker or warp on account of their

aeolotachic contraction.

125. _Heating Furnaces_ are used in iron manufacture chiefly for

bringing masses of steel or wrought iron to a temperature proper for

rolling or forging. In order to economize power in these operations, the

metal should in general be as soft and hence as hot as is consistent

with its reaching a low temperature before the rolling or forging is

finished, because, as explained in S 32, undisturbed cooling from a high

temperature injures the metal. Many of the furnaces used for this

heating are in a general way like the puddling furnace shown in fig. 14,

except that they are heated by gas, that the hearth or bottom of the

chamber in which they are heated is nearly flat, and that it is usually

very much larger than that of a puddling furnace. But in addition there

are many special kinds of furnaces arranged to meet the needs of each

case. Of these two will be shown here, the Gjers soaking pit for steel

ingots, and the Eckman or continuous furnace, as modified by C. H.

Morgan for heating billets.

[Illustration: FIG. 31.--Section of Gjers Soaking Pit.]

126. _Gjers Soaking Pit._--When the outer crust of a large ingot in

which a lot of molten steel has been cast has so far cooled that it can

be moved without breaking, the temperature of the interior is still far

above that suitable for rolling or hammering--so far above that the

surplus heat of the interior would more than suffice to reheat the now

cool crust to the rolling temperature, if we could only arrest or even

greatly retard the further escape of heat from that crust. Bringing such

an ingot, then, to the rolling temperature is not really an operation of

heating, because its average temperature is already above the rolling

temperature, but one of equalizing the temperature, by allowing the

internal excess of heat to "soak" through the mass. Gjers did this by

setting the partly-solidified ingot in a well-closed "pit" of brickwork,

preheated by the excess heat of previous lots of ingots. The

arrangement, shown in fig. 31, has three advantages--(1) that the

temperature is adjusted with absolutely no consumption of fuel; (2) that

the waste of iron due to the oxidation of the outer crust of the ingot

is very slight, because the little atmospheric oxygen initially in the

pit is not renewed, whereas in a common heating furnace the flame brings

a constant fresh supply of oxygen; and (3) that the ingot remains

upright during solidification, so that its pipe is concentrated at one

end and is thus removable. (See S 121.) In this form the system is

rather inflexible, for if the supply of ingots is delayed the pits grow

unduly cool, so that the next ensuing lot of ingots either is not heated

hot enough or is delayed too long in soaking. This defect is usually

remedied by heating the pits by the Siemens regenerative system (see S

99); the greater flexibility thus gained outweighs the cost of the fuel

used and the increased loss of iron by oxidation by the Siemens gas

flame.

127. _Continuous Heating Furnace._--The Gjers system is not applicable

to small ingots or "billets,"[5] because they lack the inner surplus

heat of large ingots; indeed, they are now allowed to cool completely.

To heat these on the intermittent plan for further rolling, i.e. to

charge a lot of them as a whole in a heating furnace, bring them as a

whole to rolling temperature, and then withdraw them as a whole for

rolling, is very wasteful of heat, because it is only in the first part

of the heating that the outside of the ingots is cool enough to abstract

thoroughly the heat from the flame. During all the latter part of the

heating, when the temperature of the ingot has approached that of the

flame, only an ever smaller and smaller part of the heat of that flame

can be absorbed by the ingots. Hence in the intermittent system most of

the heat generated within the furnace escapes from it with the products

of combustion. The continuous heating system (fig. 32) recovers this

heat by bringing the flame into contact with successively cooler and

cooler billets, A-F, and finally with quite cold ones, of consequently

great heat-absorbing capacity.

[Illustration: FIG. 32.--Diagram of C. H. Morgan's Continuous Heating

Furnace for 2-inch billets 30 ft. long.

A, Hottest billet ready for rolling.

B, Exit door.

C, Pusher, for forcing billets forward.

D, Water-cooled pipe on which billets are pushed forward.

E, Magnesite bricks on which the hot billets slide forward.

F, The billet last entered.

G, The suspended roof.

H, The incoming air preheated by G and by the pipes N and brought from

above G to between N by a flue not shown.

J, The incoming gas.

L, The flame.

M, The escaping products of combustion.

N, Pipes through which the products of combustion pass.]

As soon as a hot billet A is withdrawn by pushing it endwise out of

the exit door B, the whole row is pushed forward by a set of

mechanical pushers C, the billets sliding on the raised water-cooled

pipes D, and, in the hotter part of the furnace, on the magnesite

bricks E, on which iron slides easily when red-hot. A new cold billet

is then charged at the upper end of the hearth, and the new cycle

begins by pushing out through B a second billet, and so forth. To

lessen the loss in shape of "crop ends," and for general economy,

these billets are in some cases 30 ft. long, as in the furnace shown

in fig. 32. It is to make it wide enough to receive such long billets

that its roof is suspended, as here shown, by two sets of iron

tie-rods. As the foremost end of the billet emerges from the furnace

it enters the first of a series of roll-trains, and passes immediately

thence to others, so that before half of the billet has emerged from

the furnace its front end has already been reduced by rolling to its

final shape, that of merchant-bars, which are relatively thin, round

or square rods, in lengths of 300 ft.

In the intermittent system the waste heat can, it is true, be utilized

either for raising steam (but inefficiently and inconveniently,

because of the intermittency), or by a regenerative method like the

Siemens, fig. 19; but this would probably recover less heat than the

continuous system, first, because it transfers the heat from flame to

metal indirectly instead of directly; and, second, because the

brickwork of the Siemens system is probably a poorer heat-catcher than

the iron billets of the continuous system, because its disadvantages

of low conductivity and low specific heat probably outweigh its

advantages of roughness and porosity.

128. _Rolling, Forging, and Drawing._--The three chief processes for

shaping iron and steel, rolling, forging (i.e. hammering, pressing or

stamping) and drawing, all really proceed by squeezing the metal into

the desired shape. In forging, whether under a hammer or under a press,

the action is evidently a squeeze, however skilfully guided. In drawing,

the pull of the pincers (fig. 33) upon the protruding end, F, of the

rod, transmitted to the still undrawn part, E, squeezes the yielding

metal of the rod against the hard unyielding die, C. As when a

half-opened umbrella is thrust ferrule-foremost between the balusters of

a staircase, so when the rod is drawn forward, its yielding metal is

folded and forced backwards and centrewards by the resistance of the

unyielding die, and thus it is reduced in diameter and simultaneously

lengthened proportionally, without material change of volume or density.

[Illustration: FIG. 33.--Wire undergoing Reduction in the Die.]

[Illustration: FIG. 34.--Two-high Rolling Mill.]

129. _Methods of Rolling._--Of rolling much the same is true. The

rolling mill in its simplest form is a pair of cylindrical rollers, BB

(figs. 34 and 35) turning about their axes in opposite directions as

shown by the arrows, and supported at their ends in strong frames called

"housings," CC (fig. 35). The skin of the object, D, which is undergoing

rolling, technically called "the piece," is drawn forward powerfully by

the friction of the revolving rolls, and especially of that part of

their surface which at any given instant is moving horizontally (HH in

fig. 34), much as, the rod is drawn through the die in fig. 33, while

the vertical component of the motion of the rear part JJ of the rolls

forces the plastic metal of that part of "the piece" with which they are

in contact backwards and centrewards, reducing its area and

simultaneously lengthening it proportionally, here again as in drawing

through a die. The rolls thus both draw the piece forward like the

pincers of a wire die, and themselves are a die which like a river ever

renews or rather maintains its fixed shape and position, though its

particles themselves are moving constantly forward with "the piece"

which is passing between them.

[Illustration: FIG. 35.--Two-high Rolling Mill.]

After the piece has been reduced in thickness by its first passage or

"pass" between the rolls, it may be given a second reduction and then a

third and so on, either by bringing the two rolls nearer together, as in

case of the plain rolls BB at the left in fig. 35, or by passing the

piece through an aperture, F', smaller than the first F, as in case of

the grooved rolls, AA, shown at the right, or by both means jointly. If,

as sketched in fig. 34, the direction in which each of the rolls turns

is constant, then after the piece has passed once through the rolls to

the right, it cannot undergo a second pass till it has been brought back

to its initial position at the left. But bringing it back wastes power

and, still worse, time, heat, and metal, because the yellow- or even

white-hot piece is rapidly cooling down and oxidizing. In order to

prevent this waste the direction in which the rolls move may be

reversed, so that the piece may be reduced a second time in passing to

the left, in which case the rolls are usually driven by a pair of

reversing engines; or the rolls may be "three high," as shown in fig.

36, with the upper and the lower roll moving constantly to the right and

the middle roll constantly to the left, so that the piece first passes

to the right between the middle and lower rolls, and then to the left

between the middle and upper rolls. The advantage of the "reversing"

system is that it avoids lifting the piece from below to above the

middle roll, and again lowering it, which is rather difficult because

the white-hot piece cannot be guided directly by hand, but must be moved

by means of hooks, tongs, or even complex mechanism. The advantage of

the three-high mill is that, because each of its moving parts is always

moving in the same direction, it may be driven by a relatively small and

hence cheap engine, the power delivered by which between the passes is

taken up by a powerful fly-wheel, to be given up to the rolls during the

next pass. (See also ROLLING MILL.)

[Illustration: FIG. 36.--Three-high Rolling Mill.]

130. _Advantages and Applicability of Rolling._--Rolling uses very much

less power than drawing, because the friction against the fixed die in

the latter process is very great. For much the same reason rolling

proceeds much faster than drawing, and on both these accounts it is

incomparably the cheaper of the two. It is also very much cheaper than

forging, in large part because it works so quickly. The piece travels

through the rolls very rapidly, so that the reduction takes place over

its whole length in a very few seconds, whereas in forging, whether

under hammer or press, after one part of the piece has been compressed

the piece must next be raised, moved forward, and placed so that the

hammer or press may compress the next part of its length. This moving is

expensive, because it has to be done, or at least guided, by hand, and

it takes up much time, during which both heat and iron are wasting. Thus

it comes about that rolling is so very much cheaper than either forging

or drawing that these latter processes are used only when rolling is

impracticable. The conditions under which it is impracticable are (1)

when the piece has either an extremely large or an extremely small cross

section, and (2) when its cross section varies materially in different

parts of its length. The number of great shafts for marine engines,

reaching a diameter of 22(1/8) in. in the case of the "Lusitania," is so

small that it would be wasteful to instal for their manufacture the

great and costly rolling mill needed to reduce them from the gigantic

ingots from which they must be made, with its succession of decreasing

passes, and its mechanism for rotating the piece between passes and for

transferring it from pass to pass. Great armour plates can indeed be

made by rolling, because in making such flat plates the ingot is simply

rolled back and forth between a pair of plain cylindrical rolls, like BB

of fig. 35, instead of being transferred from one grooved pass to

another and smaller one. Moreover, a single pair of rolls suffices for

armour plates of any width or thickness, whereas if shafts of different

diameters were to be rolled, a special final groove would be needed for

each different diameter, and, as there is room for only a few large

grooves in a single set of rolls, this would imply not only providing

but installing a separate set of rolls for almost every diameter of

shaft. Finally the quantity of armour plate needed is so enormous that

it justifies the expense of installing a great rolling mill. Krupp's

armour-plate mill, with rolls 4 ft. in diameter and 12 ft. long, can

roll an ingot 4 ft. thick.

Pieces of very small cross section, like wire, are more conveniently

made by drawing through a die than by rolling, essentially because a

single draft reduces the cross section of a wire much more than a single

pass between rolls can. This in turn is because the direct pull of the

pincers on the protruding end of the wire is much stronger than the

forward-drawing pull due to the friction of the cold rolls on the wire,

which is necessarily cold because of its small section.

Pieces which vary materially in cross section from point to point in

their length cannot well be made by rolling, because the cross section

of the piece as it emerges from the rolls is necessarily that of the

aperture between the rolls from which it is emerging, and this aperture

is naturally of constant size because the rolls are cylindrical. Of

course, by making the rolls eccentric, and by varying the depth and

shape of the different parts of a given groove cut in their surface, the

cross section of the piece made in this groove may vary somewhat from

point to point. But this and other methods of varying the cross section

have been used but little, and they do not seem capable of wide

application.

The fact that rolling is so much cheaper than forging has led engineers

to design their pieces so that they can be made by rolling, i.e. to make

them straight and of uniform cross section. It is for this reason, for

instance, that railroad rails are of constant uniform section throughout

their length, instead of having those parts of their length which come

between the supporting ties deeper and stronger than the parts which

rest on the ties. When, as in the case of eye bars, it is imperative

that one part should differ materially in section from the rest, this

part may be locally thickened or thinned, or a special part may here be

welded on. When we come to pieces of very irregular shape, such as

crank-shafts, anchors, trunnions, &c., we must resort to forging, except

for purposes for which unforged castings are good enough.

[Illustration: FIG. 37.--Steam Hammer.

A, Round bar to be hammered.

B, Anvil.

C, Anvil block or foundation.

D, Falling tup.

E, Steam piston.

F, Piston-rod for lifting tup and driving it down.

G, Steam cylinder.]

131. _Forging_ proceeds by beating or squeezing the piece under

treatment from its initial into its final shape, as for instance by

hammering a square ingot or bloom first on one corner and then on

another until it is reduced to a cylindrical shape as shown at A in fig.

37. As the ingot is reduced in section, it is of course lengthened

proportionally. Much as in the smith's forge the object forged rests on

a massive anvil and anvil block, B and C, and is struck by the tup D of

the hammer. This tup is raised and driven down by steam pressure applied

below or above the piston E of the steam cylinder mounted aloft, and

connected with the tup by means of the strong piston-rod F. The demand

for very large forgings, especially for guns and armour plate, led to

the building of enormous steam hammers. The falling parts of the largest

of these, that at Bethlehem, Pa., weigh 125 tons.

The first cost of a hammer of moderate size is much less than that of a

hydraulic press of like capacity, as is readily understood when we stop

to reflect what powerful pressure, if gradually applied, would be needed

to drive the nail which a light blow from our hand hammer forces easily

into the woodwork. Nevertheless the press uses much less power than the

hammer, because much of the force of the latter is dissipated in setting

up useless--indeed harmful, and at times destructive--vibrations in the

foundations and the surrounding earth and buildings. Moreover, the

effect of the sharp blow of the hammer is relatively superficial, and

does not penetrate to the interior of a large piece as the slowly

applied pressure of the hydraulic press does. Because of these facts the

great hammers have given place to enormous forging presses, the 125-ton

Bethlehem hammer, for instance, to a 14,000-ton hydraulic press, moved

by water under a pressure of 7000 lb. per square inch, supplied by

pumps of 16,000 horse power.

TABLE IV.--_Reduction in Cost of Iron Manufacture in America--C.

Kirchoff._

+-------------------+------------------------+-------------+----------------------------------------------------+

| | | Period | Cost, Profit and Production, at End of Period in |

| | | covered. | Percentage of that at Beginning of Period. |

| | +------+------+------------------------------------+------+--------+

| | | | | Cost. | | |

| Place represented.| Operation represented. | | +------+------+-------+------+-------+ | Produc-|

| | | | | | | | | Total |Profit|tion per|

| | | From | To | | | | |exclu- | per |Furnace |

| | | | | Ore. | Fuel.|Labour.|Total.| ding | Ton. |&c., per|

| | | | | | | | |raw Ma-| | Day |

| | | | | | | | |terial.| | |

+-------------------+------------------------+------+------+------+------+-------+------+-------+------+--------+

| A large Southern | | | | | | | | | | |

| Establishment | Manufacture of Pig Iron| 1889 | 1898 | 79 | 64.1 | 51.9 | 63.4 | .. | 47.9 | 167.7 |

| North-eastern | | | | | | | | | | |

| District | " " | 1890 | 1898 |103.7 | 97 | 61.1 | 65.8 | .. | 33.9 | 163.3 |

| Pittsburg District| " " | 1887 | 1897 | .. | .. | 46 | .. | 44 | .. | .. |

| Eastern District | Manufacture of Bessemer| | | | | | | | | |

| | Steel Ingots | 1891 | 1898 | .. | .. | 75 | 64.39| .. | .. | 107 |

| Pittsburg | " " | 1887 | 1897 | .. | .. | .. | .. | 52 | .. | .. |

| Not stated | Rolling Wire Rods | 1888 | 1898 | .. | .. | .. | 63.6 | .. | .. | 325 |

+-------------------+------------------------+------+------+------+------+-------+------+-------+------+--------+

132. _Statistics._--The cheapening of manufacture by improvements in

processes and machinery, and by the increase in the scale of

operations, has been very great. The striking examples of it shown in

Table IV. are only typical of what has been going on continuously

since 1868. Note, for instance, a reduction of some 35% in the total

cost, and an even greater reduction in the cost of labour, reaching in

one case 54%, in a period of between seven and ten years. This great

economy is not due to reduction in wages. According to Mr Carnegie, in

one of the largest American steel works the average wages in 1900 for

all persons paid by the day, including labourers, mechanics and boys,

were more than $4 (say, 16s. 6d.) a day for the 311 working days. How

economical the methods of mining, transportation and manufacture have

become is shown by the fact that steel billets have been sold at

$13.96 (L2, 17s. 8d.) per ton, and in very large quantities at $15

(L3, 2s.) per ton in the latter case, according to Mr Carnegie,

without further loss than that represented by interest, although the

cost of each ton includes that of mining 2 tons of ore and carrying

them 1000 miles, mining and coking 1.3 tons of coal and carrying its

coke 50 m., and quarrying one-third of a ton of limestone and carrying

it 140 m., besides the cost of smelting the ore, converting the

resultant cast iron into steel, and rolling that steel into rails.