Chapter XV: Part 15

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.

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.

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

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.

A, Superficial blowholes.
B, Deep-seated blowholes.
C, Pipe.]

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.

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.

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.

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.

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.)

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.

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._