Chapter XI: Part 11

4. _Historical Sketch._--The iron oxide of which the ores of iron consist would be so easily deoxidized and thus brought to the metallic state by the carbon, i.e. by the glowing coals of any primeval savage's wood fire, and the resulting metallic iron would then differ so strikingly from any object which he had previously seen, that its very early use by our race is only natural. The first observing savage who noticed it among his ashes might easily infer that it resulted from the action of burning wood on certain extremely heavy stones. He could pound it out into many useful shapes. The natural steps first of making it intentionally by putting such stones into his fire, and next of improving his fire by putting it and these stones into a cavity on the weather side of some bank with an opening towards the prevalent wind, would give a simple forge, differing only in size, in lacking forced blast, and in details of construction, from the Catalan forges and bloomaries of to-day. Moreover, the coals which deoxidized the iron would inevitably carburize some lumps of it, here so far as to turn it into the brittle and relatively useless cast iron, there only far enough to convert it into steel, strong and very useful even in its unhardened state. Thus it is almost certain that much of the earliest iron was in fact steel. How soon after man's discovery, that he could beat iron and steel out while cold into useful shapes, he learned to forge it while hot is hard to conjecture. The pretty elaborate appliances, tongs or their equivalent, which would be needed to enable him to hold it conveniently while hot, could hardly have been devised till a very much later period; but then he may have been content to forge it inconveniently, because the great ease with which it mashes out when hot, perhaps pushed with a stout stick from the fire to a neighbouring flat stone, would compensate for much inconvenience. However this may be, very soon after man began to practise hot-forging he would inevitably learn that sudden cooling, by quenching in water, made a large proportion of his metal, his steel, extremely hard and brittle, because he would certainly try by this very quenching to avoid the inconvenience of having the hot metal about. But the invaluable and rather delicate art of tempering the hardened steel by a very careful and gentle reheating, which removes its extreme brittleness though leaving most of its precious hardness, needs such skilful handling that it can hardly have become known until very long after the art of hot-forging.

The oxide ores of copper would be deoxidized by the savage's wood fire even more easily than those of iron, and the resulting copper would be recognized more easily than iron, because it would be likely to melt and run together into a mass conspicuous by its bright colour and its very great malleableness. From this we may infer that copper and iron probably came into use at about the same stage in man's development, copper before iron in regions which had oxidized copper ores, whether they also had iron ores or not, iron before copper in places where there were pure and easily reduced ores of iron but none of copper. Moreover, the use of each metal must have originated in many different places independently. Even to-day isolated peoples are found with their own primitive iron-making, but ignorant of the use of copper.

If iron thus preceded copper in many places, still more must it have preceded bronze, an alloy of copper and tin much less likely than either iron or copper to be made unintentionally. Indeed, though iron ores abound in many places which have neither copper nor tin, yet there are but few places which have both copper and tin. It is not improbable that, once bronze became known, it might replace iron in a measure, perhaps even in a very large measure, because it is so fusible that it can be cast directly and easily into many useful shapes. It seems to be much more prominent than iron in the Homeric poems; but they tell us only of one region at one age. Even if a nation here or there should give up the use of iron completely, that all should is neither probable nor shown by the evidence. The absence of iron and the abundance of bronze in the relics of a prehistoric people is a piece of evidence to be accepted with caution, because the great defect of iron, its proneness to rust, would often lead to its complete disappearance, or conversion into an unrecognizable mass, even though tools of bronze originally laid down beside it might remain but little corroded. That the ancients should have discovered an art of hardening bronze is grossly improbable, first because it is not to be hardened by any simple process like the hardening of steel, and second because, if they had, then a large proportion of the ancient bronze tools now known ought to be hard, which is not the case.

Because iron would be so easily made by prehistoric and even by primeval man, and would be so useful to him, we are hardly surprised to read in Genesis that Tubal Cain, the sixth in descent from Adam, discovered it; that the Assyrians had knives and saws which, to be effective, must have been of hardened steel, i.e. of iron which had absorbed some carbon from the coals with which it had been made, and had been quenched in water from a red heat; that an iron tool has been found embedded in the ancient pyramid of Kephron (probably as early as 3500 B.C.); that iron metallurgy had advanced at the time of Tethmosis (Thothmes) III. (about 1500 B.C.) so far that bellows were used for forcing the forge fire; that in Homer's time (not later than the 9th century B.C.) the delicate art of hardening and tempering steel was so familiar that the poet used it for a simile, likening the hissing of the stake which Ulysses drove into the eye of Polyphemus to that of the steel which the smith quenches in water, and closing with a reference to the strengthening effect of this quenching; and that at the time of Pliny (A.D. 23-79) the relative value of different baths for hardening was known, and oil preferred for hardening small tools. These instances of the very early use of this metal, intrinsically at once so useful and so likely to disappear by rusting away, tell a story like that of the single foot-print of the savage which the waves left for Robinson Crusoe's warning. Homer's familiarity with the art of tempering could come only after centuries of the wide use of iron.

3. _Three Periods._--The history of iron may for convenience be divided into three periods: a first in which only the direct extraction of wrought iron from the ore was practised; a second which added to this primitive art the extraction of iron in the form of carburized or cast iron, to be used either as such or for conversion into wrought iron; and a third in which the iron worker used a temperature high enough to melt wrought iron, which he then called molten steel. For brevity we may call these the periods of wrought iron, of cast iron, and of molten steel, recognizing that in the second and third the earlier processes continued in use. The first period began in extremely remote prehistoric times; the second in the 14th century; and the third with the invention of the Bessemer process in 1856.

6. _First Period._--We can picture to ourselves how in the first
period the savage smith, step by step, bettered his control over his
fire, at once his source of heat and his deoxidizing agent. Not
content to let it burn by natural draught, he would blow it with his
own breath, would expose it to the prevalent wind, would urge it with
a fan, and would devise the first crude valveless bellows, perhaps the
pigskin already familiar as a water-bottle, of which the psalmist
says: "I am become as a bottle in the smoke." To drive the air out of
this skin by pressing on it, or even by walking on it, would be easy;
to fill it again with air by pulling its sides apart with his fingers
would be so irksome that he would soon learn to distend it by means of
strings. If his bellows had only a single opening, that through which
they delivered the blast upon the fire, then in inflating them he
would draw back into them the hot air and ashes from the fire. To
prevent this he might make a second or suction hole, and thus he would
have a veritable engine, perhaps one of the very earliest of all.
While inflating the bellows he would leave the suction port open and
close the discharge port with a pinch of his finger; and while blowing
the air against the fire he would leave the discharge port open and
pinch together the sides of the suction port.

The next important step seems to have been taken in the 4th century
when some forgotten Watt devised valves for the bellows. But in spite
of the activity of the iron manufacture in many of the Roman
provinces, especially England, France, Spain, Carinthia and near the
Rhine, the little forges in which iron was extracted from the ore
remained, until the 14th century, very crude and wasteful of labour,
fuel, and iron itself: indeed probably not very different from those
of a thousand years before. Where iron ore was found, the local smith,
the _Waldschmied_, converted it with the charcoal of the surrounding
forest into the wrought iron which he worked up. Many farmers had
their own little forges or smithies to supply the iron for their
tools.

The fuel, wood or charcoal, which served both to heat and to deoxidize
the ore, has so strong a carburizing action that it would turn some of
the resultant metal into "natural steel," which differs from wrought
iron only in containing so much carbon that it is relatively hard and
brittle in its natural state, and that it becomes intensely hard when
quenched from a red heat in water. Moreover, this same carburizing
action of the fuel would at times go so far as to turn part of the
metal into a true cast iron, so brittle that it could not be worked at
all. In time the smith learnt how to convert this unwelcome product
into wrought iron by remelting it in the forge, exposing it to the
blast in such a way as to burn out most of its carbon.

7. _Second Period._--With the second period began, in the 14th
century, the gradual displacement of the direct extraction of wrought
iron from the ore by the intentional and regular use of this indirect
method of first carburizing the metal and thus turning it into cast
iron, and then converting it into wrought iron by remelting it in the
forge. This displacement has been going on ever since, and it is not
quite complete even to-day. It is of the familiar type of the
replacing of the simple but wasteful by the complex and economical,
and it was begun unintentionally in the attempt to save fuel and
labour, by increasing the size and especially the height of the forge,
and by driving the bellows by means of water-power. Indeed it was the
use of water-power that gave the smith pressure strong enough to force
his blast up through a longer column of ore and fuel, and thus enabled
him to increase the height of his forge, enlarge the scale of his
operations, and in turn save fuel and labour. And it was the
lengthening of the forge, and the length and intimacy of contact
between ore and fuel to which it led, that carburized the metal and
turned it into cast iron. This is so fusible that it melted, and,
running together into a single molten mass, freed itself mechanically
from the "gangue," as the foreign minerals with which the ore is mixed
are called. Finally, the improvement in the quality of the iron which
resulted from thus completely freeing it from the gangue turned out to
be a great and unexpected merit of the indirect process, probably the
merit which enabled it, in spite of its complexity, to drive out the
direct process. Thus we have here one of these cases common in the
evolution both of nature and of art, in which a change, made for a
specific purpose, has a wholly unforeseen advantage in another
direction, so important as to outweigh that for which it was made and
to determine the path of future development.

With this method of making molten cast iron in the hands of a people
already familiar with bronze founding, iron founding, i.e. the casting
of the molten cast iron into shapes which were useful in spite of its
brittleness, naturally followed. Thus ornamental iron castings were
made in Sussex in the 14th century, and in the 16th cannons weighing
three tons each were cast.

The indirect process once established, the gradual increase in the
height and diameter of the high furnace, which has lasted till our own
days, naturally went on and developed the gigantic blast furnaces of
the present time, still called "high furnaces" in French and German.
The impetus which the indirect process and the acceleration of
civilization in the 15th and 16th centuries gave to the iron industry
was so great that the demands of the iron masters for fuel made
serious inroads on the forests, and in 1558 an act of Queen
Elizabeth's forbade the cutting of timber in certain parts of the
country for iron-making. Another in 1584 forbade the building of any
more iron-works in Surrey, Kent, and Sussex. This increasing scarcity
of wood was probably one of the chief causes of the attempts which the
iron masters then made to replace charcoal with mineral fuel. In 1611
Simon Sturtevant patented the use of mineral coal for iron-smelting,
and in 1619 Dud Dudley made with this coal both cast and wrought iron
with technical success, but through the opposition of the charcoal
iron-makers all of his many attempts were defeated. In 1625 Stradda's
attempts in Hainaut had no better success, and it was not till more
than a century later that iron-smelting with mineral fuel was at last
fully successful. It was then, in 1735, that Abraham Darby showed how
to make cast iron with coke in the high furnace, which by this time
had become a veritable blast furnace.

The next great improvement in blast-furnace practice came in 1811,
when Aubertot in France used for heating steel the furnace gases rich
in carbonic oxide which till then had been allowed to burn uselessly
at the top of the blast furnace. The next was J. B. Neilson's
invention in 1828 of heating the blast, which increased the production
and lessened the fuel-consumption of the furnace wonderfully. Very
soon after this, in 1832, the work of heating the blast was done by
means of the waste gases, at Wasseralfingen in Bavaria.

Meanwhile Henry Cort had in 1784 very greatly simplified the
conversion of cast iron into wrought iron. In place of the old forge,
in which the actual contact between the iron and the fuel, itself an
energetic carburizing agent, made decarburization difficult, he
devised the reverberatory puddling furnace (see fig. 14 below), in
which the iron lies in a chamber apart from the fire-place, and is
thus protected from the carburizing action of the fuel, though heated
by the flame which that fuel gives out.

The rapid advance in mechanical engineering in the latter part of this
second period stimulated the iron industry greatly, giving it in 1728
Payn and Hanbury's rolling mill for rolling sheet iron, in 1760 John
Smeaton's cylindrical cast-iron bellows in place of the wooden and
leather ones previously used, in 1783 Cort's grooved rolls for rolling
bars and rods of iron, and in 1838 James Nasmyth's steam hammer. But
even more important than these were the advent of the steam engine
between 1760 and 1770, and of the railroad in 1825, each of which gave
the iron industry a great impetus. Both created a great demand for
iron, not only for themselves but for the industries which they in
turn stimulated; and both directly aided the iron master: the steam
engine by giving him powerful and convenient tools, and the railroad
by assembling his materials and distributing his products.

About 1740 Benjamin Huntsman introduced the "crucible process" of
melting steel in small crucibles, and thus freeing it from the slag,
or rich iron silicate, with which it, like wrought iron, was
mechanically mixed, whether it was made in the old forge or in the
puddling furnace. This removal of the cinder very greatly improved the
steel; but the process was and is so costly that it is used only for
making steel for purposes which need the very best quality.

8. _Third Period._--The third period has for its great distinction the
invention of the Bessemer and open-hearth processes, which are like
Huntsman's crucible process in that their essence is their freeing
wrought iron and low carbon steel from mechanically entangled cinder,
by developing the hitherto unattainable temperature, rising to above
1500 deg. C., needed for melting these relatively infusible products.
These processes are incalculably more important than Huntsman's, both
because they are incomparably cheaper, and because their products are
far more useful than his.

Thus the distinctive work of the second and third periods is freeing
the metal from mechanical impurities by fusion. The second period, by
converting the metal into the fusible cast iron and melting this, for
the first time removed the gangue of the ore; the third period by
giving a temperature high enough to melt the most infusible forms of
iron, liberated the slag formed in deriving them from cast iron.

In 1856 Bessemer not only invented his extraordinary process of making
the heat developed by the rapid oxidation of the impurities in pig
iron raise the temperature above the exalted melting-point of the
resultant purified steel, but also made it widely known that this
steel was a very valuable substance. Knowing this, and having in the
Siemens regenerative gas furnace an independent means of generating
this temperature, the Martin brothers of Sireuil in France in 1864
developed the open-hearth process of making steel of any desired
carbon-content by melting together in this furnace cast and wrought
iron. The great defect of both these processes, that they could not
remove the baneful phosphorus with which all the ores of iron are
associated, was remedied in 1878 by S. G. Thomas, who showed that, in
the presence of a slag rich in lime, the whole of the phosphorus could
be removed readily.

9. After the remarkable development of the blast furnace, the
Bessemer, and the open-hearth processes, the most important work of
this, the third period of the history of iron, is the birth and growth
of the science and art of iron metallography. In 1868 Tschernoff
enunciated its chief fundamental laws, which were supplemented in 1885
by the laws of Brinell. In 1888 F. Osmond showed that the wonderful
changes which thermal treatment and the presence of certain foreign
elements cause were due to allotropy, and from these and like
teachings have come a rapid growth of the use of the so-called "alloy
steels" in which, thanks to special composition and treatment, the
iron exists in one or more of its remarkable allotropic states. These
include the austenitic or gamma non-magnetic manganese steel, already
patented by Robert Hadfield in 1883, the first important known
substance which combined great malleableness with great hardness, and
the martensitic or beta "high speed tool steel" of White and Taylor,
which retains its hardness and cutting power even at a red heat.

10. _Constitution of Iron and Steel._--The constitution of the various classes of iron and steel as shown by the microscope explains readily the great influence of carbon which was outlined in SS 2 and 3. The metal in its usual slowly cooled state is a conglomerate like the granitic rocks. Just as a granite is a conglomerate or mechanical mixture of distinct crystalline grains of three perfectly definite minerals, mica, quartz, and felspar, so iron and steel in their usual slowly cooled state consist of a mixture of microscopic particles of such definite quasi-minerals, diametrically unlike. These are cementite, a definite iron carbide, Fe3C, harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress; and ferrite, pure or nearly pure metallic [alpha]-iron, soft, weak, with high electric conductivity, and in general like copper except in colour. In view of the fact that the presence of 1% of carbon implies that 15% of the soft ductile ferrite is replaced by the glass-hard cementite, it is not surprising that even a little carbon influences the properties of the metal so profoundly.

But carbon affects the properties of iron not only by giving rise to varying proportions of cementite, but also both by itself shifting from one molecular state to another, and by enabling us to hold the iron itself in its unmagnetic allotropic forms, [beta]- and [gamma]-iron, as will be explained below. Thus, sudden cooling from a red heat leaves the carbon not in definite combination as cementite, but actually dissolved in [beta]- and [gamma]-allotropic iron, in the conditions known as martensite and austenite, not granitic but glass-like bodies, of which the "hardened" and "tempered" steel of our cutting tools in large part consists. Again, if more than 2% of carbon is present, it passes readily into the state of pure graphitic carbon, which, in itself soft and weak, weakens and embrittles the metal as any foreign body would, by breaking up its continuity.

11. The _Roberts-Austen_ or _carbon-iron diagram_ (fig. 1), in which vertical distances represent temperatures and horizontal ones the percentage of carbon in the iron, aids our study of these constituents of iron. If, ignoring temporarily and for simplicity the fact that part of the carbon may exist in the state of graphite, we consider the behaviour of iron in cooling from the molten state, AB and BC give the temperature at which, for any given percentage of carbon, solidification begins, and A_a_, _a_B, and B_c_ that at which it ends. But after solidification is complete and the metal has cooled to a much lower range of temperature, usually between 900 deg. and 690 deg. C., it undergoes a very remarkable series of transformations. GHSa gives the temperature at which, for any given percentage of carbon, these transformations begin, and PSP' that at which they end.

These freezing-point curves and transformation curves thus divide the diagram into 8 distinct regions, each with its own specific state or constitution of the metal, the molten state for region 1, a mixture of molten metal and of solid austenite for region 2, austenite alone for region 4 and so on. This will be explained below. If the metal followed the laws of equilibrium, then whenever through change of temperature it entered a new region, it would forthwith adopt the constitution normal to that region. But in fact the change of constitution often lags greatly, so that the metal may have the constitution normal to a region higher than that in which it is, or even a patchwork constitution, representing fragments of those of two or more regions. It is by taking advantage of this lagging that thermal treatment causes such wonderful changes in the properties of the cold metal.

12. With these facts in mind we may now study further these different constituents of iron.

_Austenite, gamma_ ([gamma]) _iron._--Austenite is the name of the
solid solution of an iron carbide in allotropie [gamma]-iron of which
the metal normally consists when in region 4. In these solid
solutions, as in aqueous ones, the ratios in which the different
chemical substances are present are not fixed or definite, but vary
from case to case, not _per saltum_ as between definite chemical
compounds, but by infinitesimal steps. The different substances are as
it were dissolved in each other in a state which has the
indefiniteness of composition, the absolute merging of identity, and
the weakness of reciprocal chemical attraction, characteristic of
aqueous solutions.

On cooling into region 6 or 8 austenite should normally split up into
ferrite and cementite, after passing through the successive stages of
martensite, troostite and sorbite, Fe_xC = Fe3C + Fe_(x-3). But this
change may be prevented so as to preserve the austenite in the cold,
either very incompletely, as when high-carbon steel is "hardened,"
i.e. is cooled suddenly by quenching in water, in which case the
carbon present seems to act as a brake to retard the change; or
completely, by the presence of a large quantity of manganese, nickel,
tungsten or molybdenum, which in effect sink the lower boundary GHS_a_
of region 4 to below the atmospheric temperature. The important
manganese steels of commerce and certain nickel steels are
manganiferous and niccoliferous austenite, unmagnetic and hard but
ductile.

Austenite may contain carbon in any proportion up to about 2.2%. It is
non-magnetic, and, when preserved in the cold either by quenching or
by the presence of manganese, nickel, &c., it has a very remarkable
combination of great malleability with very marked hardness, though it
is less hard than common carbon steel is when hardened, and probably
less hard than martensite. When of eutectoid composition, it is called
"hardenite." Suddenly cooled carbon steel, even if rich in austenite,
is strongly magnetic because of the very magnetic [alpha]-iron which
inevitably forms even in the most rapid cooling from region 4. Only in
the presence of much manganese, nickel, or their equivalent can the
true austenite be preserved in the cold so completely that the steel
remains non-magnetic.

13. _Beta_ ([beta]) _iron_, an unmagnetic, intensely hard and brittle
allotropic form of iron, though normal and stable only in the little
triangle GHM, is yet a state through which the metal seems always to
pass when the austenite of region 4 changes into the ferrite and
cementite of regions 6 and 8. Though not normal below MHSP', yet like
[gamma]-iron it can be preserved in the cold by the presence of about
5% of manganese, which, though not enough to bring the lower boundary
of region 4 below the atmospheric temperature and thus to preserve
austenite in the cold, is yet enough to make the transformation of
[beta] into [alpha] iron so sluggish that the former remains
untransformed even during slow cooling.

Again, [beta]-iron may be preserved incompletely as in the "hardening
of steel," which consists in heating the steel into the austenite
state of region 4, and then cooling it so rapidly, e.g. by quenching
it in cold water, that, for lack of the time needed for the completion
of the change from austenite into ferrite and cementite, much of the
iron is caught in transit in the [beta] state. According to our
present theory, it is chiefly to beta iron, preserved in one of these
ways, that all of our tool steel proper, i.e. steel used for cutting
as distinguished from grinding, seems to owe its hardness.

14. _Martensite_, _Troostite_ and _Sorbite_ are the successive stages
through which the metal passes in changing from austenite into ferrite
and cementite. _Martensite_, very hard because of its large content of
[beta]-iron, is characteristic of hardened steel, but the two others,
far from being definite substances, are probably only roughly bounded
stages of this transition. _Troostite_ and _sorbite_, indeed, seem to
be chiefly very finely divided mixtures of ferrite and cementite, and
it is probably because of this fineness that sorbitic steel has its
remarkable combination of strength and elasticity with ductility which
fits it for resisting severe vibratory and other dynamic stresses,
such as those to which rails and shafting are exposed.

15. _Alpha_ ([alpha]) _iron_ is the form normal and stable for regions
5, 6 and 8, i.e. for all temperatures below MHSP'. It is the common,
very magnetic form of iron, in itself ductile but relatively soft and
weak, as we know it in wrought iron and mild or low-carbon steel.

16. _Ferrite_ and _cementite_, already described in S 10, are the
final products of the transformation of austenite in slow-cooling.
[beta]-ferrite and austenite are the normal constituents for the
triangle GHM, [alpha]-ferrite (i.e. nearly pure [alpha]-iron) with
austenite for the space MHSP, cementite with austenite for region 7,
and [alpha]-ferrite and cementite jointly for regions 6 and 8. Ferrite
and cementite are thus the normal and usual constituents of slowly
cooled steel, including all structural steels, rail steel, &c., and of
white cast iron (see S 18).

17. _Pearlite._--The ferrite and cementite present interstratify
habitually as a "eutectoid"[2] called "pearlite" (see ALLOYS, Pl.,
fig. 11), in the ratio of about 6 parts of ferrite to 1 of cementite,
and hence containing about 0.90% of carbon. Slowly cooled steel
containing just 0.90% of carbon (S in fig. 1) consists of pearlite
alone. Steel and white cast iron with more than this quantity of
carbon consist typically of kernels of pearlite surrounded by
envelopes of free cementite (see ALLOYS, Pl., fig. 13) sufficient in
quantity to represent their excess of carbon over the eutectoid ratio;
they arc called "hyper-eutectoid," and are represented by region 8 of
Fig. 1. Steel containing less than this quantity of carbon consists
typically of kernels of pearlite surrounded by envelopes of ferrite
(see ALLOYS, Pl., fig. 12) sufficient in quantity to represent their
excess of iron over this eutectoid ratio; is called "hypo-eutectoid";
and is represented by region 6 of Fig. 1. This typical "envelope and
kernel" structure is often only rudimentary.

The percentage of pearlite and of free ferrite or cementite in these
products is shown in fig. 2, in which the ordinates of the line ABC
represent the percentage of pearlite corresponding to each percentage
of carbon, and the intercept ED, MN or KF, of any point H, P or L,
measures the percentage of the excess of ferrite or cementite for
hypo- and hyper-eutectic steel and white cast iron respectively.

18. _The Carbon-Content, i.e. the Ratio of Ferrite to Cementite, of
certain typical Steels._--Fig. 3 shows how, as the carbon-content
rises from 0 to 4.5%, the percentage of the glass-hard cementite,
which is 15 times that of the carbon itself, rises, and that of the
soft copper-like ferrite falls, with consequent continuous increase of
hardness and loss of malleableness and ductility. The tenacity or
tensile strength increases till the carbon-content reaches about
1.25%, and the cementite about 19%, and then in turn falls, a result
by no means surprising. The presence of a small quantity of the hard
cementite ought naturally to strengthen the mass, by opposing the
tendency of the soft ferrite to flow under any stress applied to it;
but more cementite by its brittleness naturally weakens the mass,
causing it to crack open under the distortion which stress inevitably
causes. The fact that this decrease of strength begins shortly after
the carbon-content rises above the eutectoid or pearlite ratio of
0.90% is natural, because the brittleness of the cementite which, in
hyper-eutectoid steels, forms a more or less continuous skeleton
(ALLOYS, Pl., fig. 13) should be much more effective in starting
cracks under distortion than that of the far more minute particles of
cementite which lie embedded, indeed drowned, in the sixfold greater
mass of ferrite with which they are associated in the pearlite itself.
The large massive plates of cementite which form the network or
skeleton in hyper-eutectoid steels should, under distortion, naturally
tend to cut, in the softer pearlite, chasms too serious to be healed
by the inflowing of the plastic ferrite, though this ferrite flows
around and immediately heals over any cracks which form in the small
quantity of cementite interstratified with it in the pearlite of
hypo-eutectoid steels.

As the carbon-content increases the welding power naturally decreases
rapidly, because of the rapid fall of the "solidus curve" at which
solidification is complete (Aa of fig. 1), and hence of the range in
which the steel is coherent enough to be manipulated, and, finally, of
the attainable pliancy and softness of the metal. Clearly the mushy
mixture of solid austenite and molten iron of which the metal in
region 2 consists cannot cohere under either the blows or the pressure
by means of which welding must be done. Rivet steel, which above all
needs extreme ductility to endure the distortion of being driven home,
and tube steel which must needs weld easily, no matter at what
sacrifice of strength, are made as free from carbon, i.e. of as nearly
pure ferrite, as is practicable. The distortion which rails undergo in
manufacture and use is incomparably less than that to which rivets are
subjected, and thus rail steel may safely be much richer in carbon and
hence in cementite, and therefore much stronger and harder, so as to
better endure the load and the abrasion of the passing wheels. Indeed,
its carbon-content is made small quite as much because of the violence
of the shocks from these wheels as because of any actual distortion to
be expected, since, within limits, as the carbon-content increases
the shock-resisting power decreases. Here, as in all cases, the
carbon-content must be the result of a compromise, neither so small
that the rail flattens and wears out like lead, nor so great that it
snaps like glass. Boiler plates undergo in shaping and assembling an
intermediate degree of distortion, and therefore they must be given an
intermediate carbon-content, following the general rule that the
carbon-content and hence the strength should be as great as is
consistent with retaining the degree of ductility and the
shock-resisting power which the object will need in actual use. Thus
the typical carbon-content may be taken as about 0.05% for rivets and
tubes, 0.20% for boiler plates, and 0.50 to 0.75% for rails, implying
the presence of 0.75% of cementite in the first two, 3% in the third
and 7.5% to 11.25% in the last.

19. _Carbon-Content of Hardened Steels._--Turning from these cases in
which the steel is used in the slowly cooled state, so that it is a
mixture of pearlite with ferrite or cementite, i.e. is pearlitic, to
those in which it is used in the hardened or martensitic state, we
find that the carbon-content is governed by like considerations.
Railway car springs, which are exposed to great shock, have typically
about 0.75% of carbon; common tool steel, which is exposed to less
severe shock, has usually between 0.75 and 1.25%; file steel, which is
subject to but little shock, and has little demanded of it but to bite
hard and stay hard, has usually from 1.25 to 1.50%. The carbon-content
of steel is rarely greater than this, lest the brittleness be
excessive. But beyond this are the very useful, because very fusible,
cast irons with from 3 to 4% of carbon, the embrittling effect of
which is much lessened by its being in the state of graphite.

20. _Slag or Cinder_, a characteristic component of wrought iron,
which usually contains from 0.20 to 2.00% of it, is essentially a
silicate of iron (ferrous silicate), and is present in wrought iron
simply because this product is made by welding together pasty granules
of iron in a molten bath of such slag, without ever melting the
resultant mass or otherwise giving the envelopes of slag thus
imprisoned a chance to escape completely.

21. _Graphite_, nearly pure carbon, is characteristic of "gray cast
iron," in which it exists as a nearly continuous skeleton of very thin
laminated plates or flakes (fig. 27), usually curved, and forming from
2.50% to 3.50% of the whole. As these flakes readily split open, when
a piece of this iron is broken rupture passes through them, with the
result that, even though the graphite may form only some 3% of the
mass by weight (say 10% by volume), practically nothing but graphite
is seen in the fracture. Hence the weakness and the dark-grey fracture
of this iron, and hence, by brushing this fracture with a wire brush
and so detaching these loosely clinging flakes of graphite, the colour
can be changed nearly to the very light-grey of pure iron. There is
rarely any important quantity of graphite in commercial steels. (See S
26.)

22. _Further Illustration of the Iron-Carbon Diagram._--In order to
illustrate further the meaning of the diagram (fig. 1), let us follow
by means of the ordinate QUw the undisturbed slow cooling of molten
hyper-eutectoid steel containing 1% of carbon, for simplicity assuming
that no graphite forms and that the several transformations occur
promptly as they fall due. When the gradually falling temperature
reaches 1430 deg. (q), the mass begins to freeze as [gamma]-iron or
austenite, called "primary" to distinguish it from that which forms
part of the eutectic. But the freezing, instead of completing itself
at a fixed temperature as that of pure water does, continues until the
temperature sinks to r on the line Aa. Thus the iron has rather a
freezing-range than a freezing-point. Moreover, the freezing is
"selective." The first particles of austenite to freeze contain about
0.33% of carbon (p). As freezing progresses, at each successive
temperature reached the frozen austenite has the carbon-content of the
point on Aa which that temperature abscissa cuts, and the still molten
part or "mother-metal" has the carbon-content horizontally opposite
this on the line AB. In other words, the composition of the frozen
part and that of the mother-metal respectively are p and q at the
beginning of the freezing, and r and t' at the end; and during
freezing they slide along Aa and AB from p to r and from q to t'.
This, of course, brings the final composition of the frozen austenite
when freezing is complete exactly to that which the molten mass had
before freezing began.

The heat evolved by this process of solidification retards the fall of
temperature; but after this the rate of cooling remains regular until
T (750 deg.) on the line Sa (Ar3) is reached, when a second
retardation occurs, due to the heat liberated by the passage within
the pasty mass of part of the iron and carbon from a state of mere
solution to that of definite combination in the ratio Fe3C, forming
microscopic particles of cementite, while the remainder of the iron
and carbon continue dissolved in each other as austenite. This
formation of cementite continues as the temperature falls, till at
about 690 deg. C., (U, called Ar_(2-1)) so much of the carbon (in this
case about 0.10%) and of the iron have united in the form of
cementite, that the composition of the remaining solid-solution or
"mother-metal" of austenite has reached that of the eutectoid,
hardenite; i.e. it now contains 0.90 % of carbon. The cementite which
has thus far been forming may be called "pro-eutectoid" cementite,
because it forms before the remaining austenite reaches the eutectoid
composition. As the temperature now falls past 690 deg., this
hardenite mother-metal in turn splits up, after the fashion of
eutectics, into alternate layers of ferrite and cementite grouped
together as pearlite, so that the mass as a whole now becomes a
mixture of pearlite with cementite. The iron thus liberated, as the
ferrite of this pearlite, changes simultaneously to [alpha]-ferrite.
The passage of this large quantity of carbon and iron, 0.90% of the
former and 12.6 of the latter, from a state of mere solution as
hardenite to one of definite chemical union as cementite, together
with the passage of the iron itself from the [gamma] to the [alpha]
state, evolves so much heat as actually to heat the mass up so that it
brightens in a striking manner. This phenomenon is called the
"recalescence."

This change from austenite to ferrite and cementite, from the [gamma]
through the [beta] to the [alpha] state, is of course accompanied by
the loss of the "hardening power," i.e. the power of being hardened by
sudden cooling, because the essence of this hardening is the retention
of the [beta] state. As shown in ALLOYS, Pl., fig. 13, the slowly
cooled steel now consists of kernels of pearlite surrounded by
envelopes of the cementite which was born of the austenite in cooling
from T to U.

23. To take a second case, molten hypo-eutectoid steel of 0.20% of
carbon on freezing from K to x passes in the like manner to the state
of solid austenite, [gamma]-iron with this 0.20% of carbon dissolved
in it. Its further cooling undergoes three spontaneous retardations,
one at K' (Ar3 about 820 deg.), at which part of the iron begins to
isolate itself within the austenite mother-metal in the form of
envelopes of [beta]-ferrite, i.e. of free iron of the [beta]
allotropic modification, which surrounds the kernels or grains of the
residual still undecomposed part of the austenite. At the second
retardation, K" (Ar2, about 770 deg.) this ferrite changes to the
normal magnetic [alpha]-ferrite, so that the mass as a whole becomes
magnetic. Moreover, the envelopes of ferrite which began forming at
Ar3 continue to broaden by the accession of more and more ferrite born
from the austenite progressively as the temperature sinks, till, by
the time when Ar1 (about 690 deg.) is reached, so much free ferrite
has been formed that the remaining mother-metal has been enriched to
the composition of hardenite, i.e. it now contains 0.90% of carbon.
Again, as the temperature in turn falls past Ar1 this hardenite
mother-metal splits up into cementite and ferrite grouped together as
pearlite, with the resulting recalescence, and the mass, as shown in
Alloys, Pl., fig. 12, then consists of kernels of pearlite surrounded
by envelopes of ferrite. All these phenomena are parallel with those
of 1.00% carbon steel at this same critical point Ar1. As such steel
cools slowly past Ar3, Ar2 and Ar1, it loses its hardening power
progressively.

In short, from Ar3 to Ar1 the excess substance ferrite or cementite,
in hypo- and hyper-eutectoid steels respectively, progressively
crystallizes out as a network or skeleton within the austenite
mother-metal, which thus progressively approaches the composition of
hardenite, reaching it at Ar1, and there splitting up into ferrite and
cementite interstratified as pearlite. Further, any ferrite liberated
at Ar3 changes there from [gamma] to [beta], and any present at Ar2
changes from [beta] to [alpha]. Between H and S, Ar3 and Ar2 occur
together, as do Ar2 and Ar1 between S and P' and Ar3, Ar2 and Ar1 at S
itself; so that these critical points in these special cases are
called Ar_(3-2), Ar_(2-1) and Ar_(3-2-1) respectively. The
corresponding critical points which occur during rise of temperature,
with the reverse transformations, are called Ac1, Ac2, Ac3, &c. A
(Tschernoff) is the generic name, r refers to falling temperature
(_refroidissant_) and c to rising temperature (_chauffant_, Osmond).

24. The freezing of molten cast iron of 2.50% of carbon goes on
selectively like that of these steels which we have been studying,
till the enrichment of the molten mother-metal in carbon brings its
carbon-contents to B, 4.30%, the eutectic[3] carbon-content, i.e. that
of the greatest fusibility or lowest melting-point. At this point
selection ceases; the remaining molten metal freezes as a whole, and
in freezing splits up into a conglomerate eutectic of (1) austenite of
about 2.2 % of carbon, and therefore saturated with that element, and
(2) cementite; and with this eutectic is mixed the "primary" austenite
which froze out as the temperature sank from v to v'. The white-hot,
solid, but soft mass is now a conglomerate of (1) "primary" austenite,
(2) "eutectic" austenite and (3) "eutectic" cementite. As the
temperature sinks still farther, pro-eutectoid cementite (see S 22)
forms progressively in the austenite both primary and eutectic, and
this pro-eutectoid cementite as it comes into existence tends to
assemble in the form of a network enveloping the kernels or grains of
the austenite from which it springs. The reason for its birth, of
course, is that the solubility of carbon in austenite progressively
decreases as the temperature falls, from about 2.2% at 1130 deg. (a),
to 0.90% at 690 deg. (Ar1), as shown by the line aS, with the
consequence that the austenite keeps rejecting in the form of this
pro-eutectoid cementite all carbon in excess of its saturation-point
for the existing temperature. Here the mass consists of (1) primary
austenite, (2) eutectic austenite and cementite interstratified and
(3) pro-eutectoid cementite.

This formation of cementite through the rejection of carbon by both
the primary and the eutectic austenite continues quite as in the case
of 1.00% carbon steel, with impoverishment of the austenite to the
hardenite or eutectoid ratio, and the splitting up of that hardenite
into pearlite at Ar1, so that the mass when cold finally consists of
(1) the primary austenite now split up into kernels of pearlite
surrounded by envelopes of pro-eutectoid cementite, (2) the eutectic
of cementite plus austenite, the latter of which has in like manner
split up into a mixture of pearlite plus cementite. Such a mass is
shown in fig. 4. Here the black bat-like patches are the masses of
pearlite plus pro-eutectoid cementite resulting from the splitting up
of the primary austenite. The magnification is too small to show the
zebra striping of the pearlite. In the black-and-white ground mass the
white is the eutectic cementite, and the black the eutectic austenite,
now split up into pearlite and pro-eutectoid cementite, which cannot
here be distinguished from each other.

25. As we pass to cases with higher and higher carbon-content, the
primary austenite which freezes in cooling across region 2 forms a
smaller and smaller proportion of the whole, and the
austenite-cementite eutectic which forms at the eutectic
freezing-point, 1130 deg. (aB), increases in amount until, when the
carbon-content reaches the eutectic ratio, 4.30%, there is but a
single freezing-point, and the whole mass when solid is made up of
this eutectic. If there is more than 4.30% of carbon, then in cooling
through region 3 the excess of carbon over this ratio freezes out as
"primary" cementite. But in any event the changes which have just been
described for cast iron of 2.50% of carbon occur in crossing region 7,
and at Ar1 (PSP').

Just as variations in the carbon-content shift the temperature of the
freezing-range and of the various critical points, so do variations in
the content of other elements, notably silicon, phosphorus, manganese,
chromium, nickel and tungsten. Nickel and manganese lower these
critical points, so that with 25% of nickel Ar3 lies below the common
temperature 20 deg. C. With 13% of manganese Ar3 is very low, and the
austenite decomposes so slowly that it is preserved practically intact
by sudden cooling. These steels then normally consist of [gamma]-iron,
modified by the large amount of nickel or manganese with which it is
alloyed. They are non-magnetic or very feebly magnetic. But the
critical points of such nickel steel though thus depressed, are not
destroyed; and if it is cooled in liquid air below its Ar2, it passes
to the [alpha] state and becomes magnetic.

26. _Double Nature of the Carbon-Iron Diagram._--The part played by
graphite in the constitution of the iron-carbon compounds, hitherto
ignored for simplicity, is shown in fig. 5. Looking at the matter in a
broad way, in all these carbon-iron alloys, both steel and cast irons,
part of the carbon may be dissolved in the iron, usually as austenite,
e.g. in regions 2, 4, 5 and 7 of Fig. 1; the rest, i.e. the carbon
which is not dissolved, or the "undissolved carbon," forms either the
definite carbide, cementite, Fe3C, or else exists in the free state as
graphite. Now, just as fig. 1 shows the constitution of these
iron-carbon alloys for all temperatures and all percentages of carbon
when the undissolved carbon exists as cementite, so there should be a
diagram showing this constitution when all the undissolved carbon
exists as graphite. In short, there are two distinct carbon-iron
diagrams, the iron-cementite one shown in fig. 1 and studied at length
in SS 22 to 25, and the iron-graphite one shown in fig. 5 in unbroken
lines, with the iron-cementite diagram reproduced in broken lines for
comparison. What here follows represents our present rather
ill-established theory. These two diagrams naturally have much the
same general shape, but though the boundaries of the several regions
in the iron-cementite diagram are known pretty accurately, and though
the relative positions of the boundaries of the two diagrams are
probably about as here shown, the exact topography of the
iron-graphite diagram is not yet known. In it the normal constituents
are, for region II., molten metal + primary austenite; for region
III., molten metal + primary graphite; for region IV., primary
austenite; for region VII., eutectic austenite, eutectic graphite, and
a quantity of pro-eutectoid graphite which increases as we pass from
the upper to the lower part of the region, together with primary
austenite at the left of the eutectic point B' and primary graphite at
the right of that point. Thus when iron containing 2.50% of carbon (v.
fig. 1) solidifies, its carbon may form cementite following the
cementite-austenite diagram so that white, i.e. cementitiferous, cast
iron results; or graphite, following the graphite-austenite diagram,
so that ultra-grey, i.e. typical graphitic cast iron results; or, as
usually happens, certain molecules may follow one diagram while the
rest follow the other diagram, so that cast iron which has both
cementite and graphite results, as in most commercial grey cast iron,
and typically in "mottled cast iron," in which there are distinct
patches of grey and others of white cast iron.

Though carbon passes far more readily under most conditions into the
state of cementite than into that of graphite, yet of the two graphite
is the more stable and cementite the less stable, or the "metastable"
form. Thus cementite is always tending to change over into graphite by
the reaction Fe3C = 3Fe + Gr, though this tendency is often held in
check by different causes; but graphite never changes back directly
into cementite, at least according to our present theory. The fact
that graphite may dissolve in the iron as austenite, and that when
this latter again breaks up it is more likely to yield cementite than
graphite, is only an apparent and not a real exception to this law of
the greater stability of graphite than of cementite.

Slow cooling, slow solidification, the presence of an abundance of
carbon, and the presence of silicon, all favour the formation of
graphite; rapid cooling, the presence of sulphur, and in most cases
that of manganese, favour the formation of cementite. For instance,
though in cast iron, which is rich in carbon, that carbon passes
comparatively easily into the state of graphite, yet in steel, which
contains much less carbon, but little graphite forms under most
conditions. Indeed, in the common structural steels which contain only
very little carbon, hardly any of that carbon exists as graphite.

27. _Thermal Treatment._--The hardening, tempering and annealing of
steel, the chilling and annealing of cast iron, and the annealing of
malleable cast iron are explained readily by the facts just set forth.

28. _The hardening of steel_ consists in first transforming it into
austenite by heating it up into region 4 of fig. 1, and then quenching
it, usually in cold water, so as to cool it very suddenly, and thus to
deny the time which the complete transformation of the austenite into
ferrite and cementite requires, and thereby to catch much of the iron
in transit in the hard brittle [beta] state. In the cold this
transformation cannot take place, because of molecular rigidity or
some other impediment. The suddenly cooled metal is hard and brittle,
because the cold [beta]-iron which it contains is hard and brittle.

The degree of hardening which the steel undergoes increases with its
carbon-content, chiefly because, during sudden cooling, the presence
of carbon acts like a brake to impede the transformations, and thus to
increase the quantity of [beta]-iron caught in transit, but probably
also in part because the hardness of this [beta]-iron increases with
its carbon-content. Thus, though sudden cooling has very little effect
on steel of 0.10% of carbon, it changes that of 1.50% from a somewhat
ductile body to one harder and more brittle than glass.

29. _The Tempering and Annealing of Steel._--But this sudden cooling
goes too far, preserving so much [beta]-iron as to make the steel too
brittle for most purposes. This brittleness has therefore in general
to be mitigated or "tempered," unfortunately at the cost of losing
part of the hardness proper, by reheating the hardened steel slightly,
usually to between 200 deg. and 300 deg. C., so as to relax the
molecular rigidity and thereby to allow the arrested transformation to
go on a little farther, shifting a little of the [beta]-iron over into
the [alpha] state. The higher the tempering-temperature, i.e. that to
which the hardened steel is thus reheated, the more is the molecular
rigidity relaxed, the farther on does the transformation go, and the
softer does the steel become; so that, if the reheating reaches a
dull-red heat, the transformation from austenite into ferrite and
cementite completes itself slowly, and when now cooled the steel is as
soft and ductile as if it had never been hardened. It is now said to
be "annealed."