FOOTNOTE:_Encyclopaedia Britannica, 11th Edition, "Ireland" to "Isabey, Jean Baptiste" Volume 14, Slice 7

[1] By solution in concentrated hydrochloric acid, a yellow liquid is

obtained, which on concentration over sulphuric acid gives yellow

deliquescent crusts of ferroso-ferric chloride, Fe3Cl8.18H2O.

IRON AGE, the third of the three periods, Stone, Bronze and Iron Ages,

into which archaeologists divide prehistoric time; the weapons, utensils

and implements being as a general rule made of iron (see ARCHAEOLOGY).

The term has no real chronological value, for there has been no

universal synchronous sequence of the three epochs in all quarters of

the world. Some countries, such as the islands of the South Pacific, the

interior of Africa, and parts of North and South America, have passed

direct from the Stone to the Iron Age. In Europe the Iron Age may be

said to cover the last years of the prehistoric and the early years of

the historic periods. In Egypt, Chaldaea, Assyria, China, it reaches far

back, to perhaps 4000 years before the Christian era. In Africa, where

there has been no Bronze Age, the use of iron succeeded immediately the

use of stone. In the Black Pyramid of Abusir (VIth Dynasty), at least

3000 B.C., Gaston Maspero found some pieces of iron, and in the funeral

text of Pepi I. (about 3400 B.C.) the metal is mentioned. The use of

iron in northern Europe would seem to have been fairly general long

before the invasion of Caesar. But iron was not in common use in Denmark

until the end of the 1st century A.D. In the north of Russia and Siberia

its introduction was even as late as A.D. 800, while Ireland enters upon

her Iron Age about the beginning of the 1st century. In Gaul, on the

other hand, the Iron Age dates back some 800 years B.C.; while in

Etruria the metal was known some six centuries earlier. Homer represents

Greece as beginning her Iron Age twelve hundred years before our era.

The knowledge of iron spread from the south to the north of Europe. In

approaching the East from the north of Siberia or from the south of

Greece and the Troad, the history of iron in each country eastward is

relatively later; while a review of European countries from the north

towards the south shows the latter becoming acquainted with the metal

earlier than the former It is suggested that these facts support the

theory that it is from Africa that iron first came into use. The finding

of worked iron in the Great Pyramids seems to corroborate this view. The

metal, however, is singularly scarce in collections of Egyptian

antiquities. The explanation of this would seem to lie in the fact that

the relics are in most cases the paraphernalia of tombs, the funereal

vessels and vases, and iron being considered an impure metal by the

ancient Egyptians it was never used in their manufacture of these or for

any religious purposes. This idea of impurity would seem a further proof

of the African origin of iron. It was attributed to Seth, the spirit of

evil who according to Egyptian tradition governed the central deserts of

Africa. The Iron Age in Europe is characterized by an elaboration of

designs in weapons, implements and utensils. These are no longer cast

but hammered into shape, and decoration is elaborate curvilinear rather

than simple rectilinear, the forms and character of the ornamentation

of the northern European weapons resembling in some respects Roman arms,

while in others they are peculiar and evidently representative of

northern art. The dead were buried in an extended position, while in the

preceding Bronze Age cremation had been the rule.

See Lord Avebury, _Prehistoric Times_ (1865; 1900); Sir J. Evans,

_Ancient Stone Implements_ (1897); _Horae Ferales, or Studies in the

Archaeology of Northern Nations_, by Kemble (1863); Gaston C. C.

Maspero, _Guide du Musee de Boulaq_, 296; _Scotland in Pagan

Times--The Iron Age_, by Joseph Anderson (1883).

IRON AND STEEL.[1] 1. Iron, the most abundant and the cheapest of the

heavy metals, the strongest and most magnetic of known substances, is

perhaps also the most indispensable of all save the air we breathe and

the water we drink. For one kind of meat we could substitute another;

wool could be replaced by cotton, silk or fur; were our common silicate

glass gone, we could probably perfect and cheapen some other of the

transparent solids; but even if the earth could be made to yield any

substitute for the forty or fifty million tons of iron which we use each

year for rails, wire, machinery, and structural purposes of many kinds,

we could not replace either the steel of our cutting tools or the iron

of our magnets, the basis of all commercial electricity. This usefulness

iron owes in part, indeed, to its abundance, through which it has led us

in the last few thousands of years to adapt our ways to its properties;

but still in chief part first to the single qualities in which it

excels, such as its strength, its magnetism, and the property which it

alone has of being made at will extremely hard by sudden cooling and

soft and extremely pliable by slow cooling; second, to the special

combinations of useful properties in which it excels, such as its

strength with its ready welding and shaping both hot and cold; and

third, to the great variety of its properties. It is a very Proteus. It

is extremely hard in our files and razors, and extremely soft in our

horse-shoe nails, which in some countries the smith rejects unless he

can bend them on his forehead; with iron we cut and shape iron. It is

extremely magnetic and almost non-magnetic; as brittle as glass and

almost as pliable and ductile as copper; extremely springy, and

springless and dead; wonderfully strong, and very weak; conducting heat

and electricity easily, and again offering great resistance to their

passage; here welding readily, there incapable of welding; here very

infusible, there melting with relative ease. The coincidence that so

indispensable a thing should also be so abundant, that an iron-needing

man should be set on an iron-cored globe, certainly suggests design. The

indispensableness of such abundant things as air, water and light is

readily explained by saying that their very abundance has evolved a

creature dependent on them. But the indispensable qualities of iron did

not shape man's evolution, because its great usefulness did not arise

until historic times, or even, as in case of magnetism, until modern

times.

These variations in the properties of iron are brought about in part by

corresponding variations in mechanical and thermal treatment, by which

it is influenced profoundly, and in part by variations in the

proportions of certain foreign elements which it contains; for, unlike

most of the other metals, it is never used in the pure state. Indeed

pure iron is a rare curiosity. Foremost among these elements is carbon,

which iron inevitably absorbs from the fuel used in extracting it from

its ores. So strong is the effect of carbon that the use to which the

metal is put, and indeed its division into its two great classes, the

malleable one, comprising steel and wrought iron, with less than 2.20%

of carbon, and the unmalleable one, cast iron, with more than this

quantity, are based on carbon-content. (See Table I.)

TABLE 1.--_General Classification of Iron and Steel according (1) to

Carbon-Content and (2) to Presence or Absence of Inclosed Slag._

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

| | 0.30%). | Carbon (say, between | 2.2 to 5%). |

| | | 0.30 and 2.2%). | |

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

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

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

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

* The term "Alloy Cast Irons" is not actually in frequent use, not

because of any question as to its fitness or meaning, but because

the need of such a generic term rarely arises in the industry.

2. _Nomenclature._--Until about 1860 there were only three important

classes of iron--wrought iron, steel and cast iron. The essential

characteristic of wrought iron was its nearly complete freedom from

carbon; that of steel was its moderate carbon-content (say between 0.30

and 2.2%), which, though great enough to confer the property of being

rendered intensely hard and brittle by sudden cooling, yet was not so

great but that the metal was malleable when cooled slowly; while that of

cast iron was that it contained so much carbon as to be very brittle

whether cooled quickly or slowly. This classification was based on

carbon-content, or on the properties which it gave. Beyond this, wrought

iron, and certain classes of steel which then were important,

necessarily contained much slag or "cinder," because they were made by

welding together pasty particles of metal in a bath of slag, without

subsequent fusion. But the best class of steel, crucible steel, was

freed from slag by fusion in crucibles; hence its name, "cast steel."

Between 1860 and 1870 the invention of the Bessemer and open-hearth

processes introduced a new class of iron to-day called "mild" or

"low-carbon steel," which lacked the essential property of steel, the

hardening power, yet differed from the existing forms of wrought iron in

freedom from slag, and from cast iron in being very malleable. Logically

it was wrought iron, the essence of which was that it was (1) "iron" as

distinguished from steel, and (2) malleable, i.e. capable of being

"wrought." This name did not please those interested in the new product,

because existing wrought iron was a low-priced material. Instead of

inventing a wholly new name for the wholly new product, they

appropriated the name "steel," because this was associated in the public

mind with superiority. This they did with the excuse that the new

product resembled one class of steel--cast steel--in being free from

slag; and, after a period of protest, all acquiesced in calling it

"steel," which is now its firmly established name. The old varieties of

wrought iron, steel and cast iron preserve their old names; the new

class is called steel by main force. As a result, certain varieties,

such as blister steel, are called "steel" solely because they have the

hardening power, and others, such as low-carbon steel, solely because

they are free from slag. But the former lack the essential quality,

slaglessness, which makes the latter steel, and the latter lack the

essential quality, the hardening power, which makes the former steel.

"Steel" has come gradually to stand rather for excellence than for any

specific quality. These anomalies, however confusing to the general

reader, in fact cause no appreciable trouble to important makers or

users of iron and steel, beyond forming an occasional side-issue in

litigation.

3. _Definitions._--_Wrought iron_ is slag-bearing malleable iron,

containing so little carbon (0.30% or less), or its equivalent, that it

does not harden greatly when cooled suddenly.

_Steel_ is iron which is malleable at least in some one range of

temperature, and also is either (a) cast into an initially malleable

mass, or (b) is capable of hardening greatly by sudden cooling, or (c)

is both so cast and so capable of hardening. (Tungsten steel and certain

classes of manganese steel are malleable only when red-hot.) Normal or

carbon steel contains between 0.30 and 2.20% of carbon, enough to make

it harden greatly when cooled suddenly, but not enough to prevent it

from being usefully malleable when hot.

_Cast iron_ is, generically, iron containing so much carbon (2.20% or

more) or its equivalent that it is not usefully malleable at any

temperature. Specifically, it is cast iron in the form of castings other

than pigs, or remelted cast iron suitable for such castings, as

distinguished from pig iron, i.e. the molten cast iron as it issues from

the blast furnace, or the pigs into which it is cast.

_Malleable cast iron_ is iron which has been cast in the condition of

cast iron, and made malleable by subsequent treatment without fusion.

_Alloy steels_ and _cast irons_ are those which owe their properties

chiefly to the presence of one or more elements other than carbon.

_Ingot iron_ is slagless steel with less than 0.30% of carbon.

_Ingot steel_ is slagless steel containing more than 0.30% of carbon.

_Weld steel_ is slag-bearing iron malleable at least at some one

temperature, and containing more than 0.30% of carbon.

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.

[Illustration: FIG. 1.--Roberts-Austen or Carbon-Iron diagram. The

Cementite-Austenite or Metastable form.]

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.

[Illustration: FIG. 2.--Relation between the carbon-content and the

percentage of the several constituents of slowly cooled steel and

white cast iron.]

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.

[Illustration: FIG. 3.--Physical properties and assumed microscopic

constitution of the pearlite series, graphiteless steel slowly cooled

and white cast iron. By "total ferrite" is meant both that which forms

part of the pearlite and that which is in excess of the pearlite,

taken jointly. So with the "total cementite."]

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.

[Illustration: FIG. 4.--The constitution of hypo-eutectic white or

cementitiferous cast iron (washed metal), W. Campbell. The black

bat-like areas are the primary austenite, the zebra-marked ground mass

the eutectic, composed of white stripes of cementite and black stripes

of austenite. Both the primary and eutectic austenite have changed in

cooling into a mixture of pearlite and pro-eutectoid cementite, too

fine to be distinguished here.]

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.

[Illustration: FIG. 5.--Graphite-austenite or stable carbon-iron,

diagram.]

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."

30. _Chilling cast iron_, i.e. hastening its cooling by casting it in

a cool mould, favours the formation of cementite rather than of

graphite in the freezing of the eutectic at aBc, and also, in case of

hyper-eutectic iron, in the passage through region 3. Like the

hardening of steel, it hinders the transformation of the austenite,

whether primary or eutectic, into pearlite + cementite, and thus

catches part of the iron in transit in the hard [beta] state. The

annealing of such iron may occur in either of two degrees--a small

one, as in making common chilled cast iron objects, such as railway

car wheels, or a great one, as in making malleable cast iron. In the

former case, the objects are heated only to the neighbourhood of Ac1,

say to 730 deg. C., so that the [beta]-iron may slip into the a state,

and the transformation of the austenite into pearlite and cementite

may complete itself. The joint effect of such chilling and such

annealing is to make the metal much harder than if slowly cooled,

because for each 1% of graphite which the chilling suppresses, 15% of

the glass-hard cementite is substituted. Thus a cast iron which, if

cooled slowly, would have been "grey," i.e. would have consisted

chiefly of graphite with pearlite and ferrite (which are all

relatively soft bodies), if thus chilled and annealed consists of

cementite and pearlite. But in most such cases, in spite of the

annealing, this hardness is accompanied by a degree of brittleness too

great for most purposes. The process therefore is so managed that only

the outer shell of the casting is chilled, and that the interior

remains graphitic, i.e. grey cast iron, soft and relatively malleable.

31. In making _malleable castings_ the annealing, i.e. the change

towards the stable state of ferrite + graphite, is carried much

farther by means of a much longer and usually a higher heating than in

the manufacture of chilled castings. The castings, initially of white

cast iron, are heated for about a week, to a temperature usually above

730 deg. C. and often reaching 900 deg. C. (1346 deg. and 1652 deg.

F.). For about 60 hours the heat is held at its highest point, from

which it descends extremely slowly. The molecular freedom which this

high temperature gives enables the cementite to change gradually into

a mixture of graphite and austenite with the result that, after the

castings have been cooled and their austenite has in cooling past Ac1

changed into pearlite and ferrite, the mixture of cementite and

pearlite of which they originally consisted has now given place to one

of fine or "temper" graphite and ferrite, with more or less pearlite

according to the completeness of the transfer of the carbon to the

state of graphite.

Why, then, is this material malleable, though the common grey cast

iron, which is made up of about the same constituents and often in

about the same proportion, is brittle? The reason is that the

particles of temper graphite which are thus formed within the solid

casting in its long annealing are so finely divided that they do not

break up the continuity of the mass in a very harmful way; whereas in

grey cast iron both the eutectic graphite formed in solidifying, and

also the primary graphite which, in case the metal is hyper-eutectic,

forms in cooling through region 3 of fig. 1, surrounded as it is by

the still molten mother-metal out of which it is growing, form a

nearly continuous skeleton of very large flakes, which do break up in

a most harmful way the continuity of the mass of cast iron in which

they are embedded.

In carrying out this process the castings are packed in a mass of iron

oxide, which at this temperature gradually removes the fine or

"temper" graphite by oxidizing that in the outer crust to carbonic

oxide, whereon the carbon farther in begins diffusing outwards by

"molecular migration," to be itself oxidized on reaching the crust.

This removal of graphite doubtless further stimulates the formation of

graphite, by relieving the mechanical and perhaps the osmotic

pressure. Thus, first, for the brittle glass-hard cementite there is

gradually substituted the relatively harmless temper graphite; and,

second, even this is in part removed by surface oxidation.

32. _Fineness of Structure._--Each of these ancient processes thus

consists essentially in so manipulating the temperature that, out of

the several possible constituents, the metal shall actually consist of

a special set in special proportions. But in addition there is another

very important principle underlying many of our thermal processes,

viz. that the state of aggregation of certain of these constituents,

and through it the properties of the metal as a whole, are profoundly

affected by temperature manipulations. Thus, prior exposure to a

temperature materially above Ac3 coarsens the structure of most steel,

in the sense of giving it when cold a coarse fracture, and enlarging

the grains of pearlite, &c., later found in the slowly cooled metal.

This coarsening and the brittleness which accompanies it increase with

the temperature to which the metal has been exposed. Steel which after

a slow cooling from about 722 deg. C. will bend 166 deg. before

breaking, will, after slow cooling from about 1050 deg. C., bend only

18 deg. before breaking. This injury fortunately can be cured either

by _reheating_ the steel to Ac3 when it "refines," i.e. returns

spontaneously to its fine-grained ductile state (_cooling_ past Ar3

does not have this effect); or by breaking up the coarse grains by

_mechanical distortion_, e.g. by forging or rolling. For instance, if

steel has been coarsened by heating to 1400 deg. C., and if, when it

has cooled to a lower temperature, say 850 deg. C. we forge it, its

grain-size and ductility when cold will be approximately those which

it would have had if heated only to 850 deg. Hence steel which has

been heated very highly, whether for welding, or for greatly softening

it so that it can be rolled to the desired shape with but little

expenditure of power, ought later to be refined, either by reheating

it from below Ar3 to slightly above Ac3 or by rolling it after it has

cooled to a relatively low temperature, i.e. by having a low

"finishing temperature." Steel castings have initially the extremely

coarse structure due to cooling without mechanical distortion from

their very high temperature of solidification; they are "annealed,"

i.e. this coarseness and the consequent brittleness are removed, by

reheating them much above Ac3, which also relieves the internal

stresses due to the different rates at which different layers cool,

and hence contract, during and after solidification. For steel

containing less than about 0.13% of carbon, the embrittling

temperature is in a different range, near 700 deg. C., and such steel

refines at temperatures above 900 deg. C.

33. _The Possibilities of Thermal Treatment._--When we consider the

great number of different regions in fig. 1, each with its own set of

constitutents, and remember that by different rates of cooling from

different temperatures we can retain in the cold metal these different

sets of constituents in widely varying proportions; and when we further

reflect that not only the proportion of each constituent present but

also its state of aggregation can be controlled by thermal treatment, we

see how vast a field is here opened, how great a variety of different

properties can be induced in any individual piece of steel, how enormous

the variety of properties thus attainable in the different varieties

collectively, especially since for each percentage of carbon an

incalculable number of varieties of steel may be made by alloying it

with different proportions of such elements as nickel, chromium, &c. As

yet there has been only the roughest survey of certain limited areas in

this great field, the further exploration of which will enormously

increase the usefulness of this wonderful metal.

34. _Alloy steels_ have come into extensive use for important special

purposes, and a very great increase of their use is to be expected. The

chief ones are nickel steel, manganese steel, chrome steel and

chrome-tungsten steel. The general order of merit of a given variety or

specimen of iron or steel may be measured by the degree to which it

combines strength and hardness with ductility. These two classes of

properties tend to exclude each other, for, as a general rule, whatever

tends to make iron and steel hard and strong tends to make it

correspondingly brittle, and hence liable to break treacherously,

especially under shock. Manganese steel and nickel steel form an

important exception to this rule, in being at once very strong and hard

and extremely ductile. _Nickel steel_, which usually contains from 3 to

3.50% of nickel and about 0.25% of carbon, combines very great tensile

strength and hardness, and a very high limit of elasticity, with great

ductility. Its combination of ductility with strength and hardening

power has given it very extended use for the armour of war-vessels. For

instance, following Krupp's formula, the side and barbette armour of

war-vessels is now generally if not universally made of nickel steel

containing about 3.25% of nickel, 0.40% of carbon, and 1.50% of

chromium, deeply carburized on its impact face. Here the merit of nickel

steel is not so much that it resists perforation, as that it does not

crack even when deeply penetrated by a projectile. The combination of

ductility, which lessens the tendency to break when overstrained or

distorted, with a very high limit of elasticity, gives it great value

for shafting, the merit of which is measured by its endurance of the

repeated stresses to which its rotation exposes it whenever its

alignment is not mathematically straight. The alignment of marine

shafting, changing with every passing wave, is an extreme example. Such

an intermittently applied stress is far more destructive to iron than a

continuous one, and even if it is only half that of the limit of

elasticity, its indefinite repetition eventually causes rupture. In a

direct competitive test the presence of 3.25% of nickel increased nearly

sixfold the number of rotations which a steel shaft would endure before

breaking.

35. As actually made, _manganese steel_ contains about 12% of manganese

and 1.50% of carbon. Although the presence of 1.50% of manganese makes

steel relatively brittle, and although a further addition at first

increases this brittleness, so that steel containing between 4 and 5.5%

can be pulverized under the hammer, yet a still further increase gives

very great ductility, accompanied by great hardness--a combination of

properties which was not possessed by any other known substance when

this remarkable alloy, known as Hadfield's manganese steel, was

discovered. Its ductility, to which it owes its value, is profoundly

affected by the rate of cooling. Sudden cooling makes the metal

extremely ductile, and slow cooling makes it brittle. Its behaviour in

this respect is thus the opposite of that of carbon steel. But its great

hardness is not materially affected by the rate of cooling. It is used

extensively for objects which require both hardness and ductility, such

as rock-crushing machinery, railway crossings, mine-car wheels and

safes. The burglar's blow-pipe locally "draws the temper," i.e. softens

a spot on a hardened carbon steel or chrome steel safe by simply heating

it, so that as soon as it has again cooled he can drill through it and

introduce his charge of dynamite. But neither this nor any other

procedure softens manganese steel rapidly. Yet this very fact that it is

unalterably hard has limited its use, because of the great difficulty of

cutting it to shape, which has in general to be done with emery wheels

instead of the usual iron-cutting tools. Another defect is its

relatively low elastic limit.

36. _Chrome steel_, which usually contains about 2% of chromium and 0.80

to 2% of carbon, owes its value to combining, when in the "hardened" or

suddenly cooled state, intense hardness with a high elastic limit, so

that it is neither deformed permanently nor cracked by extremely violent

shocks. For this reason it is the material generally if not always used

for armour-piercing projectiles. It is much used also for certain

rock-crushing machinery (the shoes and dies of stamp-mills) and for

safes. These are made of alternate layers of soft wrought iron and

chrome steel hardened by sudden cooling. The hardness of the hardened

chrome steel resists the burglar's drill, and the ductility of the

wrought iron the blows of his sledge.

Vanadium in small quantities, 0.15 or 0.20%, is said to improve steel

greatly, especially in increasing its resistance to shock and to

often-repeated stress. But the improvement may be due wholly to the

considerable chromium content of these so-called vanadium steels.

37. _Tungsten steel_, which usually contains from 5 to 10% of tungsten

and from 1 to 2% of carbon, is used for magnets, because of its great

retentivity.

38. _Chrome-tungsten or High-speed Steel._--Steel with a large content

of both chromium and tungsten has the very valuable property of

"red-hardness," i.e. of retaining its hardness and hence its power of

cutting iron and other hard substances, even when it is heated to dull

redness, say 600 deg. C. (1112 deg. F.) by the friction of the work

which it is doing. Hence a machinist can cut steel or iron nearly six

times as fast with a lathe tool of this steel as with one of carbon

steel, because with the latter the cutting speed must be so slow that

the cutting tool is not heated by the friction above say 250 deg. C.

(482 deg. F.), lest it be unduly softened or "tempered" (S 29). This

effect of chromium, tungsten and carbon jointly consists essentially in

raising the "tempering temperature," i.e. that to which the metal, in

which by suitable thermal treatment the iron molecules have been brought

to the allotropic [gamma] or [beta] state or a mixture of both, can be

heated without losing its hardness through the escape of that iron into

the [alpha] state. In short, these elements seem to impede the

allotropic change of the iron itself. The composition of this steel is

as follows:--

The usual limits. Apparently the best.

Carbon 0.32 to 1.28 0.68 to 0.67

Manganese 0.03 " 0.30 0.07 " 0.11

Chromium 2.23 " 7.02 5.95 " 5.47

Tungsten 9.25 " 25.45 17.81 " 18.19

39. _Impurities._--The properties of iron and steel, like those of most

of the metals, are profoundly influenced by the presence of small and

sometimes extremely small quantities of certain impurities, of which the

most important are phosphorus and sulphur, the former derived chiefly

from apatite (phosphate of lime) and other minerals which accompany the

iron ore itself, the latter from the pyrite found not only in most iron

ores but in nearly all coal and coke. All commercial iron and steel

contain more or less of both these impurities, the influence of which is

so strong that a variation of 0.01%, i.e. of one part in 10,000, of

either of them has a noticeable effect. The best tool steel should not

contain more than 0.02% of either, and in careful practice it is often

specified that the phosphorus and sulphur respectively shall not exceed

0.04 and 0.05% in the steel for important bridges, or 0.06 and 0.07% in

rail steel, though some very prudent engineers allow as much as .085% or

even 0.10% of phosphorus in rails.

40. The specific effect of _phosphorus_ is to make the metal cold-short,

i.e. brittle in the cold, apparently because it increases the size and

the sharpness of demarcation of the crystalline grains of which the mass

is made up. The specific effect of _sulphur_ is to make the metal

red-short, i.e. brittle, when at a red heat, by forming a network of

iron sulphide which encases these crystalline grains and thus plays the

part of a weak link in a strong chain.

41. _Oxygen_, probably dissolved in the iron as ferrous oxide FeO, also

makes the metal red-short.

42. _Manganese_ by itself rather lessens than increases the

malleableness and, indeed, the general merit of the metal, but it is

added intentionally, in quantities even as large as 1.5% to palliate the

effects of sulphur and oxygen. With sulphur it forms a sulphide which

draws together into almost harmless drops, instead of encasing the

grains of iron. With oxygen it probably forms manganous oxide, which is

less harmful than ferrous oxide. (See S 35.)

43. _Ores of Iron._--Even though the earth seems to be a huge iron

meteor with but a thin covering of rocks, the exasperating proneness of

iron to oxidize explains readily why this metal is only rarely found

native, except in the form of meteorites. They are four important iron

ores, magnetite, haematite, limonite and siderite, and one of less but

still considerable importance, pyrite or pyrites.

44. _Magnetite_, Fe3O4, contains 72.41% of iron. It crystallizes in

the cubical system, often in beautiful octahedra and rhombic

dodecahedra. It is black with a black streak. Its specific gravity is

5.2, and its hardness 5.5 to 6.5. It is very magnetic, and sometimes

polar.

45. _Haematite_, or red haematite, Fe2O3, contains 70% of iron. It

crystallizes in the rhombohedral system. Its colour varies from

brilliant bluish-grey to deep red. Its streak is always red. Its

specific gravity is 5.3 and its hardness 5.5 to 6.5.

46. _Limonite_, 2Fe2O3, 3H2O, contains 59.9% of iron. Its colour

varies from light brown to black. Its streak is yellowish-black, its

specific gravity 3.6 to 4.0, and its hardness 5 to 5.5. Limonite and

the related minerals, turgite, 2Fe2O3 + H2O, and gothite, Fe2O3 + H2O,

are grouped together under the term "brown haematite."

47. _Siderite_, or spathic iron ore, FeCO3, crystallizes in the

rhombohedral system and contains 48.28% of iron. Its colour varies

from yellowish-brown to grey. Its specific gravity is 3.7 to 3.9, and

its hardness 3.5 to 4.5. The clayey siderite of the British coal

measures is called "clay band," and that containing bituminous matter

is called "black band."

48. _Pyrite_, FeS2, contains 46.7% of iron. It crystallizes in the

cubic system, usually in cubes, pentagonal dodecahedra or octahedra,

often of great beauty and perfection. It is golden-yellow, with a

greenish or brownish-black streak. Its specific gravity is 4.83 to

5.2, its hardness 6 to 6.5. Though it contains far too much sulphur to

be used in iron manufacture without first being desulphurized, yet

great quantities of slightly cupriferous pyrite, after yielding nearly

all their sulphur in the manufacture of sulphuric acid, and most of

the remainder in the wet extraction of their copper, are then used

under the name of "blue billy" or "purple ore," as an ore of iron, a

use which is likely to increase greatly in importance with the gradual

exhaustion of the richest deposits of the oxidized ores.

49. _The Ores actually Impure._--As these five minerals actually exist

in the earth's crust they are usually more or less impure chemically,

and they are almost always mechanically mixed with barren mineral

matter, such as quartz, limestone and clay, collectively called "the

gangue." In some cases the iron-bearing mineral, such as magnetite or

haematite, can be separated from the gangue after crashing, either

mechanically or magnetically, so that the part thus enriched or

"concentrated" alone need be smelted.

50. _Geological Age._--The Archaean crystalline rocks abound in deposits

of magnetite and red haematite, many of them very large and rich. These

of course are the oldest of our ores, and from deposits of like age,

especially those of the more readily decomposed silicates, has come the

iron which now exists in the siderites and red and brown haematites of

the later geological formations.

51. _The World's Supply of Iron Ore._--The iron ores of the earth's

crust will probably suffice to supply our needs for a very long period,

perhaps indeed for many thousand years. It is true that an official

statement, which is here reproduced, given in 1905 by Professor

Tornebohm to the Swedish parliament, credited the world with only

10,000,000,000 tons of ore, and that, if the consumption of iron should

continue to increase hereafter as it did between 1893 and 1906, this

quantity would last only until 1946. How then can it be that there is a

supply for thousands of years? The two assertions are not to be

reconciled by pointing out that Professor Tornebohm underestimated, for

instance crediting the United States with only 1.1 billion tons, whereas

the United States Geological Survey's expert credits that country with

from ten to twenty times this quantity; nor by pointing out that only

certain parts of Europe and a relatively small part of North America

have thus far been carefully explored for iron ore, and that the rest of

these two continents and South America, Asia and Africa may reasonably

be expected to yield very great stores of iron, and that pyrite, one of

the richest and most abundant of ores, has not been included. Important

as these considerations are, they are much less important than the fact

that a very large proportion of the rocks of the earth's crust contain

more or less iron, and therefore are potential iron ores.

Encyclopaedia Britannica, 11th Edition, "Ireland" to "Isabey, Jean Baptiste" Volume 14, Slice 7 · 第 15 / 27 题