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1911 Encyclopedia Britannica

Iron And Steel

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IRON AND STEEL.' 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; but still in chief part first to the single qualities in which it 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.

Containing very little Carbon (say,

less than 0.30 / o).

Containing an Intermediate

Quantity of Carbon (say, between

o)

O.30 an d2.2/o.

Containing much Carbon (say,

from 2.2 to 5 /o).

Slag-bearing or

WROUGHT IRON.

WELD STEEL.

" Weld-metal " Series.

Puddled and bloomary, or Charcoal-

hearth iron belong here,

Puddled and blister steel

belong here.

LOW-CARBON or MILD STEEL,

sometimes called " ingot-iron."

HALF-HARD and HIGH-CARBON

STEELS, sometimes called

CAST IRON.

" ingot-steel."

Slagless or " Ingot-

It may be either Bessemer, open-

hearth, or crucible steel.

They may be either Bessemer,

open-hearth, or crucible steel.

Normal cast iron, " washed " metal,

and most " malleable cast iron "

Metal '.' Series.

M

Malleable cast iron also often

belongs here.

belong here.

ALLOY STEELS.

ALLOY CAST IRONS.*

Nickel, manganese, tungsten, and

chrome steels belong here.

Spiegeleisen, ferro-manganese, and

silico-spiegel belong here.

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 I]]. - General Classification of Iron and Steel according (I) to Carbon-Content and (2) to Presence or Absence of Inclosed Slag. * 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.

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 1 The word " iron " was in 0. Eng. iren, isern or isen, cf. Ger. Eisen, Dut. ysen, Swed. jam, Dan. jern; the original Teut. base is isern, and cognates are found in Celtic, Ir. iarun, Gael. iarunn, Breton, houarn, &c. The ulterior derivation is unknown; connexion has been suggested without much probability with is, ice, from its hard bright surface, or with Lat. aes, aeris, brass. The change from isen to iren (in 16th cent. yron ) is due to rhotacism, but whether direct from isen or through isern, irern is doubtful. " Steel " represents the 0. Eng. stel or stele (the true form; only found, however, with spelling style, cf. styl-ecg, steel-edged), cognate with Ger. Stahl, Dut. and Dan. staal, &c.; the word is not found outside Teutonic. Skeat ( Etym. Dict., 1898) finds the ultimate origin in the Indo-European base stale-, to be firm or still, and compares Lat. stagnum, standing-water.

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 " carbon wcarbon 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 (I) " iron " as distinguished from steel, and XIV. 26 (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.

1 3. Definitions

2 4. Historical Sketch

3 5. Three Periods

4 6. First Period

5 7. Second Period

6 8. Third Period

7 17. Pearlite

8 32. Fineness of Structure

9 33. The Possibilities of Thermal Treatment

10 51. The World's Supply of Iron Ore

11 53. Future Cost of Ore

12 54. Ore Supply of the Chief Iron-making Countries

13 56. veographical Distribution of the British Works

14 61. Richness of Iron Ores

15 63. Shaping and Adjusting Processes

16 68. Shape and Size of the Blast - Furnace

17 70. Means of Heating the Blast

18 72. Blast furnace Gas Engines

19 73. Mechanical Appliances.

20 77. Direct Metal and the Mixer

21 78. Conversion or Purifying Processes for converting Cast Iron into Steel or Wrought Iron

22 86. Recarburizing

23 87. Darby's Process

24 89. Source of Heat

25 93. Control of the Basic Bessemer Process

26 96. Range in Size of Converters

27 97. The Bessemer Process for making Steel Castings

28 104. Case Hardening

29 105. Deep Carburizing; Harvey and Krupp Processes

30 120. Defects in Steel Ingots

31 124. Castings and Forgings

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 ifs 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 150o B.C.) so far that bellows were used for forcing the forge fire; that in Homer's time (not later than the gth 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.

5. 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 lengthen ing 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 ironsmelting 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° 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 andthe 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 b y 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 §§ 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 quasiminerals, diametrically unlike. These are cementite, a definite iron carbide, Fe 3 C, harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress; and ferrite, pure or nearly pure metallic a-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, 0and 7-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 (3and 7-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.

I 1. 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 Aa, aB, and Bc that at which it ends. But after solidification is complete and the metal has cooled to a much lower range of temperature, usually between 9 00 and 690 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 0.5. .. ... .

Percentage Composition FIG. I.-Roberts-Austen or Carbon-Iron diagram. The Cementite-Austenite or Metastable form.

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 ('y ) iron.-Austenite is the name of the solid solution of an iron carbide in allotropic y-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 0 C= Fe 3 C +Fe(i 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 GHSa 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, Steep Cast Iron no; d ` r t1 ?at J ustenite+ Cementite. Eutectic here freezes 7 Austenite+Cementite Pro-eutectoid Cementite forms progressiuely 882930 1200 0 U a 5 6 ? Pearli te T v 3 A here splits up '(and cementite 140 C C P Ferrite ? PearIite?

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eutectic,a earIitei-Cementit .(/a primarg, Oxide ,Cementite(t 400 300 20 even if rich in austenite, is strongly magnetic because of the very magnetic a-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 (13) 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 -y-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 (3 into a iron so sluggish that the former remains untransformed even during slow cooling.

Again, (3-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, 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 (3 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 (3-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 (a) 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 § 10, are the final products of the transformation of austenite in slow-cooling. (3ferrite and austenite are the normal constituents for the triangle GHM, a-ferrite (i.e. nearly pure a-iron) with austenite for the space Mhsp, cementite with austenite for region 7, and a-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 § 18).

17. Pearlite

The ferrite and cementite present interstratify habitually as a " eutectoid " 1 called " pearlite " (see Alloys, Pl., fig. I 1), in the ratio of about 6 parts of ferrite to I of cementite, and hence containing about 0.90% of carbon. Slowly cooled steel containing just 0.90% of carbon (S in fig. I) 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 are 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.

1 A " eutectic " is the last-freezing part of an alloy, and corresponds to what the mother-liquor of a saline solution would become if such a solution, after the excess of saline matter had been crystallized out, were finally completely frozen. It is the mother-liquor or " bittern " frozen. Its striking characteristics are: (1) that for given metals alloyed together its composition is fixed, and does not vary with the proportions in which those metals are present, because any " excess metal," i.e. so much of either metal as is present in excess over the eutectic ratio, freezes out before the eutectic; (2) that though thus constant, its composition is not in simple atomic proportions; (3) that its freezing-point is constant; and (4) that, when first formed, it habitually consists of interstratified plates of the metals which compose it. If the alloy has a composition very near that of its own eutectic, then when solidified it of course contains a large proportion of the eutectic, and only a small proportion of the excess metal. If it differs widely from the eutectic in composition, then when solidified it consists of only a small quantity of eutectic and a very large quantity of the excess metal. But, far below the freezing-point, transformations may take place in the solid metal, and follow a course quite parallel with that of freezing, though with no suggestion of liquidity. A " eutectoid " is to such a transformation in solid metal what a eutectic is to freezing proper. It is the last part of the metal to undergo this transformation and, when thus transformed, it is of constant though not atomic composition, and habitually consists of interstratified plates of its component metals.

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, FIG. 2. - Relation between the carbon-content and the percentage of the several constituents of slowly cooled steel and white cast iron.

measures the percentage of the excess of ferrite or cementite for hypoand 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 O 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 copperlike 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 carboncontent rises above the eutectoid or pearlite ratio of o

90% is natural, because the brittleness of the cementite which, in hypereutectoid 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 Steel White Cast Iron 100 75 K 0 ?

0 0 0.5 1.0 1

5 2.0 2.5 3.0 _ _ _ Hardness ._Per 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." immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.

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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. I), 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 1 0 20 24 2 32 30 4.0 4.3 4. 4' 4.6 8'2 5. 6 6.0 0. 4 0.07 ' of 160000 120000 a H a?

80000 -in C o 40000 ti 3.5404 5 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 lil'° g? Boiler plates undergo in sharing and assembling an intermediate degree of distort' 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 o

05% for rivets and tubes, 0.20% for boiler plates, and 0.50 to 0.75% for rails, implying the presence of o

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

5 o %. The carboncontent 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 to % 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 § 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-eutectiod 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° (q), the mass begins to freeze as -y-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 freezingrange than a freezing-point. Moreover, the freezing is " selective." The first particles of austenite to freeze contain about o

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°) on the line Sa (Ar 3) 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 Fe 3 C, 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° 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 " mothermetal " 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 reacnes the eutectoid composition. As the temperature now falls past 690°, 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 a-ferrite. The passage of this large quar`it-y 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 y to the a 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 y through the # to the a 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 (3 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, -y-iron with this 0.20% of carbon dissolved in it. Its further cooling undergoes three spontaneous retardations, one at K' (Ar 3 about 820°), at which part of the iron begins to isolate itself within the austenite mother-metal in the form of envelopes of 0-ferrite, i.e. of free iron of the /3 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°) this ferrite changes to the normal magnetic a-ferrite, so that the mass as a whole becomes magnetic. Moreover, the envelopes of ferrite which began forming at Ar 3 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 Ar t (about 690°) 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 Ar l 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

oo % carbon steel at this same critical point Ar l. As such steel cools slowly past Ar3, Ar 2 and Ar 1, it loses its hardening power progressively.

In short, from Ar 3 to Ar t the excess substance ferrite or cementite, in hypoand hyper-eutectoid steels respectively, progressively crystallizes out as a network or skeleton within the austenite mothermetal, which thus progressively approaches the composition of hardenite, reaching it at Ar t, and there splitting up into ferrite and cementite interstratified as pearlite. Further, any ferrite liberated at Ar 3 changes there from -y to a, and any present at Ar 2 changes from (3 to a. Between H and S, Ar 3 and Ar 2 occur together, as do Ar 2 and Ar l between S and P' and Ar 3, Ar 2 and Ar t at S itself; so that these critical points in these special cases are called Ar 3 _ 2, Ar2_1 and Ar 3 _ 2 _ 1 respectively. The corresponding critical points which occur during rise of temperature, with the reverse transformations, are called Ac1, Ace, Ac 3, &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 1 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 k1) " primary " austenite, (2) " eutectic " austenite and (3) " eutectic " cementite. As the temperature sinks still farther, pro-eutectoid cementite (see § 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° (a), to 0.90% at 690° (Ar 1), 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 Ari, so that the mass when cold finally consists of (1) 1 Note the distinction between the " eutectic " or alloy of lowest freezing-point, 1130°, B, with 4.30% of carbon, and the " eutectoid," hardenite and pearlite, or alloy of lowest transformation-point, 690° S, with 0.90% of carbon. (See § 17.) 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


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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Iron And Steel'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/bri/i/iron-and-steel.html. 1910.

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