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IRON AND STEEL
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See also:IRON AND See also: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 See also:save the See also:air we breathe and the See also:water we drink. For one See also:kind of See also:meat we could substitute another; See also:wool could be replaced by See also:cotton, See also:silk or See also:fur; were our See also:common silicate See also:glass gone, we could probably perfect and cheapen some other of the transparent solids; but even if the See also:earth could be made to yield any substitute for the See also:forty or fifty million tons of iron which we use each See also:year for rails, See also: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 See also:electricity. This usefulness iron owes in See also: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 See also:chief part first to the single qualities in which itvery weak; conducting See also:heat and electricity easily, and again offering See also:great resistance to their passage; here See also: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 See also:man should be set on an iron-cored globe, certainly suggests See also:design. The indispensableness of such abundant things as air, water and See also: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 See also:evolution, because its great usefulness did not arise until historic times, or even, as in See also:case of See also:magnetism, until See also:modern times. These See also:variations in the properties of iron are brought about in part by corresponding variations in See also:mechanical and thermal treatment, by which it is influenced profoundly, and in part by variations in the proportions of certain See also:foreign elements which it contains; for, unlike most of the other metals, it is never used in the pure See also:state. Indeed pure iron is a rare curiosity. Foremost among these elements is See also:carbon, which iron inevitably absorbs from the See also:fuel used in extracting it from its ores. Se strong is the effect of carbon that the use to which the See also:metal is put, and indeed its See also: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, See also:cast iron, with more than this quantity, are based on carbon-content. (See Table I.) Containing very little Carbon (say, Containing an Intermediate Containing much Carbon (say, less than li0.30 ttle %). C Contaifrom 2.2 to 5 %). Quantity of Carbon (say, between 0.30 and 2.2 /°). Slag-bearing or ... WROUGHT IRON. WELD STEEL. " Weld-metal " See also:Series. Puddled and bloomary, or See also:Charcoal- Puddled and See also:blister steel See also:hearth iron belong here. belong here. See also:LOW-CARBON Or MILD STEEL, See also:HALF-HARD and HIGH-CARBON CAST IRON. sometimes called " See also:ingot-iron." STEELS, sometimes called " ingot-steel." Slagless or "Ingot- It may be either See also:Bessemer, open- They may be either Bessemer, Normal cast iron, " washed " metal, Metal " Series. hearth, or crucible steel. open-hearth, and most " malleable cast iron " belong here. or crucible steel. iron also often Malleable cast belongs here. ALLOY STEELS. ALLOY CAST IRONS.*
See also:Nickel, See also:manganese, See also:tungsten, and Spiegeleisen, ferro-manganese, and
chrome steels belong here. silico-spiegel belong here.
* The See also: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 See also:industry.
excels, such as its strength, its magnetism, and the See also: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 See also:special combinations of useful properties in which it excels, such as its strength with its ready welding and shaping both hot and See also:cold; and third, to the great variety of its properties. It is a very See also:Proteus. It is extremely hard in our files and razors, and extremely soft in our See also:horse-See also:shoe nails, which in some countries the See also: The See also:change from isen to iren in 16th cent. yron) is due to rhotacism, but whether See also:direct from isen or through isern, irern is doubtful. " Steel "• represents the O. Eng. stel or See also:stele (the true See also:form ; only found, however, with spelling See also:style, cf. st yl-ecg, steel-edged ). cognate with Ger. See also:Stahl, Dut. and Dan. slant, &c.; the word is not found outside See also:Teutonic. See also:Skeat (Etym. See also:Diet., 1898) finds the ultimate origin in the Indo-See also:European base sick-, to be See also:firm or still, and compares Lat. stagnunz, See also:standing-water. a I C. 2(' 2. Nomenclature.—Until about 186o there were only three important classes of iron—wrought iron, steel and cast iron. The essential characteristic of wrought iron was its nearly See also: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 See also: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 See also:bath of slag, without subsequent See also:fusion.. But the best class of steel, crucible steel, was freed from slag by fusion in crucibles; hence its name, " cast steel." Between 186o and 187o the invention of the Bessemer and open-hearth processes introduced a new class of iron to-See also:day called " mild " or " low-carbon steel," which lacked the essential property of steel, the hardening See also: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 (r) "iron" as distinguished from steel, and II (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 See also:free from slag; and, after a See also: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 See also: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 See also:general reader, in fact cause no appreciable trouble to important makers or users of iron and steel, beyond forming an occasional See also:side-issue in litigation. 3. See also:Definitions.—Wrought iron is slag-bearing malleable iron, containing so little carbon (0.30% or less), or its See also: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 See also: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 See also:pig iron, i.e. the molten cast iron as it issues from the blast See also:furnace, or the pigs into which it is cast. Malleable cast iron is iron which has been cast in the See also: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 o.3o% of carbon. Weld steel is slag-bearing iron malleable at least at some one temperature, and containing more than 0.30% of carbon. 4. See also:Historical See also:Sketch.—The iron See also: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 See also:savage's See also:wood See also:fire, and the resulting metallic iron would then differ so strikingly from any See also:object which he had previously seen, that its very See also:early use by our See also:race is only natural. The first observing savage who noticed it among his ashes might easily infer that it resulted from the See also:action of burning wood on certain extremely heavy stones. He could See also:pound it out into many useful shapes. The natural steps first of making it intention-ally by putting such stones into his fire, and next of improving his fire by putting it and these stones into a cavity on the See also:weather side of some See also:bank with an opening towards the prevalent See also:wind, would give a See also:simple forge, differing only in See also: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 afterman's See also:discovery, that he could See also:beat iron and steel out while cold into useful shapes, he learned to forge it while hot is hard to conjecture. The See also:pretty elaborate appliances, See also: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 See also:flat See also: 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 See also: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 See also:evidence. The See also:absence of iron and the abundance of bronze in the See also:relics of a prehistoric See also:people is a piece of evidence to be accepted with caution, because the great defect of iron, its proneness to See also:rust, would often See also:lead to its complete disappearance, or See also: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 See also:process like the hardening of steel, and second because, if they had, then a large proportion of the See also: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 See also:Genesis that Tubal See also:Cain, the See also:sixth in descent from See also: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 See also:tool has been. found embedded in the ancient See also:pyramid of Kephron (probably as early as 3500 B.C.); that iron metallurgy had advanced at the See also:time of Tethmosis (Thothmes) III. (about r500 B.C.) so far that See also:bellows were used for forcing the forge fire; that in See also:Homer's time (not later than the 9th See also:century B.c.) the delicate art of hardening and tempering steel was so See also:familiar that the poet used it for a simile, likening the hissing of the stake which Ulysses drove into the See also:eye of See also: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 See also:Pliny (A.D. 23–79) the relative value of different See also: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 See also:story like that of the single See also:foot-See also:print of the savage which the waves See also:left for See also: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 See also: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 See also: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 See also:control over his fire, at once his source of heat and his deoxidizing See also:agent. Not See also:con-See also:tent to let it See also:burn by natural See also:draught, he would See also:blow it with his own breath, would expose it to the prevalent wind, would urge it with a See also:fan, and would devise the first crude valveless bellows, perhaps the pigskin already familiar as a water-See also:bottle, of which the psalmist says: " I am become as a bottle in the See also: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 See also:engine, perhaps one of the very earliest of all. While inflating the bellows he would leave the suction See also:port open and See also:close the See also:discharge port with a pinch of his See also: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 See also:Watt devised valves for the bellows. But in spite of the activity of the iron manufacture in many of the See also:Roman provinces, especially See also:England, See also:France, See also:Spain, See also:Carinthia and near the See also: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 See also:local smith, the Waldschmied, converted it with the charcoal of the surrounding See also:forest into the wrought iron which he worked up. Many farmers had their own little forges or smithies to See also: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 See also:gradual displacement of the direct extraction of wrought iron from the ore by the intentional and See also: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 re-placing of the simple but wasteful by the complex and economical, and it was begun unintentionally in the See also:attempt to save fuel and labour, by increasing the size and especially the height of the forge, and by See also: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 See also:column of ore and fuel, and thus enabled him to increase the height of his forge, enlarge the See also:scale of his operations, and in turn save fuel and labour. And it was the lengthen-See also: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, See also:running together into a single molten mass, freed itself mechanically from the " See also: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 See also: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 See also: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 See also: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 See also:diameter of the high furnace, which has lasted till our own days, naturally went on and See also:developed the gigantic blast furnaces of the See also:present time, still called " high furnaces " in See also:French and See also:German. The impetus which the indirect process and the See also:acceleration of See also: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 See also:act of See also:Queen See also: B. See also:Neilson's invention in 1828 of heating the blast, which increased the See also:production and lessened the fuel-See also:consumption of the furnace wonder-fully. Very soon after this, in 1832, the See also:work of heating the blast was done by means of the See also:waste gases, at Wasseralfingen in See also:Bavaria.
Meanwhile See also: 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 See also:pro-ducts. These processes are incalculably more important than Huntsman's, both because they are incomparably cheaper, and because their products are far more >pneflil than his.
Thus the distinctive work of thr.Liseccind, and, third periods is freein
804
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 See also:Siemens regenerative See also:gas furnace an See also:independent means of generating this temperature, the See also: In 1888 F. Osmond showed that the wonderful changes which thermal treatment andthe presence of certain foreign elements cause were due to See also: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 See also:composition and treatment, the iron exists in one or more of its remarkable allotropic states. These include the austenitic or See also:gamma non-magnetic manganese steel, already patented by See also:Robert Hadfield in I:;t33, the first important known substance which combined great IUSlleableness with great hardness, and the martensitic or beta " high See also:speed tool steel " of See also: r i. The See also:Roberts-See also:Austen or carbon-iron See also:diagram (fig. 1), in which See also:vertical distances represent temperatures and See also:horizontal ones the percentage of carbon in the iron, See also: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 See also: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 See also:lower range of temperature, ,zoo- Austenite 4 Iron "0°' typo- taw( " 0°' Austen, te+Cemen t/terEufectic See also:hen re r,tectoid <Wect nid, , f tie See also:Soap- 7 .e°~• t Austenite-}Cementite o. B~° Cr , sp~~e Pro-eutectoid Cementite forms ,roeressiuel0 U 'o~ ro R~~ e ,Austenite hen splits up into Pearlite=e'trectoid ferrite See also:anti cementite ,600- 6 t• soo. Pearlite+, , I Pr9 eutec-; S t .0° told Ferrite 1 Pearlite•} autecyic,and t See also:ems Pearlite {.Cementitepe0•e tactoid) (o tna'y 1 SOOfOxida ` ~ementae{eute tectm.and 1 Snowxide pro-eutectoid soo- O m o• Carbon 0 0.6 11.0 1.0 0.0 310 6.0 019 e•n 6.0 5.0 o o 9.0 6.00'e3 Iron 100 006 00.0 09.6 6e•0 57•6 67.0 69.6 66.0 95.9 60.0 96.5 06.0 93.39033 Percentage Composition The Cementite-Austenite or Metastable form. usually between 9000 and 69o° 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 See also: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 See also: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 1100 130 ,500 MOIten Cast Iron typo-eutectic N4per-eefeatia Steel Cast Iron 6 ,00 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 See also: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 See also: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, Fe0C=FesC+Fe(z 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 See also:brake to retard the change; or completely, by the presence of a large quantity of manganese, nickel, tungsten or See also:molybdenum, which in effect sink the lower boundary GHSa of region 4 to below the atmospheric temperature. The important manganese steels of See also: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 See also:marten site. 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 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 (f) iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and See also: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 °o 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, 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 13 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 a-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 See also: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. See also: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 § to, are the final products of the transformation of austenite in slow-cooling. 13-ferrite and austenite are the normal constituents for the triangle See also:OHM, 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, See also:rail steel, &c., and of white cast iron (see § 18). 17. Pearlite.—The ferrite and cementite present interstratify habitually as a " eutectoid called " pearlite " (see Annoys, 1'1., fig. I I), 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 See also: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 also:ace At1.o-s, 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. I. This typical " envelope and See also:kernel " structure is often only rudimentary. ' .A '• eutectic " is the last-freezing part of an alloy, and corresponds to what the See also:mother-liquor of a saline solution would become if such a solution, after the excess of saline See also:matter had been crystallized out, were finally completely frozen. It is the mother-liquor or " See also: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 See also: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 solidi-tied 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 See also: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 See also:line See also: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, measurescne percentage of the excess of ferrite or cementite for hypo-and hyper-eutectic steel and white cast iron respectively. 18. The Carbon-Content, i.e. the Ratio of Ferrite to Cementite, of certain typical Steels.—Fig. 3 shows how, as the carbon-content rises from 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 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 hypereutectoid steels, forms a more or less continuous See also:skeleton (ALLOYS, Pl., fig. 13) should be much more effective in starting cracks under distortion than that of the far more See also:minute particles of cementite which See also: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 a I! i- +•5 2.0 2.5 3.0 Percentage of Carbon Tenacity - Ductility Hardness ~.~._Per cent Ferrite or Cementite immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels. As the carbon-content increases the welding power naturally decreases rapidly, because of the rapid fall of the " solidus See also: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. See also:Rivet steel, which above all needs extreme ductility to endure the distortion of being driven See also:home, and See also:tube steel which must needs weld easily, no matter at what See also: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 See also: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 0.4 0•#' 1.2 1.6 2.0 2.4 9.8 3.2 3.0 a•oa'se•a 4.8 8'2 5.0 0.0 O.4 (POT Percentage of Combined Carbon Qm aE ti 1a ~ ~ soon( 2000( K O 60 ag , a=dues -- ~~ .. 80000 ~ or• Cementite a0000 - l __ - ..... .. mrnw- F 0 0.8 1.0 3.5 4.0 "5 carbon-content increases the See also:shock-resisting power decreases. Here, as in all cases, the carbon-content must be the result of a See also:compromise, neither so small that the rail flattens and wears out like lead, nor so great that it snaps like glass. See also: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 See also: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 See also: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 o•75 and 1.25%; See also: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 See also:chance to See also:escape completely.
21. Graphite, nearly pure carbon, is characteristic of " See also: 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 See also:weight (say to % by See also:volume), practically nothing but graphite is seen in the fracture. Hence the weakness and the dark-See also:grey fracture of this iron, and hence, by brushing this fracture with a wire See also: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 See also:Illustration of the Iron-Carbon Diagram.—In See also:order to illustrate further the meaning of the diagram (fig. 1), let us follow by means of the See also: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° (q), the mass begins to freeze as 7-iron or austenite, called " See also: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 See also: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 See also:rate of cooling remains regular until T (75o°) on the line Sa (Ara) 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 See also:mere solution to that of definite combination in the ratio Fe3C, forming microscopic particles of cementite, while the See also: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 Ara_,) so much of the carbon (in this case about 0.10%) and of the iron have See also: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 he called " pro-eutectoid " cementite, because it forms before the remaining austenite reacnes the eutectoid composition. As the temperature now falls past 69o° this hardenite mother-metal in turn splits up, after the See also: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 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 See also: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 13 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 13 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 See also: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, 7-iron with this 0-20% of carbon dissolved in it. Its further cooling undergoes three spontaneous retardations, one at K' (Ara about 82o''), at which part of the iron begins to isolate itself within the austenite mother-metal in the form of envelopes of 13-ferrite, i.e. of free iron of the p allotropic modification, which surrounds the kernels or grains of the residual still undecomposed part of the austenite. At the second retardation, K" (Ara, 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 Ara continue to broaden by the See also:accession of more and more ferrite born from the austenite progressively as the temperature sinks, till, by the time when Ar, (about 69o°) 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 See also:Ari this hardenite mother-metal splits up into cementite and ferrite grouped together as pearlite, with the resulting recalescence, and the mass, as shown in Annoys, 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 See also:critical point Arl. As such steel cools slowly past Ara, Ara and Ari, it loses its hardening power progressively. In See also:short, from Ara to Ari 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 Ari, and there splitting up into ferrite and cementite interstratified as pearlite. Further, any ferrite liberated at Ara changes there from y to ,8, and any present at Ara changes from (3 to a. Between H and S, Ara and Ara occur together, as do Ara and Ari between S and P' and Ara, Ara and Ari at S itself; so that these critical points in these special cases are called Ara-2, Arai and Ara_a_i respectively. The corresponding critical points which occur during rise of temperature, with the See also:reverse transformations, are called Aci, Aca, Aca, &c. A (Tschernoff) is the generic name, r refers to falling temperature (ref roidissant) and c to rising temperature (chauff See also:ant, 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' 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 See also: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 ,i) " 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 See also: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° (An), 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 (I) 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 .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 (t) ' See also:Note the distinction between the " eutectic" or alloy of lowest freezing-point. 113o°, B, with 4.30 % of carbon, and the " eutectoid." hardenite and pearlite, or alloy of lowest transformation-point, 69o° 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 a mixture of pearlite plus cementite. Such a mass is shown in fig. 4. Here the See also:black See also:bat-like patches are the masses of pearlite plus proeutectoid cementite resulting from the splitting up of the primary austenite. The magnification is too small to show the See also:zebra striping of the pearlite. In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other. 25. As we pass to cases with higher and higher carbon-content, the primary austenite which freezes in cooling across region 2 forms a smaller and smaller proportion of the whole, and the austenitecementite eutectic which forms at the eutectic freezing-point, 1130° (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 See also:crossing region 7, and at Ara (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 See also:silicon, phosphorus, manganese, See also:chromium, nickel and tungsten. Nickel and manganese lower these critical points, so that with 25% of nickel Ara lies below the common temperature 2o° C. With 13 % of manganese Ara is very low, and the austenite decomposes so slowly that it is preserved practically intact by sudden cooling. These steels then normally consist of 'y-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 Ara, it passes to the a state and becomes magnetic. 26. See also: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 See also:rest, i.e. the carbon which is not dissolved, or the " undissolved carbon," forms either the definite carbide, cementite, FeaC, 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. I and studied at length in §§ 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 See also: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 thetwo diagrams are probably about as here shown, the exact See also: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-I-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. I) 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 FeaC=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 See also: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 See also:sulphur, and in most cases that of manganese, favour the formation of cementite. For in-stance, 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 See also: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. I, 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 state. In the cold this trans-formation cannot take place, because of molecular rigidity or some
Steel Cast Iron
See also:Legend
Oraphlt —A detentt* diagram.
Combrtlte—Austenite diagram
shonr for comparison 100-
o, ) See also:Carte O.6 1.0 be Q.0 be 84 0.6 40 M6 4
MO 6 1.0 .664, Iron See also: AusMn i VII B Solid Primers and wM6ts Prlmar6 and eutectic 000 -.'k{( ifAustanita•wtectic Orepk#( Orc Olt. +eutectic Austen/ti t o o,. . . .. - - - - - - - - - - - - - - - - - - - - - - - - - - 5 aoo. S 000 P' coo aoo See also:Clue Oxide 500 -See also:Straw 04de usually to between 200° and 300° C., so as to relax the molecular either by reheating the steel to Aca when it " refines," i.e. returns rigidity and thereby to allow the arrested transformation to go on a little farther, shifting a little of the 0-iron over into the a 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 See also: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 state. The annealing of such iron may occur in either of two degrees—a small one, as in making common chilled cast iron See also: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 Act, say to 730° C., so that the a-iron may slip into the a state, and the trans-formation of the austenite into pearlite and cementite may complete itself. The See also:joint effect of such chilling and such annealing is to make the metal much harder than if slowly cooled, because for each i °is 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 See also:outer See also: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 See also:week, to a temperature usually above 730° C. and often reaching 900° C. (1346° and 1652° F.). For about 6o See also: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 Aci changed into pearlite and ferrite, the mixture of cementite and pearlite of which they originally consisted has now given place to one of See also:fine or See also:temper " graphite and ferrite, with more or less pearlite according to the completeness of the See also: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 See also: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 See also:aggregation of certain of these constituents, and through it the properties of the metal as a whole, are profoundly affected by temperature manipulations. Thus, See also:prior exposure to a temperature materially above Aca 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° C. will bend 166° before breaking, will, after slow cooling from about 1050° C., bend only 18° before breaking. This injury fortunately can he cured spontaneously to its fine-grained ductile state (cooling past Ara 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° C.., and if, when it has cooled to a lower temperature, say 850° C. we forge it, its See also:rain-size and ductility when cold will be approximately those which it would have had if heated only to 850°. 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 See also:expenditure of power, ought later to be refined, either by reheating it from below An to slightly above Aca or by rolling it after it has cooled to a relatively low temperature, i.e. by having a low '' See also: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 Aca, which also relieves the See also:internal stresses due to the different rates at which different layers cool, and hence See also: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° C., and such steel refines at temperatures above goo° C.
33. The Possibilities of Thermal Trectment.—When we consider the great number of different regions in fig. i, 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 See also: Its combination of ductility with strength and hardening power has given it very extended use for the See also:armour of See also:war-vessels. For instance, following See also:Krupp's See also:formula, the side and See also: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 See also: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-tke 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 See also:alignment is not mathematically straight. The alignment of marine shafting, changing with every passing See also:wave, is an extreme example. Such an intermittently applied stress is far more destructive to iron than a continuous "se, 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 See also:shaft would endure before breaking. 35. As actually made, manganese steel contains about 12% of manganese and 1.5o% 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. Nits 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 See also:rock-crushing machinery, railway crossings, mine-car wheels and See also:safes. The burglar's blow-See also: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 See also:drill through it and introduce his See also:charge of See also:dynamite. But neither this nor any other See also: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 See also: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 o•8o 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 itis 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 See also:dies of See also:stamp-See also: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. See also:Vanadium in small quantities, o• 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° C. (1112° F.) by the See also:friction of the work which it is doing. Hence a machinist can cut steel or iron nearly six times as fast with a See also: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° C. (482° F.), lest it be unduly softened or " tempered " (§ 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 y or 13 state or a mixture of both, can be heated without losing its hardness through the escape of that iron into the a 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.ferrous oxide. (See § 35.) 43. Ores of Iron.—Even though the earth seems to be a huge iron See also: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, See also:magnetite, See also:haematite, See also:limonite and siderite, and one of less but still considerable importance, pyrite or See also:pyrites. 44. Magnetite, Fe304, contains 72.41 % of iron. It crystallizes in the cubical See also: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, Fe203, contains 70% of iron. It crystallizes in the See also: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, 3H20, contains 59.9% of iron. Its colour varies from light See also: Its specific gravity is 3.7 to 3.9, and its hardness 3.5 to 4.5. The clayey siderite of the See also:British coal See also:measures is called See also:clay See also: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 See also: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 See also:acid, and most of the remainder in the wet extraction of their copper, are then used under the name of " See also:blue billy " or " See also: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. 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 See also: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 o•or %, i.e. of one part in ro,00o, 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 o•04 and 0'05% in the steel for important See also:bridges, or o•o6 and o•07 % in, rail steel, though some very prudent See also:engineers allow as much as •o85 % or even o•ro% 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 See also:link in a strong See also:chain. 41. See also: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 Carbon 0.32 to 1.28 0.68 to 0.67 . . 0.07 49. The Ores actually Impure.—As these five minerals actually Manganese . exist in the earth's crust they are usually more or less impure 0.03 „ 0.30 „ 0.11 Chromium . 2.23 „ 7.02 9.95 ,. 5.47 Tungsten 9.25 ,, 25.45 17.81 , 18.I9 chemically, and they are almost always mechanically mixed with barren mineral matter, such as quartz, See also: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 crushing, either mechanically or magnetically, so that the part thus enriched or " concentrated " alone need be smelted. so. See also: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 See also:oldest of our ores, and from deposits of like age, especially those of the more readily decomposed ilicates, has come the iron which now exists in the siderites andired and brown haematites of the later geological formations. 51. The See also: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 See also:official statement, which is here reproduced, Ore Supply. Country . Workable See also:Annual Annual Con- Deposits. Output. sumption. tons. tons. tons. United States 1,100,000,000 35,000,000 35,000,000 Great See also:Britain I,000,000,000 14,000,000 20,000,000 See also:Germany 2,200,000,000 21,000,000 24,000,000 Spain 500,000,000 8,000,000 I,000,000 See also:Russia and See also:Finland 1,500,000,000 4,000,000 6,000,000 France . 1,500,000,000 6,000,000 8,00o,000 See also:Sweden I ,000,000,000 4,000,000 1,000,000 See also:Austria-See also:Hungary 1,200,000,000 3,000,000 4,000,000 Other countries 5,00o,000 1,000,000 See also:Total 10,000,000,000 100,000,000 100,000,000 Note to Table.—Though this estimate seems to be near the truth as regards the British ores, it does not See also:credit the United States with one-tenth, if indeed with one-twentieth, of their true quantity as estimated by that country's Geological Survey in 1907. given in 1905 by See also:Professor Tornebohm to the See also:Swedish See also:parliament, credited the world with only 1o,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 See also:expert credits that country with from ten to twenty times this quantity; nor by pointing out that only certain parts of See also:Europe and a relatively small part of See also:North See also:America have thus far been carefully explored for iron ore, and that the rest of these two continents and See also:South America, See also:Asia and See also: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. 52. What Constitutes an Iron Ore.—Whether a ferruginous rock is or is not ore is purely a question of current demand and supply. That is ore from which there is reasonable See also:hope that metal can be extracted with profit, if not to-day, then within a reasonable length of time. Rock containing 21% of See also:gold is an extraordinarily rich gold ore; that with 21% of copper is a profitable one to-day; that containing 21% of iron is not so to-day, for the See also:sole reason that its iron cannot be extracted with profit in competition with the existing richer ores. But it will become a profitable ore as soon as the richer ore shall have been exhausted. Very few of the ores which. are See also:mined to-day contain less than 25% of iron, and some of them contain over 6o%. As these richest ores are exhausted, poorer and, poorer ones will be used, and the cost of iron will increase progressively if measured either in See also:units of the actual See also:energy used in miningand smelting it, or in its power of purchasing See also:animal and See also:vegetable products, cotton, wool, See also:corn, &c., the supply of which is renewable and indeed capable of very great increase, but probably not if measured in its power of purchasing the various mineral products, e.g. the other metals, coal, See also:petroleum and the precious stones, of which the supply is limited. This is simply one instance of the inevitable progressive increase in cost of the irrecreatable mineral relatively to the recreatable animal and vegetable. When, in the course of centuries, the exhaustion of richer ores shall have forced us to mine, crush and concentrate mechanically or by magnetism the ores which contain only 2 or 3% of iron, then the cost of iron in the ore, measured in terms of the energy needed to mine and concentrate it, will be comparable with the actual cost of the copper in the ore of the copper-mines of to-day. But, intermediate in richness between these two extremes, the iron ores mined to-day and these 2 and 3% ores, there is an incalculably great quantity of ore capable of mechanical concentration, and another perhaps vaster See also:store of ore which we do not yet know how to concentrate mechanically, so that the day when a pound of iron in the ore will cost as much as a pound of copper in the ore See also:costs to-day is immeasurably distant. 53. Future Cost of Ore.—The cost of iron ore is likely to rise much less rapidly than that of coal, because the additions to our known supply are likely to be very much greater in the case of ore than in that of coal, for the reason that, while rich and great iron ore beds may exist anywhere, those of coal are confined chiefly to the Carboniferous formation, a fact which has led to the systematic survey and measurement of this formation in most countries. In short, a very large part of the earth's coal supply is known and measured, but its iron ore supply is hardly to be guessed. On the other See also:hand, the cost of iron ore is likely to rise much faster than that of the potential See also:aluminium ores, clay and its derivatives, because of the vast extent and richness of the deposits of this latter class. It is possible that, at some remote day, aluminium, or one of its alloys, may become the great structural material, and iron be used chiefly for those objects for which it is especially fitted, such as magnets, springs and cutting tools. In passing, it may be noted that the cost of the ore itself forms a relatively small part of the cost even of the cruder forms of steel, hardly a See also:quarter of the cost of such simple products as rails, and an insignificant part of the cost of many most important finished objects, such as magnets, cutting tools, springs and wire, for which iron is almost indispensable. Thus, if the use of ores very much poorer than those we now treat, and the need of concentrating them mechanically, were to double the cost of a pound of iron in the concentrated ore ready for smelting, that would increase the cost of rails by only one quarter. Hence the addition to the cost of finished steel objects which is due to our being forced to use progressively poorer and poorer ores is likely to be much less than the addition due to the progressive rise in the cost of coal and in the cost of labour, because of the ever-rising scale of living. The effect of each of these additions will be lessened by the future improvements in processes of manufacture, and more particularly by the progressive replacement of that ephemeral source of energy, coal, by the See also:secular See also:sources, the winds, waves, tides, See also:sunshine, the earth's heat and, greatest of all, its momentum. 54. Ore Supply of the Chief Iron-making Countries: The United States mine nearly all of their iron ores, Austria-Hungary, Russia and France mine the greater part of theirs, but none of these countries exports much ore. Great Britain and Germany, besides See also:mining a great See also:deal of ore, still have to import much from Spain, Sweden and in the case of Germany from See also:Luxemburg, although, because of the customs arrangement between these last two countries, this importation is not usually reported: See also:Belgium imports nearly all of its ore, while Sweden and Spain export most of the ore which they mine. 55. Great Britain has many valuable ore beds, some rich in iron, many of them near to beds of coal and to the See also:sea-See also:coast, to canals or to navigable See also:rivers. They extend from See also:Northamptonshire to near See also:Glasgow. About two-thirds of the ore mined is clayey siderite. In 1905 the See also:Cleveland See also:district in North See also:Yorkshire supplied 41 % of the total British product of iron ores; See also:Lincolnshire, 14.8%; Northamptonshire, 13.9%; See also:Leicestershire, 4.7 %; See also:Cumberland, 8.6%; North See also:Lancashire, 2.7%; See also:Staffordshire, 6.1 %; and See also:Scotland, 51%. The annual production of British iron ore reached 18,031,957 tons in 1882, but in 1905 it had fallen to 14,590,703 tons, valued at £3,482,184. In addition 7,344,786 tons, or about half as much as was mined in Great Britain, were imported, 78.5 % of it from Spain. The most important British ore See also:deposit is the Lower Cleveland See also:bed of oolitic siderite in the See also:Middle See also:Lias, near Middles-See also:borough. It is from to to 17 ft. thick, and its ore contains about 3o of iron. 56. See also:Geographical See also:Distribution of the British Works.—Most of the British iron works lie in and near the important coal-See also:fields in Scotland between the mouth of the See also:Clyde and the Forth, in Cleveland and See also:Durham, in Cumberland and Lancashire, in south Yorkshire, See also:Derbyshire, and Lincolnshire, in Staffordshire and Northamptonshire, and in south \Vales in spite of its lack of ore. The most important See also:group is that of Cleveland and Durham, which makes about one-third of all the British pig iron. It has the great Cleveland ore bed and the excellent Durham coal near See also:tide-water at See also:Middlesbrough. The most important seat of the manufacture of See also:cutlery and the finer kinds of steel is at See also:Sheffield. 57. The United States have great deposits of ore in many different places. The rich beds near See also:Lake See also:Superior, chiefly red haematite, yielding at present about 55 % of iron, are thought to contain between t and 2 billion tons, and the red and brown haematites of the See also:southern states about to billion tons. The middle states, New See also:York, New See also:Jersey and See also:Pennsylvania, are known to have many great deposits of rich magnetite, which supplied a very large pro-portion of the See also:American ores till the discovery of the very cheaply mined ores of Lake Superior. In 1906 these latter formed 8o% of the American production, and the southern states supplied about 13% of it, while the rich deposits of the middle states are husbanded in accordance with the law that ore bodies are See also:drawn on in the order of their apparent profitableness. The most important American iron-making district is in and about See also:Pittsburg, to whose cheap coal the rich Lake Superior ores are brought nearly 1000 m., about four-fifths of the distance in the large ore steamers of the Great Lakes. See also:Chicago, nearer to the Lake ores, though rather far from the Pittsburg coal-field, is a very important centre.for rail-making for the railroads of the western states. See also:Ohio, the Lake See also:Erie end of New York State, eastern Pennsylvania and See also:Maryland have very important works, the ore for which comes in part from Lake Superior and in part from Pennsylvania, New York and See also:Cuba, and the fuel from Pennsylvania and its neighbourhood. See also:Tennessee and See also:Alabama in the south rely on southern ore and fuel. 58. Germany gets about two-thirds of her total ore supply from the great See also:Jurassic " Minette " ore deposit of Luxemburg and See also:Lorraine, which reaches also into France and Belgium. In spite of its containing only about 36% of iron, this deposit is of very great value because of its great size, and of the consequent small cost of mining. It stretches through an See also:area of about 8 m. wide and 40 M. long, and in some places it is nearly 6o ft. thick. There are valuable deposits also in Siegerland and in many other parts of the country. 59. Sweden has abundant, rich and very pure iron ores, but her lack of coal has restricted her iron manufacture chiefly to the very purest and best classes of iron and steel, in making which her thrifty and intelligent people have developed very rare skill. The magnetite ore bodies which supply this industry lie in a band about 18o m. long, reaching from a little north of See also:Stockholm See also:westerly toward the See also:Norwegian frontier, between the latitudes 59° and 61 ° N. In Swedish See also:Lapland, near the See also:Arctic circle, are the great See also:Gellivara, Kirunavara and Luossavara magnetite beds, among the largest in Europe. From these beds, which in some parts are about 300 ft. thick, much ore is sent to Germany and Great Britain. 60. Other Countries.—S See also:pain has large, rich and pure iron ore beds, near both her See also:northern and her southern sea coast. She exports about 90 % of See also:alI the iron ore which she mines, most of it to England. France draws most of her iron ore from her own part of the great Minette ore deposit, and from those parts of it which were taken from her when she lost See also:Alsace and Lorraine. Russia's most valuable ore deposit is the very large and easily mined one of Krivoi Rog in the south, from which comes about half of the See also:Russian iron ore. It is near the Donetz coal-field, the largest in Europe. There are also important ore beds in the Urals, near the border of Finland, and at the south of See also:Moscow. In Austria-Hungary, besides the famous Styrian Erzberg, with its siderite ore bed about 450 ft. thick, there are cheaply mined but poor and impure ores near See also:Prague, and important ore beds in both northern and southern Hungary. See also:Algeria, See also:Canada, Cuba and See also:India have valuable ore bodies. 61. Richness of Iron Ores.—The American ores now mined are decidedly richer than those of most European countries. To make a ton of pig iron needs only about I.9 tons of ore in the United States, 2 tons in Sweden and Russia, 2.4 tons in Great Britain and Germany, and about 2.7 tons in France and Belgium, while about 3 tons of the native British ores are needed per ton of pig iron. 62. The general See also:scheme of iron manufacture is shown diagrammatically in fig. 6. To put the iron contained in iron ore into a state in which it can be used as a metal requires essentially, first its deoxidation, and second its separation from they other mineral matter, such as clay, quartz, &c.. with which it is found associated. These two things arc done simultaneously by heatingand melting the ore in contact with coke, charcoal or See also:anthracite, in the iron blast furnace, from which issue intermittently two molten streams, the iron now deoxidized and incidentally carburized by the fuel with which it has been in contact, and the mineral matter, now called " slag." This crude cast iron, called " pig iron," may be run from the blast furnace directly Ore into moulds, which give the metal the final shape in which it is to be used in the arts; but it is almost always either remelted, following path 1 of fig. 6, and then cast into castings of cast iron, or converted into wrought iron or steel by purifying it, following path 2. If it is to follow path t, the castings into which it is made may be either (a) grey or (b) chilled or (c) malleable. Grey iron castings are made by remelting the pig iron either in a small shaft or '` See also:cupola " furnace, or in a reverberatory or " air " furnace, with very little change of chemical composition, and then casting it directly into suitable moulds, usually of either " baked," i.e. See also:oven-dried, or " See also:green," i.e. moist undried, See also:sand, but sometimes of iron covered with a refractory coating to protect it from being melted or over-heated by the molten cast iron. The general procedure in the manufacture of chilled and of •malleable castings has been described in §§ 30 and 31. If the pig iron is to follow path 2, the See also:purification which converts it into wrought iron or steel consists chiefly in oxidizing and thereby removing its carbon, phosphorus and other impurities, while it is molten, either by means of the oxygen of atmospheric air blown through it as in the Bessemer process; or by the oxygen of iron ore stirred into it as in the puddling and See also:Bell-Krupp processes, or by both together as in the open hearth process. On its way from the blast furnace to the converter or open hearth furnace the. pig iron is often passed through a great See also:reservoir called a " mixer," which acts also as an equalizer, to lessen the variation in composition of the cast iron, and as a purifier, removing part of the sulphur and silicon. 63. Shaping and Adjusting Processes.—Besides these ex-See also:traction and purification processes there are those of See also:adjustment and shaping. The adjusting processes adjust either the ultimate composition, e.g. carburizing wrought iron by long heating in contact with charcoal (cementation), or the proximate composition or constitution, as in the hardening, tempering and annealing of steel already described (§§ 28, 29), or both, as in the process of making malleable cast iron (§ 31). The shaping processes include the mechanical ones, such as rolling, forging and wire-See also:drawing, and the remelting ones such as the crucible process of melting wrought iron or steel in crucibles and casting it in ingots for the manufacture of the best kinds of tool steel. Indeed, the remelting of cast iron to make grey iron castings belongs here. This classification, though it See also:helps to give a general See also:idea of the subject, yet like most of its kind cannot be applied rigidly. Thus the crucible process in its American form both carburizes and remelts, and the open hearth process is often used rather for remelting than for purifying. 64. The iron blast furnace, a crude but very efficient piece of apparatus, is an enormous shaft usually about 8o ft. high and 20 ft. wide at its widest part. It is at all times full from top to bottom, somewhat as sketched in See also:figs. 7 and 8, of a solid column of lumps of fuel, ore and limestone, which are charged through a hopper at the top, and descend slowly as the lower end of the column is eaten off through the burning away of its coke by means of very hot air or " blast " blown through d` Remelting Processes-s Conversion Processes GG, Flanges on the ore bucket; P, Cinder notch; HH, Fixed flanges on the top of RR', Water cooled boxes; the furnace; S, Blast pipe; J. Counterweighted false bell; T, See also:Cable for allowing conical K, Alain bell; hot torn of bucket to 0. Tuvere; drop. holes or " tuyeres " near the bottom or " hearth," and through the melting away, by the heat thus generated, both of the iron itself which has been deoxidized in its descent, and of the other minerals of the ore, called the " gangue," which unite with the Drops of Slag i Drops of Iron Layer of Molten Slag--•??¢ Layer of Molten Iron- - * The ore and lime actually exist here in See also:powder. They are shown in lump form because of the difficulty of presenting to the eye their powdered state. lime of the limestone and the ash of the fuel to form a complex molten silicate called the " cinder " or " slag." Interpenetrating this descending column of solid ore, limestone and coke, there is an upward rushing column of hot gases, the atmospheric See also:nitrogen of the blast from the tuyeres, and the Fin, q.—Method of transferring charge from bucket to main charging bell, without permitting escape of furnace gas (lettering as to fig. 7). carbonic oxide from the See also:combustion of the coke by that blast. The upward ascent of the column of gases is as See also:swift as the descent of the solid charge is slow. The former occupies but a very few seconds, the latter from 12 to 15 hours. Lumps of Coke Lumps»f/ron Ore • Lumps of Lune - - - - V In the upper part of the furnace the carbonic oxide deoxidizes the iron oxide of the ore by such reactions as xCO±FeOs = Fe+xCO2• Part of the resultant carbonic acid is again de-oxidized to carbonic oxide by the surrounding fuel, See also:CO2+C=2CO, and the carbonic oxide thus formed deoxidizes more iron oxide, &c. As indicated in fig. 7, before the iron ore has descended very far it has given up nearly the whole of its oxygen, and thus lost its power of oxidizing the rising carbonic oxide, so that from here down the See also:atmosphere of the furnace consists essentially of carbonic oxide and nitrogen. But the transfer of heat from the rising gases to the sinking solids, which has been going on in the upper part of the furnace, continues as the solid column gradually sinks downward to the hearth, till at the " fusion level " (A in fig. 7) the solid matter has become so hot that the now deoxidized iron melts, as does the slag as fast as it is formed by the union of its three constituents, the gangue, the lime resulting from the decomposition of the limestone and the ash of the fuel. Hence from this level down the only solid matter is the coke, in lumps which are burning rapidly and hence shrinking, while between them the molten iron and slag trickle, somewhat as sketched in fig. 8, to collect in the hearth in two layers as distinct as water and oil, the iron below, the slag above. As they collect, the molten iron is drawn off at intervals through a hole A (fig. 8), temporarily stopped with clay, at the very bottom, and the slag through another hole a little higher up, called the " cinder notch." Thus the furnace may be said to have four zones, those of (r) deoxidation, (2) heating, (3) melting, and (4) See also:collecting, though of course the heating is really going on in all four of them. In its slow descent the deoxidized iron nearly saturates itself with carbon, of which it usually contains between 3'5 and 4%, taking it in part from the fuel with which it is in such intimate contact, and in part from the finely divided carbon deposited within the very lumps of ore, by the reaction 2CO= C+CO2. This carburizing is an indispensable part of the process, because through it alone can the iron be made fusible enough to melt at the temperature which can be generated in the furnace, and only when liquid can it be separated readily and completely from the slag. In fact, the molten iron is heated so far above its melting point that, instead of being run at once into pigs as is usual, it may, without solidifying, be carried even several See also:miles in large clay-lined ladles to the mill where it is to be converted into steel. 65. The fuel has, in addition to its duties of deoxidizing and carburizing the iron and yielding the heat needed for melting both the iron and slag, the further task of desulphurizing the iron, probably by the reaction FeS-j-CaO+C=Fe+CaS+CO. The desulphurizing effect of this transfer of the sulphur from union with iron to union with See also:calcium is due to the fact that, whereas iron sulphide dissolves readily in the molten metallic iron, calcium sulphide, in the presence of a slag rich in lime, does not, but by preference enters the slag, which may thus absorb even as much as 3 °o of sulphur. This action is of great importance whether the metal is to be used as cast iron or is to be converted into wrought iron or steel. In the former case there is no later chance to remove sulphur, a minute quantity of which does great harm by leading to the formation of cementite instead of graphite and ferrite, and thus making the cast-iron castings too hard to be cut to exact shape with steel tools; in the latter case the converting or purifying processes, which are essentially oxidizing ones, though they remove the other impurities,, carbon, silicon, phosphorus and manganese, are not well adapted to desulphurizing, which needs rather deoxidizing conditions, so as to cause the formation of calcium sulphide, than oxidizing ones. 66. The See also:duty of the limestone (CaCO3) is to furnish enough lime to form with the gangue of the ore and the ash of the fuel a lime silicate or slag of such a composition (I) that it will melt at the temperature which it reaches at about level A, of fig. 7, (2) that it will be fluid enough to run out through the cinder notch, and (3) that it will be rich enough in lime to supply that needed for the desulphurizing reaction FeS+ CaO+C=Fe+CaS+CO. In short, its duty is to "See also:flux" the gangue and ash, and See also:wash out the sulphur. 67. In order that the slag shall have these properties its composition usually lies between the following limits: See also:silica, 26 to 35%; lime, , plus 1.4 times the See also:magnesia, 45 to 55%; alumina, 5 to 20%. Of these the silica and alumina are chiefly those which the gangue of the ore and the ash of the fuel intro-duce, whereas the lime is that added intentionally to form with these others a slag of the needed See also:physical properties. Thus the more gangue the ore contains, i.e. the poorer it is in iron, the more limestone must in general be added, and hence the more slag results, though of course an ore the gangue of which initially contains much lime and little silica needs a much smaller addition of limestone than one of which the gangue is chiefly silica. Further, the more sulphur there is to remove, the greater must be the quantity of slag needed to dissolve it as calcium sulphide. In smelting the rich Lake Superior ores the quantity of slag made was formerly as small as 28% of that of the pig iron, whereas in smelting the Cleve-See also:land ores of Great Britain it is usually necessary to make as much as 11 tons of slag for each ton of iron. 68. Shape and Size of the Blast-Furnace.—Large size has here, as in most metallurgical operations, not only its usual advantage of See also:economy of See also:installation, labour and See also:administration per unit of product, but the further very important one that it lessens the proportion which the outer heat-radiating and hence heat-wasting surface bears to the whole. The limits set to the furnace builder's natural See also:desire to make his furnace as large as possible, and its present shape (an obtuse inverted See also:cone set below an acute upright one, both of them truncated), have been reached in part empirically, and in part by reasoning which is open to question, as indeed are the reasons which will now be offered reservedly for both size and shape. First the width at the tuyeres (fig. 7) has generally been limited to about 122 ft. by the fear that, if it were greater, the blast would penetrate so feebly to the centre that the difference in conditions between centre and circumference would be so great as to cause serious unevenness of working. Of See also:late furnaces have been built even as wide as 17 ft. in the hearth, and it may prove that a width materially greater than 12i ft. can profitably be used. With the width at the bottom thus limited, the furnace builder naturally tries to gain volume as rapidly as possible by flaring or " battering " his walls outwards, i.e. by making the " bosh " or lower part of his furnace an inverted cone as obtuse as is consistent with the free descent of the solid charge. In practice a furnace may be made to work regularly if its boshes make an See also:angle of between 730 and 76° with the horizontal, and we may assume that one element of this regularity is the regular easy sliding of the charge over this steep slope. A still steeper one not only gives less available See also:room, but actually leads to irregular working, perhaps because it unduly favours the passage of the rising gas along the walls instead of up and through the charge, and thus causes the deoxidation of the central core to lag behind that of the periphery of the column, with the consequence that this central core arrives at the bot tom incompletely deoxidized. In the very swift-running furnaces of the Pittsburg type this outward flare of the boshes ceases at about 12 ft. above the tuyeres, and is there reversed, as in fig. 7, so that the furnace above this is a very acute upright cone, the walls of which make an angle of about 40 with the vertical, instead of an obtuse inverted cone. In explanation or See also:justification of this it has been said that a much easier descent must be provided above this level than is needed below it. Below this level the solid charge descends easily, because it consists of coke alone or nearly alone, and this in turn because the temperature here is so high as to melt not only the iron now de-oxidized and brought to the metallic state, but also the gangue of the ore and the limestone, which here unite to form the molten slag, and run freely down between the lumps of coke. This coke descends freely even through this fast-narrowing space, because it is perfectly solid. and dry without a trace of pastiness. But immediately above this level the charge is relatively viscous, because here the temperature has fallen so far that it is. now at the melting or formation point of the slag, which therefore is pasty, liable to weld the whole mass together as so much See also:tar would, and thus to obstruct the descent of the charge, or in short to " See also:scaffold." The reason why at this level the walls must form an upright instead of an inverted cone, why the furnace must widen downward instead of narrowing, is, according to some metallurgists, that this shape is needed in order that, in spite of the pastiness of the slag in this formative period of incipient fusion, this layer may descend freely as the lower part of the column is gradually eaten away. To this very plausible theory it may be objected that in many slow-running furnaces, which work very regularly and show no sign of scaffolding, the outward flare of the boshes continues (though steepened) far above this region of pastiness, indeed nearly half-way to the top of the furnace. This proves that the regular descent of the material in its pasty state can take place even in a space which is narrowing downwards. To this objection it may in turn be answered that, though this degree of freedom of descent may suffice for a slow-running furnace, particularly if the slag is given such a composition that it passes quickly from the solid state to one of decided fluidity, yet it is not enough for swift-running ones, especially if the composition of the slag is such that, in melting, it remains long in a very sticky condition. In limiting the diameter at the tuyeres to 12' ft., the height of the boshes to one which will keep their upper end below the region of pastiness, and their slope to one over which the burning coke will descend freely, we limit the width of the furnace at the top of the boshes and thus complete the outline of the lower part of the furnace. The height of the furnace is rarely as great as too ft., and in the belief of many metallurgists it should not be much more than 8o ft. There are some very evident disadvantages of excessive height; for instance, that the weight of an excessively high column of solid coke, ore and limestone tends to crush the coke and jam the charge in the lower and narrowing part of the furnace, and that the frictional resistance of a long column calls for a greater consumption of power for driving the blast up through it. Moreover, this resistance increases much more rapidly than the height of the furnace, even if the rapidity with which the blast is forced through is constant; and it still further increases if the additional space gained by lengthening the furnace is made useful by increasing proportionally the rate of production, as indeed would naturally be done, because the chief See also:motive for gaining this additional space is to increase production. The reason why the frictional resistance would be further increased is the very simple one that the increase in the rate of production implies directly a corresponding increase in the quantity of blast forced through, and hence in the velocity of the rising gases, because the chemical work of the blast furnace needs a certain quantity of blast for each ton of iron made. In short, to increase the rate of production by lengthening the furnace increases the frictional resistance of the rising gases, both by increasing their quantity and hence their velocity and by lengthening their path. Indeed, one important reason for the difficulties in working very high furnaces, e.g. those too ft. high, may be that this frictional resistance becomes so great as actually to interrupt the even descent of the charge, parts of which are at times suspended like a See also:ball in the rising See also:jet of a See also:fountain, to fall perhaps with destructive violence when some shifting condition momentarily lessens the friction. We see how powerful must be the lifting effect of the rising gases when we reflect that their velocity in a too ft. furnace rapidly driven is probably at least as great as 2000 ft. per minute, or that of a " high wind." Conceive these gases passing at this great velocity through the narrow openings between the adjoining lumps of coke and ore. Indeed, the velocity must be far greater than this where the edge or corner of one lump touches the side of another, and the only room for the passage of this enormous quantity of gas is that left by the roughness and irregularity of the individual lumps. The furnace is made rather narrow at the top or " stock line," in order that the entering ore, fuel and flux may readily be distributed evenly. But extreme narrowness would not only cause the escaping gases to move so swiftly that they would sweep much of the fine ore out of the furnace, but would also throw needless work on the blowing engines by throttling back the rising gases, and would lessen unduly the space available for the charge in the upper part of the furnace. From its top down, the walls of the furnace slope outward at an angle of between 3° and 8°, partly in order to ease the descent of the charge, here impeded by the swelling of the individual particles of ore caused by the deposition within them of great quantities of fine carbon, by the reaction of 2CO=C+CO2. To widen it more abruptly would indeed increase the volume of the furnace, but would probably lead to See also:grave irregularities in the distribution of the gas and charge, and hence in the working of the furnace. When we have thus fixed the height of the furnace, its diameter at its ends, and the slope of its upper and lowerparts, we have completed its outline closely enough for our purpose here. 69. Hot Blast and Dry Blast.—On its way from the blowing engine to the tuyeres of the blast-furnace, the blast, i.e. the air forced in for the purpose of burning the fuel, is usually pre-heated, and in some of the most progressive works is dried by Gayley's See also:refrigerating process. These steps lead to a saving of fuel so great as to be astonishing at first sight—indeed in case of Gayley's blast-drying process incredible to most writers, who proved easily and promptly to their own See also:satisfaction that the actual saving was impossible. But the explanation is really so very simple that it is rather the incredulity of these writers that is astonishing. In the hearth of the blast furnace the heat made latent by the fusion of the iron and slag must of course be supplied by some body which is itself at a temperature above the melting point of these bodies, which for simplicity of exposition we may call the critical temperature of the blast-furnace process, because heat will flow only from a hotter to a cooler object. Much the same is true of the heat needed for the deoxidation of the silica, SiO2+2C=Si+2CO2. Now the heat developed by the combustion of coke to carbonic oxide with cold air containing the usual quantity of moisture, develops a temperature only slightly above this critical point; and it is only the heat represented by this narrow temperature-margin that is available for doing this critical work of fusion and deoxidation. That is the crux of the matter. If by pre-heating the blast we add to the sum of the heat available; or if by drying it we subtract from the work to be done by that heat the quantity needed for decomposing the atmospheric moisture; or if by removing part of its nitrogen we lessen the mass over which the heat developed has to be spread—if by any of these means we raise the temperature developed by the combustion of the coke, it is clear that we increase the proportion of the total heat which is available for this critical work in exactly the way in which we should increase the proportion of the water of a stream, initially too in. deep, which should flow over a waste See also:weir initially t in. beneath the stream's surface, by raising the upper surface of the water to in. and thus increasing the See also:depth of the water to t to in. Clearly this raising the level of the water by to% increases tenfold, or by r000%, the volume of water which is above the level of the weir. The special conditions of the blast-furnace actually exaggerate the saving due to this widening of the available temperature-margin, and beyond this drying the blast does great See also:good by preventing the serious irregularities in working the furnace caused by changes in the humidity of the air with varying weather. 7°. Means of Heating the Blast.—After the ascending column of gases has done its work of heating and deoxidizing the ore, it still necessarily contains so much carbonic oxide, usually between 20 and 26% by weight, that it is a very valuable fuel, part of which is used for raising steam for generating the blast itself and driving the rolling mill engines, &c., or directly in gas engines, and the rest for heating the blast. This heating was formerly done by burning part of the gases, after their escape from the furnace top, in a large combustion chamber, around a series of cast iron pipes through which the blast passed on its way from the blowing engine to the tuyeres. But these " iron pipe stoves " are fast going out of use, chiefly because they are destroyed quickly if an attempt is made to heat the blast above r000° F. (538° C.), often a very important thing. In their place the regenerative stoves of the Whitwell and See also:Cowper types (figs. so and II) are used. With these the regular temperature of the blast at some works is about 14000 F. (760°C.), and the usual blast temperature lies between goo° and t 20o° F. 480° and 65o° C.). Like the Siemens furnace, described in § 99, they have two distinct phases: one, " on gas," during which part of the waste gas of the blast-furnace is burnt within the See also:stove, highly heating the great surface of See also:brickwork which for that purpose is provided within it; the other, " on wind," during which the blast is heated by passing it back over these very surfaces which have thus been heated. They are heat-filters or heat-traps for impounding the heat developed by the combustion of the furnace gas, and later returning it to the blast. Each blast-furnace is now provided with three or even four of these stoves, which collectively may be nearly thrice as large as the furnace itself. At any given time one of these is " on wind " and the others " on gas." The Whitwell stove (fig. 1o), by means of the surface of several fire-See also:brick walls, catches in one phase the heat evolved by the burning gas as it sweeps through, and in the other phase returns that heat to the entering blast as it sweeps through from left to right. In the original Whitwell stove, which lacks the chimneys shown at the top of fig. to, both the burning gas and the blast pass up and down repeatedly. In the H. See also:Kennedy modification, shown in fig. to, the gas and air in one phase enter at the bottom of all three of the large vertical See also:chambers, burn in passing up-wards, and escape at once at the top, as shown by the broken arrows. In the other phase the cold blast, forced in at A, passes four times up and down, as shown by the unbroken arrows, and escapes as hot blast at B. This, then, is a " one-pass " stove when on gas but a " four-pass " one when on wind. The Cowper stove means of broken arrows. Hence this is a four- t"'iatth, the gconsequence for iven out- pass stove when on wind, but a one-pass thside dimensions, it stove when on gas. offers about one-half more heating surface than the true Whitwell stove; and (2) in that the gas and the blast pass only once up and once down through it, instead of twice up and twice down as in the modern true Whitwell stoves. As regards frictional resistance, this smaller number of reversals of direction compensates in a measure for the smaller area of the Cowper flues. The large combustion chamber B permits thorough combustion of the gas. 71. Preservation of the Furnace Walls.—The combined fluxing and abrading action of the descending charge tends to See also:wear away the lining of the furnace where it is hottest, which of course is near its lower end, thus changing its shape materially, lessening its efficiency, and in particular increasing its consumption of fuel. The walls, therefore, are now made thin, and are thoroughly cooled by water, which circulates through pipes or boxes bedded in them. James Gayley's method of cooling, shown in fig. 7, is to set in the brick-work walls several horizontal rows of flat water-cooled bronze boxes, RR', extending nearly to the interior of the furnace, and tapered so that they can readily be withdrawn and replaced in case they burn through. The brick-work may wear back to the front edges of these boxes, or even, as is shown at R', a little farther. But in the latter case their edges still determine'the effective See also:profile of the furnace walls because the depressions at the hack of these edges become filledwith carbon and scoriaceous rtiiit.ter when the furnace is in normal working. Each of these rows, of Which five are shown in fig. 7, consists of a great number of short segmental boxes. 72. Blast furnace Gas Engines.—When the gas which escapes from the furnace top is used in gas engines it generates about four times as much power as when it is used for raising steam. It has been calculated that the gas from a pair of old-fashioned blast-furnaces making r600 tons of iron per week would in this way yield some s6,000 horse-power in excess of their own needs, and that all the available blast-furnace gas in the United States would develop about 1,500,000 horse-power, to develop which by raising steam would need about 20,000,000 tons of coal a year. Of this power about half would be used at the blast-furnaces them- selves, leaving 750,000 horse- power available •t•"'•' for driving the t machinery of the rolling mills, &c. This use of the gas engine is likely to have far-reaching results. In order to utilize this power, the converting mill, in which the pig iron is converted into steel, and the rolling mills must adjoin the blast -furnace. The numerous converting mills which treat pig iron made at a distance will now have the crushing See also:burden of providing in other ways the power which their rivals get from the blast-furnace, in addition to the severe disadvantage under which they already suffer, of wasting the initial heat of the molten cast iron as it runs from the blast-furnace. Before its use in the gas engine, the blast-furnace gas has to be freed care-fully from the large quantity of fine ore dust which it carries in suspension. 73. Mechanical Appliances.— Moving the raw materials and the pro-ducts: In order to move economically , the great quantity FIG. 1r.--Diagram of Cowper Hot-Blast of materials which Stove at See also:Duquesne. (AfterJ .Kennedy.) Broken enter and issue from arrows show the .path of the gas. and air each furnace daily, twhile the See also:hat of the blast while iittais " on wind arrows mechanical appli- ances have at many A, Entrance for blast-furnace gas. works displaced c' Ch B, Coimnmeybustvailonvechamber. , . hand labour wholly, D, Cold blast main. and indeed that any E, Hollow bricks. of the materials should be shovelled by hand is not to be thought of in designing new works. The arrangement at the See also:Carnegie See also:Company's Duquesne works (fig. 12) may serve as an example of modern methods of handling. The See also:standard-See also:gauge cars which bring the ore and coke to Duquesne pass over one of three very long rows of bins, A, B, and C (fig. 12), of which A and B receive the materials (ore, coke and limestone) wind for immediate use, while C receives those to be stored for See also:winter f use. From A and B the materials are drawn as they are needed into large buckets D standing on cars, which carry them to the foot of the hoist track EE, up which they are hoisted to the top of the furnace. Arrived here, the material is introduced into the furnace by an ingenious piece of mechanism which completely prevents the furnace gas from escaping into the air. The hoist-engineer in the See also:house F at the foot of the furnace, when informed by means of an See also:indicator that the bucket has arrived at the top, lowers it so that its flanges GG (fig. 7) rest on the corresponding fixed flanges HH, as shown in fig. 9. The farther descent of the bucket being thus arrested, the special cable T is now slackened, so that the conical bottom of the bucket drops down, pressing down by its weight the the See also:string of moulds, each thus containing a pig, moves slowly forward, the pigs solidify and cool, the more quickly because in transit they are sprayed with water or even submerged in A and B, Bins for stock for immediate use. F, Hoist-engine house. N, N, N, Ladles carrying the molten C, Receiving See also:bin for winter stock See also:pile. LL, Travelling See also:crane commanding stock pile. cast iron to the works, where D, D, Ore bucket. M, Ore bucket receiving ore for stock pile. it is converted into steel by the EE, Hoist-track. M', Bucket removing ore from stock pile. open hearth process. See also:counter-weighted false See also:cover J of the furnace, so that the contents of the bucket slide down into the space between this false cover and the true charging bell, K. The special cable T is now tightened again, and lifts the bottom of the bucket so as both to close it and to close the space between J ,and K, by allowing J to rise back to its initial place. The bucket then descends along the hoist-track to make way for the next succeeding one, and K is lowered, dropping the charge into the furnace. Thus some 1700 tons of materials are charged daily into each of these furnaces without being shovelled at all, running by gravity from bin to bucket and from bucket to furnace, and being hoisted and charged into the furnace by a single engineer below, without any assistance or supervision at the furnace- toThe winter stock of materials is drawn from the left-hand See also:row of bins, and distributed over immense stock piles by means of the water in the tank EE. Arrived at the farther sheave C, the now cool pigs are dumped into a railway car. Besides a great saving of labour, only partly offset by the cost of See also:repairs, these See also:machines have the great merit of making the management independent of a very troublesome set of labourers, the hand pig-breakers, who were not only absolutely indispensable for every cast and every day, because the pig iron most be removed promptly to make way.f or the next succeeding cast of iron, but very difficult to replace because of the great physical endurance which their work requires. 75. Direct Processes for making Wrought Iron and Steel.—The A, Ladle bringing the cast iron from the blast-furnace. EE, Tank in which the moulds are submerged. BB, The moulds. F, Car into which the cooled pigs are dropped. C, D, Sheaves carrying the endless chain of moulds. G, Distributing See also:funnel. great crane LL (fig. 12), which transfers it as it is needed to the row A of bins, whence it is carried to the furnace, as already explained. 74, Casting the Molten Pig Iron.—The molten pig iron at many works is still run directly from the furnace into sand or iron moulds arranged in a way which suggests a See also:nursing See also:litter of pigs; hence the name " pig iron." These gigs are then usually broken by hand. The Uehling casting See also:machine (fig. 13) has displaced this method in many works. It consists essentially of a series of thin-walled moulds, BB, carried by endless chains past the See also:lip of a great ladle A. This pours into them the molten cast iron which it has just received directly from the blast-furnace. Aspresent way of getting the iron of the ore into the form of wrought iron and steel by first making cast iron and then purifying it, i.e. by first putting carbon and silicon into the iron and then taking them out again at great expense, at first sight seems so unreasonably roundabout that many " direct " processes of extracting the iron without thus charging it with carbon and silicon have been proposed, and some of them have at times been important. But to-day they have almost ceased to exist. That the blast-furnace process must be followed by a purifying one, that carburization must at once be undone by decarburization, is clearly a disadvantage, but it is one which is far outweighed by five important incidental advantages. (I) The strong deoxidizing action incidental to this carburizing removes the sulphur easily and cheaply, a thing hardly to be expected of any direct process so far as we can see. (2) The carburizing incidentally carburizes the brickwork of the furnace, and thus protects it against corrosion by the molten slag. (3) It protects the molten iron against reoxidation, the greatest stumbling See also:block in the way of the direct processes hitherto. (4) This same strong deoxidizing action leads to the practically complete deoxidation and hence extraction of the iron. (5) In that carburizing lowers the melting point of the iron greatly, it lowers somewhat the temperature to which the mineral matter of the ore has to be raised in order that the iron may be separated from it, because this separation requires that both iron and slag shall be very fluid. Indeed, few if any of the direct processes have attempted to make this separation, or to make it complete, leaving it for some subsequent operation, such as the open hearth process. In addition, the blast-furnace uses a very cheap source of energy, coke, anthracite, charcoal, and even certain kinds of raw bituminous coal, and owing first to the intimacy of contact between this fuel and the ore on which it works, and second to the thoroughness of the transfer of heat from the products of that fuel's combustion in their long upward See also:journey through the descending charge, even this cheap energy is used most effectively. Thus we have reasons enough why the blast-furnace has displaced all competing processes, without taking into See also:account its further advantage in lending itself easily to working on an enormous scale and with trifling consumption of labour, still further lessened by the general practice of transferring the molten cast iron in enormous ladles into the vessels in which its conversion into steel takes place. Nevertheless, a direct process may yet be made profitable under conditions which specially favour it, such as the lack of any fuel suitable for the blast-furnace, coupled with an abundance of cheap fuel suitable for a direct process and of cheap rich ore nearly free from sulphur. 76. The chief difficulty in the way of modifying the blast-furnace process itself so as to make it accomplish what the direct processes aim at, by giving its product less carbon and silicon than pig iron as now made contains, is the removal of the sulphur. The processes for converting cast iron into steel can now remove phosphorus easily, but the removal of sulphur in them is so difficult that it has to be accomplished for the most part in the blast-furnace itself. As desulphurizing seems to need the direct and energetic action of carbon on the molten iron itself, and as molten iron absorbs carbon most greedily, it is hard to see how the blast-furnace is to desulphurize without carburizing almost to saturation, i.e. without making cast iron. 77. Direct Metal and the Mixer.—Until relatively lately the cast iron for the Bessemer and open-hearth processes was nearly always allowed to solidify in pigs, which were next broken up by hand and remelted at great cost. It has long been seen that there would be a great saving if this remelting could be avoided and " direct See also:meta]," i.e. the molten cast iron direct from the blast-furnace, could be treated in the conversion process. The obstacle is that, owing to unavoidable irregularities in the blast-furnace process, the silicon- and sulphur-content of the cast iron vary to a degree and with an abruptness which are inconvenient for any conversion process and intolerable for the Bessemer process. For the acid variety of this process, which does not remove sulphur, this most harmful element must be held below a limit which is always low, though it varies somewhat with the use to which the steel is to be put. Further, the point at which the process should be arrested is recognized by the See also:appearance of the flame which issues from the converter's mouth, and variations in the silicon-content of the cast iron treated alter this appearance, so that the indications of the flame become confusing, and control over the process is lost. Moreover, the quality of the resultant steel depends upon the temperature of the process, and this in turn depends upon the proportion of silicon, the combustion of which is the chief source of the heat developed. Hence the importance of having the silicon-content constant. In the basic Bessemer process, also, unforeseen variations in the silicon-content are harmful, because the quantity of lime added should be just that needed to neutralize the resultant silica and the phosphoric acid and no more. Hence the importance of having the silicon-content See also:uniform. This uniformity is now given by the use of the " mixer " invented by See also:Captain W. R. See also: Should several furnaces simultaneously make iron too rich in silicon, this may be diluted by pouring into the mixer some low-silicon iron melted for this purpose in a cupola furnace. This See also:device not only makes the cast iron much more uniform, but also removes much of its sulphur by a curious slow reaction. Many metals have the power of dissolving their own oxides and sulphides, but not those of other metals. Thus iron, at least highly carburetted, i.e. cast iron, dissolves its own sulphide freely, but not that of either calcium or manganese. Consequently, when we deoxidize calcium in the iron blast-furnace, it greedily absorbs the sulphur which has been dissolved in the iron as iron sulphide, and the sulphide of calcium thus formed separates from the iron. In like manner, if the molten iron in the mixer contains manganese, this metal unites with the sulphur present, and the manganese sulphide, insoluble in the iron, slowly rises to the surface, and as it reaches the air, its sulphur oxidizes to sulphurous acid, which escapes. Further, an important part of the silicon may be removed in the mixer by keeping it very hot and covering the metal with a rather basic slag. This is very useful if the iron is intended for either the basic Bessemer or the basic open-hearth process, for both of which silicon is harmful. 78. Conversion or Purifying Processes for converting Cast Iron into Steel or Wrought Iron.—As the essential difference between cast iron on one hand and wrought iron and steel on the other is that the former contains necessarily much more carbon, usually more silicon, and often more phosphorus that are suit-able or indeed permissible in the latter two, the chief work of all these conversion processes is to remove the excess of these several foreign elements by oxidizing them to carbonic oxide CO, silica SiO2, and phosphoric acid P2Os, respectively. Of these the first escapes immediately as a gas, and the others unite with iron oxide, lime, or other strong base present to form a molten silicate or silico-phosphate called " cinder " or " slag," which floats on the molten or pasty metal. The ultimate source of the oxygen may be the air, as in the Bessemer process, or rich iron oxide as in the puddling process, or both as in the open-hearth process; but in any case iron oxide is the chief immediate source, as is to be expected, because the oxygen of the air would naturally unite in much greater proportion with some of the great quantity of iron offered to it than with the small quantity of these impurities. The iron oxide thus formed immediately oxidizes these foreign elements, so that the iron is really a See also:carrier of oxygen from air to impurity. The typical reactions are something like the following: Fe3O4+4C=4C0+3Fe; Fe304+C=3FeO+CO; 2P + 5FeaO4=12FeO + 3FeO,P2Os; Si + 2Fe304=3FeO,SiO2 + 3FeO. Beside, this their chief and easy work of oxidizing carbon, silicon and phosphorus, the conversion processes have the harder task• of removing sulphur, chiefly by converting it into calcium sulphide, CaS, or manganous sulphide, MnS, which rise to the top of the molten metal and there enter the overlying slag, from which the sulphur may escape by oxidizing to the gaseous See also:compound, sulphurous acid, SO2. 79. In the puddling process molten cast iron is converted into wrought iron, i.e. low-carbon slag-bearing iron, by oxidizing its carbon, silicon and phosphorus, by means of iron oxide stirred into it as it lies in a thin shallow layer in the " hearth " or flat See also:basin of a reverberatory furnace (fig. 14), itself lined with iron ore. As the iron oxide is stirred into the molten metal laboriously by the workman or "puddler " with his See also:hook or "See also:rabble," it oxidizes the silicon to silica and the phosphorus to phosphoric acid, and unites with both these products, forming with them a basic iron silicate rich in phosphorus, called " puddling " or " tap cinder." It oxidizes the carbon also, which escapes in purple jets of burning carbonic oxide. As the melting point of the metal is gradually raised by the progressive decarburization, it at length passes above the temperature of the furnace, about 1400° C., with the consequence that the metal, now below its melting point, solidifies in pasty grains, or " comes to nature." These grains the puddler welds together by means of his rabble into rough 8o-lb balls, each like a sponge of metallic iron particles with its pores filled with the still molten cinder. These balls are next worked into merchantable shape, and the cinder is simultaneously expelled in large part, first by hammering them one at a time under a steam hammer (fig. 37) or by squeezing them, and next by rolling them. The squeezing is usually done in the way shown in fig. 15. Here BB is a large fixed iron See also:cylinder, corrugated within, and C an excentric cylinder, also corrugated, which, in turning to the right, by the friction of its See also:cor- rugated surface rotates the puddled ball D which has just entered at A, so that, turning around its own See also:axis, it travels to the right and is gradually changed from a ball into a See also:bloom, a rough cylindrical mass of white hot iron, still dripping with cinder. This bloom is See also:im- mediately rolled down into a long flat See also:bar, called " muck bar," and this in turn is cut into short lengths which, piled one on another, are reheated and again rolled down, FIG. t See also:Plan of Burden's sometimes with repeated cutting, 5 piling and re-rolling, into the Excentric Revolving Squeezer final shape in which It is actuall y for Puddled Balls. to be used. But, See also:roll and re-roll as often as we like, much cinder remains imbedded in the iron, in the form of threads and rods drawn out in the direction of rolling, and of course weakening the metal in the transverse direction. 80. Machine Puddling.—The few men who have, and are willing to exercise, the great strength and endurance which the puddler needs when he is stirring the pasty iron and balling it up, command such high See also:wages, and with their little Soo-lb charges turn out their iron so slowly, that many ways of puddling by machinery have been tried. None has succeeded permanently, though indeed one offered by J. P. See also:Roe is not without promise. The essential difficulty has been that none of them could sub-divide the rapidly solidifying charge into the small balls which the workman dexterously forms by hand, and that if the charge is not thus subdivided but drawn as a single ball, the cinder cannot be squeezed out of it thoroughly enough. 81. Direct Puddling.—In common practice the cast iron as it runs from the blast-furnace is allowed to solidify and cool completely in the form of pigs, which are then graded by their fracture, and remelted in the puddling furnace itself. At Hourpes, in order to save the expense of this remelting, the molten cast iron as it comes from the blast-furnace is poured directly into the puddling furnace, in large charges of about 2200 lb, which are thus about four times as large as those of common puddling furnaces. These large charges are puddled by two gangs of four men each, and a great saving in fuel and labour is effected. Attractive as are these advances in puddling, they have not been widely adopted, for two chief reasons: First, owners of puddling works have been reluctant to spend See also:money freely in plant for a process of which the future is so uncertain, and this unwillingness has been the more natural because these very men are in large part the more conservative fraction, which has resisted the temptation to abandon puddling and adopt the steel-making processes. Second, in puddling iron which is to be used as a raw material for making very fine steel by the crucible process, quality is the thingof first importance. Now in the series of operations, the blast-furnace, puddling and crucible processes, through which the iron passes from the state of ore to that of crucible tool steel, it is so difficult to detect just which are the conditions essential to excellence in the final product that, once a given procedure has been found to yield excellent steel, every one of its details is adhered to by the more cautious ironmasters, often with surprising conservatism. Buyers of certain excellent classes of Swedish iron have been said to object even to the substitution of electricity for water-power as a means of driving the machinery of the forge. In case of direct puddling and the use of larger charges this conservatism has some See also:foundation. because the established See also:custom of allowing the cast iron to solidify gives a better opportunity of examining its fracture, and thus of rejecting unsuitable iron, than is afforded in direct puddling. So, too, when several puddlers are jointly responsible for the thoroughness of their work, as happens in puddling large charges, they will not exercise such care (nor indeed will a given degree of care be so effective) as when responsibility for each charge rests on one man. 82. The removal of phosphorus, a very important duty of the puddling process, requires that the cinder shall be " basic," i.e. that it shall have a great excess of the strong base, ferrous oxide, FeO, for the phosphoric acid to unite with, lest it be deoxidized by the carbon of the iron as fast as it forms, and so return to the iron, following the general rule that oxidized bodies enter the slag and unoxidized ones the metallic iron. But this basicity implies that for each part of the silica or silicic acid which inevitably results from the oxidation of the silicon of the pig iron, the cinder shall contain some three parts of iron oxide, itself a valuable and expensive substance. Hence, in order to save iron oxide the pig iron used should be nearly free from silicon. It should also be nearly free from sulphur, because of the great difficulty of removing this element in the puddling process. But the strong deoxidizing conditions needed in the blast-furnace to remove sulphur tend strongly to deoxidize silica and thus to make the pig iron rich in silicon. 83. The " refinery process " of fitting pig iron for the puddling process by removing the silicon without the carbon, is sometimes used because of this difficulty in making a pig iron initially low in both sulphur and silicon. In this process molten pig iron with much silicon but little sulphur has its silicon oxidized to silica and thus slagged off, by means of a blast of air playing on the iron through a blanket of burning coke which covers it. The coke thus at once supplies by its combustion the heat needed for melting the iron and keeping it hot, and by itself dissolving in the molten metal returns carbon to it as fast as this element is burnt out by the blast, so that the " refined " cast iron which results, though still rich in carbon and therefore easy to melt in the puddling process, has relatively little silicon. 84. In the Bessemer or " pneumatic" process, which indeed might be called the " fuel-less " process, molten pig iron is converted into steel by having its carbon, silicon and manganese, and often its phosphorus and sulphur, oxidized and thus removed by air forced through it in so many fine streams and hence so rapidly that the heat generated by the oxidation of these impurities suffices in and by itself, unaided by burning any other fuel, not only to keep the iron molten, but even to raise its temperature from a point initially but little above the melting point of cast iron, say 1150° to 1250° C., to one well above the melting point of the resultant steel, say 1500° C. The " Bessemer converter " or " See also:vessel " (fig. 16) in which this wonderful process is carried out is a huge See also:retort, lined with clay, See also:dolomite or other refractory material, hung aloft and turned on trunnions, DD, through the right-hand one of which the blast is carried to the gooseneck E, which in turn delivers it to the tuyeres Q at the bottom. There are two distinct varieties of this process, the original undephosphorizing or " acid " Bessemer process, so called because the converter is lined with acid materials, i.e. those rich in silicic acid, such as quartz and clay, and because the slag is consequently acid, i.e. siliceous; and the dephosphorizing or " Thomas " or " basic Bessemer " process, so called because the converter is lined with basic materials, usually calcined dolomite, a mixture of lime and magnesia, See also:bound together with tar, and because the slag is made very basic by adding much lime to it. In the basic Bessemer process phosphorus is readily removed by oxidation, because the product of its oxidation, phosphoric acid, P205, in the presence of an excess of base forms stable See also:phosphates of lime and iron which pass into the slag, making it valuable as an artificial manure. But this dephosphorization by oxidation can be carried out only in the case slag is basic. If it is acid, i.e. if it holds much more than 20% of, FIG. 16.—I2-15 ton Bessemer Converter. A, Trunnion-See also:ring. 0, Tuyere-See also:plate. B, Main shell. P, False plate. Upper part of shell. Q, Tuyeres. D, Trunnions. R, Keys holding lid of tuyere- E, See also:Goose-See also:neck. See also:box. F, Tuyere-box. S, Refractory lining.
N, Lid of tuyere-box. U, See also: The oxygen of the blast having been thus taken up by the molten metal, its nitrogen issues from the mouth of the converter as a See also:pale spark-bearing cone. Under normal conditions the silicon oxidizes first. Later, when most of it has been oxidized, the carbon begins to oxidize to carbonic oxide, which in turn See also:burns to carbonic acid as it meets the outer air on escaping from the mouth of the converter, and generates a true flame which grows bright, then brilliant, then almost See also:blinding, as it rushes and roars, then " drops," i.e. shortens and suddenly grows quiet when the last of the carbon has burnt away, and no flame-forming substance remains. Thus may a zo-ton charge of cast iron be converted into steel in ten minutes.' It is by the appearance of the flame that the operator or " blower " knows when to end the process, judging by its brilliancy, colour, See also:sound, See also:sparks, smoke and other indications. 86. Recarburizing.—The process may be interrupted as soon as the carbon-content has fallen to that which the final product is to have, or it may be continued till nearly the whole of the carbon has been burned out, and then the needed carbon may be added by " recarburizing." The former of these ways is followed by the very skilful and intelligent blowers in Sweden, who, with the temperature and all other conditions well under control, and with their minds set on the quality rather than on the quantity of their product, can thus make steel of any desired carbon-content from o• r o to 1.25%. But even with all their skill and care, while the carbon-content is still high the indications of the flame are not so decisive as to justify them in omitting to test the steel before removing it from the converter, as a check on the accuracy of their blowing. The delay which this test causes is so unwelcome that in all other countries the blower continues the blow until decarburization is nearly complete, because of the very great accuracy with which he can then read the indications of the flame, an accuracy which leaves little to be desired. Then, without waiting to test the product, he " recarburizes " it, i.e. adds enough carbon to give it the content desired, and then immediately pours the steel into a great clay-lined casting ladle by turning the converter over, and through a nozzle in the bottom of this ladle pours the steel into its ingot moulds. In making very low-carbon steel this recarburizing proper is not needed; but in any event a considerable quantity of manganese must be added unless the pig iron initially contains much of that metal, in order to remove from the molten steel the oxygen which it has absorbed from the blast, lest this make it redshort. If the carbon-content is not to be raised materially, this manganese is added in the form of preheated lumps of " ferro-manganese," which contains about . 8o% of manganese, 5% of carbon and 15''/, of iron, with a little silicon and other impurities. If, on the other hand, the carbon-content is to be raised, then carbon and manganese are usually added together in the form of a manganiferous molten pig iron, called spiegeleisen, i.e. " See also:mirror-iron," from the brilliancy of its facets, and usually containing somewhere about 12% of manganese and 4% of carbon, though the proportion between these two elements has to be adjusted so as to introduce the desired quantity- of each into the molten steel. Part of the carbon of this spiegeleisen unites with the oxygen occluded in the molten iron to form carbonic oxide, and again a bright flame, greenish with manganese, escapes from the converter. 87. Darby's Process.—Another way of introducing the carbon is Darby's process of throwing large paper bags filled with - anthracite, coke or gas-carbon into the casting ladle as the molten steel is pouring into it. The steel dissolves the carbon of this fuel even more quickly than water would dissolve See also:salt under like conditions. 88. Bessemer and Mushet.—Bessemer had no very wide knowledge of metallurgy, and after overcoming many stupendous I The length of the blow varies very greatly, in general increasing with the proportion of silicon and with the size of charge. Thus the small Swedish charges with but little silicon may be blown in 5 minutes, but for a 20-ton charge the time is more likely to reach or exceed 10 minutes, and sometimes reaches 20 minutes or even more. by-product. In order that the phosphoric acid may be the more fully liberated by the humic acid, &c., of the earth, a little silicious sand is mixed with the still molten slag after it liar been poured off from the molten steel. The slag is used in See also:agriculture with no further preparation, save very fine grinding. 92. The lining of the converter is made of 90% of the mixture of lime and magnesia which results from calcining dolomite, (Ca,Mg)CO3, at a very high temperature, and ro°/o of coal tar freed from its water by heating. This mixture may be rammed in place, or baked blocks of it may be laid up like a See also:masonry See also:wall. In either case such a lining is expensive, and has but a short See also:life, in few works more than 200 charges, and in some only too, though the silicious lining of the acid converter lasts thousands of charges. Hence, for the basic process, spare converters must be provided, so that there may always be some of them re-lining, either while standing in the same place as when in use, or, as in Holley's arrangement, in a See also:separate repair house, to which these gigantic vessels are removed bodily. 93• Control of the Basic Bessemer Process.—The removal of the greater part of the phosphorus takes place after the carbon has been oxidized and the flame has consequently " dropped," probably because the lime, which is charged in solid lumps, is taken up by the slag so slowly that not until late in the operation does the slag become so basic as to be retentive of phosphoric acid. Hence in making steel rich in carbon it is not possible, as in the acid Bessemer process, to end the operation as soon as the carbon in the metal has fallen to the point sought, but it is necessary to remove practically all of the carbon, then the phosphorus, and then " recarburize," i.e. add whatever carbon the steel is to contain. The quantity of phosphorus in the pig iron is usually known accurately, and the dephosphorization takes place so regularly that the quantity of air which it needs can be foretold closely. The blower therefore stops the process when he has blown a predetermined quantity of air through, counting from the drop of the flame; but as a check on his forecast he usually tests the blown metal before recar- difficulties he was greatly embarrassed by the brittleness or " redshortness " of his steel, which he did not know how to cure. But two remedies were quickly offered, one by the skilful Swede, Goransson. who used a pig iron initially rich in manganese and stopped his blow before much oxygen had been taken up; and the other by a British steel maker, Robert Mushet, who proposed the use of the manganiferous cast iron called spiegeleisen, and thereby removed the only remaining serious obstacle to the rapid spread of the process. From this many have claimed for Mushet a part almost or even quite equal to Bessemer's in the development of the Bessemer process, even calling it the " Bessemer-Mushet process." But this seems most unjust. Mushet had no such exclusive knowledge of the effects of manganese that he alone could have helped Bessemer; and even if nobody had then proposed the use of spiegeleisen, the development of the Swedish Bessemer practice would have gone on, and, the process thus established and its value and great economy thus shown in Sweden, it would have been only a question of time how soon somebody would have proposed the addition of manganese. Mushet's aid was certainly valuable, but not more than Goransson's, who, besides thus offering a preventive of redshortness, further helped the process on by raising its temperature by the simple expedient of further subdividing the blast, thus increasing the surface of contact between blast and metal, and thus in turn hastening the oxidation. The two great essential discoveries were first that the rapid passage of air through molten cast iron raised its temperature above the melting point of low-carbon steel, or as it was then called " malleable iron," and second that this low-carbon steel, which Bessemer was the first to make in important quantities, was in fact an extraordinarily valuable substance when made under proper conditions. 8A. Source of Heat.—The carbon of the pig iron, burning as it does only to carbonic oxide within the converter, does not by itself generate a temperature high enough for the needs of the process. The oxidation of manganese is capable of generating a very high temperature, but it has the very serious disadvantage of causing such thick clouds of smoky oxide of manganese as to hide the flame from the blower, and prevent him from recognizing the moment when the blow should be ended. Thus it comes about that the temperature is regulated primarily by adjusting the quantity of silicon in the pig iron treated, i% of this element usually sufficing. If any individual blow proves to be too hot, it may be cooled by throwing cold " scrap " steel such as the waste ends of rails and other pieces, into the converter, or by injecting with the blast a little steam, which is decomposed by the iron by the endothermic reaction H20+Fe=2H+FeO. If the temperature is not high enough, it is raised by managing the blast in such a way as to oxidize some of the iron itself permanently, and thus to generate much heat. 9o. The basic or dephosphorizing variety of the Bessemer process, called in Germany the " Thomas " process, differs from the acid process in four chief points: (r) that its slag is made very basic and hence dephosphorizing by adding much lime to it; (2) that the lining is basic, because an acid lining would quickly be destroyed by such a basic slag; (3) that the process is arrested not at the " drop of the flame " (§85) but at a pre-determined length of time after it; and (4) that phosphorus instead of silicon is the chief source of heat. Let us consider these in turn. 91. The slag, in order that it may have such an excess of base that this will retain the phosphoric acid as fast as it is formed by the oxidation of the phosphorus of the pig iron, and prevent it from being re-deoxidized and re-absorbed by the iron, should, according to von Ehrenwerth's rule which is generally followed, contain enough lime to form approximately a tetra-calcic silicate, 4CaO,SiO2 with the silica which results from the oxidation of the silicon of the pig iron and tri-calcic phosphate, 3CaO,P2O5, with the phosphoric acid which forms. The danger of this " rephosphorization " is greatest at the end of the blow, when the recarburizing additions are made. This lime is charged in the form of common quicklime, CaO, resulting from the calcination of a pure limestone, CaCO3, which should be as free as possible from silica. The usual composition of this slag is iron oxide, io to 16%; lime, 40 to 50%; magnesia, 5%; silica, 6 to 9%; phosphoric acid, 16 to 2o%. Its phosphoric acid makes it so valuable as a fertilizer that it is a most importantburizing it. 94. Source of Heat.—Silicon cannot here be used as the chief source of heat as it is in the acid Bessemer process, because most of the heat which its oxidation generates is consumed in heating the great quantities of lime needed for neutralizing the resultant silica. Fortunately the phosphorus, turned from a curse into a blessing, develops by its oxidation the needed temperature, though the fact that this requires at least 1.8o% of phosphorus limits the use of the process, because there are few ores which can be made to yield so phosphoric a pig iron. Further objections to the presence of silicon are that the resultant silica (r) corrodes the lining of the converter, (2) makes the slag froth so that it both throws much of the charge out and blocks up the See also:nose of the converter, and (3) leads to rephosphorization. These effects are so serious that until very lately it was thought that the silicon could not safely be much in excess of 1%. But Massenez and See also:Richards, following the plan outlined by Pourcel in 1879, have found that even 3% of silicon is permissible if, by adding iron ore, the resultant silica is made into a fuid slag, and if this is removed in the early cool part of the process, when it attacks the lining of the converter but slightly. Manganese to the extent of i•8o% is desired as a means of preventing the resultant steel from being redshort, i.e. brittle at a red or forging heat. The pig iron should be as nearly free as possible from sulphur, because the removal of any large quantity, of this injurious element in the process itself is both difficult and expensive. 95. The car casting system deserves description chiefly because it shows how, when the scale of operations is as enormous as it is in the Bessemer process, even a slight simplification and a slight heat-saving may be of great economic importance. Whatever be the form into which the steel is to be rolled, it must in general first be poured from the Bessemer converter in which it is made into a large clay-lined ladle, and thence cast in vertical pyramidal ingots. To bring them to a temperature suitable for rolling, these ingots must be set in heating or soaking furnaces (§ See also:i25), and this should be done as soon as possible after they are cast, both to lessen the loss of their initial heat, and to make way advantage in that it can develop a much higher temperature than is attainable in either of its competitors, the crucible and the open-hearth processes. Indeed, no limit has yet been found to the temperature which can be reached, if matters are so arranged that not only the carbon and silicon of the pig iron, but also a considerable part of the metallic iron which is the iron itself, are oxidized by the blast; or if, as in the Walrand-Legenisel modification, after the combustion of the initial carbon and silicon of the pig iron has already raised the charge to a very high temperature, a still further rise of temperature is brought about by adding more silicon in the form of ferro-silicon, and oxidizing it by further blowing. But in the crucible and the open-hearth processes the temperature attainable is limited by the danger of melting the furnace itself, both because some essential parts of it, which, unfortunately, are of a destructible shape, are placed most unfavourably in that they are surrounded by the heat on all sides, and because the furnace is necessarily hotter than the steel made within it. But no part of the Bessemer converter is of a shape easily affected by the heat, no part of it is exposed to the heat on more than one side, and the converter itself is necessarily cooler than the metal within it, because the heat is generated within the metal itself by the combustion of its silicon and other calorific elements. In it the steel heats the converter, whereas in the open-hearth and crucible processes the furnace heats the steel. 98. The open-hearth process consists in making molten steel out of pig or cast iron and " scrap," i.e. waste pieces of steel and iron melted together on the " open hearth," i.e. the uncovered basin-like bottom of a reverberatory furnace, under conditions of which fig. 18 may give a general idea. The con- xuIlllllllglillllulglllllllil Ilullllll{,,,1111111 I, ", ~„ ullNlllil!umIIl iilllii{lir , , I,~,iI,See also:IIII / for the next succeeding See also:lot of ingots, a matter of great importance, because the charges of steel follow each other at such very brief intervals. A pair of working converters has made 4958 charges of 10 tons each, or a total of 50,547 tons, in one See also:month, or at an See also:average rate of a charge every seven minutes and twenty-four seconds throughout every working day. It is this extraordinary rapidity that makes the process so economical and determines the .way in which its details must be carried out. Moreover, since the mould acts as a covering to retard the loss of heat, it should not be removed from the ingot until just before the latter is to be placed in its soaking furnace. These conditions are fulfilled by the car casting system of F. W. Wood, of Sparrows Point, Md., in which the moulds, while receiving the steel, stand on a See also:train of cars, which are immediately run to the side of the soaking furnace. Here, as soon as the ingots have so far solidified that they can be lifted without breaking, their moulds are removed and set on an adjoining train of cars, and the ingots are charged directly into the soaking furnace. The mould-train now carries its empty moulds to a cooling yard, and, as soon as they are cool enough to be used again, carries them back to the neighbourhood of the converters to receive a new lot of steel. In this system there is for each ingot and each mould only one handling in which it is moved as a separate unit, the mould from one train to the other, the ingot from its train into the furnace. In the other movements, all the moulds and ingots of a given charge of steel are grouped as a train, which is moved as a unit by a loco-motive. The difficulty in the way of this system was that, in pouring the steel from ladle to mould, more or less of itoccasionally spatters, and these spatterings, if they strike the rails or the running See also:gear of the cars, obstruct and foul them, preventing the See also:movement of the train, because the solidified steel is extremely tenacious. But this cannot be tolerated, because the economy of the process requires extreme promptness in each of its steps. On account of this difficulty the moulds formerly stood, not on cars, but directly on the See also:floor of a casting See also:pit while receiving the molten steel. When the ingots had so tar solidified that they could be handled, the moulds were removed and set on the floor to cool, the ingots were set on a car and carried to the soaking furnace, and the moulds were then replaced in the casting pit. Here each mould and each ingot was handled as a separate unit twice, instead of only once as in the car casting system; the ingots radiated away great quantities of heat in passing naked from the converting mill to the soaking furnaces, and the heat which they and the moulds radiated while in the converting mill was not only wasted, but made this mill, open-doored as it was, so intolerably hot, that the cost of labour there was materially in-creased. Mr Wood met this difficulty by the simple device of so shaping the cars that they completely- protect both their own running gear and the track from all possible spattering, a device which, simple as it is, has materially lessened the cost of the steel and greatly increased the production. How great the increase has been, from this and many other causes, is shown in Table III. 1870 . 188o 1889 . . 1899 (average for a month) 1903 . . Thus in See also:thirty-three years the rate of production per pair of vessels increased more than sixty-See also:fold. The production of European Bessemer works is very much less than that of American. Indeed, the whole German production of acid Bessemer steel in 1899 was at a rate but slightly greater than that here given for one pair of American converters; and three pairs, if this rate were continued, would make almost exactly as much steel as all the sixty-five active British Bessemer converters, acid and basic together, made in 1899. 96. Range in Size of Converters.—In the Bessemer process, and indeed in most high-temperature processes, to operate on a large scale has, in addition to the usual economies which it offers in other industries, a special one, arising from the fact that from a large hot furnace or hot mass in general a very much smaller proportion ,^f its heat dissipates through See also:radiation and like causes than from a smaller body, just as a thin red-hot wire cools in the air much faster han a thick bar equally hot. Hence the progressive increase which has occurred in the size of converters, until now some of them can treat a 20-ton charge, is not surprising. But, on the other hand, when only a relatively small quantity of a special kind of steel is needed, very much smaller charges, in some cases weighing even less than half a ton, have been treated with technical success. 97. The Bessemer Process for making Steel Castings.—This has been particularly true in the manufacture of steel castings, i.e. objects usually of more or less intricate shape, which are cast initially in the form in which they are to be used, Instead of being forged or rolled to that form from steel cast originally in ingots. For making castings, especially those which are so thin and intricate that, in order that the molten steel may remain molten long enough to run into the thin parts of the mould, it must be heated initially very far above its melting-point, the Bessemer process has a very great Half See also:Section showing condition Half Section showing condition of charge when boiling very of charge when boiling violently gently. during oreing. See also:Gross Tons per Week. 254 3,433 8,549 11,233 15,704 version of cast iron into steel, of course, consists in lessening its content of the several foreign elements, carbon, silicon, phosphorus, &c. The open-hearth process does this by two distinct steps: (1) by oxidizing and removing these elements by means I of the flame of the furnace, usually aided by the oxygen of light charges of iron ore, and (2) by diluting them with scrap steel or its equivalent. The '" pig and ore " or " Siemens variety of the process works chiefly by oxidation, the " pig and scrap " or " Siemens-Martin " variety chiefly by dilution, sometimes indeed by extreme dilution, as when zo parts of cast iron are diluted with 90 parts of scrap. Both varieties may be carried out in the basic and dephosphorizing way, i.e. id presence of a basic slag and in a basic- or neutral-lined furnace; or in the acid and undephosphorizing way, in presence of an acid, i.e. silicious slag, and in a furnace with a silicious lining. The charge may be melted down on the " open hearth " itself, or, as in the more advanced practice, the pig iron may be brought in the molten state from the blast furnace in which it is made. Then the furnacernan, controlling the decarburization and purification of the molten charge by his examination of test ingots taken from time to time, gradually oxidizes and so removes the foreign elements, and thus brings the metal simultaneously to approximately the composition needed and to a temperature far enough above its present melting= point to permit of its being cast into ingots or, other castings. He then pours or taps the molten charge from the furnace into a large clay-lined casting ladle, giving it the final additions of manganese, usually with carbon and often with silicon, needed to give it exactly the desired composition. He then casts it into its final form through a nozzle in the bottom of the casting ladle, as in the Bessemer process. The oxidation of the foreign elements must be very slow, lest the effervescence due to the escape of carbonic oxide from the carbon of the metal throw the charge out of the doors and ports of the furnace, which itself must be shallow in order to hold the flame down close to the charge. It is in large part because of this shallowness, which contrasts so strongly with the height and roominess of the Bessemer converter, that the process lasts hours where the Bessemer process lasts minutes, though there is the further difference that in the opens-hearth process the transfer of heat from flame to charge through the intervening layer of slag is necessarily slow, whereas in the Bessemer process the heat, generated as it is in and by the metallic bath itself, raises the temperature very rapidly. The slowness of this rise of the temperature compels us to make the removal of the carbon slow for a very simple reason. That removal progressively raises the melting-point of the metal, after line Aa of fig. 1, i.e. makes the charge more and more infusible; and this progressive rise of the melting-point of the charge must not be allowed to outrun the actual rise of temperature, or in other words the charge must always be kept molten, because once solidified it is very hard to remelt. Thus the necessary slowness of the heating up of the molten charge would compel us to make the removal of the carbon slow, even if this slowness were not already forced on us by the danger of having the charge froth so much as to run out of the furnace. The general plan of the open-hearth process was certainly conceived by See also:Josiah See also:Marshall See also:Heath in 1845, if not indeed by See also:Reaumur in 1722, but for lack of a furnace in which a high enough temperature could be generated it could not' be carried out until the development of the Siemens regenerative gas furnace about 186o. It was in large part through the efforts of Le Chatelier that this process, so long conceived, was at last, in 1864, put into actual use, by the brothers Martin, of Sireuil in France. 99. Siemens Open-Hearth Furnace.—These furnaces are usually stationary, but in that shown in figs. 19 to 22 the working chamber or furnace body, G of fig. 22, rotates about its own axis, rolling on the rollers M shown in fig. 21. In this working chamber, a long quasi-cylindrical vessel of brickwork, heated by burning within it pre-heated gas with pre-heated air, the charge is melted and brought to the desired composition and temperature. The working chamber indeed is the furnace proper, in which the whole of the open-hearth process is carried out, and the See also:function of all the rest of the apparatus, apart from the tilting mechanism, is simply to pre-heat the air and gas, and to lead them to the furnace proper and thence to the See also:chimney. How this is done may be understood more easily if figs. 19 and 20 are regarded for a moment as forming a single diagrammatic figure instead of sections in different planes. The unbroken arrows show the direction of the incoming gas and air, the broken ones the direction of the escaping products of their combustion. The air and gas, the latter coming from the gas producers or other source, arrive through H and J respectively, and their path thence is deter-mined by the position of the See also:reversing valves K and K'. In the position shown in solid lines, these valves deflect the air and gas into the left-hand pair of " regenerators " or spacious heat-transferring chambers. In these, bricks in great See also:numbers are piled loosely, in such a way that, while they leave ample passage for the gas and air, yet they offer to them a very great extent of surface, and therefore readily transfer to them the heat which they have as readily sucked out of the escaping products of combustion in the last preceding phase. The gas and air thus separately pre-heated to about Itoo° C. (2012° F.) rise thence as two separate streams through the uptakes (fig. 22), and first mix at the moment of entering the working chamber through the ports L and L' (fig. 19). As they are so hot at starting, their combustion of course yields a very much higher temperature than if they had been cold before burning, and they form an enormous flame, which fills the great working chamber. The products of combustion are sucked by the pull of the chimney through the farther or right-hand end of this chamber, out through the exit ports, as shown by the dotted arrows, down through the right-hand pair of regenerators, heating to perhaps 13oo° C. the upper part of the loosely-piled masses of brickwork within them, and thence past the valves K and K' to the chimney, flue O. During this phase the incoming gas and air have been withdrawing heat from the left-hand regenerators, which have thus been cooling.down, while the escaping products of combustion have been depositing heat in the right-hand pair of regenerators, which have thus been heating up. After some thirty minutes this condition of things is reversed by turning the valves K and K' 900into the positions shown in dotted lines, when they deflect the incoming gas and air into the right-hand regenerators, so that they may absorb in passing the heat which has just been stored there; thence they pass tip through the right-hand uptakes and ports into the working chamber, n-here as before they n1ia, hnrn and heat the charge. Thence they are sucked out by the chimney-draught through the left-hand ports,down through the uptakes and regenerators, here again See also:meeting and heating the loose mass of " regenerator " brickwork, and finally escape by the chimney-flue O. After another thirty minutes the G, Furnace body. N, See also:Hydraulic cylinder for tilting H, Air supply. the furnace. J, Gas supply. O, Flue leading to chimney. K, Air reversing See also:valve. p, Slag pockets. K', Gas reversing valve. R, Charging boxes. w, Water-cooled See also:joints between 1., Air port. L', Gas port. furnace proper, G, and ports M, Rollers on which the furnace L, L'. tilts. current is again reversed to its initial direction, and so on. Additional information and CommentsThere are no comments yet for this article.
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