6.First Period.—We can picture to ourselves how in the first period the savage smith, step by step, bettered his control over his fire, at once his source of heat and his deoxidizing agent. Not content to let it burn by natural draught, he would blow it with his own breath, would expose it to the prevalent wind, would urge it with a fan, and would devise the first crude valveless bellows, perhaps the pigskin already familiar as a water-bottle, of which the psalmist says: “I am become as a bottle in the smoke.” To drive the air out of this skin by pressing on it, or even by walking on it, would be easy; to fill it again with air by pulling its sides apart with his fingers would be so irksome that he would soon learn to distend it by means of strings. If his bellows had only a single opening, that through which they delivered the blast upon the fire, then in inflating them he would draw back into them the hot air and ashes from the fire. To prevent this he might make a second or suction hole, and thus he would have a veritable engine, perhaps one of the very earliest of all. While inflating the bellows he would leave the suction port open and close the discharge port with a pinch of his finger; and while blowing the air against the fire he would leave the discharge port open and pinch together the sides of the suction port.The next important step seems to have been taken in the 4th century when some forgotten Watt devised valves for the bellows. But in spite of the activity of the iron manufacture in many of the Roman provinces, especially England, France, Spain, Carinthia and near the Rhine, the little forges in which iron was extracted from the ore remained, until the 14th century, very crude and wasteful of labour, fuel, and iron itself: indeed probably not very different from those of a thousand years before. Where iron ore was found, the local smith, theWaldschmied, converted it with the charcoal of the surrounding forest into the wrought iron which he worked up. Many farmers had their own little forges or smithies to supply the iron for their tools.The fuel, wood or charcoal, which served both to heat and to deoxidize the ore, has so strong a carburizing action that it would turn some of the resultant metal into “natural steel,” which differs from wrought iron only in containing so much carbon that it is relatively hard and brittle in its natural state, and that it becomes intensely hard when quenched from a red heat in water. Moreover, this same carburizing action of the fuel would at times go so far as to turn part of the metal into a true cast iron, so brittle that it could not be worked at all. In time the smith learnt how to convert this unwelcome product into wrought iron by remelting it in the forge, exposing it to the blast in such a way as to burn out most of its carbon.7.Second Period.—With the second period began, in the 14th century, the gradual displacement of the direct extraction of wrought iron from the ore by the intentional and regular use of this indirect method of first carburizing the metal and thus turning it into cast iron, and then converting it into wrought iron by remelting it in the forge. This displacement has been going on ever since, and it is not quite complete even to-day. It is of the familiar type of the replacing of the simple but wasteful by the complex and economical, and it was begun unintentionally in the attempt to save fuel and labour, by increasing the size and especially the height of the forge, and by driving the bellows by means of water-power. Indeed it was the use of water-power that gave the smith pressure strong enough to force his blast up through a longer column of ore and fuel, and thus enabled him to increase the height of his forge, enlarge the scale of his operations, and in turn save fuel and labour. And it was the lengthening of the forge, and the length and intimacy of contact between ore and fuel to which it led, that carburized the metal and turned it into cast iron. This is so fusible that it melted, and, running together into a single molten mass, freed itself mechanically from the “gangue,” as the foreign minerals with which the ore is mixed are called. Finally, the improvement in the quality of the iron which resulted from thus completely freeing it from the gangue turned out to be a great and unexpected merit of the indirect process, probably the merit which enabled it, in spite of its complexity, to drive out the direct process. Thus we have here one of these cases common in the evolution both of nature and of art, in which a change, made for a specific purpose, has a wholly unforeseen advantage in another direction, so important as to outweigh that for which it was made and to determine the path of future development.With this method of making molten cast iron in the hands of a people already familiar with bronze founding, iron founding,i.e.the casting of the molten cast iron into shapes which were useful in spite of its brittleness, naturally followed. Thus ornamental iron castings were made in Sussex in the 14th century, and in the 16th cannons weighing three tons each were cast.The indirect process once established, the gradual increase in the height and diameter of the high furnace, which has lasted till our own days, naturally went on and developed the gigantic blast furnaces of the present time, still called “high furnaces” in French and German. The impetus which the indirect process and the acceleration of civilization in the 15th and 16th centuries gave to the iron industry was so great that the demands of the iron masters for fuel made serious inroads on the forests, and in 1558 an act of Queen Elizabeth’s forbade the cutting of timber in certain parts of the country for iron-making. Another in 1584 forbade the building of any more iron-works in Surrey, Kent, and Sussex. This increasing scarcity of wood was probably one of the chief causes of the attempts which the iron masters then made to replace charcoal with mineral fuel. In 1611 Simon Sturtevant patented the use of mineral coal for iron-smelting, and in 1619 Dud Dudley made with this coal both cast and wrought iron with technical success, but through the opposition of the charcoal iron-makers all of his many attempts were defeated. In 1625 Stradda’s attempts in Hainaut had no better success, and it was not till more than a century later that iron-smelting with mineral fuel was at last fully successful. It was then, in 1735, that Abraham Darby showed how to make cast iron with coke in the high furnace, which by this time had become a veritable blast furnace.The next great improvement in blast-furnace practice came in 1811, when Aubertot in France used for heating steel the furnace gases rich in carbonic oxide which till then had been allowed to burn uselessly at the top of the blast furnace. The next was J. B. Neilson’s invention in 1828 of heating the blast, which increased the production and lessened the fuel-consumption of the furnace wonderfully. Very soon after this, in 1832, the work of heating the blast was done by means of the waste gases, at Wasseralfingen in Bavaria.Meanwhile Henry Cort had in 1784 very greatly simplified the conversion of cast iron into wrought iron. In place of the old forge, in which the actual contact between the iron and the fuel, itself an energetic carburizing agent, made decarburization difficult, he devised the reverberatory puddling furnace (see fig. 14 below), in which the iron lies in a chamber apart from the fire-place, and is thus protected from the carburizing action of the fuel, though heated by the flame which that fuel gives out.The rapid advance in mechanical engineering in the latter part of this second period stimulated the iron industry greatly, giving it in 1728 Payn and Hanbury’s rolling mill for rolling sheet iron, in 1760 John Smeaton’s cylindrical cast-iron bellows in place of the wooden and leather ones previously used, in 1783 Cort’s grooved rolls for rolling bars and rods of iron, and in 1838 James Nasmyth’s steam hammer. But even more important than these were the advent of the steam engine between 1760 and 1770, and of the railroad in 1825, each of which gave the iron industry a great impetus. Both created a great demand for iron, not only for themselves but for the industries which they in turn stimulated; and both directly aided the iron master: the steam engine by giving him powerful and convenient tools, and the railroad by assembling his materials and distributing his products.About 1740 Benjamin Huntsman introduced the “crucible process” of melting steel in small crucibles, and thus freeing it from the slag, or rich iron silicate, with which it, like wrought iron, was mechanically mixed, whether it was made in the old forge or in the puddling furnace. This removal of the cinder very greatly improved the steel; but the process was and is so costly that it is used only for making steel for purposes which need the very best quality.8.Third Period.—The third period has for its great distinction the invention of the Bessemer and open-hearth processes, which are like Huntsman’s crucible process in that their essence is their freeing wrought iron and low carbon steel from mechanically entangled cinder, by developing the hitherto unattainable temperature, rising to above 1500° C., needed for melting these relatively infusible products. These processes are incalculably more important than Huntsman’s, both because they are incomparably cheaper, and because their products are far more useful than his.Thus the distinctive work of the second and third periods is freeingthe metal from mechanical impurities by fusion. The second period, by converting the metal into the fusible cast iron and melting this, for the first time removed the gangue of the ore; the third period by giving a temperature high enough to melt the most infusible forms of iron, liberated the slag formed in deriving them from cast iron.In 1856 Bessemer not only invented his extraordinary process of making the heat developed by the rapid oxidation of the impurities in pig iron raise the temperature above the exalted melting-point of the resultant purified steel, but also made it widely known that this steel was a very valuable substance. Knowing this, and having in the Siemens regenerative gas furnace an independent means of generating this temperature, the Martin brothers of Sireuil in France in 1864 developed the open-hearth process of making steel of any desired carbon-content by melting together in this furnace cast and wrought iron. The great defect of both these processes, that they could not remove the baneful phosphorus with which all the ores of iron are associated, was remedied in 1878 by S. G. Thomas, who showed that, in the presence of a slag rich in lime, the whole of the phosphorus could be removed readily.9. After the remarkable development of the blast furnace, the Bessemer, and the open-hearth processes, the most important work of this, the third period of the history of iron, is the birth and growth of the science and art of iron metallography. In 1868 Tschernoff enunciated its chief fundamental laws, which were supplemented in 1885 by the laws of Brinell. In 1888 F. Osmond showed that the wonderful changes which thermal treatment and the presence of certain foreign elements cause were due to allotropy, and from these and like teachings have come a rapid growth of the use of the so-called “alloy steels” in which, thanks to special composition and treatment, the iron exists in one or more of its remarkable allotropic states. These include the austenitic or gamma non-magnetic manganese steel, already patented by Robert Hadfield in 1883, the first important known substance which combined great malleableness with great hardness, and the martensitic or beta “high speed tool steel” of White and Taylor, which retains its hardness and cutting power even at a red heat.
6.First Period.—We can picture to ourselves how in the first period the savage smith, step by step, bettered his control over his fire, at once his source of heat and his deoxidizing agent. Not content to let it burn by natural draught, he would blow it with his own breath, would expose it to the prevalent wind, would urge it with a fan, and would devise the first crude valveless bellows, perhaps the pigskin already familiar as a water-bottle, of which the psalmist says: “I am become as a bottle in the smoke.” To drive the air out of this skin by pressing on it, or even by walking on it, would be easy; to fill it again with air by pulling its sides apart with his fingers would be so irksome that he would soon learn to distend it by means of strings. If his bellows had only a single opening, that through which they delivered the blast upon the fire, then in inflating them he would draw back into them the hot air and ashes from the fire. To prevent this he might make a second or suction hole, and thus he would have a veritable engine, perhaps one of the very earliest of all. While inflating the bellows he would leave the suction port open and close the discharge port with a pinch of his finger; and while blowing the air against the fire he would leave the discharge port open and pinch together the sides of the suction port.
The next important step seems to have been taken in the 4th century when some forgotten Watt devised valves for the bellows. But in spite of the activity of the iron manufacture in many of the Roman provinces, especially England, France, Spain, Carinthia and near the Rhine, the little forges in which iron was extracted from the ore remained, until the 14th century, very crude and wasteful of labour, fuel, and iron itself: indeed probably not very different from those of a thousand years before. Where iron ore was found, the local smith, theWaldschmied, converted it with the charcoal of the surrounding forest into the wrought iron which he worked up. Many farmers had their own little forges or smithies to supply the iron for their tools.
The fuel, wood or charcoal, which served both to heat and to deoxidize the ore, has so strong a carburizing action that it would turn some of the resultant metal into “natural steel,” which differs from wrought iron only in containing so much carbon that it is relatively hard and brittle in its natural state, and that it becomes intensely hard when quenched from a red heat in water. Moreover, this same carburizing action of the fuel would at times go so far as to turn part of the metal into a true cast iron, so brittle that it could not be worked at all. In time the smith learnt how to convert this unwelcome product into wrought iron by remelting it in the forge, exposing it to the blast in such a way as to burn out most of its carbon.
7.Second Period.—With the second period began, in the 14th century, the gradual displacement of the direct extraction of wrought iron from the ore by the intentional and regular use of this indirect method of first carburizing the metal and thus turning it into cast iron, and then converting it into wrought iron by remelting it in the forge. This displacement has been going on ever since, and it is not quite complete even to-day. It is of the familiar type of the replacing of the simple but wasteful by the complex and economical, and it was begun unintentionally in the attempt to save fuel and labour, by increasing the size and especially the height of the forge, and by driving the bellows by means of water-power. Indeed it was the use of water-power that gave the smith pressure strong enough to force his blast up through a longer column of ore and fuel, and thus enabled him to increase the height of his forge, enlarge the scale of his operations, and in turn save fuel and labour. And it was the lengthening of the forge, and the length and intimacy of contact between ore and fuel to which it led, that carburized the metal and turned it into cast iron. This is so fusible that it melted, and, running together into a single molten mass, freed itself mechanically from the “gangue,” as the foreign minerals with which the ore is mixed are called. Finally, the improvement in the quality of the iron which resulted from thus completely freeing it from the gangue turned out to be a great and unexpected merit of the indirect process, probably the merit which enabled it, in spite of its complexity, to drive out the direct process. Thus we have here one of these cases common in the evolution both of nature and of art, in which a change, made for a specific purpose, has a wholly unforeseen advantage in another direction, so important as to outweigh that for which it was made and to determine the path of future development.
With this method of making molten cast iron in the hands of a people already familiar with bronze founding, iron founding,i.e.the casting of the molten cast iron into shapes which were useful in spite of its brittleness, naturally followed. Thus ornamental iron castings were made in Sussex in the 14th century, and in the 16th cannons weighing three tons each were cast.
The indirect process once established, the gradual increase in the height and diameter of the high furnace, which has lasted till our own days, naturally went on and developed the gigantic blast furnaces of the present time, still called “high furnaces” in French and German. The impetus which the indirect process and the acceleration of civilization in the 15th and 16th centuries gave to the iron industry was so great that the demands of the iron masters for fuel made serious inroads on the forests, and in 1558 an act of Queen Elizabeth’s forbade the cutting of timber in certain parts of the country for iron-making. Another in 1584 forbade the building of any more iron-works in Surrey, Kent, and Sussex. This increasing scarcity of wood was probably one of the chief causes of the attempts which the iron masters then made to replace charcoal with mineral fuel. In 1611 Simon Sturtevant patented the use of mineral coal for iron-smelting, and in 1619 Dud Dudley made with this coal both cast and wrought iron with technical success, but through the opposition of the charcoal iron-makers all of his many attempts were defeated. In 1625 Stradda’s attempts in Hainaut had no better success, and it was not till more than a century later that iron-smelting with mineral fuel was at last fully successful. It was then, in 1735, that Abraham Darby showed how to make cast iron with coke in the high furnace, which by this time had become a veritable blast furnace.
The next great improvement in blast-furnace practice came in 1811, when Aubertot in France used for heating steel the furnace gases rich in carbonic oxide which till then had been allowed to burn uselessly at the top of the blast furnace. The next was J. B. Neilson’s invention in 1828 of heating the blast, which increased the production and lessened the fuel-consumption of the furnace wonderfully. Very soon after this, in 1832, the work of heating the blast was done by means of the waste gases, at Wasseralfingen in Bavaria.
Meanwhile Henry Cort had in 1784 very greatly simplified the conversion of cast iron into wrought iron. In place of the old forge, in which the actual contact between the iron and the fuel, itself an energetic carburizing agent, made decarburization difficult, he devised the reverberatory puddling furnace (see fig. 14 below), in which the iron lies in a chamber apart from the fire-place, and is thus protected from the carburizing action of the fuel, though heated by the flame which that fuel gives out.
The rapid advance in mechanical engineering in the latter part of this second period stimulated the iron industry greatly, giving it in 1728 Payn and Hanbury’s rolling mill for rolling sheet iron, in 1760 John Smeaton’s cylindrical cast-iron bellows in place of the wooden and leather ones previously used, in 1783 Cort’s grooved rolls for rolling bars and rods of iron, and in 1838 James Nasmyth’s steam hammer. But even more important than these were the advent of the steam engine between 1760 and 1770, and of the railroad in 1825, each of which gave the iron industry a great impetus. Both created a great demand for iron, not only for themselves but for the industries which they in turn stimulated; and both directly aided the iron master: the steam engine by giving him powerful and convenient tools, and the railroad by assembling his materials and distributing his products.
About 1740 Benjamin Huntsman introduced the “crucible process” of melting steel in small crucibles, and thus freeing it from the slag, or rich iron silicate, with which it, like wrought iron, was mechanically mixed, whether it was made in the old forge or in the puddling furnace. This removal of the cinder very greatly improved the steel; but the process was and is so costly that it is used only for making steel for purposes which need the very best quality.
8.Third Period.—The third period has for its great distinction the invention of the Bessemer and open-hearth processes, which are like Huntsman’s crucible process in that their essence is their freeing wrought iron and low carbon steel from mechanically entangled cinder, by developing the hitherto unattainable temperature, rising to above 1500° C., needed for melting these relatively infusible products. These processes are incalculably more important than Huntsman’s, both because they are incomparably cheaper, and because their products are far more useful than his.
Thus the distinctive work of the second and third periods is freeingthe metal from mechanical impurities by fusion. The second period, by converting the metal into the fusible cast iron and melting this, for the first time removed the gangue of the ore; the third period by giving a temperature high enough to melt the most infusible forms of iron, liberated the slag formed in deriving them from cast iron.
In 1856 Bessemer not only invented his extraordinary process of making the heat developed by the rapid oxidation of the impurities in pig iron raise the temperature above the exalted melting-point of the resultant purified steel, but also made it widely known that this steel was a very valuable substance. Knowing this, and having in the Siemens regenerative gas furnace an independent means of generating this temperature, the Martin brothers of Sireuil in France in 1864 developed the open-hearth process of making steel of any desired carbon-content by melting together in this furnace cast and wrought iron. The great defect of both these processes, that they could not remove the baneful phosphorus with which all the ores of iron are associated, was remedied in 1878 by S. G. Thomas, who showed that, in the presence of a slag rich in lime, the whole of the phosphorus could be removed readily.
9. After the remarkable development of the blast furnace, the Bessemer, and the open-hearth processes, the most important work of this, the third period of the history of iron, is the birth and growth of the science and art of iron metallography. In 1868 Tschernoff enunciated its chief fundamental laws, which were supplemented in 1885 by the laws of Brinell. In 1888 F. Osmond showed that the wonderful changes which thermal treatment and the presence of certain foreign elements cause were due to allotropy, and from these and like teachings have come a rapid growth of the use of the so-called “alloy steels” in which, thanks to special composition and treatment, the iron exists in one or more of its remarkable allotropic states. These include the austenitic or gamma non-magnetic manganese steel, already patented by Robert Hadfield in 1883, the first important known substance which combined great malleableness with great hardness, and the martensitic or beta “high speed tool steel” of White and Taylor, which retains its hardness and cutting power even at a red heat.
10.Constitution of Iron and Steel.—The constitution of the various classes of iron and steel as shown by the microscope explains readily the great influence of carbon which was outlined in §§ 2 and 3. The metal in its usual slowly cooled state is a conglomerate like the granitic rocks. Just as a granite is a conglomerate or mechanical mixture of distinct crystalline grains of three perfectly definite minerals, mica, quartz, and felspar, so iron and steel in their usual slowly cooled state consist of a mixture of microscopic particles of such definite quasi-minerals, diametrically unlike. These are cementite, a definite iron carbide, Fe3C, harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress; and ferrite, pure or nearly pure metallic α-iron, soft, weak, with high electric conductivity, and in general like copper except in colour. In view of the fact that the presence of 1% of carbon implies that 15% of the soft ductile ferrite is replaced by the glass-hard cementite, it is not surprising that even a little carbon influences the properties of the metal so profoundly.
But carbon affects the properties of iron not only by giving rise to varying proportions of cementite, but also both by itself shifting from one molecular state to another, and by enabling us to hold the iron itself in its unmagnetic allotropic forms, β- and γ-iron, as will be explained below. Thus, sudden cooling from a red heat leaves the carbon not in definite combination as cementite, but actually dissolved in β- and γ-allotropic iron, in the conditions known as martensite and austenite, not granitic but glass-like bodies, of which the “hardened” and “tempered” steel of our cutting tools in large part consists. Again, if more than 2% of carbon is present, it passes readily into the state of pure graphitic carbon, which, in itself soft and weak, weakens and embrittles the metal as any foreign body would, by breaking up its continuity.
11. TheRoberts-Austenorcarbon-iron diagram(fig. 1), in which vertical distances represent temperatures and horizontal ones the percentage of carbon in the iron, aids our study of these constituents of iron. If, ignoring temporarily and for simplicity the fact that part of the carbon may exist in the state of graphite, we consider the behaviour of iron in cooling from the molten state, AB and BC give the temperature at which, for any given percentage of carbon, solidification begins, and Aa,aB, and Bcthat at which it ends. But after solidification is complete and the metal has cooled to a much lower range of temperature, usually between 900° and 690° C., it undergoes a very remarkable series of transformations. GHSagives the temperature at which, for any given percentage of carbon, these transformations begin, and PSP′ that at which they end.
These freezing-point curves and transformation curves thus divide the diagram into 8 distinct regions, each with its own specific state or constitution of the metal, the molten state for region 1, a mixture of molten metal and of solid austenite for region 2, austenite alone for region 4 and so on. This will be explained below. If the metal followed the laws of equilibrium, then whenever through change of temperature it entered a new region, it would forthwith adopt the constitution normal to that region. But in fact the change of constitution often lags greatly, so that the metal may have the constitution normal to a region higher than that in which it is, or even a patchwork constitution, representing fragments of those of two or more regions. It is by taking advantage of this lagging that thermal treatment causes such wonderful changes in the properties of the cold metal.
12. With these facts in mind we may now study further these different constituents of iron.
Austenite, gamma(γ)iron.—Austenite is the name of the solid solution of an iron carbide in allotropie γ-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, notper saltumas between definite chemical compounds, but by infinitesimal steps. The different substances are as it were dissolved in each other in a state which has the indefiniteness of composition, the absolute merging of identity, and the weakness of reciprocal chemical attraction, characteristic of aqueous solutions.On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, FexC = Fe3C + Fe(x−3). But this change may be prevented so as to preserve the austenite in the cold, either very incompletely, as when high-carbon steel is “hardened,”i.e.is cooled suddenly by quenching in water, in which case the carbon present seems to act as a brake to retard the change; or completely, by the presence of a large quantity of manganese, nickel, tungsten or molybdenum, which in effect sink the lower boundary GHSaof region 4 to below the atmospheric temperature. The important manganese steels of commerce and certain nickel steels are manganiferous and niccoliferous austenite, unmagnetic and hard but ductile.Austenite may contain carbon in any proportion up to about 2.2%. It is non-magnetic, and, when preserved in the cold either by quenching or by the presence of manganese, nickel, &c., it has a very remarkable combination of great malleability with very marked hardness, though it is less hard than common carbon steel is when hardened, and probably less hard than martensite. When of eutectoid composition, it is called “hardenite.” Suddenly cooled carbon steel,even if rich in austenite, is strongly magnetic because of the very magnetic α-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(β)iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and stable only in the little triangle GHM, is yet a state through which the metal seems always to pass when the austenite of region 4 changes into the ferrite and cementite of regions 6 and 8. Though not normal below MHSP′, yet like γ-iron it can be preserved in the cold by the presence of about 5% of manganese, which, though not enough to bring the lower boundary of region 4 below the atmospheric temperature and thus to preserve austenite in the cold, is yet enough to make the transformation of β into α iron so sluggish that the former remains untransformed even during slow cooling.Again, β-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 β 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,TroostiteandSorbiteare 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 β-iron, is characteristic of hardened steel, but the two others, far from being definite substances, are probably only roughly bounded stages of this transition.Troostiteandsorbite, indeed, seem to be chiefly very finely divided mixtures of ferrite and cementite, and it is probably because of this fineness that sorbitic steel has its remarkable combination of strength and elasticity with ductility which fits it for resisting severe vibratory and other dynamic stresses, such as those to which rails and shafting are exposed.15.Alpha(α)ironis 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.Ferriteandcementite, already described in § 10, are the final products of the transformation of austenite in slow-cooling. β-ferrite and austenite are the normal constituents for the triangle GHM, α-ferrite (i.e.nearly pure α-iron) with austenite for the space MHSP, cementite with austenite for region 7, and α-ferrite and cementite jointly for regions 6 and 8. Ferrite and cementite are thus the normal and usual constituents of slowly cooled steel, including all structural steels, rail steel, &c., and of white cast iron (see § 18).17.Pearlite.—The ferrite and cementite present interstratify habitually as a “eutectoid”2called “pearlite” (seeAlloys, Pl., fig. 11), in the ratio of about 6 parts of ferrite to 1 of cementite, and hence containing about 0.90% of carbon. Slowly cooled steel containing just 0.90% of carbon (S in fig. 1) consists of pearlite alone. Steel and white cast iron with more than this quantity of carbon consist typically of kernels of pearlite surrounded by envelopes of free cementite (seeAlloys, Pl., fig. 13) sufficient in quantity to represent their excess of carbon over the eutectoid ratio; they arc called “hyper-eutectoid,” and are represented by region 8 of Fig. 1. Steel containing less than this quantity of carbon consists typically of kernels of pearlite surrounded by envelopes of ferrite (seeAlloys, Pl., fig. 12) sufficient in quantity to represent their excess of iron over this eutectoid ratio; is called “hypo-eutectoid”; and is represented by region 6 of Fig. 1. This typical “envelope and kernel” structure is often only rudimentary.The percentage of pearlite and of free ferrite or cementite in these products is shown in fig. 2, in which the ordinates of the line ABC represent the percentage of pearlite corresponding to each percentage of carbon, and the intercept ED, MN or KF, of any point H, P or L, measures the percentage of the excess of ferrite or cementite for hypo- and hyper-eutectic steel and white cast iron respectively.Fig. 2.—Relation between the carbon-content and the percentage of the several constituents of slowly cooled steel and white cast iron.18.The Carbon-Content, i.e. the Ratio of Ferrite to Cementite, of certain typical Steels.—Fig. 3 shows how, as the carbon-content rises from 0 to 4.5%, the percentage of the glass-hard cementite, which is 15 times that of the carbon itself, rises, and that of the soft copper-like ferrite falls, with consequent continuous increase of hardness and loss of malleableness and ductility. The tenacity or tensile strength increases till the carbon-content reaches about 1.25%, and the cementite about 19%, and then in turn falls, a result by no means surprising. The presence of a small quantity of the hard cementite ought naturally to strengthen the mass, by opposing the tendency of the soft ferrite to flow under any stress applied to it; but more cementite by its brittleness naturally weakens the mass, causing it to crack open under the distortion which stress inevitably causes. The fact that this decrease of strength begins shortly after the carbon-content rises above the eutectoid or pearlite ratio of 0.90% is natural, because the brittleness of the cementite which, in hyper-eutectoid steels, forms a more or less continuous skeleton (Alloys, Pl., fig. 13) should be much more effective in starting cracks under distortion than that of the far more minute particles of cementite which lie embedded, indeed drowned, in the sixfold greater mass of ferrite with which they are associated in the pearlite itself. The large massive plates of cementite which form the network or skeleton in hyper-eutectoid steels should, under distortion, naturally tend to cut, in the softer pearlite, chasms too serious to be healed by the inflowing of the plastic ferrite, though this ferrite flows around and immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.Fig. 3.—Physical properties and assumed microscopic constitution of the pearlite series, graphiteless steel slowly cooled and white cast iron. By “total ferrite” is meant both that which forms part of the pearlite and that which is in excess of the pearlite, taken jointly. So with the “total cementite.”As the carbon-content increases the welding power naturally decreases rapidly, because of the rapid fall of the “solidus curve” at which solidification is complete (Aaof fig. 1), and hence of the range in which the steel is coherent enough to be manipulated, and, finally, of the attainable pliancy and softness of the metal. Clearly the mushy mixture of solid austenite and molten iron of which the metal in region 2 consists cannot cohere under either the blows or the pressure by means of which welding must be done. Rivet steel, which above all needs extreme ductility to endure the distortion of being driven home, and tube steel which must needs weld easily, no matter at what sacrifice of strength, are made as free from carbon,i.e.of as nearly pure ferrite, as is practicable. The distortion which rails undergo in manufacture and use is incomparably less than that to which rivets are subjected, and thus rail steel may safely be much richer in carbon and hence in cementite, and therefore much stronger and harder, so as to better endure the load and the abrasion of the passing wheels. Indeed, its carbon-content is made small quite as much because of the violence of the shocks from these wheels as because of any actual distortion to be expected, since, within limits, as thecarbon-content increases the shock-resisting power decreases. Here, as in all cases, the carbon-content must be the result of a compromise, neither so small that the rail flattens and wears out like lead, nor so great that it snaps like glass. Boiler plates undergo in shaping and assembling an intermediate degree of distortion, and therefore they must be given an intermediate carbon-content, following the general rule that the carbon-content and hence the strength should be as great as is consistent with retaining the degree of ductility and the shock-resisting power which the object will need in actual use. Thus the typical carbon-content may be taken as about 0.05% for rivets and tubes, 0.20% for boiler plates, and 0.50 to 0.75% for rails, implying the presence of 0.75% of cementite in the first two, 3% in the third and 7.5% to 11.25% in the last.19.Carbon-Content of Hardened Steels.—Turning from these cases in which the steel is used in the slowly cooled state, so that it is a mixture of pearlite with ferrite or cementite,i.e.is pearlitic, to those in which it is used in the hardened or martensitic state, we find that the carbon-content is governed by like considerations. Railway car springs, which are exposed to great shock, have typically about 0.75% of carbon; common tool steel, which is exposed to less severe shock, has usually between 0.75 and 1.25%; file steel, which is subject to but little shock, and has little demanded of it but to bite hard and stay hard, has usually from 1.25 to 1.50%. The carbon-content of steel is rarely greater than this, lest the brittleness be excessive. But beyond this are the very useful, because very fusible, cast irons with from 3 to 4% of carbon, the embrittling effect of which is much lessened by its being in the state of graphite.20.Slag or Cinder, a characteristic component of wrought iron, which usually contains from 0.20 to 2.00% of it, is essentially a silicate of iron (ferrous silicate), and is present in wrought iron simply because this product is made by welding together pasty granules of iron in a molten bath of such slag, without ever melting the resultant mass or otherwise giving the envelopes of slag thus imprisoned a chance to escape completely.21.Graphite, nearly pure carbon, is characteristic of “gray cast iron,” in which it exists as a nearly continuous skeleton of very thin laminated plates or flakes (fig. 27), usually curved, and forming from 2.50% to 3.50% of the whole. As these flakes readily split open, when a piece of this iron is broken rupture passes through them, with the result that, even though the graphite may form only some 3% of the mass by weight (say 10% by volume), practically nothing but graphite is seen in the fracture. Hence the weakness and the dark-grey fracture of this iron, and hence, by brushing this fracture with a wire brush and so detaching these loosely clinging flakes of graphite, the colour can be changed nearly to the very light-grey of pure iron. There is rarely any important quantity of graphite in commercial steels. (See § 26.)22.Further Illustration of the Iron-Carbon Diagram.—In order to illustrate further the meaning of the diagram (fig. 1), let us follow by means of the ordinate QUw the undisturbed slow cooling of molten hyper-eutectoid steel containing 1% of carbon, for simplicity assuming that no graphite forms and that the several transformations occur promptly as they fall due. When the gradually falling temperature reaches 1430° (q), the mass begins to freeze as γ-iron or austenite, called “primary” to distinguish it from that which forms part of the eutectic. But the freezing, instead of completing itself at a fixed temperature as that of pure water does, continues until the temperature sinks to r on the line Aa. Thus the iron has rather a freezing-range than a freezing-point. Moreover, the freezing is “selective.” The first particles of austenite to freeze contain about 0.33% of carbon (p). As freezing progresses, at each successive temperature reached the frozen austenite has the carbon-content of the point on Aa which that temperature abscissa cuts, and the still molten part or “mother-metal” has the carbon-content horizontally opposite this on the line AB. In other words, the composition of the frozen part and that of the mother-metal respectively are p and q at the beginning of the freezing, andrandt′ at the end; and during freezing they slide along Aaand AB fromptorand fromqtot′. This, of course, brings the final composition of the frozen austenite when freezing is complete exactly to that which the molten mass had before freezing began.The heat evolved by this process of solidification retards the fall of temperature; but after this the rate of cooling remains regular until T (750°) on the line Sa(Ar3) is reached, when a second retardation occurs, due to the heat liberated by the passage within the pasty mass of part of the iron and carbon from a state of mere solution to that of definite combination in the ratio Fe3C, forming microscopic particles of cementite, while the remainder of the iron and carbon continue dissolved in each other as austenite. This formation of cementite continues as the temperature falls, till at about 690° C., (U, called Ar2−1) so much of the carbon (in this case about 0.10%) and of the iron have united in the form of cementite, that the composition of the remaining solid-solution or “mother-metal” of austenite has reached that of the eutectoid, hardenite;i.e.it now contains 0.90 % of carbon. The cementite which has thus far been forming may be called “pro-eutectoid” cementite, because it forms before the remaining austenite reaches the eutectoid composition. As the temperature now falls past 690°, this hardenite mother-metal in turn splits up, after the fashion of eutectics, into alternate layers of ferrite and cementite grouped together as pearlite, so that the mass as a whole now becomes a mixture of pearlite with cementite. The iron thus liberated, as the ferrite of this pearlite, changes simultaneously to α-ferrite. The passage of this large quantity of carbon and iron, 0.90% of the former and 12.6 of the latter, from a state of mere solution as hardenite to one of definite chemical union as cementite, together with the passage of the iron itself from the γ to the α 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 γ through the β to the α 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 β state. As shown inAlloys, Pl., fig. 13, the slowly cooled steel now consists of kernels of pearlite surrounded by envelopes of the cementite which was born of the austenite in cooling from T to U.23. To take a second case, molten hypo-eutectoid steel of 0.20% of carbon on freezing from K to x passes in the like manner to the state of solid austenite, γ-iron with this 0.20% of carbon dissolved in it. Its further cooling undergoes three spontaneous retardations, one at K′ (Ar3about 820°), at which part of the iron begins to isolate itself within the austenite mother-metal in the form of envelopes of β-ferrite,i.e.of free iron of the β allotropic modification, which surrounds the kernels or grains of the residual still undecomposed part of the austenite. At the second retardation, K″ (Ar2, about 770°) this ferrite changes to the normal magnetic α-ferrite, so that the mass as a whole becomes magnetic. Moreover, the envelopes of ferrite which began forming at Ar3continue to broaden by the accession of more and more ferrite born from the austenite progressively as the temperature sinks, till, by the time when Ar1(about 690°) 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 Ar1this hardenite mother-metal splits up into cementite and ferrite grouped together as pearlite, with the resulting recalescence, and the mass, as shown inAlloys, Pl., fig. 12, then consists of kernels of pearlite surrounded by envelopes of ferrite. All these phenomena are parallel with those of 1.00% carbon steel at this same critical point Ar1. As such steel cools slowly past Ar3, Ar2and Ar1, it loses its hardening power progressively.In short, from Ar3to Ar1the excess substance ferrite or cementite, in hypo- and hyper-eutectoid steels respectively, progressively crystallizes out as a network or skeleton within the austenite mother-metal, which thus progressively approaches the composition of hardenite, reaching it at Ar1, and there splitting up into ferrite and cementite interstratified as pearlite. Further, any ferrite liberated at Ar3changes there from γ to β, and any present at Ar2changes from β to α. Between H and S, Ar3and Ar2occur together, as do Ar2and Ar1between S and P′ and Ar3, Ar2and Ar1at S itself; so that these critical points in these special cases are called Ar3−2, Ar2−1and Ar3−2−1respectively. The corresponding critical points which occur during rise of temperature, with the reverse transformations, are called Ac1, Ac2, Ac3, &c. A (Tschernoff) is the generic name, r refers to falling temperature (refroidissant) and c to rising temperature (chauffant, Osmond).24. The freezing of molten cast iron of 2.50% of carbon goes on selectively like that of these steels which we have been studying, till the enrichment of the molten mother-metal in carbon brings its carbon-contents to B, 4.30%, the eutectic3carbon-content,i.e.that of the greatest fusibility or lowest melting-point. At this point selection ceases; the remaining molten metal freezes as a whole, and in freezing splits up into a conglomerate eutectic of (1) austenite of about 2.2 % of carbon, and therefore saturated with that element, and (2) cementite; and with this eutectic is mixed the “primary” austenite which froze out as the temperature sank fromvtov′. The white-hot, solid, but soft mass is now a conglomerate of (1) “primary” austenite, (2) “eutectic” austenite and (3) “eutectic” cementite. As the temperature sinks still farther, pro-eutectoid cementite (see § 22) forms progressively in the austenite both primary and eutectic, and this pro-eutectoid cementite as it comes into existence tends to assemble in the form of a network enveloping the kernels or grains of the austenite from which it springs. The reason for its birth, of course, is that the solubility of carbon in austenite progressively decreases as the temperature falls, from about 2.2% at 1130° (a), to 0.90% at 690° (Ar1), as shown by the lineaS, with the consequence that the austenite keeps rejecting in the form of this pro-eutectoid cementite all carbon in excess of its saturation-point for the existing temperature. Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.This formation of cementite through the rejection of carbon by both the primary and the eutectic austenite continues quite as in the case of 1.00% carbon steel, with impoverishment of the austenite to the hardenite or eutectoid ratio, and the splitting up of that hardenite into pearlite at Ar1, so that the mass when cold finally consists of (1)the primary austenite now split up into kernels of pearlite surrounded by envelopes of pro-eutectoid cementite, (2) the eutectic of cementite plus austenite, the latter of which has in like manner split up into a mixture of pearlite plus cementite. Such a mass is shown in fig. 4. Here the black bat-like patches are the masses of pearlite plus pro-eutectoid cementite resulting from the splitting up of the primary austenite. The magnification is too small to show the zebra striping of the pearlite. In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other.Fig. 4.—The constitution of hypo-eutectic white or cementitiferous cast iron (washed metal), W. Campbell. The black bat-like areas are the primary austenite, the zebra-marked ground mass the eutectic, composed of white stripes of cementite and black stripes of austenite. Both the primary and eutectic austenite have changed in cooling into a mixture of pearlite and pro-eutectoid cementite, too fine to be distinguished here.25. As we pass to cases with higher and higher carbon-content, the primary austenite which freezes in cooling across region 2 forms a smaller and smaller proportion of the whole, and the austenite-cementite eutectic which forms at the eutectic freezing-point, 1130° (aB), increases in amount until, when the carbon-content reaches the eutectic ratio, 4.30%, there is but a single freezing-point, and the whole mass when solid is made up of this eutectic. If there is more than 4.30% of carbon, then in cooling through region 3 the excess of carbon over this ratio freezes out as “primary” cementite. But in any event the changes which have just been described for cast iron of 2.50% of carbon occur in crossing region 7, and at Ar1(PSP′).Just as variations in the carbon-content shift the temperature of the freezing-range and of the various critical points, so do variations in the content of other elements, notably silicon, phosphorus, manganese, chromium, nickel and tungsten. Nickel and manganese lower these critical points, so that with 25% of nickel Ar3lies below the common temperature 20° C. With 13% of manganese Ar3is very low, and the austenite decomposes so slowly that it is preserved practically intact by sudden cooling. These steels then normally consist of γ-iron, modified by the large amount of nickel or manganese with which it is alloyed. They are non-magnetic or very feebly magnetic. But the critical points of such nickel steel though thus depressed, are not destroyed; and if it is cooled in liquid air below its Ar2, it passes to the α state and becomes magnetic.26.Double Nature of the Carbon-Iron Diagram.—The part played by graphite in the constitution of the iron-carbon compounds, hitherto ignored for simplicity, is shown in fig. 5. Looking at the matter in a broad way, in all these carbon-iron alloys, both steel and cast irons, part of the carbon may be dissolved in the iron, usually as austenite,e.g.in regions 2, 4, 5 and 7 of Fig. 1; the rest,i.e.the carbon which is not dissolved, or the “undissolved carbon,” forms either the definite carbide, cementite, Fe3C, or else exists in the free state as graphite. Now, just as fig. 1 shows the constitution of these iron-carbon alloys for all temperatures and all percentages of carbon when the undissolved carbon exists as cementite, so there should be a diagram showing this constitution when all the undissolved carbon exists as graphite. In short, there are two distinct carbon-iron diagrams, the iron-cementite one shown in fig. 1 and studied at length in §§ 22 to 25, and the iron-graphite one shown in fig. 5 in unbroken lines, with the iron-cementite diagram reproduced in broken lines for comparison. What here follows represents our present rather ill-established theory. These two diagrams naturally have much the same general shape, but though the boundaries of the several regions in the iron-cementite diagram are known pretty accurately, and though the relative positions of the boundaries of the two diagrams are probably about as here shown, the exact topography of the iron-graphite diagram is not yet known. In it the normal constituents are, for region II., molten metal + primary austenite; for region III., molten metal + primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B′ and primary graphite at the right of that point. Thus when iron containing 2.50% of carbon (v.fig. 1) solidifies, its carbon may form cementite following the cementite-austenite diagram so that white,i.e.cementitiferous, cast iron results; or graphite, following the graphite-austenite diagram, so that ultra-grey,i.e.typical graphitic cast iron results; or, as usually happens, certain molecules may follow one diagram while the rest follow the other diagram, so that cast iron which has both cementite and graphite results, as in most commercial grey cast iron, and typically in “mottled cast iron,” in which there are distinct patches of grey and others of white cast iron.Though carbon passes far more readily under most conditions into the state of cementite than into that of graphite, yet of the two graphite is the more stable and cementite the less stable, or the “metastable” form. Thus cementite is always tending to change over into graphite by the reaction Fe3C = 3Fe + Gr, though this tendency is often held in check by different causes; but graphite never changes back directly into cementite, at least according to our present theory. The fact that graphite may dissolve in the iron as austenite, and that when this latter again breaks up it is more likely to yield cementite than graphite, is only an apparent and not a real exception to this law of the greater stability of graphite than of cementite.Slow cooling, slow solidification, the presence of an abundance of carbon, and the presence of silicon, all favour the formation of graphite; rapid cooling, the presence of sulphur, and in most cases that of manganese, favour the formation of cementite. For instance, though in cast iron, which is rich in carbon, that carbon passes comparatively easily into the state of graphite, yet in steel, which contains much less carbon, but little graphite forms under most conditions. Indeed, in the common structural steels which contain only very little carbon, hardly any of that carbon exists as graphite.27.Thermal Treatment.—The hardening, tempering and annealing of steel, the chilling and annealing of cast iron, and the annealing of malleable cast iron are explained readily by the facts just set forth.28.The hardening of steelconsists in first transforming it into austenite by heating it up into region 4 of fig. 1, and then quenching it, usually in cold water, so as to cool it very suddenly, and thus to deny the time which the complete transformation of the austenite into ferrite and cementite requires, and thereby to catch much of the iron in transit in the hard brittle β state. In the cold this transformation cannot take place, because of molecular rigidity or some other impediment. The suddenly cooled metal is hard and brittle, because the cold β-iron which it contains is hard and brittle.Fig. 5.—Graphite-austenite or stable carbon-iron, diagram.The degree of hardening which the steel undergoes increases with its carbon-content, chiefly because, during sudden cooling, the presence of carbon acts like a brake to impede the transformations, and thus to increase the quantity of β-iron caught in transit, but probably also in part because the hardness of this β-iron increases with its carbon-content. Thus, though sudden cooling has very little effect on steel of 0.10% of carbon, it changes that of 1.50% from a somewhat ductile body to one harder and more brittle than glass.29.The Tempering and Annealing of Steel.—But this sudden cooling goes too far, preserving so much β-iron as to make the steel too brittle for most purposes. This brittleness has therefore in general to be mitigated or “tempered,” unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,usually to between 200° and 300° C., so as to relax the molecular rigidity and thereby to allow the arrested transformation to go on a little farther, shifting a little of the β-iron over into the α state. The higher the tempering-temperature,i.e.that to which the hardened steel is thus reheated, the more is the molecular rigidity relaxed, the farther on does the transformation go, and the softer does the steel become; so that, if the reheating reaches a dull-red heat, the transformation from austenite into ferrite and cementite completes itself slowly, and when now cooled the steel is as soft and ductile as if it had never been hardened. It is now said to be “annealed.”30.Chilling cast iron,i.e.hastening its cooling by casting it in a cool mould, favours the formation of cementite rather than of graphite in the freezing of the eutectic at aBc, and also, in case of hyper-eutectic iron, in the passage through region 3. Like the hardening of steel, it hinders the transformation of the austenite, whether primary or eutectic, into pearlite + cementite, and thus catches part of the iron in transit in the hard β state. The annealing of such iron may occur in either of two degrees—a small one, as in making common chilled cast iron objects, such as railway car wheels, or a great one, as in making malleable cast iron. In the former case, the objects are heated only to the neighbourhood of Ac1, say to 730° C., so that the β-iron may slip into the a state, and the transformation of the austenite into pearlite and cementite may complete itself. The joint effect of such chilling and such annealing is to make the metal much harder than if slowly cooled, because for each 1% of graphite which the chilling suppresses, 15% of the glass-hard cementite is substituted. Thus a cast iron which, if cooled slowly, would have been “grey,”i.e.would have consisted chiefly of graphite with pearlite and ferrite (which are all relatively soft bodies), if thus chilled and annealed consists of cementite and pearlite. But in most such cases, in spite of the annealing, this hardness is accompanied by a degree of brittleness too great for most purposes. The process therefore is so managed that only the outer shell of the casting is chilled, and that the interior remains graphitic,i.e.grey cast iron, soft and relatively malleable.31. In makingmalleable castingsthe annealing,i.e.the change towards the stable state of ferrite + graphite, is carried much farther by means of a much longer and usually a higher heating than in the manufacture of chilled castings. The castings, initially of white cast iron, are heated for about a week, to a temperature usually above 730° C. and often reaching 900° C. (1346° and 1652° F.). For about 60 hours the heat is held at its highest point, from which it descends extremely slowly. The molecular freedom which this high temperature gives enables the cementite to change gradually into a mixture of graphite and austenite with the result that, after the castings have been cooled and their austenite has in cooling past Ac1changed into pearlite and ferrite, the mixture of cementite and pearlite of which they originally consisted has now given place to one of fine or “temper” graphite and ferrite, with more or less pearlite according to the completeness of the transfer of the carbon to the state of graphite.Why, then, is this material malleable, though the common grey cast iron, which is made up of about the same constituents and often in about the same proportion, is brittle? The reason is that the particles of temper graphite which are thus formed within the solid casting in its long annealing are so finely divided that they do not break up the continuity of the mass in a very harmful way; whereas in grey cast iron both the eutectic graphite formed in solidifying, and also the primary graphite which, in case the metal is hyper-eutectic, forms in cooling through region 3 of fig. 1, surrounded as it is by the still molten mother-metal out of which it is growing, form a nearly continuous skeleton of very large flakes, which do break up in a most harmful way the continuity of the mass of cast iron in which they are embedded.In carrying out this process the castings are packed in a mass of iron oxide, which at this temperature gradually removes the fine or “temper” graphite by oxidizing that in the outer crust to carbonic oxide, whereon the carbon farther in begins diffusing outwards by “molecular migration,” to be itself oxidized on reaching the crust. This removal of graphite doubtless further stimulates the formation of graphite, by relieving the mechanical and perhaps the osmotic pressure. Thus, first, for the brittle glass-hard cementite there is gradually substituted the relatively harmless temper graphite; and, second, even this is in part removed by surface oxidation.32.Fineness of Structure.—Each of these ancient processes thus consists essentially in so manipulating the temperature that, out of the several possible constituents, the metal shall actually consist of a special set in special proportions. But in addition there is another very important principle underlying many of our thermal processes, viz. that the state of aggregation of certain of these constituents, and through it the properties of the metal as a whole, are profoundly affected by temperature manipulations. Thus, prior exposure to a temperature materially above Ac3coarsens 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 be cured either byreheatingthe steel to Ac3when it “refines,”i.e.returns spontaneously to its fine-grained ductile state (coolingpast Ar3does not have this effect); or by breaking up the coarse grains bymechanical 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 grain-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 expenditure of power, ought later to be refined, either by reheating it from below Ar3to slightly above Ac3or by rolling it after it has cooled to a relatively low temperature,i.e.by having a low “finishing temperature.” Steel castings have initially the extremely coarse structure due to cooling without mechanical distortion from their very high temperature of solidification; they are “annealed,”i.e.this coarseness and the consequent brittleness are removed, by reheating them much above Ac3, which also relieves the internal stresses due to the different rates at which different layers cool, and hence contract, during and after solidification. For steel containing less than about 0.13% of carbon, the embrittling temperature is in a different range, near 700° C., and such steel refines at temperatures above 900° C.
Austenite, gamma(γ)iron.—Austenite is the name of the solid solution of an iron carbide in allotropie γ-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, notper saltumas between definite chemical compounds, but by infinitesimal steps. The different substances are as it were dissolved in each other in a state which has the indefiniteness of composition, the absolute merging of identity, and the weakness of reciprocal chemical attraction, characteristic of aqueous solutions.
On cooling into region 6 or 8 austenite should normally split up into ferrite and cementite, after passing through the successive stages of martensite, troostite and sorbite, FexC = Fe3C + Fe(x−3). But this change may be prevented so as to preserve the austenite in the cold, either very incompletely, as when high-carbon steel is “hardened,”i.e.is cooled suddenly by quenching in water, in which case the carbon present seems to act as a brake to retard the change; or completely, by the presence of a large quantity of manganese, nickel, tungsten or molybdenum, which in effect sink the lower boundary GHSaof region 4 to below the atmospheric temperature. The important manganese steels of commerce and certain nickel steels are manganiferous and niccoliferous austenite, unmagnetic and hard but ductile.
Austenite may contain carbon in any proportion up to about 2.2%. It is non-magnetic, and, when preserved in the cold either by quenching or by the presence of manganese, nickel, &c., it has a very remarkable combination of great malleability with very marked hardness, though it is less hard than common carbon steel is when hardened, and probably less hard than martensite. When of eutectoid composition, it is called “hardenite.” Suddenly cooled carbon steel,even if rich in austenite, is strongly magnetic because of the very magnetic α-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(β)iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and stable only in the little triangle GHM, is yet a state through which the metal seems always to pass when the austenite of region 4 changes into the ferrite and cementite of regions 6 and 8. Though not normal below MHSP′, yet like γ-iron it can be preserved in the cold by the presence of about 5% of manganese, which, though not enough to bring the lower boundary of region 4 below the atmospheric temperature and thus to preserve austenite in the cold, is yet enough to make the transformation of β into α iron so sluggish that the former remains untransformed even during slow cooling.
Again, β-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 β 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,TroostiteandSorbiteare 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 β-iron, is characteristic of hardened steel, but the two others, far from being definite substances, are probably only roughly bounded stages of this transition.Troostiteandsorbite, indeed, seem to be chiefly very finely divided mixtures of ferrite and cementite, and it is probably because of this fineness that sorbitic steel has its remarkable combination of strength and elasticity with ductility which fits it for resisting severe vibratory and other dynamic stresses, such as those to which rails and shafting are exposed.
15.Alpha(α)ironis 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.Ferriteandcementite, already described in § 10, are the final products of the transformation of austenite in slow-cooling. β-ferrite and austenite are the normal constituents for the triangle GHM, α-ferrite (i.e.nearly pure α-iron) with austenite for the space MHSP, cementite with austenite for region 7, and α-ferrite and cementite jointly for regions 6 and 8. Ferrite and cementite are thus the normal and usual constituents of slowly cooled steel, including all structural steels, rail steel, &c., and of white cast iron (see § 18).
17.Pearlite.—The ferrite and cementite present interstratify habitually as a “eutectoid”2called “pearlite” (seeAlloys, Pl., fig. 11), in the ratio of about 6 parts of ferrite to 1 of cementite, and hence containing about 0.90% of carbon. Slowly cooled steel containing just 0.90% of carbon (S in fig. 1) consists of pearlite alone. Steel and white cast iron with more than this quantity of carbon consist typically of kernels of pearlite surrounded by envelopes of free cementite (seeAlloys, Pl., fig. 13) sufficient in quantity to represent their excess of carbon over the eutectoid ratio; they arc called “hyper-eutectoid,” and are represented by region 8 of Fig. 1. Steel containing less than this quantity of carbon consists typically of kernels of pearlite surrounded by envelopes of ferrite (seeAlloys, Pl., fig. 12) sufficient in quantity to represent their excess of iron over this eutectoid ratio; is called “hypo-eutectoid”; and is represented by region 6 of Fig. 1. This typical “envelope and kernel” structure is often only rudimentary.
The percentage of pearlite and of free ferrite or cementite in these products is shown in fig. 2, in which the ordinates of the line ABC represent the percentage of pearlite corresponding to each percentage of carbon, and the intercept ED, MN or KF, of any point H, P or L, measures the percentage of the excess of ferrite or cementite for hypo- and hyper-eutectic steel and white cast iron respectively.
18.The Carbon-Content, i.e. the Ratio of Ferrite to Cementite, of certain typical Steels.—Fig. 3 shows how, as the carbon-content rises from 0 to 4.5%, the percentage of the glass-hard cementite, which is 15 times that of the carbon itself, rises, and that of the soft copper-like ferrite falls, with consequent continuous increase of hardness and loss of malleableness and ductility. The tenacity or tensile strength increases till the carbon-content reaches about 1.25%, and the cementite about 19%, and then in turn falls, a result by no means surprising. The presence of a small quantity of the hard cementite ought naturally to strengthen the mass, by opposing the tendency of the soft ferrite to flow under any stress applied to it; but more cementite by its brittleness naturally weakens the mass, causing it to crack open under the distortion which stress inevitably causes. The fact that this decrease of strength begins shortly after the carbon-content rises above the eutectoid or pearlite ratio of 0.90% is natural, because the brittleness of the cementite which, in hyper-eutectoid steels, forms a more or less continuous skeleton (Alloys, Pl., fig. 13) should be much more effective in starting cracks under distortion than that of the far more minute particles of cementite which lie embedded, indeed drowned, in the sixfold greater mass of ferrite with which they are associated in the pearlite itself. The large massive plates of cementite which form the network or skeleton in hyper-eutectoid steels should, under distortion, naturally tend to cut, in the softer pearlite, chasms too serious to be healed by the inflowing of the plastic ferrite, though this ferrite flows around and immediately heals over any cracks which form in the small quantity of cementite interstratified with it in the pearlite of hypo-eutectoid steels.
As the carbon-content increases the welding power naturally decreases rapidly, because of the rapid fall of the “solidus curve” at which solidification is complete (Aaof fig. 1), and hence of the range in which the steel is coherent enough to be manipulated, and, finally, of the attainable pliancy and softness of the metal. Clearly the mushy mixture of solid austenite and molten iron of which the metal in region 2 consists cannot cohere under either the blows or the pressure by means of which welding must be done. Rivet steel, which above all needs extreme ductility to endure the distortion of being driven home, and tube steel which must needs weld easily, no matter at what sacrifice of strength, are made as free from carbon,i.e.of as nearly pure ferrite, as is practicable. The distortion which rails undergo in manufacture and use is incomparably less than that to which rivets are subjected, and thus rail steel may safely be much richer in carbon and hence in cementite, and therefore much stronger and harder, so as to better endure the load and the abrasion of the passing wheels. Indeed, its carbon-content is made small quite as much because of the violence of the shocks from these wheels as because of any actual distortion to be expected, since, within limits, as thecarbon-content increases the shock-resisting power decreases. Here, as in all cases, the carbon-content must be the result of a compromise, neither so small that the rail flattens and wears out like lead, nor so great that it snaps like glass. Boiler plates undergo in shaping and assembling an intermediate degree of distortion, and therefore they must be given an intermediate carbon-content, following the general rule that the carbon-content and hence the strength should be as great as is consistent with retaining the degree of ductility and the shock-resisting power which the object will need in actual use. Thus the typical carbon-content may be taken as about 0.05% for rivets and tubes, 0.20% for boiler plates, and 0.50 to 0.75% for rails, implying the presence of 0.75% of cementite in the first two, 3% in the third and 7.5% to 11.25% in the last.
19.Carbon-Content of Hardened Steels.—Turning from these cases in which the steel is used in the slowly cooled state, so that it is a mixture of pearlite with ferrite or cementite,i.e.is pearlitic, to those in which it is used in the hardened or martensitic state, we find that the carbon-content is governed by like considerations. Railway car springs, which are exposed to great shock, have typically about 0.75% of carbon; common tool steel, which is exposed to less severe shock, has usually between 0.75 and 1.25%; file steel, which is subject to but little shock, and has little demanded of it but to bite hard and stay hard, has usually from 1.25 to 1.50%. The carbon-content of steel is rarely greater than this, lest the brittleness be excessive. But beyond this are the very useful, because very fusible, cast irons with from 3 to 4% of carbon, the embrittling effect of which is much lessened by its being in the state of graphite.
20.Slag or Cinder, a characteristic component of wrought iron, which usually contains from 0.20 to 2.00% of it, is essentially a silicate of iron (ferrous silicate), and is present in wrought iron simply because this product is made by welding together pasty granules of iron in a molten bath of such slag, without ever melting the resultant mass or otherwise giving the envelopes of slag thus imprisoned a chance to escape completely.
21.Graphite, nearly pure carbon, is characteristic of “gray cast iron,” in which it exists as a nearly continuous skeleton of very thin laminated plates or flakes (fig. 27), usually curved, and forming from 2.50% to 3.50% of the whole. As these flakes readily split open, when a piece of this iron is broken rupture passes through them, with the result that, even though the graphite may form only some 3% of the mass by weight (say 10% by volume), practically nothing but graphite is seen in the fracture. Hence the weakness and the dark-grey fracture of this iron, and hence, by brushing this fracture with a wire brush and so detaching these loosely clinging flakes of graphite, the colour can be changed nearly to the very light-grey of pure iron. There is rarely any important quantity of graphite in commercial steels. (See § 26.)
22.Further Illustration of the Iron-Carbon Diagram.—In order to illustrate further the meaning of the diagram (fig. 1), let us follow by means of the ordinate QUw the undisturbed slow cooling of molten hyper-eutectoid steel containing 1% of carbon, for simplicity assuming that no graphite forms and that the several transformations occur promptly as they fall due. When the gradually falling temperature reaches 1430° (q), the mass begins to freeze as γ-iron or austenite, called “primary” to distinguish it from that which forms part of the eutectic. But the freezing, instead of completing itself at a fixed temperature as that of pure water does, continues until the temperature sinks to r on the line Aa. Thus the iron has rather a freezing-range than a freezing-point. Moreover, the freezing is “selective.” The first particles of austenite to freeze contain about 0.33% of carbon (p). As freezing progresses, at each successive temperature reached the frozen austenite has the carbon-content of the point on Aa which that temperature abscissa cuts, and the still molten part or “mother-metal” has the carbon-content horizontally opposite this on the line AB. In other words, the composition of the frozen part and that of the mother-metal respectively are p and q at the beginning of the freezing, andrandt′ at the end; and during freezing they slide along Aaand AB fromptorand fromqtot′. This, of course, brings the final composition of the frozen austenite when freezing is complete exactly to that which the molten mass had before freezing began.
The heat evolved by this process of solidification retards the fall of temperature; but after this the rate of cooling remains regular until T (750°) on the line Sa(Ar3) is reached, when a second retardation occurs, due to the heat liberated by the passage within the pasty mass of part of the iron and carbon from a state of mere solution to that of definite combination in the ratio Fe3C, forming microscopic particles of cementite, while the remainder of the iron and carbon continue dissolved in each other as austenite. This formation of cementite continues as the temperature falls, till at about 690° C., (U, called Ar2−1) so much of the carbon (in this case about 0.10%) and of the iron have united in the form of cementite, that the composition of the remaining solid-solution or “mother-metal” of austenite has reached that of the eutectoid, hardenite;i.e.it now contains 0.90 % of carbon. The cementite which has thus far been forming may be called “pro-eutectoid” cementite, because it forms before the remaining austenite reaches the eutectoid composition. As the temperature now falls past 690°, this hardenite mother-metal in turn splits up, after the fashion of eutectics, into alternate layers of ferrite and cementite grouped together as pearlite, so that the mass as a whole now becomes a mixture of pearlite with cementite. The iron thus liberated, as the ferrite of this pearlite, changes simultaneously to α-ferrite. The passage of this large quantity of carbon and iron, 0.90% of the former and 12.6 of the latter, from a state of mere solution as hardenite to one of definite chemical union as cementite, together with the passage of the iron itself from the γ to the α 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 γ through the β to the α 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 β state. As shown inAlloys, Pl., fig. 13, the slowly cooled steel now consists of kernels of pearlite surrounded by envelopes of the cementite which was born of the austenite in cooling from T to U.
23. To take a second case, molten hypo-eutectoid steel of 0.20% of carbon on freezing from K to x passes in the like manner to the state of solid austenite, γ-iron with this 0.20% of carbon dissolved in it. Its further cooling undergoes three spontaneous retardations, one at K′ (Ar3about 820°), at which part of the iron begins to isolate itself within the austenite mother-metal in the form of envelopes of β-ferrite,i.e.of free iron of the β allotropic modification, which surrounds the kernels or grains of the residual still undecomposed part of the austenite. At the second retardation, K″ (Ar2, about 770°) this ferrite changes to the normal magnetic α-ferrite, so that the mass as a whole becomes magnetic. Moreover, the envelopes of ferrite which began forming at Ar3continue to broaden by the accession of more and more ferrite born from the austenite progressively as the temperature sinks, till, by the time when Ar1(about 690°) 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 Ar1this hardenite mother-metal splits up into cementite and ferrite grouped together as pearlite, with the resulting recalescence, and the mass, as shown inAlloys, Pl., fig. 12, then consists of kernels of pearlite surrounded by envelopes of ferrite. All these phenomena are parallel with those of 1.00% carbon steel at this same critical point Ar1. As such steel cools slowly past Ar3, Ar2and Ar1, it loses its hardening power progressively.
In short, from Ar3to Ar1the excess substance ferrite or cementite, in hypo- and hyper-eutectoid steels respectively, progressively crystallizes out as a network or skeleton within the austenite mother-metal, which thus progressively approaches the composition of hardenite, reaching it at Ar1, and there splitting up into ferrite and cementite interstratified as pearlite. Further, any ferrite liberated at Ar3changes there from γ to β, and any present at Ar2changes from β to α. Between H and S, Ar3and Ar2occur together, as do Ar2and Ar1between S and P′ and Ar3, Ar2and Ar1at S itself; so that these critical points in these special cases are called Ar3−2, Ar2−1and Ar3−2−1respectively. The corresponding critical points which occur during rise of temperature, with the reverse transformations, are called Ac1, Ac2, Ac3, &c. A (Tschernoff) is the generic name, r refers to falling temperature (refroidissant) and c to rising temperature (chauffant, Osmond).
24. The freezing of molten cast iron of 2.50% of carbon goes on selectively like that of these steels which we have been studying, till the enrichment of the molten mother-metal in carbon brings its carbon-contents to B, 4.30%, the eutectic3carbon-content,i.e.that of the greatest fusibility or lowest melting-point. At this point selection ceases; the remaining molten metal freezes as a whole, and in freezing splits up into a conglomerate eutectic of (1) austenite of about 2.2 % of carbon, and therefore saturated with that element, and (2) cementite; and with this eutectic is mixed the “primary” austenite which froze out as the temperature sank fromvtov′. The white-hot, solid, but soft mass is now a conglomerate of (1) “primary” austenite, (2) “eutectic” austenite and (3) “eutectic” cementite. As the temperature sinks still farther, pro-eutectoid cementite (see § 22) forms progressively in the austenite both primary and eutectic, and this pro-eutectoid cementite as it comes into existence tends to assemble in the form of a network enveloping the kernels or grains of the austenite from which it springs. The reason for its birth, of course, is that the solubility of carbon in austenite progressively decreases as the temperature falls, from about 2.2% at 1130° (a), to 0.90% at 690° (Ar1), as shown by the lineaS, with the consequence that the austenite keeps rejecting in the form of this pro-eutectoid cementite all carbon in excess of its saturation-point for the existing temperature. Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.
This formation of cementite through the rejection of carbon by both the primary and the eutectic austenite continues quite as in the case of 1.00% carbon steel, with impoverishment of the austenite to the hardenite or eutectoid ratio, and the splitting up of that hardenite into pearlite at Ar1, so that the mass when cold finally consists of (1)the primary austenite now split up into kernels of pearlite surrounded by envelopes of pro-eutectoid cementite, (2) the eutectic of cementite plus austenite, the latter of which has in like manner split up into a mixture of pearlite plus cementite. Such a mass is shown in fig. 4. Here the black bat-like patches are the masses of pearlite plus pro-eutectoid cementite resulting from the splitting up of the primary austenite. The magnification is too small to show the zebra striping of the pearlite. In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other.
25. As we pass to cases with higher and higher carbon-content, the primary austenite which freezes in cooling across region 2 forms a smaller and smaller proportion of the whole, and the austenite-cementite eutectic which forms at the eutectic freezing-point, 1130° (aB), increases in amount until, when the carbon-content reaches the eutectic ratio, 4.30%, there is but a single freezing-point, and the whole mass when solid is made up of this eutectic. If there is more than 4.30% of carbon, then in cooling through region 3 the excess of carbon over this ratio freezes out as “primary” cementite. But in any event the changes which have just been described for cast iron of 2.50% of carbon occur in crossing region 7, and at Ar1(PSP′).
Just as variations in the carbon-content shift the temperature of the freezing-range and of the various critical points, so do variations in the content of other elements, notably silicon, phosphorus, manganese, chromium, nickel and tungsten. Nickel and manganese lower these critical points, so that with 25% of nickel Ar3lies below the common temperature 20° C. With 13% of manganese Ar3is very low, and the austenite decomposes so slowly that it is preserved practically intact by sudden cooling. These steels then normally consist of γ-iron, modified by the large amount of nickel or manganese with which it is alloyed. They are non-magnetic or very feebly magnetic. But the critical points of such nickel steel though thus depressed, are not destroyed; and if it is cooled in liquid air below its Ar2, it passes to the α state and becomes magnetic.
26.Double Nature of the Carbon-Iron Diagram.—The part played by graphite in the constitution of the iron-carbon compounds, hitherto ignored for simplicity, is shown in fig. 5. Looking at the matter in a broad way, in all these carbon-iron alloys, both steel and cast irons, part of the carbon may be dissolved in the iron, usually as austenite,e.g.in regions 2, 4, 5 and 7 of Fig. 1; the rest,i.e.the carbon which is not dissolved, or the “undissolved carbon,” forms either the definite carbide, cementite, Fe3C, or else exists in the free state as graphite. Now, just as fig. 1 shows the constitution of these iron-carbon alloys for all temperatures and all percentages of carbon when the undissolved carbon exists as cementite, so there should be a diagram showing this constitution when all the undissolved carbon exists as graphite. In short, there are two distinct carbon-iron diagrams, the iron-cementite one shown in fig. 1 and studied at length in §§ 22 to 25, and the iron-graphite one shown in fig. 5 in unbroken lines, with the iron-cementite diagram reproduced in broken lines for comparison. What here follows represents our present rather ill-established theory. These two diagrams naturally have much the same general shape, but though the boundaries of the several regions in the iron-cementite diagram are known pretty accurately, and though the relative positions of the boundaries of the two diagrams are probably about as here shown, the exact topography of the iron-graphite diagram is not yet known. In it the normal constituents are, for region II., molten metal + primary austenite; for region III., molten metal + primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B′ and primary graphite at the right of that point. Thus when iron containing 2.50% of carbon (v.fig. 1) solidifies, its carbon may form cementite following the cementite-austenite diagram so that white,i.e.cementitiferous, cast iron results; or graphite, following the graphite-austenite diagram, so that ultra-grey,i.e.typical graphitic cast iron results; or, as usually happens, certain molecules may follow one diagram while the rest follow the other diagram, so that cast iron which has both cementite and graphite results, as in most commercial grey cast iron, and typically in “mottled cast iron,” in which there are distinct patches of grey and others of white cast iron.
Though carbon passes far more readily under most conditions into the state of cementite than into that of graphite, yet of the two graphite is the more stable and cementite the less stable, or the “metastable” form. Thus cementite is always tending to change over into graphite by the reaction Fe3C = 3Fe + Gr, though this tendency is often held in check by different causes; but graphite never changes back directly into cementite, at least according to our present theory. The fact that graphite may dissolve in the iron as austenite, and that when this latter again breaks up it is more likely to yield cementite than graphite, is only an apparent and not a real exception to this law of the greater stability of graphite than of cementite.
Slow cooling, slow solidification, the presence of an abundance of carbon, and the presence of silicon, all favour the formation of graphite; rapid cooling, the presence of sulphur, and in most cases that of manganese, favour the formation of cementite. For instance, though in cast iron, which is rich in carbon, that carbon passes comparatively easily into the state of graphite, yet in steel, which contains much less carbon, but little graphite forms under most conditions. Indeed, in the common structural steels which contain only very little carbon, hardly any of that carbon exists as graphite.
27.Thermal Treatment.—The hardening, tempering and annealing of steel, the chilling and annealing of cast iron, and the annealing of malleable cast iron are explained readily by the facts just set forth.
28.The hardening of steelconsists in first transforming it into austenite by heating it up into region 4 of fig. 1, and then quenching it, usually in cold water, so as to cool it very suddenly, and thus to deny the time which the complete transformation of the austenite into ferrite and cementite requires, and thereby to catch much of the iron in transit in the hard brittle β state. In the cold this transformation cannot take place, because of molecular rigidity or some other impediment. The suddenly cooled metal is hard and brittle, because the cold β-iron which it contains is hard and brittle.
The degree of hardening which the steel undergoes increases with its carbon-content, chiefly because, during sudden cooling, the presence of carbon acts like a brake to impede the transformations, and thus to increase the quantity of β-iron caught in transit, but probably also in part because the hardness of this β-iron increases with its carbon-content. Thus, though sudden cooling has very little effect on steel of 0.10% of carbon, it changes that of 1.50% from a somewhat ductile body to one harder and more brittle than glass.
29.The Tempering and Annealing of Steel.—But this sudden cooling goes too far, preserving so much β-iron as to make the steel too brittle for most purposes. This brittleness has therefore in general to be mitigated or “tempered,” unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,usually to between 200° and 300° C., so as to relax the molecular rigidity and thereby to allow the arrested transformation to go on a little farther, shifting a little of the β-iron over into the α state. The higher the tempering-temperature,i.e.that to which the hardened steel is thus reheated, the more is the molecular rigidity relaxed, the farther on does the transformation go, and the softer does the steel become; so that, if the reheating reaches a dull-red heat, the transformation from austenite into ferrite and cementite completes itself slowly, and when now cooled the steel is as soft and ductile as if it had never been hardened. It is now said to be “annealed.”
30.Chilling cast iron,i.e.hastening its cooling by casting it in a cool mould, favours the formation of cementite rather than of graphite in the freezing of the eutectic at aBc, and also, in case of hyper-eutectic iron, in the passage through region 3. Like the hardening of steel, it hinders the transformation of the austenite, whether primary or eutectic, into pearlite + cementite, and thus catches part of the iron in transit in the hard β state. The annealing of such iron may occur in either of two degrees—a small one, as in making common chilled cast iron objects, such as railway car wheels, or a great one, as in making malleable cast iron. In the former case, the objects are heated only to the neighbourhood of Ac1, say to 730° C., so that the β-iron may slip into the a state, and the transformation of the austenite into pearlite and cementite may complete itself. The joint effect of such chilling and such annealing is to make the metal much harder than if slowly cooled, because for each 1% of graphite which the chilling suppresses, 15% of the glass-hard cementite is substituted. Thus a cast iron which, if cooled slowly, would have been “grey,”i.e.would have consisted chiefly of graphite with pearlite and ferrite (which are all relatively soft bodies), if thus chilled and annealed consists of cementite and pearlite. But in most such cases, in spite of the annealing, this hardness is accompanied by a degree of brittleness too great for most purposes. The process therefore is so managed that only the outer shell of the casting is chilled, and that the interior remains graphitic,i.e.grey cast iron, soft and relatively malleable.
31. In makingmalleable castingsthe annealing,i.e.the change towards the stable state of ferrite + graphite, is carried much farther by means of a much longer and usually a higher heating than in the manufacture of chilled castings. The castings, initially of white cast iron, are heated for about a week, to a temperature usually above 730° C. and often reaching 900° C. (1346° and 1652° F.). For about 60 hours the heat is held at its highest point, from which it descends extremely slowly. The molecular freedom which this high temperature gives enables the cementite to change gradually into a mixture of graphite and austenite with the result that, after the castings have been cooled and their austenite has in cooling past Ac1changed into pearlite and ferrite, the mixture of cementite and pearlite of which they originally consisted has now given place to one of fine or “temper” graphite and ferrite, with more or less pearlite according to the completeness of the transfer of the carbon to the state of graphite.
Why, then, is this material malleable, though the common grey cast iron, which is made up of about the same constituents and often in about the same proportion, is brittle? The reason is that the particles of temper graphite which are thus formed within the solid casting in its long annealing are so finely divided that they do not break up the continuity of the mass in a very harmful way; whereas in grey cast iron both the eutectic graphite formed in solidifying, and also the primary graphite which, in case the metal is hyper-eutectic, forms in cooling through region 3 of fig. 1, surrounded as it is by the still molten mother-metal out of which it is growing, form a nearly continuous skeleton of very large flakes, which do break up in a most harmful way the continuity of the mass of cast iron in which they are embedded.
In carrying out this process the castings are packed in a mass of iron oxide, which at this temperature gradually removes the fine or “temper” graphite by oxidizing that in the outer crust to carbonic oxide, whereon the carbon farther in begins diffusing outwards by “molecular migration,” to be itself oxidized on reaching the crust. This removal of graphite doubtless further stimulates the formation of graphite, by relieving the mechanical and perhaps the osmotic pressure. Thus, first, for the brittle glass-hard cementite there is gradually substituted the relatively harmless temper graphite; and, second, even this is in part removed by surface oxidation.
32.Fineness of Structure.—Each of these ancient processes thus consists essentially in so manipulating the temperature that, out of the several possible constituents, the metal shall actually consist of a special set in special proportions. But in addition there is another very important principle underlying many of our thermal processes, viz. that the state of aggregation of certain of these constituents, and through it the properties of the metal as a whole, are profoundly affected by temperature manipulations. Thus, prior exposure to a temperature materially above Ac3coarsens 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 be cured either byreheatingthe steel to Ac3when it “refines,”i.e.returns spontaneously to its fine-grained ductile state (coolingpast Ar3does not have this effect); or by breaking up the coarse grains bymechanical 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 grain-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 expenditure of power, ought later to be refined, either by reheating it from below Ar3to slightly above Ac3or by rolling it after it has cooled to a relatively low temperature,i.e.by having a low “finishing temperature.” Steel castings have initially the extremely coarse structure due to cooling without mechanical distortion from their very high temperature of solidification; they are “annealed,”i.e.this coarseness and the consequent brittleness are removed, by reheating them much above Ac3, which also relieves the internal stresses due to the different rates at which different layers cool, and hence contract, during and after solidification. For steel containing less than about 0.13% of carbon, the embrittling temperature is in a different range, near 700° C., and such steel refines at temperatures above 900° C.
33.The Possibilities of Thermal Treatment.—When we consider the great number of different regions in fig. 1, each with its own set of constitutents, and remember that by different rates of cooling from different temperatures we can retain in the cold metal these different sets of constituents in widely varying proportions; and when we further reflect that not only the proportion of each constituent present but also its state of aggregation can be controlled by thermal treatment, we see how vast a field is here opened, how great a variety of different properties can be induced in any individual piece of steel, how enormous the variety of properties thus attainable in the different varieties collectively, especially since for each percentage of carbon an incalculable number of varieties of steel may be made by alloying it with different proportions of such elements as nickel, chromium, &c. As yet there has been only the roughest survey of certain limited areas in this great field, the further exploration of which will enormously increase the usefulness of this wonderful metal.
34.Alloy steelshave come into extensive use for important special purposes, and a very great increase of their use is to be expected. The chief ones are nickel steel, manganese steel, chrome steel and chrome-tungsten steel. The general order of merit of a given variety or specimen of iron or steel may be measured by the degree to which it combines strength and hardness with ductility. These two classes of properties tend to exclude each other, for, as a general rule, whatever tends to make iron and steel hard and strong tends to make it correspondingly brittle, and hence liable to break treacherously, especially under shock. Manganese steel and nickel steel form an important exception to this rule, in being at once very strong and hard and extremely ductile.Nickel steel, which usually contains from 3 to 3.50% of nickel and about 0.25% of carbon, combines very great tensile strength and hardness, and a very high limit of elasticity, with great ductility. Its combination of ductility with strength and hardening power has given it very extended use for the armour of war-vessels. For instance, following Krupp’s formula, the side and barbette armour of war-vessels is now generally if not universally made of nickel steel containing about 3.25% of nickel, 0.40% of carbon, and 1.50% of chromium, deeply carburized on its impact face. Here the merit of nickel steel is not so much that it resists perforation, as that it does not crack even when deeply penetrated by a projectile. The combination of ductility, which lessens the tendency to break when overstrained or distorted, with a very high limit of elasticity, gives it great value for shafting, the merit of which is measured by its endurance of the repeated stresses to which its rotation exposes it whenever its alignment is not mathematically straight. The alignment of marine shafting, changing with every passing wave, is an extreme example. Such an intermittently applied stress is far more destructive to iron than a continuous one, and even if it is only half that of the limit of elasticity, its indefinite repetition eventually causes rupture. In a direct competitive test the presence of 3.25% of nickel increased nearly sixfold thenumber of rotations which a steel shaft would endure before breaking.
35. As actually made,manganese steelcontains about 12% of manganese and 1.50% of carbon. Although the presence of 1.50% of manganese makes steel relatively brittle, and although a further addition at first increases this brittleness, so that steel containing between 4 and 5.5% can be pulverized under the hammer, yet a still further increase gives very great ductility, accompanied by great hardness—a combination of properties which was not possessed by any other known substance when this remarkable alloy, known as Hadfield’s manganese steel, was discovered. Its ductility, to which it owes its value, is profoundly affected by the rate of cooling. Sudden cooling makes the metal extremely ductile, and slow cooling makes it brittle. Its behaviour in this respect is thus the opposite of that of carbon steel. But its great hardness is not materially affected by the rate of cooling. It is used extensively for objects which require both hardness and ductility, such as rock-crushing machinery, railway crossings, mine-car wheels and safes. The burglar’s blow-pipe locally “draws the temper,”i.e.softens a spot on a hardened carbon steel or chrome steel safe by simply heating it, so that as soon as it has again cooled he can drill through it and introduce his charge of dynamite. But neither this nor any other procedure softens manganese steel rapidly. Yet this very fact that it is unalterably hard has limited its use, because of the great difficulty of cutting it to shape, which has in general to be done with emery wheels instead of the usual iron-cutting tools. Another defect is its relatively low elastic limit.
36.Chrome steel, which usually contains about 2% of chromium and 0.80 to 2% of carbon, owes its value to combining, when in the “hardened” or suddenly cooled state, intense hardness with a high elastic limit, so that it is neither deformed permanently nor cracked by extremely violent shocks. For this reason it is the material generally if not always used for armour-piercing projectiles. It is much used also for certain rock-crushing machinery (the shoes and dies of stamp-mills) and for safes. These are made of alternate layers of soft wrought iron and chrome steel hardened by sudden cooling. The hardness of the hardened chrome steel resists the burglar’s drill, and the ductility of the wrought iron the blows of his sledge.
Vanadium in small quantities, 0.15 or 0.20%, is said to improve steel greatly, especially in increasing its resistance to shock and to often-repeated stress. But the improvement may be due wholly to the considerable chromium content of these so-called vanadium steels.
37.Tungsten steel, which usually contains from 5 to 10% of tungsten and from 1 to 2% of carbon, is used for magnets, because of its great retentivity.
38.Chrome-tungsten or High-speed Steel.—Steel with a large content of both chromium and tungsten has the very valuable property of “red-hardness,”i.e.of retaining its hardness and hence its power of cutting iron and other hard substances, even when it is heated to dull redness, say 600° C. (1112° F.) by the friction of the work which it is doing. Hence a machinist can cut steel or iron nearly six times as fast with a lathe tool of this steel as with one of carbon steel, because with the latter the cutting speed must be so slow that the cutting tool is not heated by the friction above say 250° 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 γ or β state or a mixture of both, can be heated without losing its hardness through the escape of that iron into the α state. In short, these elements seem to impede the allotropic change of the iron itself. The composition of this steel is as follows:—
39.Impurities.—The properties of iron and steel, like those of most of the metals, are profoundly influenced by the presence of small and sometimes extremely small quantities of certain impurities, of which the most important are phosphorus and sulphur, the former derived chiefly from apatite (phosphate of lime) and other minerals which accompany the iron ore itself, the latter from the pyrite found not only in most iron ores but in nearly all coal and coke. All commercial iron and steel contain more or less of both these impurities, the influence of which is so strong that a variation of 0.01%,i.e.of one part in 10,000, of either of them has a noticeable effect. The best tool steel should not contain more than 0.02% of either, and in careful practice it is often specified that the phosphorus and sulphur respectively shall not exceed 0.04 and 0.05% in the steel for important bridges, or 0.06 and 0.07% in rail steel, though some very prudent engineers allow as much as .085% or even 0.10% of phosphorus in rails.
40. The specific effect ofphosphorusis 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 ofsulphuris to make the metal red-short,i.e.brittle, when at a red heat, by forming a network of iron sulphide which encases these crystalline grains and thus plays the part of a weak link in a strong chain.
41.Oxygen, probably dissolved in the iron as ferrous oxide FeO, also makes the metal red-short.
42.Manganeseby itself rather lessens than increases the malleableness and, indeed, the general merit of the metal, but it is added intentionally, in quantities even as large as 1.5% to palliate the effects of sulphur and oxygen. With sulphur it forms a sulphide which draws together into almost harmless drops, instead of encasing the grains of iron. With oxygen it probably forms manganous oxide, which is less harmful than ferrous oxide. (See § 35.)
43.Ores of Iron.—Even though the earth seems to be a huge iron meteor with but a thin covering of rocks, the exasperating proneness of iron to oxidize explains readily why this metal is only rarely found native, except in the form of meteorites. They are four important iron ores, magnetite, haematite, limonite and siderite, and one of less but still considerable importance, pyrite or pyrites.