114.Constitution of Cast Iron.—Cast iron naturally has a high carbon-content, usually between 3 and 4%, because while molten it absorbs carbon greedily from the coke with which it is in contact in the iron blast furnace in which it is made, and in the cupola furnace in which it is remelted for making most castings. This carbon may all be present as graphite, as in typical grey cast iron; or all present as cementite, Fe3C, as in typical white cast iron; or, as is far more usual, part of it may be present as graphite and part as cementite. Now how does it come about that the distribution of the carbon between these very unlike states determines the strength, hardness and many other valuable properties of the metal as a whole? The answer to this is made easy by a careful study of the effect of this same distribution on the constitution of the metal, because it is through controlling this constitution that the condition of the carbon controls these useful properties. To fix our ideas let us assume that the iron contains 4% of carbon. If this carbon is all present as graphite, so that in cooling the graphite-austenite diagram has been followed strictly (§ 26), the constitution is extremely simple; clearly the mass consists first of a metallic matrix, the carbonless iron itself with whatever silicon, manganese, phosphorus and sulphur happen to be present, in short an impure ferrite, encased in which as a wholly distinct foreign body is the graphite. The primary graphite (§ 26) generally forms a coarse, nearly continuous skeleton of curved black plates, like those shown in fig. 27; the eutectic graphite is much finer; while the pro-eutectoid and eutectoid graphite, if they exist, are probably in very fine particles. We must grasp clearly this conception of metallic matrix and encased graphite skeleton if we are to understand this subject.Fig. 27.—Graphite in Grey Cast Iron.Now this matrix itself is equivalent to a very low-carbon steel, strictly speaking to a carbonless steel, because it consists of pure ferrite, which is just what such a steel consists of; and the cast iron as a whole is therefore equivalent to a matrix of very low-carbonsteel in which is encased a skeleton of graphite plates, besides some very fine scattered particles of graphite.Next let us imagine that, in a series of cast irons all containing 4% of carbon, the graphite of the initial skeleton changes gradually into cementite and thereby becomes part of the matrix, a change which of course has two aspects, first, a gradual thinning of the graphite skeleton and a decrease of its continuity, and second, a gradual introduction of cementite into the originally pure ferrite matrix. By the time that 0.4% of graphite has thus changed, and in changing has united with 0.4 × 14 = 5.6% of the iron of the original ferrite matrix, it will have changed this matrix from pure ferrite into a mixture ofCementite0.4 + 5.6 = 6.0Ferrite96.0 − 5.6 = 90.4——96.4The residual graphite skeleton forms4 − 0.4 = 3.6——100.0But this matrix is itself equivalent to a steel of about 040% of carbon (more accurately 0.40 × 100 ÷ 96.4 = 0.415%), a rail steel, because it is of just such a mixture of ferrite and cementite in the ratio of 90.4 : 6 or 94% and 6%, that such a rail steel consists. The mass as a whole, then, consists of 96.4 parts of metallic matrix, which itself is in effect a 0.415% carbon rail steel, weakened and embrittled by having its continuity broken up by this skeleton of graphite forming 3.6% of the whole mass by weight, or say 12% by volume.As, in succeeding members of this same series of cast irons, more of the graphite of the initial skeleton changes into cementite and thereby becomes part of the metallic matrix, so the graphite skeleton becomes progressively thinner and more discontinuous, and the matrix richer in cementite and hence in carbon and hence equivalent first to higher and higher carbon steel, such as tool steel of 1% carbon, file steel of 1.50%, wire-die steel of 2% carbon and then to white cast iron, which consists essentially of much cementite with little ferrite. Eventually, when the whole of the graphite of the skeleton has changed into cementite, the mass as a whole becomes typical or ultra white cast iron, consisting of nothing but ferrite and cementite, distributed as follows (see fig. 2):—Eutectoid ferrite40.0Eutectoid cementite6.7——Eutectoid Interstratified as pearlite46.7Cementite, primary, eutectoid and pro-eutectoid53.3——100.0Total ferrite40.0Total cementite60.0——100.0Fig. 28.—Physical Properties and assumed Microscopic Constitution of Cast Iron containing 4% of carbon, as affected by the distribution of that carbon between the combined and graphitic states.The constitution and properties of such a series of cast irons, all containing 4% of carbon but with that carbon shifting progressively from the state of graphite to that of cementite as we pass from specimen to specimen, may, with the foregoing picture of a skeleton-holding matrix clearly in our minds be traced by means of fig. 28. The change from graphite into cementite is supposed to take place as we pass from left to right. BC and OH give the proportion of ferrite and cementite respectively in the matrix, DEF, KS and TU reproduced from fig. 3 give the consequent properties of the matrix, and GAF, RS and VU give, partly from conjecture, the properties of the cast iron as a whole. Above the diagram are given the names of the different classes of cast iron to which different stages in the change from graphite to cementite correspond, and above these the names of kinds of steel or cast iron, to which at the corresponding stages the constitution of the matrix corresponds, while below the diagram are given the properties of the cast iron as a whole corresponding to these stages, and still lower the purposes for which these stages fit the cast iron, first because of its strength and shock-resisting power, and second because of its hardness.115.Influence of the Constitution of Cast Iron on its Properties.—How should the hardness, strength and ductility, or rather shock-resisting power, of the cast iron be affected by this progressive change from graphite into cementite? First, the hardness (VU) should increase progressively as the soft ferrite and graphite are replaced by the glass-hard cementite. Second, though the brittleness should be lessened somewhat by the decrease in the extent to which the continuity of the strong matrix is broken up by the graphite skeleton, yet this effect is outweighed greatly by that of the rapid substitution in the matrix of the brittle cementite for the very ductile copper-like ferrite, so that the brittleness increases continuously (RS), from that of the very grey graphitic cast irons, which, like that of soapstone, is so slight that the metal can endure severe shock and even indentation without breaking, to that of the pure white cast iron which is about as brittle as porcelain. Here let us recognize that what gives this transfer of carbon from graphite skeleton to metallic matrix such very great influence on the properties of the metal is the fact that the transfer of each 1% of carbon means substituting in the matrix no less than 15% of the brittle, glass-hard cementite for the soft, very ductile ferrite. Third, the tensile strength of steel proper, of which the matrix consists, as we have already seen (fig. 3), increases with the carbon-content till this reaches about 1.25%, and then in turn decreases (fig. 28, DEF). Hence, as with the progressive transfer of the carbon from the graphitic to the cementite state in our imaginary series of cast irons, the combined carbon present in the matrix increases, so does the tensile strength of the mass as a whole for two reasons; first, because the strength of the matrix itself is increasing (DE), and second, because the discontinuity is decreasing with the decreasing proportion of graphite. With further transfer of the carbon from the graphitic to the combined state, the matrix itself grows weaker (EF); but this weakening is offset in a measure by the continuing decrease of discontinuity due to the decreasing proportion of graphite. The resultant of these two effects has not yet been well established; but it is probable that the strongest cast iron has a little more than 1% of carbon combined as cementite, so that its matrix is nearly equivalent to the strongest of the steels. As regards both tensile strength and ductility not only the quantity but the distribution of the graphite is of great importance. Thus it is extremely probable that the primary graphite, which forms large sheets, is much more weakening and embrittling than the eutectic and other forms, and therefore that, if either strength or ductility is sought, the metal should be free from primary graphite,i.e.that it should not be hyper-eutectic.The presence of graphite has two further and very natural effects. First, if the skeleton which it forms is continuous, then its planes of junction with the metallic matrix offer a path of low resistance to the passage of liquids or gases, or in short they make the metal so porous as to unfit it for objects like the cylinders of hydraulic presses, which ought to be gas-tight and water-tight. For such purposes the graphite-content should be low. Second, the very genesis of so bulky a substance as the primary and eutectic graphite while the metal is solidifying (fig. 5) causes a sudden and permanent expansion, which forces the metal into even the finest crevices in its mould, a fact which is taken advantage of in making ornamental castings and others which need great sharpness of detail, by making them rich in graphite.To sum this up, as graphite is replaced by carbon combined as cementite, the hardness, brittleness and density increase, and the expansion in solidification decreases, in both cases continuously, while the tensile strength increases till the combined carbon-content rises a little above 1%, and then in turn decreases. That strength is good and brittleness bad goes without saying; but here a word is needed about hardness. The expense of cutting castings accurately to shape, cutting on them screw threads and what not, called “machining” in trade parlance, is often a very large part of their total cost; and it increases rapidly with the hardness of the metal. On the other hand, the extreme hardness of nearly graphiteless cast iron is of great value for objects of which the chief duty is to resist abrasion, such as parts of crushing machinery. Hence objects which need much machining are made rich in graphite, so that they may be cut easily, and those of the latter class rich in cementite so that they may not wear out.116.Means of controlling the Constitution of Cast Iron.—The distribution of the carbon between these two states, so as to give the cast iron the properties needed, is brought about chiefly byadjusting the silicon-content, because the presence of this element favours the formation of graphite. Beyond this, rapid cooling and the presence of sulphur both oppose the formation of graphite, and hence in cast iron rich in sulphur, and in thin and therefore rapidly cooling castings, the silicon-content must be greater than in thick ones and in those freer from sulphur. Thus thick machinery castings usually contain between 1.50 and 2.25% of silicon, whereas thin castings and ornamental ones which must reproduce the finest details of the mould accurately may have as much as 3 or even 3.40% of it. Castings which, like hydraulic press cylinders and steam radiators, must be dense and hence must have but little graphite lest their contents leak through their walls, should not have more than 1.75% of silicon and may have even as little as 1% if impenetrability is so important that softness and consequent ease of machining must be sacrificed to it. Cast iron railroad car-wheels, the tread or rim of which must be intensely hard so as to endure the grinding action of the brakeshoe while their central parts must have good shock-resisting power, are given such moderate silicon-content, preferably between 0.50 and 0.80%, as in and by itself leaves the tendencies toward graphite-forming and toward cementite-forming nearly in balance, so that they are easily controlled by the rate of cooling. The “tread” or circumferential part of the mould itself is made of iron, because this, by conducting the heat away from the casting rapidly, makes it cool quickly, and thus causes most of the carbon here to form cementite, and thus in turn makes the tread of the wheel intensely hard; while those parts of the mould which come in contact with the central parts of the wheel are made of sand, which conducts the heat away from the molten metal so slowly that it solidifies slowly, with the result that most of its carbon forms graphite, and here the metal is soft and shock-resisting.117.Influence of Sulphur.—Sulphur has the specific harmful effects of shifting the carbon from the state of graphite to that of cementite, and thus of making the metal hard and brittle; of making it thick and sluggish when molten, so that it does not run freely in the moulds; and of making it red short,i.e.brittle at a red heat, so that it is very liable to be torn by the aeolotachic contraction in cooling from the molten state; and it has no good effects to offset these. Hence the sulphur present is, except in certain rare cases, simply that which the metallurgist has been unable to remove. The sulphur-content should not exceed 0.12%, and it is better that it should not exceed 0.08 % in castings which have to be soft enough to be machined, nor 0.05% in thin castings the metal for which must be very fluid.118.Influence of Manganese.—Manganese in many cases, but not in all, opposes the formation of graphite and thus hardens the iron, and it lessens the red shortness (§ 40), which sulphur causes, by leading to the formation of the less harmful manganese sulphide instead of the more harmful iron sulphide. Hence the manganese-content needed increases with the sulphur-content which has to be endured. In the better classes of castings it is usually between 0.40 and 0.70%, and in chilled railroad car-wheels it may well be between 0.15 and 0.30%; but skilful founders, confronted with the task of making use of cast iron rich in manganese, have succeeded in making good grey iron castings with even as much as 2.20% of this element.119.Influence of Phosphorus.—Phosphorus has, along with its great merit of giving fluidity, the grave defect of causing brittleness, especially under shock. Fortunately its embrittling effect on cast iron is very much less than on steel, so that the upper limit or greatest tolerable proportion of phosphorus, instead of being 0.10 or better 0.08% as in the case of rail steel, may be put at 0.50% in case of machinery castings even if they are exposed to moderate shocks; at 1.60% for gas and water mains in spite of the gravity of the disasters which extreme brittleness here might cause; and even higher for castings which are not exposed to shock, and are so thin that the iron of which they are made must needs be very fluid. The permissible phosphorus-content is lessened by the presence of either much sulphur or much manganese, and by rapid cooling, as for instance in case of thin castings, because each of these three things, by leading to the formation of the brittle cementite, in itself creates brittleness which aggravates that caused by phosphorus.
114.Constitution of Cast Iron.—Cast iron naturally has a high carbon-content, usually between 3 and 4%, because while molten it absorbs carbon greedily from the coke with which it is in contact in the iron blast furnace in which it is made, and in the cupola furnace in which it is remelted for making most castings. This carbon may all be present as graphite, as in typical grey cast iron; or all present as cementite, Fe3C, as in typical white cast iron; or, as is far more usual, part of it may be present as graphite and part as cementite. Now how does it come about that the distribution of the carbon between these very unlike states determines the strength, hardness and many other valuable properties of the metal as a whole? The answer to this is made easy by a careful study of the effect of this same distribution on the constitution of the metal, because it is through controlling this constitution that the condition of the carbon controls these useful properties. To fix our ideas let us assume that the iron contains 4% of carbon. If this carbon is all present as graphite, so that in cooling the graphite-austenite diagram has been followed strictly (§ 26), the constitution is extremely simple; clearly the mass consists first of a metallic matrix, the carbonless iron itself with whatever silicon, manganese, phosphorus and sulphur happen to be present, in short an impure ferrite, encased in which as a wholly distinct foreign body is the graphite. The primary graphite (§ 26) generally forms a coarse, nearly continuous skeleton of curved black plates, like those shown in fig. 27; the eutectic graphite is much finer; while the pro-eutectoid and eutectoid graphite, if they exist, are probably in very fine particles. We must grasp clearly this conception of metallic matrix and encased graphite skeleton if we are to understand this subject.
Now this matrix itself is equivalent to a very low-carbon steel, strictly speaking to a carbonless steel, because it consists of pure ferrite, which is just what such a steel consists of; and the cast iron as a whole is therefore equivalent to a matrix of very low-carbonsteel in which is encased a skeleton of graphite plates, besides some very fine scattered particles of graphite.
Next let us imagine that, in a series of cast irons all containing 4% of carbon, the graphite of the initial skeleton changes gradually into cementite and thereby becomes part of the matrix, a change which of course has two aspects, first, a gradual thinning of the graphite skeleton and a decrease of its continuity, and second, a gradual introduction of cementite into the originally pure ferrite matrix. By the time that 0.4% of graphite has thus changed, and in changing has united with 0.4 × 14 = 5.6% of the iron of the original ferrite matrix, it will have changed this matrix from pure ferrite into a mixture of
But this matrix is itself equivalent to a steel of about 040% of carbon (more accurately 0.40 × 100 ÷ 96.4 = 0.415%), a rail steel, because it is of just such a mixture of ferrite and cementite in the ratio of 90.4 : 6 or 94% and 6%, that such a rail steel consists. The mass as a whole, then, consists of 96.4 parts of metallic matrix, which itself is in effect a 0.415% carbon rail steel, weakened and embrittled by having its continuity broken up by this skeleton of graphite forming 3.6% of the whole mass by weight, or say 12% by volume.
As, in succeeding members of this same series of cast irons, more of the graphite of the initial skeleton changes into cementite and thereby becomes part of the metallic matrix, so the graphite skeleton becomes progressively thinner and more discontinuous, and the matrix richer in cementite and hence in carbon and hence equivalent first to higher and higher carbon steel, such as tool steel of 1% carbon, file steel of 1.50%, wire-die steel of 2% carbon and then to white cast iron, which consists essentially of much cementite with little ferrite. Eventually, when the whole of the graphite of the skeleton has changed into cementite, the mass as a whole becomes typical or ultra white cast iron, consisting of nothing but ferrite and cementite, distributed as follows (see fig. 2):—
The constitution and properties of such a series of cast irons, all containing 4% of carbon but with that carbon shifting progressively from the state of graphite to that of cementite as we pass from specimen to specimen, may, with the foregoing picture of a skeleton-holding matrix clearly in our minds be traced by means of fig. 28. The change from graphite into cementite is supposed to take place as we pass from left to right. BC and OH give the proportion of ferrite and cementite respectively in the matrix, DEF, KS and TU reproduced from fig. 3 give the consequent properties of the matrix, and GAF, RS and VU give, partly from conjecture, the properties of the cast iron as a whole. Above the diagram are given the names of the different classes of cast iron to which different stages in the change from graphite to cementite correspond, and above these the names of kinds of steel or cast iron, to which at the corresponding stages the constitution of the matrix corresponds, while below the diagram are given the properties of the cast iron as a whole corresponding to these stages, and still lower the purposes for which these stages fit the cast iron, first because of its strength and shock-resisting power, and second because of its hardness.
115.Influence of the Constitution of Cast Iron on its Properties.—How should the hardness, strength and ductility, or rather shock-resisting power, of the cast iron be affected by this progressive change from graphite into cementite? First, the hardness (VU) should increase progressively as the soft ferrite and graphite are replaced by the glass-hard cementite. Second, though the brittleness should be lessened somewhat by the decrease in the extent to which the continuity of the strong matrix is broken up by the graphite skeleton, yet this effect is outweighed greatly by that of the rapid substitution in the matrix of the brittle cementite for the very ductile copper-like ferrite, so that the brittleness increases continuously (RS), from that of the very grey graphitic cast irons, which, like that of soapstone, is so slight that the metal can endure severe shock and even indentation without breaking, to that of the pure white cast iron which is about as brittle as porcelain. Here let us recognize that what gives this transfer of carbon from graphite skeleton to metallic matrix such very great influence on the properties of the metal is the fact that the transfer of each 1% of carbon means substituting in the matrix no less than 15% of the brittle, glass-hard cementite for the soft, very ductile ferrite. Third, the tensile strength of steel proper, of which the matrix consists, as we have already seen (fig. 3), increases with the carbon-content till this reaches about 1.25%, and then in turn decreases (fig. 28, DEF). Hence, as with the progressive transfer of the carbon from the graphitic to the cementite state in our imaginary series of cast irons, the combined carbon present in the matrix increases, so does the tensile strength of the mass as a whole for two reasons; first, because the strength of the matrix itself is increasing (DE), and second, because the discontinuity is decreasing with the decreasing proportion of graphite. With further transfer of the carbon from the graphitic to the combined state, the matrix itself grows weaker (EF); but this weakening is offset in a measure by the continuing decrease of discontinuity due to the decreasing proportion of graphite. The resultant of these two effects has not yet been well established; but it is probable that the strongest cast iron has a little more than 1% of carbon combined as cementite, so that its matrix is nearly equivalent to the strongest of the steels. As regards both tensile strength and ductility not only the quantity but the distribution of the graphite is of great importance. Thus it is extremely probable that the primary graphite, which forms large sheets, is much more weakening and embrittling than the eutectic and other forms, and therefore that, if either strength or ductility is sought, the metal should be free from primary graphite,i.e.that it should not be hyper-eutectic.
The presence of graphite has two further and very natural effects. First, if the skeleton which it forms is continuous, then its planes of junction with the metallic matrix offer a path of low resistance to the passage of liquids or gases, or in short they make the metal so porous as to unfit it for objects like the cylinders of hydraulic presses, which ought to be gas-tight and water-tight. For such purposes the graphite-content should be low. Second, the very genesis of so bulky a substance as the primary and eutectic graphite while the metal is solidifying (fig. 5) causes a sudden and permanent expansion, which forces the metal into even the finest crevices in its mould, a fact which is taken advantage of in making ornamental castings and others which need great sharpness of detail, by making them rich in graphite.
To sum this up, as graphite is replaced by carbon combined as cementite, the hardness, brittleness and density increase, and the expansion in solidification decreases, in both cases continuously, while the tensile strength increases till the combined carbon-content rises a little above 1%, and then in turn decreases. That strength is good and brittleness bad goes without saying; but here a word is needed about hardness. The expense of cutting castings accurately to shape, cutting on them screw threads and what not, called “machining” in trade parlance, is often a very large part of their total cost; and it increases rapidly with the hardness of the metal. On the other hand, the extreme hardness of nearly graphiteless cast iron is of great value for objects of which the chief duty is to resist abrasion, such as parts of crushing machinery. Hence objects which need much machining are made rich in graphite, so that they may be cut easily, and those of the latter class rich in cementite so that they may not wear out.
116.Means of controlling the Constitution of Cast Iron.—The distribution of the carbon between these two states, so as to give the cast iron the properties needed, is brought about chiefly byadjusting the silicon-content, because the presence of this element favours the formation of graphite. Beyond this, rapid cooling and the presence of sulphur both oppose the formation of graphite, and hence in cast iron rich in sulphur, and in thin and therefore rapidly cooling castings, the silicon-content must be greater than in thick ones and in those freer from sulphur. Thus thick machinery castings usually contain between 1.50 and 2.25% of silicon, whereas thin castings and ornamental ones which must reproduce the finest details of the mould accurately may have as much as 3 or even 3.40% of it. Castings which, like hydraulic press cylinders and steam radiators, must be dense and hence must have but little graphite lest their contents leak through their walls, should not have more than 1.75% of silicon and may have even as little as 1% if impenetrability is so important that softness and consequent ease of machining must be sacrificed to it. Cast iron railroad car-wheels, the tread or rim of which must be intensely hard so as to endure the grinding action of the brakeshoe while their central parts must have good shock-resisting power, are given such moderate silicon-content, preferably between 0.50 and 0.80%, as in and by itself leaves the tendencies toward graphite-forming and toward cementite-forming nearly in balance, so that they are easily controlled by the rate of cooling. The “tread” or circumferential part of the mould itself is made of iron, because this, by conducting the heat away from the casting rapidly, makes it cool quickly, and thus causes most of the carbon here to form cementite, and thus in turn makes the tread of the wheel intensely hard; while those parts of the mould which come in contact with the central parts of the wheel are made of sand, which conducts the heat away from the molten metal so slowly that it solidifies slowly, with the result that most of its carbon forms graphite, and here the metal is soft and shock-resisting.
117.Influence of Sulphur.—Sulphur has the specific harmful effects of shifting the carbon from the state of graphite to that of cementite, and thus of making the metal hard and brittle; of making it thick and sluggish when molten, so that it does not run freely in the moulds; and of making it red short,i.e.brittle at a red heat, so that it is very liable to be torn by the aeolotachic contraction in cooling from the molten state; and it has no good effects to offset these. Hence the sulphur present is, except in certain rare cases, simply that which the metallurgist has been unable to remove. The sulphur-content should not exceed 0.12%, and it is better that it should not exceed 0.08 % in castings which have to be soft enough to be machined, nor 0.05% in thin castings the metal for which must be very fluid.
118.Influence of Manganese.—Manganese in many cases, but not in all, opposes the formation of graphite and thus hardens the iron, and it lessens the red shortness (§ 40), which sulphur causes, by leading to the formation of the less harmful manganese sulphide instead of the more harmful iron sulphide. Hence the manganese-content needed increases with the sulphur-content which has to be endured. In the better classes of castings it is usually between 0.40 and 0.70%, and in chilled railroad car-wheels it may well be between 0.15 and 0.30%; but skilful founders, confronted with the task of making use of cast iron rich in manganese, have succeeded in making good grey iron castings with even as much as 2.20% of this element.
119.Influence of Phosphorus.—Phosphorus has, along with its great merit of giving fluidity, the grave defect of causing brittleness, especially under shock. Fortunately its embrittling effect on cast iron is very much less than on steel, so that the upper limit or greatest tolerable proportion of phosphorus, instead of being 0.10 or better 0.08% as in the case of rail steel, may be put at 0.50% in case of machinery castings even if they are exposed to moderate shocks; at 1.60% for gas and water mains in spite of the gravity of the disasters which extreme brittleness here might cause; and even higher for castings which are not exposed to shock, and are so thin that the iron of which they are made must needs be very fluid. The permissible phosphorus-content is lessened by the presence of either much sulphur or much manganese, and by rapid cooling, as for instance in case of thin castings, because each of these three things, by leading to the formation of the brittle cementite, in itself creates brittleness which aggravates that caused by phosphorus.
120.Defects in Steel Ingots.—Steel ingots and other steel castings are subject to three kinds of defects so serious as to deserve notice here. They are known as “piping,” “blowholes” and “segregation.”
A, Superficial blowholes.
B, Deep-seated blowholes.
C, Pipe.
121.Piping.—In an early period of the solidification of a molten steel ingot cast in a cold iron mould we may distinguish three parts: (1) the outer layers,i.e.the outermost of the now solid metal; (2) the inner layers,i.e.the remainder of the solid metal; and (3) the molten lake,i.e.the part which still is molten. At this instant the outer layers, because of their contact with the cold mould, are cooling much faster than the inner ones, and hence tend to contract faster. But this excess of their contraction is resisted by the almost incompressible inner layers so that the outer layers are prevented from contracting as much as they naturally would if unopposed, and they are thereby virtually stretched. Later on the cooling of the inner layers becomes more rapid than that of the outer ones, and on this account their contraction tends to become greater than that of the outer ones. Because the outer and inner layers are integrally united, this excess of contraction of the inner layers makes them draw outward towards and against the outer layers, and because of their thus drawing outward the molten lake within no longer suffices to fill completely the central space, so that its upper surface begins to sink. This ebb continues, and, combined with the progressive narrowing of the molten lake as more and more of it solidifies and joins the shore layers, gives rise to the pipe, a cavity like an inverted pear, as shown at C in fig. 29. Because this pipe is due to the difference in the rates of contraction of interior and exterior, it may be lessened by retarding the cooling of the mass as a whole, and it may be prevented from stretching down deep by retarding the solidification of the upper part of the ingot, as, for instance, by preheating the top of the mould, or by covering the ingot with a mass of burning fuel or of molten slag. This keeps the upper part of the mass molten, so that it continues to flow down and feed the pipe during the early part of its formation in the lower and quicker-cooling part of the ingot. In making castings of steel this same difficulty arises; and much of the steel-founder’s skill consists either in preventing these pipes, or in so placing them that they shall not occur in the finished casting, or at least not in a harmful position. In making armour-plates from steel ingots, as much as 40% of the metal may be rejected as unsound from this cause. An ingot should always stand upright while solidifying, so that the unsound region due to the pipe may readily be cut off, leaving the rest of the ingot solid. If the ingot lay on its side while solidifying, the pipe would occur as shown in fig. 30, and nearly the whole of the ingot would be unsound.122.Blowholes.—Iron, like water and many other substances, has a higher solvent power for gases, such as hydrogen and nitrogen, when molten,i.e.liquid, than when frozen,i.e.solid. Hence in the act of solidifying it expels any excess of gas which it has dissolved while liquid, and this gas becomes entangled in the freezing mass, causing gas bubbles orblowholes, as at A and B in fig. 29. Because the volume of the pipe represents the excess of the contraction of the inner walls and the molten lake jointly over that of the outer walls, between the time when the lake begins to ebb and the time when even the axial metal is too firm to be drawn further open by this contraction, the space occupied by blowholes must, by compensating for part of this excess, lessen the size of the pipe, so that the more abundant and larger the blowholes are, the smaller will the pipe be. The interior surface of a blowhole which lies near the outer crust of the ingot, as at A in fig. 29, is liable to become oxidized by the diffusion of the atmospheric oxygen, in which case it can hardly be completely welded later, since welding implies actual contact of metal with metal; it thus forms a permanent flaw. But deep-seated blowholes like those at B are relatively harmless in low-carbon easily welding steel, because the subsequent operation of forging or rolling usually obliterates them by welding their sides firmly together.Fig. 30.—Diagram showing a Pipe so formed as to render Ingot unsound.Blowholes may be lessened or even wholly prevented by adding to the molten metal shortly before it solidifies either silicon or aluminium, or both; even as little as 0.002% of aluminium is usually sufficient. These additions seem to act in part by deoxidizing the minute quantity of iron oxide and carbonic oxide present, in part by increasing the solvent power of the metal for gas, so that even after freezing it can retain in solution the gas which it had dissolved when molten. But, because preventing blowholes increases the volume of the pipe, it is often better to allow them to form, but to control their position, so that they shall be deep-seated. This is done chiefly by casting the steel at a relatively low temperature, and by limiting the quantity of manganese and silicon which it contains. Brinell finds that, for certain normal conditions, if the sum of the percentage of manganese plus 5.2 times that of thesilicon equals 1.66, there will be no blowholes; if this sum is less, blowholes will occur, and will be injuriously near the surface unless this sum is reduced to 0.28. He thus finds that this sum should be either as great as 1.66, so that blowholes shall be absent; or as low as 0.28, so that they shall be harmlessly deep-seated. These numbers must be varied with the variations in other conditions, such as casting temperature, rapidity of solidification, &c.123.Segregation.—The solidification of an ingot of steel takes place gradually from without inwards, and each layer in solidifying tends to expel into the still molten interior the impurities which it contains, especially the carbon, phosphorus, and sulphur, which by this process are in part concentrated orsegregatedin the last-freezing part of the ingot. This is in general around the lower part of the pipe, so that here is a second motive for rejecting the piped part of the ingot. While segregation injures the metal here, often fatally, by giving it an indeterminate excess of phosphorus and sulphur, it clearly purifies the remainder of the ingot, and on this account it ought, under certain conditions, to be promoted rather than restrained. The following is an extreme case:—Carbon.Silicon.Manganese.Phosphorus.Sulphur.Composition of the initial metal per cent0.240.3360.970.0890.074Composition of the segregate1.270.411.080.7530.418The surprising fact that the degree of segregation does not increase greatly either with the slowness of solidification or with the size of the ingot, at least between the limits of 5 in. sq. and 16 in. sq., has been explained by the theory that the relative quiet due to the gentleness of the convection currents in a slowly cooling mass favours the formation of far outshooting pine-tree crystals, and that the tangled branches of these crystals landlock much of the littoral molten mother metal, and thus mechanically impede that centreward diffusion and convection of the impurities which is the essence of segregation.
121.Piping.—In an early period of the solidification of a molten steel ingot cast in a cold iron mould we may distinguish three parts: (1) the outer layers,i.e.the outermost of the now solid metal; (2) the inner layers,i.e.the remainder of the solid metal; and (3) the molten lake,i.e.the part which still is molten. At this instant the outer layers, because of their contact with the cold mould, are cooling much faster than the inner ones, and hence tend to contract faster. But this excess of their contraction is resisted by the almost incompressible inner layers so that the outer layers are prevented from contracting as much as they naturally would if unopposed, and they are thereby virtually stretched. Later on the cooling of the inner layers becomes more rapid than that of the outer ones, and on this account their contraction tends to become greater than that of the outer ones. Because the outer and inner layers are integrally united, this excess of contraction of the inner layers makes them draw outward towards and against the outer layers, and because of their thus drawing outward the molten lake within no longer suffices to fill completely the central space, so that its upper surface begins to sink. This ebb continues, and, combined with the progressive narrowing of the molten lake as more and more of it solidifies and joins the shore layers, gives rise to the pipe, a cavity like an inverted pear, as shown at C in fig. 29. Because this pipe is due to the difference in the rates of contraction of interior and exterior, it may be lessened by retarding the cooling of the mass as a whole, and it may be prevented from stretching down deep by retarding the solidification of the upper part of the ingot, as, for instance, by preheating the top of the mould, or by covering the ingot with a mass of burning fuel or of molten slag. This keeps the upper part of the mass molten, so that it continues to flow down and feed the pipe during the early part of its formation in the lower and quicker-cooling part of the ingot. In making castings of steel this same difficulty arises; and much of the steel-founder’s skill consists either in preventing these pipes, or in so placing them that they shall not occur in the finished casting, or at least not in a harmful position. In making armour-plates from steel ingots, as much as 40% of the metal may be rejected as unsound from this cause. An ingot should always stand upright while solidifying, so that the unsound region due to the pipe may readily be cut off, leaving the rest of the ingot solid. If the ingot lay on its side while solidifying, the pipe would occur as shown in fig. 30, and nearly the whole of the ingot would be unsound.
122.Blowholes.—Iron, like water and many other substances, has a higher solvent power for gases, such as hydrogen and nitrogen, when molten,i.e.liquid, than when frozen,i.e.solid. Hence in the act of solidifying it expels any excess of gas which it has dissolved while liquid, and this gas becomes entangled in the freezing mass, causing gas bubbles orblowholes, as at A and B in fig. 29. Because the volume of the pipe represents the excess of the contraction of the inner walls and the molten lake jointly over that of the outer walls, between the time when the lake begins to ebb and the time when even the axial metal is too firm to be drawn further open by this contraction, the space occupied by blowholes must, by compensating for part of this excess, lessen the size of the pipe, so that the more abundant and larger the blowholes are, the smaller will the pipe be. The interior surface of a blowhole which lies near the outer crust of the ingot, as at A in fig. 29, is liable to become oxidized by the diffusion of the atmospheric oxygen, in which case it can hardly be completely welded later, since welding implies actual contact of metal with metal; it thus forms a permanent flaw. But deep-seated blowholes like those at B are relatively harmless in low-carbon easily welding steel, because the subsequent operation of forging or rolling usually obliterates them by welding their sides firmly together.
Blowholes may be lessened or even wholly prevented by adding to the molten metal shortly before it solidifies either silicon or aluminium, or both; even as little as 0.002% of aluminium is usually sufficient. These additions seem to act in part by deoxidizing the minute quantity of iron oxide and carbonic oxide present, in part by increasing the solvent power of the metal for gas, so that even after freezing it can retain in solution the gas which it had dissolved when molten. But, because preventing blowholes increases the volume of the pipe, it is often better to allow them to form, but to control their position, so that they shall be deep-seated. This is done chiefly by casting the steel at a relatively low temperature, and by limiting the quantity of manganese and silicon which it contains. Brinell finds that, for certain normal conditions, if the sum of the percentage of manganese plus 5.2 times that of thesilicon equals 1.66, there will be no blowholes; if this sum is less, blowholes will occur, and will be injuriously near the surface unless this sum is reduced to 0.28. He thus finds that this sum should be either as great as 1.66, so that blowholes shall be absent; or as low as 0.28, so that they shall be harmlessly deep-seated. These numbers must be varied with the variations in other conditions, such as casting temperature, rapidity of solidification, &c.
123.Segregation.—The solidification of an ingot of steel takes place gradually from without inwards, and each layer in solidifying tends to expel into the still molten interior the impurities which it contains, especially the carbon, phosphorus, and sulphur, which by this process are in part concentrated orsegregatedin the last-freezing part of the ingot. This is in general around the lower part of the pipe, so that here is a second motive for rejecting the piped part of the ingot. While segregation injures the metal here, often fatally, by giving it an indeterminate excess of phosphorus and sulphur, it clearly purifies the remainder of the ingot, and on this account it ought, under certain conditions, to be promoted rather than restrained. The following is an extreme case:—
The surprising fact that the degree of segregation does not increase greatly either with the slowness of solidification or with the size of the ingot, at least between the limits of 5 in. sq. and 16 in. sq., has been explained by the theory that the relative quiet due to the gentleness of the convection currents in a slowly cooling mass favours the formation of far outshooting pine-tree crystals, and that the tangled branches of these crystals landlock much of the littoral molten mother metal, and thus mechanically impede that centreward diffusion and convection of the impurities which is the essence of segregation.
124.Castings and Forgings.—There are two distinct ways of making the steel objects actually used in the arts, such as rails, gear wheels, guns, beams, &c., out of the molten steel made by the Bessemer, open hearth, or crucible process, or in an electric furnace. The first is by “steel founding,”i.e.casting the steel as a “steel casting” in a mould which has the exact shape of the object to be made,e.g.a gear wheel, and letting it solidify there. The second is by casting it into a large rough block called an “ingot,” and rolling or hammering this out into the desired shape. Though the former certainly seems the simpler way, yet its technical difficulties are so great that it is in fact much the more expensive, and therefore it is in general used only in making objects of a shape hard to give by forging or rolling. These technical difficulties are due chiefly to the very high melting point of the metal, nearly 1500° C (2732° F.), and to the consequent great contraction which it undergoes in cooling through the long range between this temperature and that of the room. The cooling of the thinner, the outer, and in general the more exposed parts of the casting outruns that of the thicker and less exposed parts, with the consequence that, at any given instant, the different parts are contracting at very different rates,i.e.aeolotachically; and this aeolotachic contraction is very likely to concentrate severe stress on the slowest cooling parts at the time when they are passing from the molten to the solid state, when the steel is mushy, with neither the fluidity of a liquid nor the strength and ductility of a solid, and thus to tear it apart. Aeolotachic contraction further leads to the “pipes” or contraction cavities already described in § 121, and the procedure must be carefully planned first so as to reduce these to a minimum, and second so as to induce them to form either in those parts of the casting which are going to be cut off and re-melted, or where they will do little harm. These and kindred difficulties make each new shape or size a new problem, and in particular they require that for each and every individual casting a new sand or clay mould shall be made with care by a skilled workman. If a thousand like gears are to be cast, a thousand moulds must be made up, at least to an important extent by hand, for even machine moulding leaves something for careful manipulation by the moulder. It is a detail, one is tempted to say a retail, manufacture.
In strong contrast with this is the procedure in making rolled products such as rails and plates. The steel is cast in lots, weighing in some cases as much as 75 tons, in enduring cast iron moulds into very large ingots, which with their initial heat are immediately rolled down by a series of powerful roll trains into their final shape with but slight wear and tear of the moulds and the machinery. But in addition to the greater cost of steel founding as compared with rolling there are two facts which limit the use of steel castings: (1) they are not so good as rolled products, because the kneading which the metal undergoes in rolling improves its quality, and closes up its cavities; and (2) it would be extremely difficult and in most cases impracticable to cast the metal directly into any of the forms in which the great bulk of the steel of commerce is needed, such as rails, plates, beams, angles, rods, bars, and wire, because the metal would become so cool as to solidify before running far in such thin sections, and because even the short pieces which could thus be made would pucker or warp on account of their aeolotachic contraction.
125.Heating Furnacesare used in iron manufacture chiefly for bringing masses of steel or wrought iron to a temperature proper for rolling or forging. In order to economize power in these operations, the metal should in general be as soft and hence as hot as is consistent with its reaching a low temperature before the rolling or forging is finished, because, as explained in § 32, undisturbed cooling from a high temperature injures the metal. Many of the furnaces used for this heating are in a general way like the puddling furnace shown in fig. 14, except that they are heated by gas, that the hearth or bottom of the chamber in which they are heated is nearly flat, and that it is usually very much larger than that of a puddling furnace. But in addition there are many special kinds of furnaces arranged to meet the needs of each case. Of these two will be shown here, the Gjers soaking pit for steel ingots, and the Eckman or continuous furnace, as modified by C. H. Morgan for heating billets.
126.Gjers Soaking Pit.—When the outer crust of a large ingot in which a lot of molten steel has been cast has so far cooled that it can be moved without breaking, the temperature of the interior is still far above that suitable for rolling or hammering—so far above that the surplus heat of the interior would more than suffice to reheat the now cool crust to the rolling temperature, if we could only arrest or even greatly retard the further escape of heat from that crust. Bringing such an ingot, then, to the rolling temperature is not really an operation of heating, because its average temperature is already above the rolling temperature, but one of equalizing the temperature, by allowing the internal excess of heat to “soak” through the mass. Gjers did this by setting the partly-solidified ingot in a well-closed “pit” of brickwork, preheated by the excess heat of previous lots of ingots. The arrangement, shown in fig. 31, has three advantages—(1) that the temperature is adjusted with absolutely no consumption of fuel; (2) that the waste of iron due to the oxidation of the outer crust of the ingot is very slight, because the little atmospheric oxygen initially in the pit is not renewed, whereas in a common heating furnace the flame brings a constant fresh supply of oxygen; and (3) that the ingot remains upright during solidification, so that its pipe is concentrated at one end and is thus removable. (See § 121.) In this form the system is rather inflexible, for if the supply of ingots is delayed the pits grow unduly cool, so that the next ensuing lot of ingots either is not heated hot enough or is delayed too long in soaking. This defect is usually remedied by heating the pits by the Siemens regenerative system (see § 99); the greaterflexibility thus gained outweighs the cost of the fuel used and the increased loss of iron by oxidation by the Siemens gas flame.
127.Continuous Heating Furnace.—The Gjers system is not applicable to small ingots or “billets,”5because they lack the inner surplus heat of large ingots; indeed, they are now allowed to cool completely. To heat these on the intermittent plan for further rolling,i.e.to charge a lot of them as a whole in a heating furnace, bring them as a whole to rolling temperature, and then withdraw them as a whole for rolling, is very wasteful of heat, because it is only in the first part of the heating that the outside of the ingots is cool enough to abstract thoroughly the heat from the flame. During all the latter part of the heating, when the temperature of the ingot has approached that of the flame, only an ever smaller and smaller part of the heat of that flame can be absorbed by the ingots. Hence in the intermittent system most of the heat generated within the furnace escapes from it with the products of combustion. The continuous heating system (fig. 32) recovers this heat by bringing the flame into contact with successively cooler and cooler billets, A-F, and finally with quite cold ones, of consequently great heat-absorbing capacity.
A, Hottest billet ready for rolling.
B, Exit door.
C, Pusher, for forcing billets forward.
D, Water-cooled pipe on which billets are pushed forward.
E, Magnesite bricks on which the hot billets slide forward.
F, The billet last entered.
G, The suspended roof.
H, The incoming air preheated by G and by the pipes N and brought from above G to between N by a flue not shown.
J, The incoming gas.
L, The flame.
M, The escaping products of combustion.
N, Pipes through which the products of combustion pass.
As soon as a hot billet A is withdrawn by pushing it endwise out of the exit door B, the whole row is pushed forward by a set of mechanical pushers C, the billets sliding on the raised water-cooled pipes D, and, in the hotter part of the furnace, on the magnesite bricks E, on which iron slides easily when red-hot. A new cold billet is then charged at the upper end of the hearth, and the new cycle begins by pushing out through B a second billet, and so forth. To lessen the loss in shape of “crop ends,” and for general economy, these billets are in some cases 30 ft. long, as in the furnace shown in fig. 32. It is to make it wide enough to receive such long billets that its roof is suspended, as here shown, by two sets of iron tie-rods. As the foremost end of the billet emerges from the furnace it enters the first of a series of roll-trains, and passes immediately thence to others, so that before half of the billet has emerged from the furnace its front end has already been reduced by rolling to its final shape, that of merchant-bars, which are relatively thin, round or square rods, in lengths of 300 ft.Fig. 33.—Wire undergoing Reduction in the Die.Fig. 34.—Two-high Rolling Mill.In the intermittent system the waste heat can, it is true, be utilized either for raising steam (but inefficiently and inconveniently, because of the intermittency), or by a regenerative method like the Siemens, fig. 19; but this would probably recover less heat than the continuous system, first, because it transfers the heat from flame to metal indirectly instead of directly; and, second, because the brickwork of the Siemens system is probably a poorer heat-catcher than the iron billets of the continuous system, because its disadvantages of low conductivity and low specific heat probably outweigh its advantages of roughness and porosity.
As soon as a hot billet A is withdrawn by pushing it endwise out of the exit door B, the whole row is pushed forward by a set of mechanical pushers C, the billets sliding on the raised water-cooled pipes D, and, in the hotter part of the furnace, on the magnesite bricks E, on which iron slides easily when red-hot. A new cold billet is then charged at the upper end of the hearth, and the new cycle begins by pushing out through B a second billet, and so forth. To lessen the loss in shape of “crop ends,” and for general economy, these billets are in some cases 30 ft. long, as in the furnace shown in fig. 32. It is to make it wide enough to receive such long billets that its roof is suspended, as here shown, by two sets of iron tie-rods. As the foremost end of the billet emerges from the furnace it enters the first of a series of roll-trains, and passes immediately thence to others, so that before half of the billet has emerged from the furnace its front end has already been reduced by rolling to its final shape, that of merchant-bars, which are relatively thin, round or square rods, in lengths of 300 ft.
In the intermittent system the waste heat can, it is true, be utilized either for raising steam (but inefficiently and inconveniently, because of the intermittency), or by a regenerative method like the Siemens, fig. 19; but this would probably recover less heat than the continuous system, first, because it transfers the heat from flame to metal indirectly instead of directly; and, second, because the brickwork of the Siemens system is probably a poorer heat-catcher than the iron billets of the continuous system, because its disadvantages of low conductivity and low specific heat probably outweigh its advantages of roughness and porosity.
128.Rolling, Forging, and Drawing.—The three chief processes for shaping iron and steel, rolling, forging (i.e.hammering, pressing or stamping) and drawing, all really proceed by squeezing the metal into the desired shape. In forging, whether under a hammer or under a press, the action is evidently a squeeze, however skilfully guided. In drawing, the pull of the pincers (fig. 33) upon the protruding end, F, of the rod, transmitted to the still undrawn part, E, squeezes the yielding metal of the rod against the hard unyielding die, C. As when a half-opened umbrella is thrust ferrule-foremost between the balusters of a staircase, so when the rod is drawn forward, its yielding metal is folded and forced backwards and centrewards by the resistance of the unyielding die, and thus it is reduced in diameter and simultaneously lengthened proportionally, without material change of volume or density.
129.Methods of Rolling.—Of rolling much the same is true. The rolling mill in its simplest form is a pair of cylindrical rollers, BB (figs. 34 and 35) turning about their axes in opposite directions as shown by the arrows, and supported at their ends in strong frames called “housings,” CC (fig. 35). The skin of the object, D, which is undergoing rolling, technically called “the piece,” is drawn forward powerfully by the friction of the revolving rolls, and especially of that part of their surface which at any given instant is moving horizontally (HH in fig. 34), much as, the rod is drawn through the die in fig. 33, while the vertical component of the motion of the rear part JJ of the rolls forces the plastic metal of that part of “the piece” with which they are in contact backwards and centrewards, reducing its area and simultaneously lengthening it proportionally, here again as in drawing through a die. The rolls thus both draw the piece forward like the pincers of a wire die, and themselves are a die which like a river ever renews or rather maintains its fixed shape and position, though its particles themselves are moving constantly forward with “the piece” which is passing between them.
After the piece has been reduced in thickness by its first passage or “pass” between the rolls, it may be given a second reduction and then a third and so on, either by bringing the two rolls nearer together, as in case of the plain rolls BB at the left in fig. 35, or by passing the piece through an aperture, F′, smaller than the first F, as in case of the grooved rolls, AA, shown at the right, or by both means jointly. If, as sketched in fig. 34, the direction in which each of the rolls turns is constant, then after the piece has passed once through the rolls to the right, it cannot undergo a second pass till it has been brought back to its initial position at the left. But bringing it back wastes power and, still worse, time, heat, and metal, because the yellow- or even white-hot piece is rapidly cooling down and oxidizing. In order to prevent this waste the direction in which the rolls move may be reversed, so that the piece may be reduced a second time in passing to the left, in which case the rolls are usually driven by a pair of reversing engines; or the rolls maybe “three high,” as shown in fig. 36, with the upper and the lower roll moving constantly to the right and the middle roll constantly to the left, so that the piece first passes to the right between the middle and lower rolls, and then to the left between the middle and upper rolls. The advantage of the “reversing” system is that it avoids lifting the piece from below to above the middle roll, and again lowering it, which is rather difficult because the white-hot piece cannot be guided directly by hand, but must be moved by means of hooks, tongs, or even complex mechanism. The advantage of the three-high mill is that, because each of its moving parts is always moving in the same direction, it may be driven by a relatively small and hence cheap engine, the power delivered by which between the passes is taken up by a powerful fly-wheel, to be given up to the rolls during the next pass. (See alsoRolling Mill.)
130.Advantages and Applicability of Rolling.—Rolling uses very much less power than drawing, because the friction against the fixed die in the latter process is very great. For much the same reason rolling proceeds much faster than drawing, and on both these accounts it is incomparably the cheaper of the two. It is also very much cheaper than forging, in large part because it works so quickly. The piece travels through the rolls very rapidly, so that the reduction takes place over its whole length in a very few seconds, whereas in forging, whether under hammer or press, after one part of the piece has been compressed the piece must next be raised, moved forward, and placed so that the hammer or press may compress the next part of its length. This moving is expensive, because it has to be done, or at least guided, by hand, and it takes up much time, during which both heat and iron are wasting. Thus it comes about that rolling is so very much cheaper than either forging or drawing that these latter processes are used only when rolling is impracticable. The conditions under which it is impracticable are (1) when the piece has either an extremely large or an extremely small cross section, and (2) when its cross section varies materially in different parts of its length. The number of great shafts for marine engines, reaching a diameter of 221⁄8in. in the case of the “Lusitania,” is so small that it would be wasteful to instal for their manufacture the great and costly rolling mill needed to reduce them from the gigantic ingots from which they must be made, with its succession of decreasing passes, and its mechanism for rotating the piece between passes and for transferring it from pass to pass. Great armour plates can indeed be made by rolling, because in making such flat plates the ingot is simply rolled back and forth between a pair of plain cylindrical rolls, like BB of fig. 35, instead of being transferred from one grooved pass to another and smaller one. Moreover, a single pair of rolls suffices for armour plates of any width or thickness, whereas if shafts of different diameters were to be rolled, a special final groove would be needed for each different diameter, and, as there is room for only a few large grooves in a single set of rolls, this would imply not only providing but installing a separate set of rolls for almost every diameter of shaft. Finally the quantity of armour plate needed is so enormous that it justifies the expense of installing a great rolling mill. Krupp’s armour-plate mill, with rolls 4 ft. in diameter and 12 ft. long, can roll an ingot 4 ft. thick.
Pieces of very small cross section, like wire, are more conveniently made by drawing through a die than by rolling, essentially because a single draft reduces the cross section of a wire much more than a single pass between rolls can. This in turn is because the direct pull of the pincers on the protruding end of the wire is much stronger than the forward-drawing pull due to the friction of the cold rolls on the wire, which is necessarily cold because of its small section.
Pieces which vary materially in cross section from point to point in their length cannot well be made by rolling, because the cross section of the piece as it emerges from the rolls is necessarily that of the aperture between the rolls from which it is emerging, and this aperture is naturally of constant size because the rolls are cylindrical. Of course, by making the rolls eccentric, and by varying the depth and shape of the different parts of a given groove cut in their surface, the cross section of the piece made in this groove may vary somewhat from point to point. But this and other methods of varying the cross section have been used but little, and they do not seem capable of wide application.
The fact that rolling is so much cheaper than forging has led engineers to design their pieces so that they can be made by rolling,i.e.to make them straight and of uniform cross section. It is for this reason, for instance, that railroad rails are of constant uniform section throughout their length, instead of having those parts of their length which come between the supporting ties deeper and stronger than the parts which rest on the ties. When, as in the case of eye bars, it is imperative that one part should differ materially in section from the rest, this part may be locally thickened or thinned, or a special part may here be welded on. When we come to pieces of very irregular shape, such as crank-shafts, anchors, trunnions, &c., we must resort to forging, except for purposes for which unforged castings are good enough.
A, Round bar to be hammered.
B, Anvil.
C, Anvil block or foundation.
D, Falling tup.
E, Steam piston.
F, Piston-rod for lifting tup and driving it down.
G, Steam cylinder.
131.Forgingproceeds by beating or squeezing the piece under treatment from its initial into its final shape, as for instance by hammering a square ingot or bloom first on one corner and then on another until it is reduced to a cylindrical shape as shown at A in fig. 37. As the ingot is reduced in section, it is of course lengthened proportionally. Much as in the smith’s forge the object forged rests on a massive anvil and anvil block, B and C, and is struck by the tup D of the hammer. This tup is raised and driven down by steam pressure applied below or above the piston E of the steam cylinder mounted aloft, and connected with the tup by means of the strong piston-rod F. The demand for very large forgings, especially for guns and armour plate, led to the building of enormous steam hammers. The falling parts of the largest of these, that at Bethlehem, Pa., weigh 125 tons.
The first cost of a hammer of moderate size is much less than that of a hydraulic press of like capacity, as is readily understood when we stop to reflect what powerful pressure, if gradually applied, would be needed to drive the nail which a light blow from our hand hammer forces easily into the woodwork. Nevertheless the press uses much less power than the hammer, because much of the force of the latter is dissipated in setting up useless—indeed harmful, and at times destructive—vibrations in the foundations and the surrounding earth and buildings. Moreover, the effect of the sharp blow of the hammer is relatively superficial, and does not penetrate to the interior of a large piece as the slowly applied pressure of the hydraulic press does. Because of these facts the great hammers have given place to enormous forging presses, the 125-ton Bethlehem hammer, for instance, to a 14,000-ton hydraulic press, moved by water under a pressure of7000 ℔ per square inch, supplied by pumps of 16,000 horse power.
Table IV.—Reduction in Cost of Iron Manufacture in America—C. Kirchoff.