Iron, one of the elementary substances, possessing when pure the following characters: specific gravity, 8.1393 (Percy); hardness, 4.5; crystalline form, isometric; color, silver-gray; lustre, metallic; atomic weight, 56 (0 = 16); specific heat, 0.113795. Its symbol is Fe (fer-rum). Although seldom found native, and never pure, iron is the most universally and extensively distributed of metals. It occurs in large deposits in the form of oxide, and constitutes an ingredient of nearly all rocks, soils, and natural waters. So-called chalybeate mineral springs contain it in relatively large amounts. As a consequence of this wide distribution in the inorganic world, it is found also in vegetable and animal organisms, constituting 0.07 per cent. of the blood, or 5.5 to 8.5 per cent. of the ash of blood. Pure iron is unknown in the arts; and, owing to the difficulty of procuring it on a large scale, its properties have been but slightly investigated. Peligot states that iron prepared by the reduction of its protochloride by hydrogen, is filamentous, compact, malleable, and almost as white as silver.
Iron deposited by the galvanic battery is grayish white and susceptible of a high polish; it is scarcely attacked by sulphuric or muriatic acid at ordinary temperatures, but is dissolved on application of heat, evolving hydrogen free from fetid odor (in contradistinction from manufactured iron). Its malleability is not affected by rapid cooling after exposure to a high temperature. Iron may be rendered strongly magnetic by induction, but loses its magnetic power, when pure, as soon as the source of magnetism is removed. Throughout a wide range of temperature, from red heat to near its melting point, iron is more or less plastic. At red heat it is readily forged under the hammer, and at white heat two masses of iron can be firmly and intimately incorporated with each other (welded) by hammering or pressure. Welding, though not exclusively a property of iron, is possessed by no other metal to so great a degree. It is volatilized in the heat of the voltaic arch. - Iron is a metal of active chemical affinities, and enters into a large number of compounds.
It combines with oxygen in four proportions, as follows:
COMPOUNDS WITH OXYGEN.
Iron, per c't.
Oxygen per c't.
Ferrous oxide (protoxide of iron)....
Ferric oxide (sesquioxide of iron).......
Ferroso-ferric oxide (protosesquioxide of iron;magnetic oxide..........
Metallic iron rusts when exposed to moist air, and is gradually and completely converted into oxide. Mr. Grace Calvert, investigating the conditions necessary or favorable to the rusting of iron, has found that it is not acted upon by pure, dry oxygen or carbonic acid, while it is feebly attacked by moist oxygen or carbonic acid, and most rapidly by moist oxygen containing traces of carbonic acid, which forms first oxide, then ferrous carbonate, and finally hydrated sesquioxide, with admixtures of ferrous oxide and carbonate. Carbonic acid and water likewise act with energy. Solutions of alkaline hydrates, carbonates, or bicarbonates prevent the rusting of iron, while a solution of sugar promotes it. The oxidation of iron may be hindered by attaching it to a more electro-positive metal, such as zinc, or promoted by the presence of a more electro-negative metal, such as copper. Under ordinary circumstances zinc will protect iron when it covers only 1/100 of the surface of the latter, but in a solution of sugar the proportion of surface covered by zinc must be 1 to 15. The following analysis by Grace Calvert gives the composition of rust from Llangollen, Wales:
Iron decomposes steam at a red heat and is converted into oxide, hydrogen being liberated. But hydrogen passed over oxide of iron at a red heat reduces it to metallic iron, water being formed. The character of the action is here determined by the relative amounts of free hydrogen and steam. If the former predominates, reduction takes place; if the latter, oxidation. Dilute mineral acids dissolve iron, converting it into a ferrous salt, hydrogen being evolved. Under certain circumstances iron becomes "passive," and is not attacked by strong acids. This condition is brought about in various ways, and seems to be connected with a superficial oxidation of the iron. Iron burns with brilliancy in oxygen gas. Reduced by hydrogen from finely pulverized oxide, it burns readily in the air, taking fire spontaneously when the temperature of reduction has not been too high, but otherwise requiring to be first ignited. Ferrous oxide possesses so strong an affinity for oxygen that it is isolated with difficulty. Its salts are permanent when crystallized, but rapidly absorb oxygen when exposed to the air in solution. Ferrous carbonate occurs abundantly in nature.
The most important ferrous salt is the sulphate, commonly called green vitriol or copperas, obtained as an incidental product in many metallurgical operations, and applied to manifold uses in the arts. It forms a number of double salts with other sulphates. Ferric oxide occurs abundantly in nature. (See Iron Ores.) It may be prepared artificially by precipitating the hydrate from solution and subsequently igniting it, and also by simple ignition of the sulphate or nitrate; its powder is red. Ferric oxide and its salts are stable in the air, but part with a portion of oxygen when in contact with organic matter; a familiar instance is the rotting of fabrics of cotton or linen by "iron mould." On this property depends the disinfecting power of iron compounds. Ferric oxide acts also as a carrier of oxygen. Bischof has shown that spongy metallic iron is a powerful disinfectant, probably first becoming oxidized itself, and subsequently parting with its oxygen to the organic matter, then becoming again oxidized, and so on. Ferric oxide is largely used in the polishing of metals and glass. It forms salts which do not crystallize as readily as the ferrous salts.
Ferroso-ferric oxide, generally called magnetic oxide, is abundant in nature, and may be regarded as a compound of the two oxides, and its salts as compounds of ferrous and ferric salts; it is perfectly stable; its powder is black. Ferric acid is formed by heating together ferric oxide with saltpetre; it forms salts which are very unstable. - Iron combines with sulphur in two proportions, forming a proto- and a bisulphide. The former is largely used in the preparation of sulphuretted hydrogen for chemical purposes. The latter, known as py-rite or iron pyrites, occurs abundantly in nature, and is used largely as a source of sulphur in the preparation of sulphuric acid. Iron forms a definite compound with nitrogen, Fe4N2; but it is doubtful whether nitrogen plays any part in the manufacture of iron or steel. Compounds of carbon, phosphorus, and silicon with iron also exist; the effect of these substances on the properties of iron is discussed below. The compounds of iron with chlorine and cyanogen are of great importance in chemistry and in the arts.
There are two chlorides, FeCl2 and Fe2Cl6, corresponding to the two oxides, and two double compounds with cyanogen, potassic ferrocyanide or yellow prussiate of potash, K4FeC6N6, and potassic ferricyanide, or red prussiate of potash, K6FeC6N6, which are valuable chemical reagents. The ferrocyanide of iron, or Prussian blue, Fe4, FeC6N6, is formed by precipitation of a solution of a ferric salt with yellow prussiate of potash. - Iron is used in medicine as a reconstructive tonic. It is an important constituent of the animal tissues, and under ordinary circumstances the supply normally present in the food is equal to the demand; but when the number of red blood corpuscles, which contain much iron and are the special carriers of oxygen, is diminished, then their re-formation may be promoted by the administration of iron preparations. This condition (anaemia) is the real indication for iron, most of the special diseases in which it is used being dependent upon or accompanied by this condition. Hence it is largely used not only in anaemia, but in neuralgic affections, dropsy, Bright's disease, scrofulous affections, incipient phthisis, haemorrhages, the various diseases of females, in the convalescence from acute diseases, and in the protean forms of debility and weakness.
A few of the salts are astringent, and a portion of their effect is probably due to their local action upon the stomach. These may be used not only as reconstructive agents, but to check discharges and arrest haemorrhage. Iron is absorbed in small quantity, so that a considerable proportion of every dose passes through the bowels unappropriated. It may be detected in the urine, and the amount normally present has been found increased in the milk of animals to which iron has been administered. The bodily temperature is raised, the pulse quickened and strengthened, and the appetite and nutrition improved by its administration. Headache and constipation are the consequences of too large doses or too long continued use. The number of preparations in use is exceedingly and unnecessarily large, and constantly increasing. Among them are metallic iron, a grayish powder; the protocarbonate, in pills and mixture; the sulphate; hydrated oxide, usually called subcarbonate; tincture of the chloride; wine of iron; tartrate of iron and potassa; phosphate, lactate, and iodide; citrat of iron and quinia; iron and strychnia, etc. The astringent preparations are the perchlo-ride, subsulphate, and ferric alum. The freshly precipitated sesquioxide is the best antidote for arsenic.
In cases of decided anaemia, the stronger preparations of iron are indicated, as the tincture of the chloride, the perchloride, and the sulphate. In other cases the milder salts are generally preferred. The iodide of iron is especially adapted to scrofulous affections. The addition to iron of nux vomica or strychnia, or of bitter tonics like gentian, often enhances its therapeutic action. The administration of iron is contra-indicated by gastric or gastro-enteric inflammation, by plethora, fever, and febrile conditions generally. The dose of iron varies with the preparation used. It is a sort of food, and is best given with or near meals. During its use, the faeces are colored dark by it. - In the arts, iron occurs in three forms, as wrought iron, cast iron, and steel. Wrought iron is nearly pure, and highly malleable, ductile, and weldable. It is fused with difficulty, and its finished forms are therefore generally wrought at a welding heat. It contains invariably a small amount of chemically combined carbon, 0.25 per cent. or less, and intermingled cinder. Its specific gravity varies from 7.3 to 7.8. Its temperature of fusion is about 1800° 0. or 3240° F. Cast or pig iron is in most respects the opposite of wrought iron.
It is not in the slightest degree malleable, ductile, or weldable. It is readily fusible, and is therefore always cast in moulds. It is much harder than wrought iron, and is relatively rigid and brittle. There are many varieties of cast iron, exhibiting great diversity of properties. In color, the extremes are white and black, with a number of intermediate shades of gray. The hardness and brittleness vary through wide limits. White cast iron is the hardest, most rigid, and most brittle; it resists the action of the file and drill, while many of the dark varieties can be tooled with ease. The fusibility of the different varieties of cast iron likewise differs greatly. The dark irons generally require a high heat for fusion and become thinly liquid; they fill forms well, and, as they expand in cooling, make sharp castings, and are hence often called foundery irons. The lighter shades do not become so thinly liquid when fused, and as they contract on cooling are not adapted for castings; they usually contain a smaller amount of foreign matters, and hence, being adapted to conversion into wrought iron, are called forge irons.
The specific gravity of cast iron varies from 6.9 to 7.7; its fusing point is about 1500° C. or 2700° F. The difference between gray and white iron is strongly marked in the molten condition, as they flow from the furnace. Dark cast iron flows quickly and sets without any movement of the surface; when hard, the upper surface is smooth and convex. White iron emits an abundance of brilliant sparks, and its surface is vigorously agitated by the formation of crystals; the forms of the crystals are characteristic of the grade of the iron; when hard, the surface is honeycombed and depressed. There are two other varieties of cast iron: specular iron, or Spiegeleisen, and silvery or glazy iron. They are both white, but differ in character and composition from each other and from ordinary white iron. Chemically, cast iron is further removed than wrought iron from the pure metal; it always contains from 2 to 5 per cent. of carbon. The union of the carbon with the iron may be either chemical or mechanical, and usually both conditions are present in the same mass.
The result of the chemical union of iron and carbon is white iron, while the mechanical mixture of iron with black scales of graphite is dark-colored; the preponderance of one or the other of these conditions gives the various shades from black to white. Sometimes cast iron is composed of a mixture of white and gray iron in patches; the iron is then called mottled. The different grades of pig iron are generally designated by numbers. No. 1 stands for highly graphitic open-grained iron, and Nos. 2, 3, and 4 for the lighter and more compact varieties, passing toward white. Mottled and white irons are generally designated by name. Malleable castings (see Ikon Manufacture) are cast iron which has been rendered partially malleable without alteration of form. - Steel holds, both in physical properties and in chemical composition, an intermediate position between cast and wrought iron. It may be considered as a compound of iron with 0.25 to 2 per cent. of carbon. The limits given for carbon in wrought iron, steel, and cast iron, viz., 0.25 per cent. or less in wrought iron, 0.25 to 2 per cent. in steel, and from 2 to 5 per cent. in cast iron, are to be regarded as approximate only.
No sharp and accurate distinction, based on chemical composition, can be drawn between these three varieties of iron. Steel with the minimum of carbon (mild or soft steel) can scarcely be distinguished from wrought iron; it exhibits the properties of malleability, ductility, and weldability nearly to an equal degree with wrought iron; while steel with the maximum of carbon (strong or hard steel) approximates so nearly to cast iron that the above properties are almost entirely wanting. The fusing point of steel is intermediate between those of cast and wrought iron. The properties that preeminently characterize steel are connected with hardening and tempering. When heated to redness and suddenly cooled by plunging into water or other liquid, it becomes hard in proportion to the amount of carbon it contains, the temperature to which it has been raised, and the rate of cooling. Such hardened steel, being again heated and allowed to cool slowly in the air, loses its hardness to a degree proportional to the temperature to which it was reheated. This process is called tempering. Hardening and tempering are generally regarded as peculiar to steel; but, properly considered, they belong to all compounds of iron and carbon.
Wrought .iron contains too little carbon to show much hardening when rapidly cooled from a high temperature, but it is generally rendered more rigid by such treatment. Oast iron becomes very hard and brittle on sudden cooling, but, since it is much more complex'in composition than steel, the circumstances controlling the hardening are not so well understood. Cast iron which has been hardened may by a process of tempering be rendered soft again. The precise nature of the hardening process is not understood. It has been supposed to result from a chemical union of the carbon with the iron, formed at high temperature, and maintained under rapid, but resolved by slow cooling. It has also been ascribed to a state of tension or polarity of the particles, which is relieved by tempering. In the case of cast iron a change in the condition of the carbon may be often observed; some dark graphitic irons become perfectly white (chill) on sudden cooling. As to the character of the union of iron and carbon in cast iron, a difference of opinion exists.
Gurlt, Mayrhofer, Hahn, and others, have endeavored to establish the existence of definite combinations of iron and carbon, such as FeC, Fe2C, Fe4C, Fe8C, and suppose the different varieties of cast iron to be compounds or mixtures of these definite carburets with iron. The formula of spiegeleisen, in which the carbon is all combined, was supposed by Karsten to be expressed by Fe4C, which would require 5.8 per cent. carbon, but this amount is never found in reality. Gurlt proposed a lower carbide, Fe8C, which he supposed to stand in the same relation to gray iron as Karsten's tetracarbide did to white iron. These formulas, although interesting and attractive in a theoretical point of view, must be regarded as purely imaginary. Isolated analyses may seem to indicate their existence, but extended investigations show that the variations of composition in cast iron are too great to admit of any definite formulas. In the molten condition all the carbon is most probably combined with the iron. The separation of carbon as graphite takes place on cooling, and the amount separated is, other things being equal, determined by the rate of cooling.
When we consider the number of factors that enter into the case, it is not surprising that we fail to detect any regularity in the composition of cast iron. Durre proposes a classification of cast iron based on physical characters. He considers all pig irons to be mixtures of two different substances, namely, graphite and a white or light gray matrix or ground mass. He recognizes three types of iron, represented by spiegeleisen, in which the ground mass forms bold, brilliant, reed-like bundles of crystals; Swedish cannon iron, in which it appears as thin thread-like bundles; and Scotch iron, in which it presents short interlaced figures, almost obscured by the graphite. - The manifold properties possessed by iron in its various forms constitute its great value in the arts. No other metal or metallic combination possesses such a wide range of properties. The hardness and rigidity of pig iron, and the facility with which it can be cast into any desired form, adapt it to use in construction for the resistance of a crushing weight, and also to an infinite variety of utensils. The purer kinds often possess moreover great toughness, and are available for ordnance.
Wrought iron, having a high degree of tenacity and elasticity combined with malleability and ductility, is applicable to numberless uses in every-day life, particularly those which require not only strength, but the ability to resist shock. Steel is stronger than wrought or cast iron, but is intermediate between the two in rigidity. It replaces wrought iron advantageously in construction where strength is required in small bulk; but it is excluded, except in the softest varieties, where shocks are to be encountered. Its property of hardening, combined with malleability and ductility, adapts it for the manufacture of cutting tools. Until the comparatively recent introduction of the Bessemer process and the Siemens regenerative heating furnace (see Furnace), it was impossible to melt wrought iron on the large scale; and the distinction between wrought iron and cast steel was therefore well marked in their physical characters, steel showing a homogeneous crystalline, and wrought iron a more or less fibrous structure, due to the intermingled cinder.
This distinction in physical characters disappears when soft iron (that is, iron with 0.25 per cent. or less of carbon) is melted and cast in moulds; and the tendency of metallurgists at the present time is to call this product steel, without regard to its contents in carbon or its susceptibility to hardening. Bessemer and open-hearth (Martin) steels include products varying from hard steel to soft iron; they have, however, the common property of homogeneity, whence the name sometimes applied to them of "homogeneous metal." In both steel and wrought iron, therefore, the distinction is to be observed between welded and cast products. (See Steel.) - Manufactured iron has thus far been considered in the present article merely as a compound of iron and carbon. It is generally, however, much more complex in composition, and we will now consider each kind separately in greater detail. I. Cast Ikon. This is the product of the blast furnace (see Iron Manufacture), and contains a number of elementary substances derived from the ore, flux, and fuel used in its production.
The substances most commonly met with (besides carbon, which must be regarded as essential) are silicon, sulphur, phosphorus, manganese, and more rarely, or in smaller quantities, chromium, copper, nickel, cobalt, titanium, arsenic, antimony, aluminum, calcium, and magnesium. The following analyses will serve as examples:
* • ■ •
The localities of the samples furnishing the above analyses, and their description as far as given, are as follows: White iron. . 1. Maria. zell, Styria; charcoal; sp. gr. 7.729. 2. Re. schitza, Hungary; charcoal. 3. Cleveland, England; coke. 4. Medellin, Colombia, used for stamp heads; sp. gr. 7.45. 5. Primor, Tyrol; piegeleisen. 6. Sava, Carniola, Austria. 7. Froschnitz, Styria. 8. Bieber, Prussia. 9. Mtl. sen, Prussia. Mottled iron. . 10. Cleveland, England. 11. Styria: a, white portion, sp. gr. 7.069; b, gray portion, sp. gr. 6.928. Gray iron. . 12. No locality; analysis by Fresenius. 13. Cleveland, England. 14, 15, 16. Bessemer iron: Reschitza in Hungary, Neuburg in Styria, and English (hematite). 17. American gun iron. 18. Austrian gun iron. 19. "Glazy" iron, English. The influence which each of these elementary substances exerts on the physical properties of pig iron cannot be stated with precision. The subject is one of great complexity, and has not been thoroughly investigated. The following comprises what is known about it. When iron is fused in the presence of carbon, in a reducing atmosphere . conditions which obtain in the blast furnace - it combines with a portion of carbon, forming a readily fusible compound.
The condition of the carbon in the molten iron is not certainly known, but it is probably in chemical combination. When, however, this carburetted compound solidifies, the carbon may either remain in combination, giving the iron a white color; or it may assume the form of graphitic scales, mechanically disseminated throughout the mass, giving it a black color; or both conditions of the carbon may coexist, imparting to the iron a shade of gray. So far as is known, these conditions are determined by the rate of cooling of the iron. When the iron is quickly cooled, as on being cast in iron moulds or poured into water, the carbon will remain in combination; when the iron is cooled slowly, the carbon has time to separate, and a part at least will appear as graphite. Snelus ("Journal of the Iron and Steel Institute," vol. i., p. 28) has proved conclusively the separate existence of graphite in pig iron. Bell's experiments (" Chemical Phenomena of Iron Smelting ") seem to show that there is no difference in the amount or condition of the carbon in gray and white pig iron made at the Clarence furnace in the Cleveland district, England; and he considers the difference of color to be due to the fact that in the white varieties the graphitic scales are so minute as to be no longer visible.
His experiments contradict results obtained by investigators in other localities, and have not as yet been confirmed. The highly graphitic variety of pig iron is usually produced at a higher temperature than the white; and it has been noticed that when white iron is exposed to the temperature of a furnace making gray iron, it is changed into gray. This has led to the opinion that the temperature of production is the sole cause of the difference in the two varieties. A more correct statement of the fact would perhaps be, that the color of the pig metal, or in other words the amount of graphite separated, is due, other things being equal, to the time or rate of cooling. White iron caused to solidify very slowly becomes gray; and gray iron cooled quickly becomes white. The cause of this difference is conceivable on one supposition only, viz.: that there is a limited range of temperature, probably near the point of solidification of the metal, within which the separation of the carbon from the iron takes place, and that the amount of carbon separated in any given instance is proportional to the time consumed by the cooling pig iron in passing through this range of temperature.
It is evident that the time required for the metal to cool a given number of degrees, near its point of solidification, must depend partly upon the temperature of the surrounding moulds. The hotter the metal leaving the furnace, the more will it have heated the moulds as it approaches solidification, and consequently the slower will have become the rate of cooling, the longer will be the period during which carbon can separate, and the larger will be the amount of graphitic carbon in the final product. Carbon, as already remarked, increases the fusibility of iron. In chemical combination it renders iron brittle, the brittleness decreasing in proportion as the carbon separates as graphite. - Silicon is nearly always present in pig iron. White iron is occasionallv almost free from it, but the darker sorts may contain as much as 8 per cent.; 1/2 to 3 per cent. is usual. The conditions favoring the production of a highly siliconized pig iron are slow working, a high temperature in the furnace, and a cinder rich in silica. Silicon, like carbon, renders iron more fusible. The temperature of solidification of pig iron rich in silicon is therefore relatively low; and this fact, combined with the high temperature of production, affords ample opportunity for the carbon to separate as graphite.
We consequently find such pig iron always highly graphitic, and very difficult to chill, or convert into white iron by sudden cooling. In many articles made of cast iron, such as rolls, car wheels, etc, it is desired to combine toughness of structure with a hard wearing surface. This is effected by casting the object in a suitable mould of iron, so that the molten iron shall be suddenly solidified on the outside, and rendered white to a moderate depth, while the mass of the casting remains gray. The casting is subsequently annealed, to relieve the tension caused by the unequal cooling. The irons most suitable for this purpose are produced with charcoal and with a cold or but moderately, heated blast, and are of exceptional purity. Any considerable amount of silicon prevents the iron from chilling. Silicon renders iron brittle and weak. When present in very large quantity it makes the iron worthless both for castings and for conversion into wrought iron. It plays an important part in the pig iron employed for making Bessemer steel, supplying by its oxidation the greater part of the heat required to retain the metal in a molten condition. The amount of silicon in Bessemer pig iron varies from 1 to 3 per cent.
Silvery or glazy pig iron, occasionally produced when the furnace is working very hot with an excess of fuel, is white, but has none of the properties of white iron properly so called, and is weak and worthless for all purposes. It has not been thoroughly investigated. An analysis (No. 19) given above shows it to contain over 5 per cent. of silicon. - Sulphur is present in many ores of iron and in almost all mineral coals. The hotter the furnace and the more basic the cinder, the more sulphur will be removed in the cinder. Where the opposite conditions exist a large part of the sulphur in the charge will be found in the pig iron. The influence it exerts on pig iron has not been determined with precision. According to Eg-gertz, 0.4 per cent. of sulphur renders pig iron stronger and more mottled. Swedish cannon iron contains from 0.07 to 0.1 per cent. of sulphur. It is the general impression among iron founders that sulphur renders pig iron harder, whiter, and more infusible; but experimental proof is wanting on this point. Phosphorus is almost always present in cast iron.
Few iron ores or limestones are absolutely free from phosphorus; and almost the entire amount of this element present in the charge is absorbed by the iron, which it renders thinly liquid when fused, and crystalline and hard when solid. Such iron is well adapted to form ornamental and intricate castings, since it fills the mould well and brings out the fine outlines with sharpness. Less than 0.5 per cent. of phosphorus does not materially affect the physical properties of pig iron; and more than 5.5 per cent. renders it too weak and brittle to be used. The following table shows the amount of sulphur and phosphorus in some well known brands of English pig:
Phosphorus, per cent.
Sulphur, per cent.
In the conversion of pig into wrought iron by puddling, the phosphorus and sulphur are to a considerable extent eliminated. In the conversion into Bessemer steel, however, these elements remain with the iron, and therefore only the purest pig irons can be used in this process. The maximum amount of phosphorus (the most deleterious element) that Bessemer pig iron may contain is 0.1 per cent. - Manganese resembles iron in many of its chemical properties, and generally accompanies it in its ores. The amount in spathic iron ores is often large, in other ores usually small. Manganese is reduced from its oxide with much more difficulty than iron. When the manganese forms an integral part of the iron ore, it is reduced to a large extent together with the iron; but when the oxide as such is present in the charge, it passes mainly into the slag, unreduced. The effect of manganese on cast iron is peculiar. Specular iron, generally known by its German name Spiegeleisen, made from spathic ores rich in manganese, contains from 4 to 12 (exceptionally as high as 20) per cent. of the latter metal, and also nearly 5 per cent. of carbon, all chemically combined, and but a fraction of 1 per cent. of silicon. On its freshly fractured surface it is white and resplendent, with large crystalline faces.
It is very hard. Gray pig iron may contain as much as 6 per cent. of manganese without showing any tendency to whiteness; the effect of manganese may here be counteracted by the silicon. Pig iron containing manganese is preeminently adapted for conversion into wrought iron and steel. The part that manganese plays in these processes is not well understood. It replaces silicon as a heat producer in pig iron employed in the Bessemer process. Spiegeleisen is generally very pure, and is almost exclusively used in steel-making. The effect of the other substances mentioned above on the physical properties of pig iron is not known definitely. II. Wrought Iron. By far the largest amount of wrought iron in the arts is made from pig iron by the removal of the carbon, silicon, etc, through oxidation in a reverberatory furnace. (See Iron Manufacture.) The iron is removed from the furnace in the form of a white-hot ball or bloom, composed of small particles of soft iron, intimately mingled with cinder. The cinder is expelled and the particles of iron are united by squeezing, hammering, or rolling. Slabs of iron thus made are welded by exposing them to a white heat and rolling them out together.
The homogeneity of the product depends on the thoroughness of the working, and this in turn on the temperature and the fusibility of the cinder. The fibrous character of wrought iron is due to the elongation of the granules or crystals of iron by rolling with intermingled cinder. Fibre, however, is not a condition inherent in iron, nor is it necessarily characteristic of good or strong iron, as is often supposed. Iron from which the cinder has been removed by thorough working, or by fusion, exhibits the granular crystalline fracture proper to the metal itself; and such iron is, other things being equal, stronger than that showing well developed fibre. But fibrous structure is evidence of good quality in iron, in so far as it shows the absence of substances (notably phosphorus) which tend to make it crystalline and brittle. It is to be observed that all iron, even the most fibrous, shows a crystalline character when broken short off; and that some varieties of crystalline iron may appear fibrous when bent and broken slowly. The specific gravity of wrought iron differs according to treatment.
The following determinations are by Kirkaldy: rolled, 7.7626 to 7.2898; hammered, 7.8067 to 7.7206; angle iron, 7.7310 to 7.5297; sheet iron, 7.7419 to 7.5381. The physical properties of wrought iron are intimately connected with its chemical composition. Carbon is almost always present in minute quantity; without it, iron is liable to take up oxide of iron and become rotten or " burnt." In the analysis of wrought iron it is often difficult to determine whether a substance is present in the iron itself or in the cinder. This is especially the case with silicon. The amount of silicon in wrought iron is never large (disregarding cinder), as it is the element most readily removed by oxidation in the operation of puddling. It is supposed to render the iron weak and brittle. Sulphur and phosphorus are frequently met with in wrought iron, and their effects have been tolerably well determined. Sulphur makes iron "red short" or " hot short," that is, brittle at a red heat. Phosphorus, on the contrary, makes iron "cold short," that is, brittle when cold. A red short iron can be worked cold, and a cold short iron hot. According to Eggertz, iron with 0.04 per cent. of sulphur can still be punched hot. More than this amount renders iron perceptibly red short.
The effect of phosphorus on wrought iron differs according to the treatment it has received. Its tendency, even in proportions as low as 0.1 per cent., is to make iron coarsely crystalline in texture; this tendency is increased by prolonged heating. The strength and extensibility are thereby decreased and the hardness increased. If however the phosphorus is in not too large quantity and the iron is drawn out to such an extent that on slow fracture it exhibits a fibrous structure, the metal becomes both strong and tenacious. The presence of cinder facilitates the formation of fibre; and iron with an abundance of cinder (2 to 3 per cent.) has been found by Knut Styffe to be tenacious with 0.25 to 0.35 per cent. of phosphorus. He considers that phosphorus, like carbon, raises the elasticity and strength within the crystalline particles of the iron (whence results its superior hardness), but that it does not increase the cohesion between the separate crystals. The general impression among iron workers is that phosphorus and sulphur neutralize each other in iron, so that a "neutral" iron can be made from a mixture of cold and hot short irons. Whether the effect produced by such a mixture is one of neutralization, strictly speaking, or merely of dilution, remains to be determined.
Little is known of the effect of other elementary substances on wrought iron. Manganese, if present in pig iron, is almost entirely removed on conversion into wrought iron. Chromium, titanium, tungsten, manganese, and other substances are sometimes added to steel in the process of manufacture, and appear to modify its properties materially. - Prof. Graham ("Chemical Journal," vol. v., 1867), in his investigations on the occlusion of gases in metals, found that wrought iron contained many times its own bulk of gas, notably carbonic oxide. Mr. John Parry, of the Ebbw Vale iron works, Wales, has studied the subject more closely, and found that all iron contains occluded gas. In his experiments the amount of gas evolved was not determined, but its composition is as follows:
The amount of hydrogen that gray pig iron is capable of absorbing when heated in an atmosphere of this gas has been found by Parry to be 20 times its volume. By increasing the heat the gas thus absorbed is given off. Of the effect of gases on the physical properties of iron nothing is known, and the part they play in the various manufacturing processes is only beginning to be investigated. (See Iron Manufacture.) - Compounds of iron with potassium, aluminum, manganese, nickel, lead, antimony, tin, and copper are known; but none of them have found important application in the arts, except ferro-manganese, which is often used instead of spiegeleisen in the Bessemer and Martin processes. It contains sometimes over 40 per cent of manganese, with a very small amount of carbon, is acknowledged to give better results than spiegeleisen, and would supersede the latter but for its cost. - Strength of Iron. The strength of cast and wrought iron varies through wide limits. Cast iron is inferior to wrought iron in strength when exposed to tensile, torsional, or transverse strain, but shows a very high resistance to compression. Owing to its rigidity, it stretches but slightly under stress, while wrought iron elongates considerably.
In estimating tensile strength, therefore, regard must be had to the fractured as well as to the original area. The softest and purest irons elongate most, and consequently show a low tensile strength when referred to the original area, but a high degree of resistance when referred to the fractured section. A gradual increase in the amount of extension under strain is noticed from cast iron through steel to the softest wrought iron. The following tables, compiled from the experimental results of Hodgkinson, Fairbairn, Kirkaldy, Thalen, Rod-man, and others, show the limits of strength of cast and wrought iron and steel under different conditions and treatment. (Steel is included here to facilitate comparison. For more detailed information with regard to this substance, see Steel.)
Tensile and compressive Strength of various descriptions of English Cast Iron. (Hodgkinson.)
Tensile strength per sq in.
Crushing weight in tons per sq. in.
Height of specimen,
1 1/2 in.
Lowmoor, No. 1...
" " 2....
Clyde, No. 1......
" " 2......
" " 3........
Blaenavon, No. 1.......
" " 2......
Brymbo, No. 1......
" " 3......
Yniscedwyr, No. 1...
Anthracite, No. 2......
The tensile strength of Austrian gun iron is from 30,000 to 38,000 lbs.; of Russian, about 27,000 lbs.; and of Swedish, about 34,000 lbs. Experiments made by Capt. Rodman of the United States ordnance corps, with Greenwood, Springfield, and Salisbury pig irons (charcoal), and mixtures of the same, showed in 16 determinations a minimum specific gravity of 7.099 and a maximum of 7.307; a minimum tensile strength of 22,179 lbs. and a maximum of 42,884 lbs. to the square inch. The following determinations of the strength of Richmond (Mass.) charcoal pig were made at the South Boston foundery. This iron is smelted from pure red hematite, with temperature of blast varying from 100° to 350° F., and is largely used for ordnance:
Tensile strength, in lbs. per sq. in.
Remelting in a reverberatory furnace raised the average specific gravity of this iron to 7.3135, and the average tensile strength to 40,022 lbs. per square inch. Fairbairn has determined the transverse strength of cast iron rectangular bars from nearly all the British iron works. In 51 experiments on all shades of gray pig iron, the minimum breaking weight for bars 4 ft. 6 in. between supports was 357 lbs. to the square inch, corresponding to a specific gravity of 6.916, and the maximum 581 lbs., corresponding to a specific gravity of 7.122. Irons intermediate in strength, however, often show a higher specific gravity. From experiments made by Fairbairn to determine the effect of hot blast on the strength of pig iron, he concluded that No. 1 irons had been deteriorated, No. 2 slightly injured by it, and No. 3 improved by the use of hot blast. According to experiments on the resistance to torsion in cast iron, the length of the bar submitted to torsion being about eight diameters, the ultimate strength of seven samples varied from 6,176 to 10,467 lbs. to the square inch. The force requisite to give the bar a permanent set of half a degree is about nine tenths of that which will break it.
The following table shows the effect of successive remeltings on the strength of cast iron, from determinations by Fairbairn; the iron used was Eglinton No. 3, hot blast:
NO. OF MELTINGS.
Mean breaking w'ght in lbs. per sq. in. for bars 4 ft. 6 in. between supports.
Resistance to compression for cubes 1 in.
• • • •
Melting per se cannot have any effect on the physical properties of iron, and any change consequent on melting must be referred to change in chemical composition, leaving out of consideration accidental imperfections of casting. Melting may be effected in three ways: in a crucible, in a cupola or shaft furnace, or in a reverberatory or air furnace. In a crucible, by exclusion of air, the iron should be unchanged; in a cupola the atmosphere is reducing, and an increase of carbon or silicon may result; while in the oxidizing atmosphere of an air furnace the silicon and carbon are gradually removed. The following tables give the strength of wrought iron of various manufacture, composition, etc, under varying strains; steel is added for comparison:
Tensile Strength of Iron Bars. (Kirkaldy.)
Breaking weight per sq. in. in pounds.
Difference between original and fractured area, per ct.
Staffordshire B. B. scrap....
Scotch extra best best.....
" best best.......
Russian C. C. N. D........
Tensile Strength of Iron Plates. (Kirkaldy.)
Breaking w'ght per sq. inch, lengthwise.
Difference of original and fract'd area, per cent.
Breaking w'ght per sq. inch, crosswise.
Difference of original and fract'd area, per cent.
Scotch best boiler..
Staffordshire best best..........
No. of bars.
Elongation by rupt're.
Breaking weight per sq. in. on the original mean area.
Breaking weight per sq. in. on the section of rupture.
Hard puddled steel.
Middling hard puddled steel......
Soft puddled steel....
The elongation given in this table is very low, owing to the fact that the experimenter, Herr Thalen, did not measure the elongation in that foot of length in which the fracture took place, but in the other 4 ft. of the length of the bar.
Results of Experiments on the Tensile Strength of Bessemer Steel and Cast Steel at 60" F.
(From Knut Styffds "Iron and Steel")
Amount of carbon per cent.
Amount of phosphorus per cent.
Elongation by rupture.
Load per sq.in. at the limit of elasticity, lbs.
Original area, lbs.
Fractured area, lbs.
Tilted Bessemer steel from Hogbo........
Boiled Bessemer steel from Carlsdal..........
Boiled cast steel from Wikmanshyttan (Uchatius steel)
Tilted cast steel from Krupp.................
Results of Experiments on the Tensile Strength of Iron at 60 F°. (From Knut Styfe's "Iron and Steel")
Amount of carbon per cent.
Amount of phosphorus per cent.
BREAKING WEIGHT PES SQUARE INCH.
Elongation by rupture per cent.
Load per sq.
inch at the limit of elasticity, lbs.
On original area, lbs.
On fractured area, lbs.
Boiled puddled iron from Lowmoor.....
" " " from Cleveland.......
" " " from Dudley........
" " " from Motala works, Sweden..
Boiled iron made in charcoal hearth, Sweden......
" " " in Lancashire hearth, Sweden........
Results of Experiments on the effect of Hardening on the Extensibility and Strength of Iron and Steel.
(From Knut Styffe's "Iron and Steel."")
Treatment of bar before experiment.
Amount of carbon per cent.
Breaking weight per sq. inch, original area, lbs.
Ratio bet'n area of fracture and orig. mean area.
Bessemer steel from Hogbo...
Heated to redness and hardened in water.....
Uchatius steel from Wikmanshyttan.......
Heated to redness and slowly cooled in warm coal dust.
Strongly heated and hardened in oil.......
Heated to redness and slowly cooled in warm.....
Puddled iron from Surahammar..........
Heated to redness and hardened in water....
Heated to redness and slowly cooled in hot coal dust....
Iron made in charcoal hearth from Lesjoforss........
Heated to redness and hardened in water........
Heated to redness and slowly cooled in hot coal dust....
Results of Experiments on the Strength of Iron rolled cold. (Fairbairn.)
CONDITION OF BAR.
Br'king weight per sq. in., tons.
The resistance of cold rolled iron to tension, compression, and transverse strain, and also its hardness, are increased in nearly the same ratio as its breaking weight. The following table exhibits the results of experiments to determine the strength of the iron from two exploded boilers, compared with other brands of American iron and English Lowmoor boiler plate:
Average breaking weight per sq.in.,lbs.
Latter less than former per cent.
Exploded boiler, steamer "Westfield, 33 experiments......
Sligo boiler plate, 16 experiments........
American flange iron, 15 experiments.........
Tank iron, 6 experiments, 3 makers.........
Exploded boiler, steamer Red Jacket, 2 experiments.......
English best Lowmoor boiler plate, 5 experiments..........
The great variation in strength in the iron from the exploded boilers was supposed to be due to the wrenching and twisting accompanying the explosion. From the foregoing tables it will be seen that the physical properties of iron, strength, elasticity, etc, vary according to composition and treatment. The following are some of the conclusions of Knut Styffe ("The Elasticity, Extensibility, and Tensile Strength of Iron and Steel," translated by C. P. Sandberg, London, 1869): "The limit of elasticity, the absolute strength, and the extensibility are to a great extent dependent, in both iron and steel, on the mechanical treatment to which the material has been submitted, and on the temperature to which it has been exposed, either during working or subsequently. By cold-hammering, cold-rolling, and other forms of mechanical treatment applied at a low temperature, both the limit of elasticity and the absolute strength are increased; while by the same treatment the extensibility is diminished. In these respects heating produces an opposite effect. When the proportion of carbon in iron or steel is increased, while the other conditions remain the same, the limit of elasticity, as well as the absolute strength, is to a certain extent increased; but the extensibility, on the contrary, is diminished.
The absolute strength, which in good soft iron may be estimated in round numbers at 48,034 lbs. or 21.44 tons per square inch, seems to attain its maximum in steel containing about 1.2 per cent. of carbon, and is then in good cast steel or Bessemer steel about 137,240 lbs. or 61.26 tons per square inch. A small proportion of phosphorus in iron generally raises the limit of elasticity and the absolute strength, and therefore also the hardness of the metal; but at the same time it diminishes its extensibility, provided that the iron during its manufacture has been so much drawn out that on slow rupture it exhibits a fibrous fracture. By admixture, however, of slag (which always makes the iron unsound and difficult to be re-formed when heated, but which facilitates the development of a fibrous structure), an iron containing 0.25 per cent. of phosphorus seems capable of acquiring nearly the same extensibility as an iron which contains only traces of phosphorus. The presence of slag also seems to oppose the tendency of the iron to become when strongly heated crystalline, and therefore cold-short. By heating and sudden cooling (hardening), the limit of elasticity is raised, while the extensibility is diminished, not only in steel, but also in iron.
The absolute strength likewise is increased by hardening, if this be performed in a manner adapted to the quality of the material. Hardening in water without subsequent moderate heating (tempering) generally diminishes the strength of hard steel to a very considerable extent; while hardening in oil does not occasion this inconvenience, provided the heat previous to hardening has not been too high." Styffe likewise gives the result of an elaborate series of experiments on the strength of iron and steel at different temperatures from - 40° F. to 418° F., from which he deduces the following conclusions: "The absolute strength of iron and steel is not diminished by cold, but even at the lowest temperature which ever occurs in Sweden, it is at least as great as at the ordinary temperature (about 60° F.). At temperatures between 212° and 392° F., the absolute strength of steel is nearly the same as at the ordinary temperature; but in soft iron it is always greater. In neither steel nor iron is the extensibility less in severe cold than at the ordinary temperature; but from 266° to 320° F. it is generally diminished, not to any great extent in steel, indeed, but considerably in iron.
The limit of elasticity in both steel and iron lies higher in severe cold; but at about 284° F. it is lower, at least in iron, than at the ordinary temperature." In the experiments on which these conclusions are based, the strength was determined by a gradually increasing strain. The result is quite different if the strain is applied suddenly, that is, if the iron or steel is submitted to shock, as is shown in the following experiments made by 0. P. Sandberg (appendix to the work of Knut Styffe):
Height of Fall of Ball (weighing 9 cwt.) required to break each Rail (Iron) at different temperatures. Distance between supports 4 ft.; length of rail 10 ft. 5 in.
WORKS WHERE RAILS WERE MADE.
84° F., ft.
35° F., ft.
10° F., ft.
Aberdare, Wales ............
,, ,, ...................
,, ,, ..................
,, ,, .................
,, ,, ..................
,, ,, ...................
,, ,, ..............
Le Creusot, France ............
,, ,, .......................
,, ,, ............
,, ,, ............
,, ,, ........
,, ,, ...........
Average of 7 English rails........
" 5 French " ........
" 2 Belgian " ........
3 English, 5 French, and 2 Belgium rails....
Mr. Sandberg concludes from these experiments that for such iron as is usually employed for rails in Wales, France, and Belgium, the breaking strain, as tested by sudden blows or shocks, is considerably influenced by cold; such iron exhibiting at 10° F. only from one third to one fourth of the strength which it possesses at 80°. The ductility and flexibility of such iron he finds also much affected by cold; rails broken at 10° showing on an average a permanent deflection of less than one inch, while the other halves of the same rails, broken at 84°, showed a set of more than four inches before fracture. He says that at summer heat the strength of Aberdare rails was 20 per cent. greater than that of Creusot rails; but that in winter the latter were 30 per cent. stronger than the former. The confusion in the statements regarding the strength of iron and steel at different temperatures has arisen from the fact that in the experimental determinations the difference between the effect of a gradually increased and a suddenly applied strain has been overlooked. The experiments of Mr. Sandberg are conclusive on this point, and confirm the universal experience that iron and steel tools and utensils are much more liable to break in cold than in warm weather.
The breaking of rails in winter has also been referred to the hardness and rigidity of the road bed; no experiments have yet been made that confirm this view. - A very thorough investigation of the strength Of wrought iron at different temperatures was made by a committee of the Franklin institute of the state of Pennsylvania, consisting of Prof. W. R. Johnson, Prof. A. D. Bache, and Benjamin Reeves, from 1832 to 1837. These experiments were 73 in number, at temperatures from 212° to 1317° F. A remarkable anomaly was discovered in the behavior of iron at a temperature between 500° and 600°. About 572° was found to be the temperature of the maximum strength of iron; and the best qualities then showed a tenacity 15.17 per cent. over that possessed by the same iron at ordinary temperatures. Sir William Fairbairn made a similar observation on South Staffordshire iron. It showed from 60° to 325° a regular increase of tenacity from 62,-186 lbs. to 84,046 lbs. per square inch, or 30 per cent.
Mr. Clay has determined the effect of repeated workings on the tensile strength of wrought iron as follows:
Lbs. per sq. in.
Lbs. per sq. in.
The increase of strength is doubtless due to the increase of homogeneity, and the subsequent decrease to an oxidation of the iron. - A. Wohler (Ueber die Festigkeitsversuche von Eisen und Stahl) has investigated the effect of repeated strains on iron and steel, and has shown that the rupture of a material may be effected by frequently applied strains, none of which exceed the limit of rupture; that the destruction of cohesion depends on the differences of tensions which form the limits of the oscillations of the strain; and that the absolute amount of the extreme tensions is only of importance in so far as the differences of strain which effect rupture decrease with the increasing tension. When a fibre passes from a state of tension to a state of compression, or vice versa., we should consider the tensional strain as positive, and the compressive strain as negative, so that the variation will be equal to the sum of the tensional and compressive strains. This condition, often called the "fatigue"of metals, is shown in the following table:
Ultimate breaking strain in lbs. per sq. inch.
Elongation per cent.
Strain applied in lbs. per sq. inch.
Difference In maximum and minimum.
No. of applications before fracture.
Cast-steel axle, Krupp, 1862
Piece of cast-iron cylinder, Stettin.
The effect of vibration on fibrous iron, it has been generally supposed, is to make it crystalline. Experimental evidence is however lacking on this point. Iron subjected to vibratory shocks may become weak and break from "fatigue," or by reason of poor material or bad working; but there are no facts to prove that weakening is the result of a passage from the fibrous to the crystalline condition. - Prof. Robert H. Thurston has investigated the effect of unintermitted static stress on wrought iron and steel strained beyond the limit of elasticity, and has found that they do not lose their power of resistance or yield in the slightest degree. He has further determined that iron and steel, if strained beyond the limit of elasticity, and left under the action of the distorting force which has been found just capable of equilibrating their power of resistance, gain resisting power to a degree which has a limit in amount approximating closely, if not coinciding with, the ultimate resistance of the material, and which had a limit as to time in experiments hitherto made of three or four days.
Releasing the piece entirely and again submitting it to the same force immediately does not produce this strengthening effect. - The production of iron and steel in the United States in 1872 was as follows, in tons of 2,000 lbs.:
Iron and steel rails......
Other rolled and hammered iron.......
Forges and bloomaries.......
The following is the production of England, Prussia, and Sweden for 1871, and France for
I. A S. E. county of Missouri, drained by affluents of the St. Francois and Big Black rivers; area, about 500 sq. m.; pop. in 1870, 6,278, of whom 352 were colored. Iron mountain and Pilot Knob are on the N. E. border. The surface is hilly and mountainous. There are large forests of oak, hickory, pine, and cedar. Iron ore is abundant, and other metals are found. The St. Louis and Iron Mountain railroad crosses the county. The chief productions in 1870 were 12,221 bushels of wheat, 90,385 of Indian corn, and 28,141 of oats. There were 690 horses, 919 milch cows, 1,703 other cattle, 3,178 sheep, and 4,714 swine; 5 manufactories of carriages, 1 of charcoal, 1 of pig iron, and 5 saw mills. Capital, Ironton. II. A S. county of Utah, extending from Colorado on the E. to Nevada on the W.; area, 9,200 sq. m.; pop. in 1870, 2,277. It is intersected in the E. by the Colorado river, and crossed in the W. by the Wasatch mountains. Iron ore is found in this range, and at its base is some land suitable for agriculture, but much of the county is unavailable. The chief productions in 1870 were 8,917 bushels of wheat, 2,857 of Indian corn, 21,276 of potatoes, 17,968 lbs. of wool, 21,355 of butter, and 736 tons of hay. There were 732 horses, 2,114 cattle, 4,502 sheep, and 3 saw mills.