STEEL, iron united with carbone. See IRON.
Steel has properties distinct from those of iron, which render it of superior value. From its higher degree of hardness it admits a finer polish and assumes a brighter colour. When tempered, it possesses a higher degree of elasticity, and is also more sonorous. It is more weakly attracted by the loadstone, it receives more slowly the magnetic power, but it preserves it longer. When exposed to a moist air, it does not contract rust so easily as iron.
iron. It is also heavier, increasing in weight, according to Chaptal, one hundred and seventieth part. M. Rinneman has given as the result of several accurate experiments on different kinds of steel the following specific gravity 7.795, while he makes ductile iron 7.700, and crude iron 7.251.
All iron is convertible into steel by exposing it to a certain degree of heat for a certain time along with a quantity of charcoal. Chemists differ in opinion concerning the nature and effects of this process. Some say that steel is produced by absorbing a quantity of caloric or heat in a latent state, as the older chemists had said it was formed by absorbing phlogiston. Lavosier seems to have ascribed the qualities of steel to a slight degree of oxidation, others to a combination with plumbago or black lead, and others to a union with carbone. In agreeing with those who say the formation of steel is owing to carbone, we do not differ essentially from those who attribute it to plumbago; for the art of chemistry has now found that these substances are very nearly allied. Plumbago is a true charcoal combined with a little iron. The brilliant charcoal of certain vegetable substances, more especially when formed by distillation in close vessels, possesses all the characters of plumbago. The charcoal of animal substances possesses characters still more peculiarly resembling it. Like it they are difficult to incinerate, they leave the same impression on the hands and upon paper; they likewise contain iron, and become converted into carbonic acid by combustion. When animal substances are distilled by a strong fire, a very fine powder sublimes, which attaches itself to the inner part of the neck of the retort, and this substance may be made into excellent black lead pencils.
There are two ways of making steel, namely, by fusion and by cementation. The first way is used to convert iron into steel immediately from the ore, or from crude or cast-iron. By the second way, bar-iron is exposed to a long continued heat surrounded by charcoal. Each of these ways has advantages peculiar to itself; but the same causes in fact predominate in both, for both kinds of steel are produced by heat and charcoal. The only difference between the two methods is this; in making steel by fusion the charcoal is not so equally defended from the access of air as in the other way.
Swedenborgius has given the following description of the method used in Dalecarlia for making steel from cast-iron. The ore from which the crude iron to be converted into steel is obtained is of a good kind. It is black, friable, and composed of many small grains, and it produces very tough iron. The conversion into steel is made upon a forge-hearth, something smaller than common. The sides and bottom are made of cast-iron. The tuyere is placed, with very little inclination, on one of the side-plates. The breadth of the fire-place is fourteen inches; its length is greater. The lower part of the tuyere is six inches and a half above the bottom. In the interior part of the fire-place there is an oblong opening for the flowing of the superfluous scoria. The workmen first put scoria on the bottom, then charcoal and powder of charcoal, and upon these the cast-iron run or cast into small pieces. They cover the iron with more charcoal, and excite the fire. When the pieces of iron are of a red white, and before they begin to melt, they stop the bellows, and carry the mats under a
large hammer, where they break it into pieces of three or four pounds each. The pieces are again brought to the hearth, and laid within reach of the workman, who plunges some of them into the fire, and covers them with coal. The bellows are made to blow slowly till the iron is liquefied. Then the fire is increased; and when the fusion has been long enough continued, the scoria are allowed to flow out; and at that time the iron hardens. The workman adds more of the pieces of crude iron, which he treats in the same manner; and so on a third and a fourth time, till he obtains a mass of steel of about a hundred pounds, which is generally done in about four hours. This mass is raised and carried to the hammer, where it is forged, and cut into four pieces, which are farther beat into square bars four or five feet long. When the steel is thus forged, it is thrown into water that it may be easily broken; for it is yet crude and coarse-grained. The steel is then carried to another hearth similar to the former, and there broken in pieces. These pieces are laid regularly in the fire-place, first two parallel, upon which seven or eight others are placed across; then a third row across the second, in such a manner that there is space left between those of the same row. The whole is then covered with charcoal, and the fire is excited. In about half or three quarters of an hour the pieces are made hot enough, and are then taken from the fire, one by one, to the hammer, to be forged into little bars from half a foot to two feet long, and while hot are thrown into water to be hardened. Of these pieces sixteen or twenty are put together so as to make a bundle, which is heated and welded, and afterwards forged into bars four inches thick, which are then broken into pieces of convenient length for use.
The method of converting iron into steel by cementation is a very simple process. It consists solely in exposing it for a certain time to a strong degree of heat, while closely covered with charcoal and defended from the external air. The furnaces employed for converting iron into steel (says a manufacturer of this metal) are of different sizes; some capable of converting only three or four tons weight, while others are capacious enough to contain from seven to eight or ten tons. The out-sides of these furnaces rise up in the form of a cone, or sugar-loaf, to the height of a very considerable number of feet. In the inside, opposite to each other, are placed two very long chests, made either of stone, or of bricks capable of bearing the strongest fire; which is placed between the two chests. The bars of iron, after the bottom is furnished with a necessary quantity of charcoal dust, are laid in stratum super stratum, with intermediate beds of the charcoal dust, to such a height of the chests as only to admit of a good bed at top; which is then all covered over, to prevent the admission of the common air; which, could it procure an entrance, would greatly injure the operation. The iron being thus situated, the fire is lighted; which is some time before it can be raised to a sufficient degree of heat to produce any considerable effect. After which it is continued for so many days as the operator may judge proper; only now and then drawing out what they call a proof bar. This is done by openings fit for the purpose at the ends of the chest, which are easily and with expedition stopped up again, without occasioning any injury to the contents left behind. When the opera-
tor apprehends the conversion is sufficiently completed, the fire is suffered to go out, and the furnace, with its contents, is left gradually to cool. This may take up several days: after which the furnace is discharged, by taking out the bars of steel and the remainder of the charcoal dust.
There is a manufactory established in the parish of Cramond, about five miles from Edinburgh, in which this method is practised with great success. Great quantities of steel are made there, which we have reason to believe is of as excellent a quality as any that can be procured from other countries.
When the charcoal is taken out, it is found as black as before it was introduced into the furnace, unless by accident the external air has got admittance. The bars preserve their exterior form only; the surface frequently exhibits a great number of tumors or blisters, whence they are called blistered steel.
The hardness of steel is much increased by tempering. This consists in heating it to a red heat, and then plunging it suddenly into cold water. If it be allowed to cool slowly, it still preserves its ductility; or if it be heated again after being tempered, it loses its hardness, and again becomes ductile. In heating steel for tempering it, the most remarkable circumstance is, the different colours it assumes, according to the degree of heat it has received. As it is gradually heated, it becomes white, then yellow, orange, purple, violet, and at last of a deep blue colour.
According to Reaumur, the steel which is most heated in tempering is generally the hardest. Hence it is believed, that the more violent the heat to which steel is exposed, and the more suddenly it is plunged into cold water, the harder the steel will be. Rinman, again, has deduced a conclusion directly opposite, that the steel which is naturally hardest demands the least degree of heat to temper it. Different methods have been proposed to determine what degree of heat is most proper; but the easiest method is to take a bar of steel, so long, that while one end is exposed to a violent heat, the other may be kept cold. By examining the intermediate portions, it may be found what degree of heat has produced the greatest hardness.
By tempering, steel is said to increase both in bulk and in weight. Reaumur says, that a small bar six inches long, six lines broad, and half an inch thick, was increased at least a line in length after being tempered to a reddish white colour; that is, supposing the dilatation proportional in all dimensions increasing at the rate of 48 to 49. Iron also expands when heated; but when the heat passes off, it returns to its former dimensions. That the weight of steel is also augmented by tempering, has been found by experiment. Rinman having weighed exactly in an hydrostatic balance two kinds of fine steel made by cementation, and not tempered, found their density to be that of water as 7,991 to 1; after being tempered, the density of the one was 7,553, and that of the other 7,708. M. de Morveau took three bars just of a size to enter a certain caliber 28 lines long, and each side two lines broad; one of the bars was soft iron, and the two others were taken from the same piece of fine steel. In order to communicate an equal degree of heat to each, in an earthen vessel in the midst of a wind furnace, the bar of soft iron and one of the bars of steel were thrown into
cold water; the other bar of steel was cooled slowly over some pieces of charcoal at a distance from the furnace. The bar of iron and the one of steel that was allowed to cool slowly passed easily into the caliber again; but the bar of tempered steel was lengthened almost one-ninth of a line.
There is no doubt but tempering changes the grain; that is, the appearance of the texture of a piece of steel when broken. This is the mark which is usually observed in judging of the quality of steel, or of the tempering which suits it best. The tempered bar is broken in several places after having received different degrees of heat in different places. What proves completely the effect of heat upon the grain, at least in some kinds of steel, is, that a bar of steel exposed to all the intermediate degrees of heat, from the smallest sensible heat to a red heat, is found to increase in fineness of grain from the slightly heated to the strongly heated end. The celebrated Rinman has made many experiments on the qualities of steel exposed to different degrees of heat in tempering, but particularly to three kinds, viz. steel heated to an obscure red, to a bright red, and to a red white. Hard brittle steel, made by cementation, and heated to an obscure red and tempered, exhibited a fine grain, somewhat shining, and was of a yellow white colour. When tempered at a bright red heat, the grain was coarser and more shining; when tempered at a red white heat, the grain was also coarse and shining.
With a view to determine how far steel might be improved in its grain by tempering it in different ways, M. de Morveau took a bar of blistered steel, and broke it into four parts nearly of the same weight. They were all heated to a red heat in the same furnace, and withdrawn from the fire at the same instant. One of the pieces was left at the side of the furnace to cool in the air, the second was plunged into cold water, the third into oil, and the fourth into mercury. The piece of steel that was cooled in the air resisted the hammer a long time before it was broken; it was necessary to notch it by the file, and even then it was broken with difficulty. It showed in its fracture a grain sensibly more fine and more shining than it was before. The second piece, which had been plunged into water, broke easily: its grain was rather finer than the first, and almost of the same white colour. The third piece, which was tempered in oil, appeared very hard when tried by the file; it was scarcely possible to break it. Its grain was as fine, but not quite so bright, as that which was tempered in water. The fourth piece, which was dipped into mercury, was evidently superior to all the rest in the fineness and colour of the grain. It broke into many fragments with the first stroke of the hammer, the fractures being generally transverse.
M. de Morveau was not altogether satisfied with these experiments, and therefore thought it necessary to repeat them with finer steel. He took a bar of steel two lines square, such as is used in Germany for tools by engravers and watchmakers; he divided it into four pieces, and treated them in the same way as he had done the blistered steel. The first piece, which was cooled in the air, it was very difficult to break: the fracture appeared in the midst of the grain very fine, but white and shining. The second, which was tempered in water, was broken into three fragments at the
first blow; its grain was perfectly equal, of a grey ash-colour, and of remarkable fineness. One of its sides was polished, and a drop of the nitrous acid which was poured upon it left a black spot, but not deep. But when a drop of the same acid was poured on the middle of the fracture, after it had been equally polished, it left a black spot much deeper. The third piece, which was plunged in oil, bent as easily as the piece which was cooled in the air; the file made an impression on it with difficulty; it was necessary to break it with a vice; its grain was inferior in fineness to the second, but it was of a darker colour. The fourth, which was tempered in mercury, exhibited a grain of an intermediate fineness between the second and the third. From these experiments, it appears that steel may be hardened by tempering it, not only with water, but with any other liquid which is capable of accelerating its cooling.
Steel may be unmade, or reduced to the state of iron, by a management similar to that by which it is made, that is, by cementation. But the cement used for this purpose must be composed of substances entirely free from inflammable matter, and rather capable of absorbing it, as calcareous earth or quicklime. By a cementation with calcareous earth, continued during eight or ten hours, steel is reduced to the state of iron. After it has been tempered, it may be again untempered, and softened to any degree that we think proper; for which purpose we have only to heat it more or less, and to let it cool slowly. By this method we may soften the hardest-tempered steel.