As good iron is the basis of good steel, all that has been said under the articles IRON and SMELTING in reference to the sources and means of procuring that metal in a state of purity, may be advantageously referred to as a preliminary to the present.

Steel is a carburet of iron, with probably a slight mixture of other substances which are more or less essential to its perfection, and certainly in most cases with some alloy which is not essential, but which, on the contrary, is to some extent injurious. This description would equally apply to cast-iron, which differs from steel as to its ingredients principally in the quantity of carbon, cast-iron having sometimes one-fifteenth part, and good steel seldom more than one two-hundredth part of that substance. The difference between the proportions of the carbon does little, however, to explain the difference between cast-iron and steel; for, while the condition of cast-iron is retained, it is found that diminishing the quantity of carbon renders it still less like good steel. It appears, in short, that the good qualities of steel—and they are very various—depend upon circumstances partly chemical and partly mechanical, which have hitherto defied analysis. It is not even precisely known whether the union of the iron and carbon is a chemical or mechanical union; perhaps it may be partly one and partly the other, for reasons which will presently be given.

In consequence of this ignorance as to what constitutes the essential qualities of good steel, the processes by which favourable results have been obtained have in nearly all cases been empirical, and in many instances have been real or pretended secrets. The processes are of a nature to forbid any very nice calculations, and they are liable to great and unappreciable modifications in the execution. For example, steel being, as before stated, a carburet of iron, and having generally a slight admixture of oxide of iron, must be subject to many incalculable changes during its successive exposures to violent heat in contact with carbonaceous fuel and atmospheric air. Moreover, the hammering, on which many of its good properties depend, is obviously an operation which cannot be meted out with very scrupulous nicety, and is besides liable to be very much influenced by the temperature of the metal and by the direction of the blows in reference to the mechanical structure of the mass.

A good practical essay on steel, it is therefore evident, would consist in an exceedingly minute detail of the actual operations applied to a certain description of ore, or to a known specimen of manufactured iron, which, with certain sorts of fuel, had been found uniformly to produce steel peculiarly adapted to certain purposes. Such an essay would form a volume, and it would still convey imperfectly what it professed to teach, because in all the processes there are certain stages of the conversion whose advent is judged of by the experienced eye and hand of the skilful workman, from symptoms which can be explained only to the sight and touch. Here we only propose to describe, in very general terms, some of the principal processes, so as to convey a knowledge of the theory of steel-making without professing to give the actual practice. We must premise, that the destination of the steel is of great importance in estimating even the theory of those processes, as may be well supposed when it is recollected that a lancet will fracture almost like glass, while a bricklayer's trowel is required to cut the most refractory lump of semi-vitrified clay in the shape of a brick. These two instruments are perhaps at the extremities of the scale, the perfect hardness and brittleness of the lancet contrasting with the extraordinary toughness and tenacity of the trowel.

It was at one time, indeed, thought so difficult to combine these last-mentioned qualities with sufficient hardness to sever a good stock-brick, that trowels were made of a plate of iron to supply the toughness, and an edge of steel to give the hardness. Even at the present time it is supposed that the peculiar qualities of certain sword-blades result from their being combinations of hard steel with soft tenacious iron fibre; and that the variegated or damasked surface of such blades is owing to the different appearances presented by the iron and steel. By some this effect has been supposed to result from chemical changes acting partially upon the original carburet, depositing the carbon more profusely in some parts of the iron than in others. It may arise, as already hinted, from some portions of the carburet being in a chemical, and others only in a mechanical state of union.

According to other authorities, the structure in question has been manufactured expressly by binding up portions of soft iron wire with ingots of steel, and hammering the whole into a mass at a high temperature. Such a process will, it is known, produce very similar appearances. Whatever be the truth in regard to the sword-blades, certain combinations of iron and steel in parallel laminae are advantageously employed for some purposes. The carpenter's plane-iron, for example, consists of a very thin hard steel face on an iron back; because this instrument requires to unite a cutting edge nearly equal to that of the lancet with a tenacity which shall encounter unjured the hardest knots; a trial almost as severe as that applied to the trowel.

One great cause of the uncertainty and obscurity attending the practice of steel-making, is the importance of the hammering or other mechanical parts of the operation. If the distinction between good and bad steel were principally chemical, the production of the former would long since have been rendered easy and determinate. How little this is the case may be inferred from the fact, that the elaborate series of experiments conducted a few years by Dr Faraday and Mr Stodart have scarcely added a new fact to the science of steel-making; while, on the other hand, the immense value of mechanical action is shown, among numerous instances, by the increased strength of drawn iron wire as compared with rolled iron of equal size, a difference amounting sometimes to sixty or seventy per cent.

The remarkable qualities of the trowels for which Mr Walby was celebrated about forty years since, resulted in a great measure, if not entirely, from good and rapid hammering at a moderate temperature. The object of hammering being to condense the steel, it is evident that when at a white heat, in a state approaching to fusion, the mass is so plastic, it yields so freely, that hammering is perfectly inoperative, except to change the external form; while, on the other hand, if the mass be cold, it is so unsusceptible of what may be called intestinal movement among its particles that the most violent hammering can do little more than dislocate portions of the surface, which will accordingly crack or scale off. Between these extremes a medium may be found, and was, we believe, found by Mr Walby. His hammering was principally performed at a low cherry-red heat; and, by means of a peculiar and ingeniously mounted hammer of considerable weight, he was enabled to do all that was required before the temperature was sensibly lowered. But, as we are informed, he did more than this. It is quite certain that in hammering any mass, and especially in a thin plate, the central cannot be under the same circumstances as the exterior portions. Not only will the centre retain its heat somewhat longer, but, what is of more consequence, the tendency of the central portions to spread laterally under the hammer, is resisted by the marginal parts; while these latter not being so protected by a bolt, spread freely, and perhaps separate into detached pieces. If, for example, a circular disc of steel at a low temperature were violently beaten under a flat hammer, it would be very much condensed in the middle, while the circumference would gradually separate, showing radial splits or cracks. The most perfect way to condense a circular disc of metal would obviously be to confine it in a very strongly defended cavity, whose walls should prevent all lateral spreading, and thus the full effect of each blow would be felt in condensation. Such a process is, however, inapplicable to trowel making, and perhaps to all other purposes except the striking of medals, where we see it employed; but Mr Walby obtained nearly all the effect of such an arrangement by forging each trowel considerably larger than it was ultimately intended to be, and cutting off about an inch of superfluous metal after the hammering was completed; cutting off, that is to say, the wall which had acted to restrain the spreading of the central portion of the blade, and which had probably become loose and spongy itself for want of such restraint, thus leaving only the close compact metal in the finished trowel.

Having thus endeavoured to direct the reader's attention to some of the qualities demanded in steel, and to the causes which affect their production, we shall briefly describe some of the operations.

Steel is most frequently made from rolled bars of good, by which we mean pure iron. To communicate to these bars the desired quantity of carbon, they are formed into bundles, and are placed in a large store or furnace alternating with layers of carbon (hard-wood charcoal is preferred,) and a high temperature being maintained for a week or ten days, the iron gradually absorbs the required quantity of carbon, and becomes converted into steel. The completeness of this conversion is judged of from time to time by the examination of certain of the bars, which are so disposed as to be accessible for this purpose. If the carbon has not penetrated to the centre of the metal, this will be evident on breaking the bar transversely, as the section will exhibit a colour in the centre different from that near the surface: it will show what the workmen call a pith. Towards the end of the process, the watching requires to be skilful and constant, because, if the absorption of carbon becomes very excessive, the metal may be rendered so fusible as suddenly to melt; and though this would be of little consequence in a sound crucible, it would be attended with great loss in a large stove or hearth. The surface of the bars becomes so nearly in this condition that it is always blistered by the escape or rarefaction of air or gas from the interior of the metal; and hence bars so prepared have acquired the name of blister or blistered steel. The process itself is called cementation.

The bars thus prepared do not differ very greatly from cast-iron, except in the smaller quantity of carbon which they contain, and in their freedom from impurities. They have somewhat more malleability and tenacity than cast-iron, but not so much as is imparted to that substance during its conversion to bar-iron, and which must now, except for very coarse purposes, be communicated to these bars of cementation—without, however, depriving them of their carbon. With this view the bars so prepared are broken up into short lengths, are made into bundles, heated almost to a white heat, hammered, welded together, re-broken and re-hammered till they are reduced as nearly as possible to a compact homogeneous mass of greater specific gravity than in their former state. In all these weldings, care is required to preserve the surfaces clean and unoxidized, as upon this depends the perfect union of the two surfaces.

This care is dispensed with in the processes for making cast-steel, the nature of which has been already indicated in describing that of cementation. The pure iron and a certain proportion of carbon are fused together in a crucible, and being cast into ingots, these are treated somewhat like the bundles of the cemented bars. They are hammered at a high temperature till they are rendered malleable and dense, and till a certain portion of the carbon is displaced, that substance being generally in excess. Various modes of applying the carbon have been proposed; but it is very difficult to determine in the abstract which of these is the best. One mode of application is, by the introduction of a stream of gas. Cast-steel is free from the defects which are liable to attend the imperfect welding of the bars, and is likely to supersede all others for the finer purposes of cutlery. It requires, however, the most skilful manipulation as the point of sufficient fusion is reached, and this must be performed under the most severe exposure to heat; so severe as to demand that the workmen should be protected, by clothing of wetted sackcloth, from the joint effects of the opened furnace and the glowing crucible. This combination of skill and severe labour secures high wages, and enhances the price of the article.

The experiments of Faraday and Stodart, before alluded to, were undertaken not so much to improve the mode of manufacture as to determine the effect of various alloys; it having been inferred, partly from the condensation which frequently accompanies chemical unions, and partly from the examination of certain specimens of steel which were known to possess good qualities, that a small portion of foreign matter might be beneficially introduced. With this view, alloys of steel with gold, silver, platinum, and many other metals, in various proportions and combinations, were made, without however producing any very definite results. One of the best was a mixture of 500 parts of steel with one part of silver. While the silver was in this small proportion, it appeared to diffuse itself equally throughout the mass, and the union appeared to be chemical. No remarkable increase of specific gravity accompanied the union; and the opinion formed at the time, that the steel was improved by the addition, is beginning to be shaken. It is worth mentioning, that if the quantity of silver much exceeded the five-hundredth part, the excess was distributed in small distinct fibrous masses, in such a manner as to unfit the steel for most purposes of utility.

Having by any of the means now mentioned manufactured a good steel or a good alloy, it is found to have acquired a property on which its great value in the arts entirely depends: we mean the property of becoming very hard when suddenly cooled from a high temperature. If heated to whiteness, and then plunged into water, instead of the malleable tenacious substance which had been produced under the welding hammers, it has become nearly as hard as a diamond and as brittle as glass. The sudden cooling from a white heat would, however, be found to have damaged considerably the compactness of the steel; and practically it is known, that the lower the temperature on which the sudden cooling can be made to act with effect, the better will be the quality of the metal. It is also supposed that the lower the temperature at which the steel is manufactured—that is to say, hammered into compactness and malleability—the lower will be the temperature necessary to give it hardness. When all these points have been determined to the greatest exactness, there remains another consideration which has been well pointed out by a recent writer on the subject. Since the perfection of every steel instrument depends upon its receiving a hardness very exactly proportioned to its intended use, and as the subsequent operations will equally affect every part of the hardened steel, it is most important that the hardening be equal in every part, in order that the hardness of the whole, when reduced, shall be equal. If then the heated steel, at the moment of its plunge into the water, be in some parts coated with an oxyde of iron while in other parts it is clean, the rapidity of the cooling in the various parts will be so different as to affect seriously the uniformity of the result. In this way, among others, we can understand how it happens that one part of a razor or other instrument shall be much harder than another. To avoid this inequality, it has been proposed that the instrument shall be perfectly cleansed upon a stone previously to hardening, and then heated with the utmost care to prevent the formation, or at least the unequal formation, of oxyde upon the surface. Various opinions and practices prevail as to the mode of cooling. Besides water at various temperatures, saline solutions, mercury, and a current of air, have been suggested; but it is doubtful whether any of them is preferable to water at a moderate temperature.

The steel having now been made indefinitely hard, too hard generally for any practical purposes, it requires to be tempered or softened down more or less, according to the nature and uses of the intended instrument. This is done by the application of a moderate heat, varying from $430^\circ$ to $600^\circ$ of Fahrenheit; the higher temperatures softening the metal proportionally more than the lower.

Formerly the heat applied in tempering was judged by the colour assumed by the steel, a portion being always ground clean, to enable the operator to observe exactly when the required shade of colour was obtained. These colours, it may be stated, are produced by the action of the oxygen in the air upon the heated metal; and though their indications are sufficiently accurate for most purposes, it has been found desirable in some cases to substitute a fusible metallic bath, by which a regulated temperature may with certainty be communicated.

The following table, showing the temperatures according to Fahrenheit which correspond with various colours, was drawn up by the late Mr Stodart:

1. Very pale straw yellow ........................................... $430^\circ$ 2. A shade of darker yellow ........................................... $450$ 3. Darker straw yellow .................................................. $470$ 4. Still darker straw yellow ............................................. $490$ 5. A browner yellow ...................................................... $500$ 6. Yellow slightly tinged with purple ................................ $520$ 7. Light purple .................................................................. $530$ 8. Dark purple .................................................................. $550$ 9. Deep blue .................................................................... $570$ 10. Paler blue ................................................................... $590$ 11. Still paler blue ............................................................. $610$ 12. Still paler blue, with a tinge of green, ......................... $630$

When the hardened steel is heated only within the limits here specified, it matters not how it is cooled; the softening or tempering equally takes place, and is proportioned to the temperature. The knowledge of the proper degrees of heat or colour adapted to various instruments, is obtained wholly by experience. The following table, extracted from the work before mentioned, is perhaps as accurate as any:

| Instruments | Colours | Temperatures | |-------------|---------|--------------| | Razors and instruments with a stout back and fine edge | Straw colour | $430^\circ$ to $450^\circ$ | | Scalpels and penknives | Full yellow | $470^\circ$ | | Scissors and small shears | Brown yellow | $490^\circ$ | | Pocket and pruning knives | First tinge of purple | $510^\circ$ | | Watch-springs, swords, &c. | Purple | $550^\circ$ to $560^\circ$ |

After the operation of tempering, nothing is required but the final grinding, fashioning, and polishing of the article, which it is not here requisite to discuss. (C. K.)

STEEL-YARD. See WEIGHING-MACHINES.