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will fuftain from the abundance of fcales which will fly off in the working. The way to meliorate fuch jitel as this, mull: be to diveft it of part of its falts and its "fulphurs, but peculiarly of the laft ; and Mr. Reaumur found, that the bu- rying the bars of fuch fteel in lime, or any other alkaline fubftance that would readily abforb the fulphurs, and placing it for a proper time in the fire, it would be in a manner de- compofed again, and come out a very good and perfect /ta/; and this is no trivial proof, that the account given of the manner of iron's becoming fted is the true one. By this management//^/ may again be converted* or re- duced to its primitive iron, and a body of any middle degree, between fteel and iron, may be produced, by flopping this procefs at different points of time, or continuing it till all the adventitious falts and fulphurs are drawn off or abforbed. Hardnefs and flexibility are the two great points in this ope- ration, and what the metal gets of the one of thefe^ .it af- furedly lofes of the other. The greater hardnefs is from the greater quantity of the adventitious falts and fulphurs ; and the greater flexibility, or, as the workmen call it, the greater hoM % is from the greater quantity of the metallic ; and he who is able to calculate properly the quantities of his fait, charcoal, and foot, and to regulate exactly the degree of fire, is capable to make fteel of what degree of temper or per- fection he pleafes.
Iron, impregnated with new falts and new fulphur^ U not however perfectly reduced to the {rate of what the workman expects in fteel; there wants yet the quenching it while hot in cold water, to compleat the operation. The fteel pene- trated every way by the particles of fire, and at that inftant plunged into water, is by that quenching ftopt in the very ftate in which it was, and is not fuffered to exhale the ig- neous bodies, as it would otherwife have done. It was, while in the fire, rarified and dilated, and this quenching continues it in that ftate, and it is found, on meafuring, confiderably thicker and larger than it was while unheated. It miwht be fuppofed from hence, that the particles of fire, which were lodged in the fteel while red-hot, were thus de- tained and imprifoned in it, and fo wrought the change, as they are known to remain in many calcined bodies, though they have not been fo fuddenly quenched ; but if this were the cafe, the fteel muff be encreafed in weight as well as bignefs, as many calcined bodies are, but this is not found to be fo in fact.
Recourfe muff be had therefore to fome other operation on the fteel, and this appears, in reality, to be no other than the change of its texture, or internal ftructure. If a body be naturally compofed of a number of particles, which are in themfelves hard and compact, but between which there are certain fpaces ; if one take from thofe hard and compact particles fomewhat to fill up thofe voids, it. muff appear very evidently, that though by this the proper particles are rendered lefs hard, yet the whole body muff be made harder than it was before.
In the converting of iron into fteel, Mr. Reaumur conceives that the particles of iron, being naturally very covetous of falts and fulphurs, have imbibed great quantities of thefe ad- ventitious bodies, while the fpaces between them have been able to retain very few. This being then the firft ftate of the fteel, when it is afterwards heated, in order to its being tempered by quenching, the fire drives thefe fulphureous and faline particles, fo abundant in the iron, out of its granules, and difperfes them throughout the fpaces between thefe gra- nules ; and that it is to this equal and regular diftribution of thefe falts and fulphurs, fixed in that ftate by the imme- diate quenching, that the hardnefs, and other qualities of fteel are owing.
To the advantage the metal gets by this means, there is however always joined a difadvantage, which is, that its grain naturally and necelfarily becomes coarfer by it, and the fteel has the lefs body. The hardeft fteel always muff neceflarily have the leaff body ; but according to the rules laid down by Mr. Reaumur, one may dilpofe the hardnefs in what degree one pleafes, and confequently all the other properties are in the direction of the fame laws. If the oppofition of the qualities of fteel be nicely confider- ed, we fnall find, that inftead of being hurtful, they are truly what fhould be molt wifhed for, and are productive of the moft deferable confequences. It may appear fomewhat ftrange, that hardnefs and flexibility in jted fhould be fuch directly oppofite and contradictory principles, and that the quenching, which makes fteel more powerful to relift preffure or rubbing, fhould at the fame time make it more feeble in action ; but this is plainly the cafe, and a piece of fteel wire, which, before it is tempered by quenching, will, when hung vertically, fufpend a certain weight, will not fuftain the fame weight when tempered, but will be broken even by a lefs; and that if the trial be made by tempering it up to one certain part, and no farther, the wire will affuredly break at that very part where the tempered and untempered porti- ons meet. The encreafe of fize in every grain of fteel, from the quenching, gives the plain folution of this phe- nomenon. The rupture of any body whatever, that is, the feparation of its parts, by whatever means it is done, muft j
be mere difficult, the more intimately thofe parts touch, or the more points they touch in. The granules of fteel are thefe touching parts, and it is very evident that thefe gra- nules becoming larger on the quenching, muft after this touch in fewer points, in the whole combination of the mafs ; and on the other hand, as they are larger, they alfo muft mutually touch each other in a larger fpace. Here are two contrary principles, which conftitute the facility and the difficulty of the rupture by different means ; and it cannot be, but that the one of thefe conftructions muft give the greater force to refift preflure or friction, and the other to refift drawing or traction.
It has been already obferved that fteel is the harder, accord- ing as it was more or lefs hot at the time when it was plunged into the water; and it muft be added to this, that it alfo is the more fo, the more cold the water is into which it is plunged. The degree of heat in the fteel is eafily known by its colour, and the diftinctions in it are familiar to the workmen. Several perfons have thought there was great virtue to be communicated to the water by means of different ingredients ; this Mr. Reaumur carefully tried, and the refult was, that he found common cold water the beft, and moft ufeful of all liquors : it is true, that vinegar and verjuice harden the fteel fomewhat more, and aqua fortis greatly fo, but the latter of thefe renders it too hard for fervice ; and repeated trials have evinced, that common water alone will give fteel all the hardnefs it can be wifhed to have, pro- vided only that it be plunged into it at a time when it is fufficiently hot.
If fteel, after the tempering, be found too hard, there is a very familiar way of bringing it back to what ftate one pleafes, between that and iron, which is only the heating it in the fire ; for it may be kept in the fire fo long, as to be reduced wholly to iron again. It is eafy to infer from hence, what was before obferved, that caft iron is fteel of a. peculiar kind ; its properties plainly evince, that it is fted with an over proportion of all that makes it fo, and confe- quently of all its properties. It is not malleable, is very brittle, and too hard for the file, or any other too], to cut. Thefe are the qualities of fteel which is over tempered, or, as it may be called, too much fteel: it owes thefe qualities to its being overcharged with thofe fulphurs and falts, which in a due proportion makes fteel of iron. The method of reducing this to the ftate of wrought iron, is the fame with what wc have before obferved is to be ufed to untemper too high tem- pered fteel, that is, keeping it in the fire till part of its /alts and fulphurs are driven off; and if the fire alone be thought too difficult, or too tedious a method, Mr. Reaumur has found that thofe alkaline fubftances, which naturally abforb' fulphurs, will affift the reducing this to the ftate of wrought iron, as they do to the untempering of fteel, and that the very fame fubftances of this kind, which have been found beft in the untempering fted, fucceed beft in the render- ing this malleable; fuch are lime, chalk, and the like. Caft iron may be taken for thefe operations' in two ftates ; the one, that of fimply melted iron in its firft fufion, the other that of the iron, which has been run into the mould. When a caft piece of iron, in a certain form, is heated again with the calx of animal bones, which is one common way, this being a fubftance extremely divefted of falts and fulphurs, it abforbs thofe of the metal too greedily, and divefts it of too great a quantity of them ; fo that when the piece comes afterwards to be filed, the fragments fly off in large fcales, and the beauty of the caft figure muft be loft : to remedy this difficulty, fome powdered fulphur is to be added in the heating ; for this being a fulphureous and faline body, checks and moderates the effects of the calx of bone. Chalk in the fame manner can only be employed with cer- tain regulations; it fucceeds very well, if the fire be not too violent, or too long continued, but if it be exceffive in either of thefe particulars, the chalk throws into the body of the iron falts and fulphurs of its own, which lay hid in its ftructure, and which would not have been difiodged by a fmaller fire; but which being difiodged, and thrown into the iron, act upon it, and render the fuccefs of the operation quite different from what was intended, the iron becoming more hard and more brittle by it.
One great nicety in all thefe operations, is the underftanding the figns by which it is known at what time caft iron is fuf- ficiently foftened, or over tempered fted is fufficiently re- duced toward the ftate of iron again ; but this is only to be acquired by nice obfervation, and on tins depends the whole certainty of the procefles, fince every minute's continu- ance in the fire gives the operation a different turn in de- gree.
Mr. Reaumur, in one of his experiments in the fofening a piece of caft iron of a beautiful figure, found, on taking it out, that it had loft prodigioufly.of its weight, and coming to a nicer examination, he found that it was hollow; its outer cruft having been fooner converted into malleable, that is unfufible, iron than the reft, and the violence of the fire afterwards having melted away thejnner part, which was not yet fo changed. This accident led him to experiments, by which he found that he could, at any time, mrlt out r the
