« 이전계속 »
plate, shivered into nearly forty bits, individual has thus been warmed and that are now on their owner's shelves, allowed to cool at least a thousand times. without any visible mark of a crack. At what stage in the prucess the chemiThe cement that was used is that known cal change was complete it is impossible as the “Mend-all cement"; but, of course, to tell. Very likely in twenty or thirty the skill goes for much more than the days. But slowly and gradually the choice of material.
cement has so parted with some element The artist in question, however, has of solution as to vitrify, or, at all events, long been careful to procure the smallest to unite itself with the glass in a union possible bottles of the cement; and that indissoluble by the agency of hot water, for the following reason: The material the original solvent of the preparation. is prepared for use by dipping the bottle We cannot bake walls, unless it be by containing it into hot water. It then selecting very hot weather for their conbecomes fluid, and is easily applied to struction. On the other hand, we can the edges to be joined, the fragments avoid that soaking of the bricks, if rain being themselves kept in hot water, and happen to come on during the process of taken out and wiped before being building, of which no notice is now usucemented and put into place. If a large ally taken. We know of ore costly number of mends are made, and the building, of which the color is permanwhole affair gets out of shape—which it ently and entirely ruined by this neglect. is very difficult in that case to avoid - Our object is to induce the Royal Instithe mended article is again put in very tute of British Architects, the Institution hot water, when it comes to pieces at of Civil Engineers, or others whom it once, Each piece has to be carefully may concern, to investigate this importwiped and re-cemented.
ant fact of the behavior of lime or other The objection to large bottles of cement. What is the effect of time on cement is this — hitherto a perfectly their cohesive force? Should time be empirical one:- After the cement has allowed to elapse before the use, as is been liquefied and cooled a certain num- the case in Italy? Should it be comber of times, it becomes refractory. It bined with repeated rise and fall of no longer liquefies easily, if at all. It temperature, as in the case of the Mendmakes bad mends, or is altogether useless. all cement and of the lamp-glass that we
To this long-observed fact, a second have cited? Will an exposure of a certain observation has been added, within the duration to a steady heat have the same last few days, which is highly significant. effect? And, if so, what heat, and for The bell of a “Queen's” reading lamp, how long applied? These are questions which had a large piece broken out of it of primary interest to the architect. some years ago, was mended by the Their solution may place a new material artist in question, and has been in niglit- within his grasp. He may be enabled to ly use ever since. After the fashion of rear those egg-shell domes which the immended things, this glass shade has agination of Bulwer fashioned for the outlived its more robust brothers. But i homes of the “Coming Race." We can the line of junction was visible, and the never tell how far a new idea may carry repairer thought that it would be an im- us. We strongly suspect that the build provement to wash the bell, remove the 'ers of the earliest bridge over the Tiber, old cement, and re-cement it. The bell, or of the walls of Rome-certainly that however, thought otherwise. To the the builders of the Roman wall of Lonsurprise of the operator it laughed at don—had a much clearer insight into the the hottest water, and absolutely re- relation between the time spent in prefused to allow the crack to re-open, or paration and the permanent durability the cement in any way to soften or dis- of work, than has any writer or lecturer solve.
on structures at the present day. We This phenomenon is worthy of reflec- are not now speaking of those “Jerry tion. It is impossible to disconnect it builders” of whom a contemporary justly from the behavior of the cement in the observed, àpropos of this very discovery bottle. The shade of a queen's lamp of a Roman relic, that the Romans would does not get so hot as to burn one's have thrown them from the Tarpeian fingers, but it gets warm. The mended rock. We are speaking of thoughtful
and experienced men. We are quite specifications. We know not on whom sure that Isambard Kingdom Brunel, now to place confidence for an equally were he now alive, would respond to our earnest and practical desire to reply to words by an elaborate series of experi- such a question, but we trust that each ments; and that if the result confirmed and all of our friends who have the the practice of the Italian builder, he opportunity will do their best to aid in would then and there have re-drawn his the formulation of the answer.
THE MANUFACTURE AND USES OF CAST STEEL.
From the “ English Mechanic."* The first question which the user of ers of steel who knew the trade before cast steel has to answer is, to decide which the introduction of spiegel iron into of the three great methods of making steel Sheffield, and it is remarkable how many produces a material best adapted to his of them expressed the opinion that the own wants. Sir Henry suggested to you, crucible cast steel now in use is not in his admirable lecture, that Bessemer as good as it was when they were steel would answer every purpose for young. which steel is used, with the possible To obtain sound ingots from high-class exception of the steel required to make iron it is necessary to boil the steel for Canadian axes. There can be no doubt nearly half an hour after it has become whatever that the Bessemer steel, of fluid, and then to allow it to cool down which Sir Henry gave you the first to a certain temperature before it is analysis, would have made an excellent poured into the mould. The process is Canadian axe had it contained the proper called “killing” the steel, and it is an quantity of combined carbon. The only axiom amongst them that the higher the doubt I feel is whether it could be pro- quality of the steel the more “killing" it duced of sufficient soundness without so takes. It is in this part of the process large a percentage of waste as not to of crucible cast steel-melting that the raise the price beyond that at which virtue of the process consists; and the crucible cast steel for Canadian axes is cost and quality of the cast steel pronow sold. I must confess that my ex-duced depend in a large degree upon perience of Bessemer cast steel would the skill brought to bear upon it. My incline me to say that it could not. In theory is that the reason why high-class spite of the prejudice that exists among steel has to be so long boiled is to get consumers of steel, such is the competi. rid of its occluded gas, which would tion of the present day that I am sure otherwise produce bubbles or “honeythat if steel could be produced of as combs” in its attempts to escape. The good quality, and cheaper in price, by addition of a portion of scrap steel much any other process than that of melting assists the “killing,” as would naturally in crucibles, the present melting furnaces be the case if we suppose the scrap, of Sheffield would rapidly melt away in which has been melted before, to have to old bricks and mortar. I venture to parted with its occluded gas in the first express the opinion that the reason that melting. That the presence of manganhigh-class Bessemer steel is not as good ese or silicon helps largely to “kill” the as high-class crucible cast steel is because steel, I account for on the theory that the former cannot be made sufficiently the carbonic acid unites with the mansound without the admixture of silicon ganese or silicon, and becomes a solid. and manganese, both of which substances So far, my theory appears to hold water are injurious to cast steel for most pur- pretty well; but when I come to the fact poses. I fear that the advantages sup- that low-quality cast steel—for example, posed to be derived from the use of steel melted from Bessemer rail scrap, manganese in the manufacture of cast which contains from 0.15 to 0.05 per steel are, to a large extent, illusory. I cent. of phosphorus—does not require have frequently conversed with consum. any“ killing” at all, and may be poured
into the mould as hot as the strength of * Abstract of a lecture by Mr. Henry Seebohm.
the crucible will allow, I am obliged to ened part, such as cold setts, which have admit that I am not chemist enough to to stand the blows of a very heavy hamgive you an explanation of the cause. mer. The main point which I wish to impress Die Temper (i per cent. carbon).upon you is that the much-maligned rule The most suitable temper for tools where of thumb, which insists upon the superi- the suriace only is required to be hard, ority of crucible cast steel over Bessemer and where the capacity to stand great steel for certain purposes, may have a pressure is of importance, such as stampscientific basis, and must not be hastily ing or pressing dies, boiler cups, &c. set aside as prejudice.
Both the last two tempers may be easily Having decided by what process the welded by a mechanic accustomed to steel is to be made, the next question weld cast steel. that should come before the consumer of Next to quality, by which is meant the cast steel is the percentage of carbon percentage of phosphorus, sulphur, siliwhich he wishes it to contain. When I con, manganese, &c., the most important first began. business, the “temper” of thing is temper, or percentage of carbon. steel, or the percentage of carbon which For many purposes, indeed, temper is of it contained, was concealed from the more importance than quality. Nothing consumer. The following is a list of the is more common than for steel to be most useful “tempers” of cast steel : rejected as bad in quality because it has
Razor Temper (13 per cent. carbon). - been used for a purpose for which the This steel is so easily burnt by being temper was unsuitable. overheated that it can only be placed in When the steel has arrived in the the hands of a very skillful workman. user's hands, the first process which it When properly treated, it will do twice undergoes is the forging it into the the work of ordinary tool steel for turn- shape required. This process is really ing chilled rolls, &c.
two processes. First, that of heating to Saw File Temper (13 per cent. car- make it malleable, and second, that of bon). - This steel requires careful treat- hammering it, while it is hot, into the ment; and although it will stand more required shape. The golden rule in fire than the preceding temper, should forging is to heat the steel as little as not be heated above a cherry-red. possible before it is forged, and to ham
Tool Temper (14 per cent. carbon).- mer it as much as possible in the process The most useful temper for turning of forging. It is impossible to lay down tools, drills, and planing-machine tools exact rules for each of the thousand-andin the hands of ordinary workmen. It is one tools in which steel is used. The possible to weld cast steel of this temper, treatment of each tool, in each process but not without care and skill.
wbich it undergoes, is an art that can Spindle Temper (1} per cent. carbon). only be learnt by practice, and can no -A very useful temper for mill-picks, more be taught in a lecture than the arts circular cutters, very large turning tools, of skating, riding, or swimming. The taps, screwing dies, &c. This temper utmost that can be done is to lay down requires considerable care in welding. certain general rules, and, if possible, to
Chisel Temper (1 per cent. carbon).— attempt some scientific explanation of An extremely useful temper, combining, them, to elevate them above the despised as it does, great toughness in the un position of rules of thumb. The worst hardened state with the capacity of hard fault that can be committed is to overening at a low heat. It may also be heat the steel. When steel is overheated welded without much difficulty. It is it becomes coarse grained. Its silky consequently well adapted for tools, texture is lost. If the temperature be where the unhardened part is required raised above a certain point, the steel beto stand the blow of a hammer without comes what is technically called “burnt," snapping, but where a hard cutting edge and the amount of hammering which it is required, such as cold chisels, hot would require to restore its fine grain salts, &c.
would reduce it to a size too small for Sett Temper (i per cent. carbon). - the required tool, and the steel must be This temper is adapted for tools where condemned as spoilt. Overheating in the chief punishment is on the unhard the fire is the primary cause of cracking
in the water. The quality of steel may which we have destined it, and whatever be so bad, i. e., the percentage of phos- injury we inflict upon it by overheating phorus in it may be so high, that the is irrevocable, and can no more be cured amount of heat absolutely necessary to or mitigated by the hammer. We must, forge it at all into the shape required therefore, double and redouble our care, may cause it to crack in hardening. One lest the temperature be raised above the of the principal reasons why a high-class point necessary to insure the required quality steel is required for certain pur- hardness. The part of the tool required poses is that it will suffer less injury by to be hardened must be heated through, being heated to a greater degree, or by and heated evenly, but must on no acbeing heated and reheated a greater num- count be overheated. Our tool must be ber of times than inferior qualities of finished at one blow—the blow caused steel. In heating steel the happy medi- by the sudden contraction of the steel um must be attained between heating it produced by its sudden cooling in the too much and too little, and between let- water; and if this blow is not sufficient ting it lie too long “soaking” in the fire, to give to the steel a fine grain and silky and not soaking it through: Both the texture; if, after the blow is given, the degree of temperature and the duration fracture, were it broken in the hardened of the heat must be carefully watched. part, should show a coarse grain or dull Some tools, such as circular cutters, files, color, instead of a fine grain or glossy &c., after they are forged into the shape luster, our tool is spoiled, and must be required, must have teeth cut into them. consigned to the limbo of “wasters." Before this can be successfully accom- The special dangers to be avoided in plished, a preliminary process has to be thardening each kind of tool must be gone through. The process of hammer- learnt by experience. Some tools will ing or forging the steel into the shape warp or “skeller," as we say in Yorkrequired has hardened the steel to such shire, if they are not plunged into the an extent as to make the cutting of teeth water in a certain way. Tools of one into it impossible or difficult. It must shape must cut the water like a knife, consequently be annealed. This process, those of another shape must stab it like like the preceding one, is a double pro- a dagger. Some tools must be hardened
The steel must be reheated as in a saturated solution of salt, the older carefully as before, and afterwards the better, whilst others are best hardcooled as slowly as possible. Many ened under a stream of running water. tools are only required to be hardened Most tools have a tendency to wateron a small part of their surface, and it is crack if taken out of the water before important that the unhardened parts they are absolutely cold. Where the should possess the maximum amount of edge of a tool only is hardened, care toughness, the minimum amount of should be taken to move it up and down brittleness, that can be attained. These in the water, so as continually to change tools must also be annealed after they the water level, lest the tool should are forged. The process of annealing, crack at the water level. Steel contracts or slow cooling, leaves the steel coarse in hardening, and contracts most when grained, gives it its maximum of ductili- it is cooled most suddenly. If the hardty, and causes it, in fact, to approach the ened part join on to the unhardened part properties of lead.
too suddenly, the steel at the junction We now come to the culminating point will be in a dangerous state of tension in our manufacture, where the invaluable which predisposes it to crack, and it is property which distinguishes steel from wise to lessen the amount of tension by wrought iron or cast metal is revealed, a distributing it over as great an area as process by which we suddenly change possible. In some tools where the shape our steel from lead into glass—the pro- necessitates a great difference in the cess of hardening. In this, as in all rapidity of cooling, it is wise to drill other processes which steel has to under- holes in the thicker parts where they go, we have to run the gauntlet of fire. will not interfere with the use of the We do so, however, at greater risk than tool, holes which are made neither for heretofore. The forging of our tool is use nor ornament, but solely with the finished; it has taken the final shape to view of equalizing the rapidity of the
cooling of the various parts, so as to dis- confess my absolute ignorance. I have tribute the area of tension, and thus no more idea why it is so than the man lessen the risk of cracking in hardening. in the moon, and the utmost I could do
Our difficulties are not quite over would be to mystify you in talking uninwhen the process of hardening has been telligibly about molecular rearrangement successfully accomplished. Our steel and crystalline transformations. Hardwas originally lead; it has now become ening in oil is another mode of treating glass. But we do not want glass: it is steel, which appears to a certain extent too brittle; we want whalebone. An un- to attain by one process the change from hardened knife would bend like wrought lead into whalebone without passing iron; a knife hardened only would break through the intermediate glass stage, like cast metal. We want both qualities and is of great value for certain tools. combined—the bending quality of the
There are many
kinds of steel to which iron and the resisting quality of the your attention should be called, but metal. We want the elasticity of the which can only obtain from me the briefwhalebone. Our knife must spring like est mention. A special steel for taps, -like what?—like steel. To attain this called mild-centered cast steel, is made our tool must pass through the final pro- by converting a cogged ingot of very cess—that of tempering.
mild cast steel, so that the additional If you
heat a piece of hardened steel carbon only penetrates a short distance. slightly, and allow it to cool again, it These bars are afterwards hammered or becomes tempered. It suddenly changes rolled down to the size required, and from glass to whalebone: and in the have the advantage of possessing a hard process of changing its nature, it fortu- surface without losing the toughness of nately changes its color, so that the the mild center. Another special steel, workman can judge by the hue of the somewhat analogous to mild-centered color the extent of the elasticity which it cast steel, is produced by melting a hard has acquired, and can give to each tool steel on to a slab of iron, or very mild the particular degree of temper which is steel heated hot enough to weld with the most adapted to its special purpose. molten steel, so that a bar may be proThe various colors through which tem- duced, one half of which is iron and the pered steel successively passes are as other half steel, or three-fourths iron, and follows: Straw, gold, chocolate, purple, one-fourth steel, as may be required. violet, and blue. Of course, in passing A third kind of special steel, which is from one color to another, the steel used for turning tools for chilled rolls, passes through the intermediate colors. magnets, and some other purposes, is It really passes through an infinite series made by adding a certain percentage of of colors, of which the six above men- wolfram, or, as the metal is more genertioned are arbitrarily selected as conven- ally called, tungsten, sometimes with and ient stages.
sometimes without carbon, sometimes to It must be borne in mind that the such an extent that it can be used withelasticity of tempered steel is acquired at out hardening in water. Special steel of the expense of its hardness. It is sup- this kind is the finest grained that can posed that the maximum of hardness be produced, but it is so brittle that in and elasticity combined is obtained by the hands of any but exceptionally tempering down to a straw color. In skilled workmen it is useless. The additempering steel regard must be had to tion of chromium, instead of wolfram, the quality most essential in the special has somewhat the same effect. tool to be tempered; for example; a It is much to be regretted that no turning tool is required to be very hard, easy method of testing cast steel has and is generally taken hot enough out of been invented. The amount of breaking the water to temper itself down to a strain and the extent of the contraction degree so slight that no perceptible of the area of the fracture are all very color is apparent, whilst a spring is well for steel which is not hardened, and required to be very elastic, and may be not required to be used in a hardened tempered down to a blue. If you ask state, but for hardened and tempered me to give you a scientific explanation of steel it is practically useless. It is very the process of tempering steel, I must difficult to harden and temper two pieces