Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/358

342 IRON 1 -45 parts of coal per 1 part of steel melted (the heat requisite for drying the pots being included). A good deal of the saving in fuel is dependent on the character of the pots employed, the best pots, which will stand several successive meltings, causing considerable economy, in that the fuel requisite to heat up new pots (starting comparatively cold) is saved, the fusion being effected in much less time, averaging from two-thirds to three-fourths of that requisite for new pots. Various modifications of the Siemens regenerative steel melting furnace have been introduced by other inventors ; thus the Swindell furnace has been used to a considerable extent in America. 34. Case Hardening. This operation is essentially the reverse of that by which cast iron is converted into malle able iron ( 22). In the latter the carbon is gradually removed, the outer portion being first affected ; in the former carbon is added to the exterior layer of a malleable iron article so as to give it great hardness, strength, and power of resisting wear and tear by superficial conversion into steel. As applied to larger articles in which the steely coating is required to be of more than just perceptible thickness (O l to 0*3 inch), the operation is essentially that of cementation, the iron articles being packed in an iron chest or box in charcoal reduced to very coarse powder (burnt or charred leather, hoofs, bones, &c., answering best), and then heated to a red heat for a sufficient length of time (some four to six hours for a coating of steel of O l inch or so in thickness) either judged by past experience or by withdrawing trial pieces from time to time, and breaking them so as to ascertain to what depth the acieration has proceeded. When this is effected the chest is withdrawn from the oven or furnace and the articles chilled by taking them out whilst still hot and quenching in water or oil, .&c., so as to harden the exterior coating. Since the steel exterior is thus rendered too hard for working with ordinary tools, the articles to be treated must be as far as possible finished before case hardening, so as only to require clean ing and polishing subsequently. Sometimes a portion only of an iron object is required to be case hardened. In this case a coating of loam or clay, &c., is applied to that part of the object not required to be hardened, and gradually dried on so as to form a jacket ; this prevents the ready access of carbon and carbon oxide to the coverea-up part, and hence hinders or entirely prevents acieration thereat ; instead of a clay coating moulded on, a roughly made loose iron jacket may be made from iron tube or sheet iron, &c., and the space between the two surfaces h lled in with clay well rammed in. In certain cases the article is case hardened as a whole, those portions required to be of malleable iron being made too large ; after acieration the whole is annealed, and the softened steely coating filed or lathed off from these portions, and the whole then heated and hardened. When only a thin coating of steel is required, it is unnecessary to acierate by packing in charcoal ; the iron to be hardened is heated to redness and then sprinkled with powdered ferrocyanide of potassium either by itself or mixed with other saline substances ; the salt fuses and carbonizes the surface of the metal to such an extent that after hardening the exterior film is usually hard enough to resist a file. Sometimes goods are cast in the first instance (for cheapness of manufacture) and then heated in hoematitc, &c., so as to convert them into malleable iron to a greater or lesser extent, the outer film being finally case hardened by ferrocyanide ; so that occasionally cast iron as an inner core, malleable iron as an exterior coating, and steel as an outermost film are met with in the same article. For axles, shafts, and other portions of machinery apt to encounter sudden strains which would snap a solid hard steel mass, but where certain portions (bearings, &c.) are required to be as hard as possible to diminish wear and friction, the local case hardening of the parts required to be hard is frequently practised ; and in this way certain of the advantages of both hard steel and wrought iron are combined. For case hardening rails Dodd s process has given good results ; as practised some years ago by the North-Eastern Hallway Company, charcoal, soda ash, and limestone crushed small were mixed together in the proportion of 1 cvt. of the first to 1 stone of each of the others, and charged into the case hardening furnace between successive tiers of rails. The rails remained in the furnace sixty hours ; when taken out they were covered with sand till cold. The cost of the process amoun s to about 12s. 6d. per ton (Lowthian Bell) ; but when the rails are of ordinary puddled malleable iron, a certain degree of brittleness is communicated. With rails from Danks s machine puddled iron the carbonization was found to extend inwards for nearly a quarter of an inch, the percentages of carbon in each suc cessive T V inch from the surface being found to be as follows : Outer i inch ...0740 to 1 013 Mepn of seven specimens = 862 Succeeding do. ...0-231 ,, G 696 ,, ,, ,, = 49S Third do. ...0-030 ,, 0-468 ,, ,, ,, = 253 35. Crucible Steel. The term " crucible steel," strictly applicable to the cast steel prepared by fusing cementation steel in crucibles, is often applied to denote various other somewhat different substances (also fused in crucibles), cementation cast steel being often designated " Hunts- mann s steel," from the name of its inventor. About th? beginning of the present century MusLet patented the production of a crucible steel by the direct carbonization of malleable iron by the fusion together in crucibles of bar or scrap iron and " a proper percentage of carbonace ous matter " ; and also the production of a similar product direct from the ore by substituting the ore for the malleable iron and increasing the amount of reducing matter. This latter process (which is substantially the method of assay ing iron ores in crucibles by the dry method on a somewhat larger scale, and with less reducing matter) had been previously patented in 1791 by Samuel Lucas, whilst substantially the same process was again patented in 1830 by Hawkins. But little steel, however, was made by this process until 1839, when Heath patented the use of what he termed " carburet of manganese " as an ingredient in making crucible steel, this substance being prepared by heating together manganese dioxide and carbonaceous matter. It being speedily found that the same result was produced whether this heating together of the manganese, dioxide, and carbonaceous matter was previously carried out, or whether these materials were separately added to the contents of the crucible and the whole melted together, the validity of the patent was vigorously contested, the utility of the manganese thus introduced into the resulting mass as a means of partially correcting the deleterious effects of sulphur and phosphorus being speedily apparent, and the possibility of the production of useful qualities of steel from even inferior iron being rapidly recognized as a valuable improvement. This Mushet- Heath process of fluxing together in crucibles malleable iron and steel scrap, powdered charcoal, and manganese oxide or spiegeleisen is still used to some extent ; the cast steel thus produced is apt to be somewhat vesicular and porous ; to over come this when bars are required the ingots are reheated and hammered or rolled, either with or without cutting and piling; the character of the cast steel is largely variable with the proportions of malleable iron and iron already carbonized that are used. Siemens or open hearth steels have of late years largely superseded this class of products. When blister steel is judged to be somewhat deficient in carbon, and is converted into cast steel by fusion, the amount of carbon present ill the cast steel can often be increased by adding carbon aceous matter to the fragments of steel with which the crucibles are filled, the additional carbon being taken up precisely as in Mushet s process of date 1800. The same effect is produced to a slight extent by employing a considerable quantity of blacklead in the crucible composition, the graphite being then directly dissolved during the fusion. The Chenot process of steel making (by fusion in crucibles of spongy iron and carbonaceous matter) has been already adverted to ( 30) ; Parry took out a patent in 1861 for con verting puddled iron into steel by fusing it with coke and fluxes in a kind of cupola furnace so as to recarbonize the metal ; by modify ing the blast and proportion of fuel employed it is possible to pro duce either steel or cast iron containing 2 per cent, and upwards of carbon ( 23). Apparently the cost of the fuel required for this process and other circumstances have prevented it from materially competing with the Bessemer and Siemens steel-making processes. Wootz or Indian steel was described in 1807 by Buchanan as being prepared from the steely iron obtained by heating in a rough conical furnace of clay some 2 feet wide at the base and 1 at the top the pure magnetites and other ores of India and charcoal, the ere and fuel being supplied at the top, and the combustion urged by a rude bellows made of a goat s skin stripped from the carcass without