Conversion Of Iron Into Steel
carbon process furnace charcoal cementation cast pots malleable oxide melted
CONVERSION OF IRON INTO STEEL, Cementation Process. - It has been known for a long period, some two centuries at least, that when wrought iron is enveloped in powdered charcoal and heated to redness for a long time it gradually becomes carbonized and converted into steel, the deposition of carbon commencing at the outside and gradually penetrating inwards in precisely the same way as that in which the decarbonization of iron proceeds in the manufacture of malleable cast iron (§ 22), a longer time being consequently requisite for the carbonization of thicker than of thinner bars ; the name of the inventor of the process, however, has been forgotten. In the middle of the 16th century it was known that when a bar of wrought iron was kept immersed for a long time in molten cast iron it gradually became acierated by taking up carbon from the cast iron ; this process is clearly closely allied to cementation in solid carbon, and was probably the forerunner thereof ; very likely it was in the first instance an accidental observation ; it was described as being in actual use about that period by various writers, notably Biringuccio in 1540 and Agricola (De Re llIetallica, Fez + C + Fe.0„ Fe.05+ yCO=Fe.+ yCO2, the two changes going on simultaneously. The escaping carbon dioxide, which penetrates through the metal less readily than does carbon oxide, and hence is apt to accumulate in certain parts, is probably the cause of the blistering of the surface of the steel often observed, especially with puddled bars containing small quantities of ferrous silicate disseminated through them ; Percy has shown that fused homogeneous metal free from interspersed slag does not give rise to blisters on cementation. Certain hydrocarbons, e.g., paraffin vapour and coal gas, will carbonize iron heated therein, and the manufacture of steel by cementation in the latter has been patented by Macintosh (vide infra). Probably in these cases the carbon comes from the direct splitting up of the hydrocarbon, with elimination of hydrogen ; but possibly the acieration is due to carbon oxide present in the coal gas or formed from the paraffin vapour, &c.,by the action of iron oxide disseminated through the bars or adherent to their surface. Many cyanogen compounds, especially ferrocyanide of potassium, when applied to iron in a heated state convert it exteriorly into steel (case hardening), and it has in consequence been supposed that nitrogenous substances are essential to the carbonization of iron by cementation, and that nitrogen is an essential constituent of steel. The evidence in behalf of this is, however, at present unsatisfactory; on the other hand, charcoal rich in alkalies, or a mixture of charcoal powder with a little lime and soda, will carbonize iron submitted to cementation therein more rapidly than charcoal more free from alkalies ; and, as these conditions are those favourable to the formation of alkaline cyanide from the nitrogen of the air, there is some reason for supposing that the carbon in the steel formed under such circumstances (like that produced in case hardening by means of ferrocyanide) is more or less derived either from cyanogen separated from the cyanide and occluded by the iron and gradually decomposed with formation of carbon, or from some other reaction of iron upon the cyanide. Accordingly nitrogenous organic matter, such as animal charcoal, leather, horn, &c., is often mixed with the charcoal used for cementation with a view to facilitating the conversion into steel by the formation of gaseous carbon compounds with the simultaneous presence of nitrogenous vapours.
The theory that carbon oxide is the source of the carbon communicated to wrought iron diming cementation, appears to have been first propounded by Leplay in 1846 (Ann. de Clair. et Phys.  xvii. 221), at a time when the properties of metals and other bodies in absorbing gases (i. e., the phenomena of occlusion) had not been so well studied as they have boon subsequently. Leplay appears to have considered that the carbon oxide splits up directly into carbon and carbon dioxide, the latter becoming again transformed into carbon oxide by the surrounding charcoal, and to have left out of consideration the intervention of the iron in becoming alternately oxidized and reduced. Other chemists have considered that by direct contact with carbon combination of the iron therewith takes place, the -carbon thus taken up by the outer layer quitting that and combining with the next layer, and so gradually travelling inwards, the outer layer recombining with more carbon as fast as it parts with carbon to the under layer, and so on throughout ; the carbon thus traversing the iron by a process somewhat akin to that by which a drop of mercury in contact with a piece of gold (or certain other metals) gradually passes into and permeates the mass, - this being in short a kind of capillary action exerted upon a solid substance. Percy's observation (Maallsi•gy, "Iron and Steel," p. 109) that charcoal after being intensely ignited will not carbonize iron when air is excluded by means of hydrogen (although it will do so to seine extent if still containing matters capable of being driven off by heat) negatives the possibility of the carbon being taken np by direct contact by this hypothetical kind of chemical union between solids, or solvent action of one solid on another ; it may be that carbon deposited on the outer layer by the chemical action of the iron on carbon oxide, eyanogen compounds, carbnretted hydrogen, he., permeates inwards by this supposed diffusive process; but the known phenomena of the absorption of gases by colloid bodies, diffusion, dialysis, occlusion, &c., as elucidated by Graham and his followers, render it wholly unnecessary to suppose that any such action takes place, and do away with all experimental grounds for supposing that it can take place. In order to carry out the process of cementation, the bars of iron are placed in a firebrick box or chest several feet long, layers of charcoal and iron being alternately piled in until the box is filled, when a luting of fireclay or of the sandy ferruginous mud produced in grinding and polishing steel articles after manufacture, termed "wheel swarf," is applied so as to close up the upper part of the box and prevent access of air ; two or more such chests are then arranged under the arched roof of a chamber erected over a fireplace in such a way that the flames from the fire pass under and lap round the sides of the chests, and impinge upon the roof, the gases escaping through orifices in the roof into a conical chimney built over the whole, - thc chamber constituting in fact a kind of furnace somewhat like a glass house or pottery kiln, the flame passing upwards from the bed instead of laterally from a fireplace at the side as in the ordinary reverberatory furnaces. Trial bars are arranged in the mass of charcoal in such positions that they can be withdrawn from time to time, and the progress of the operation examined by fracturing the bars after cooling, and seeing when the core of malleable iron disappears; from seven to ten days' heating according to the amount of carbonization required (averaging about 1 per cent.) is generally allowed, with a total charge of some 10 to 20 tons of iron in the furnace. When the requisite carbonization is attained the fire is raked out and the chests allowed to cool ; the blister steel is then either melted down into cast steel, or converted into shear steel by piling and forging, he.
According to Boussingault a material diminution in the amount of sulphur present takes place during cementation; thus he found malleable iron specimens containing 0.012 to 0.015 per cent. of sulphur yielded steels containing only 0.005 to 0.006 per cent. of sulphur. Indications in the same direction but not to so great an extent have also been observed by others ; no noticeable effect, however, is produced on the silicon, phosphorus, or manganese originally present, as far as the irregular way in which traces of cinder are always interspersed throughout bars of wrought iron will permit conclusions to be drawn. The following analyses indicate the effect of cementation on Swedish bar irons : - In consequence of the phosphorus originally present remaining unchanged, only the purest brands of iron as free as possible from these ingredients are converted into cementation steel, often known as "tool steel," commanding a high price in consequence of its physical properties, the most valuable of which are enormously deteriorated by minute quantities of sulphur and phosphorus. The process of cementation in an atmosphere of coal gas as patented by Macintosh of Glasgow consists of exposure of the bars of iron hanging vertically in a cylindrical chamber, the walls of which are kept at a high temperature by an annular fireplace surrounding it, a gentle stream of well-desulphurized coal gas being allowed to pass through the chamber. The expense of the process seems to have been the chief bar to its adoption, as steel of excellent quality can readily be made by it from good malleable iron.
in the melting furnace on fireclay stands, round which and the pots coke is piled, two pots being usually fixed in the same "melting hole," but sometimes more. When the pots are white hot the steel in small lumps is introduced by lifting up the cover and pouring the pieces down a long iron funnel; the covers being replaced and the fire made up, after some two or three hours the steel is fluid ; but if cast immediately it is found that a much larger quantity of gas separates during solidification, rendering the steel porous, than is evolved if the metal is dead-melted, i.e., allowed to remain melted for an extra half hour or more, presumably from the reaction of the iron oxide interspersed throughout the steel upon the carbon evolving carbon oxide during the earlier period, this evolution subsequently ceasing, owing partly to the reduction of the oxide and partly to its floating up to the top of the fused mass as scoriae. According to Bessemer the chief part of the " dead melting" effect of the extra time allowed in fusing steel for the molten metal to stand in the furnace after fusion is brought about is due to the reduction of a little silicon from the crucible materials, &c., the presence of that element greatly diminishing the tendency to evolution of gas during solidification (see § 44). When the pots are withdrawn and the casting made (frequently from the contents of many pots combined together), they are replaced in the melting holes before they cool, and used over again, a somewhat smaller quantity of metal being introduced for the second melting (and less still for the third), in order to prevent the surface of the fused scoria being at the same level as before, the pots being chiefly attacked at that place. About 3.5 parts of coke are required for 1 of mild steel melted, and somewhat less for harder steels, which melt more easily.
The Siemens regenerative furnace (§ 10), fed with gas from a producer, can be vary advantageously employed instead of the older coal or coke-fired furnaces. In such a steel melting furnace (fig. 59) the fusion chamber generally contains some two dozen pots, and is constructed in the form of a trench with overhanging sides, which are arched both horizontally and vertically to keep them from sinking in whilst in use. The floor is covered with finely ground hard coke, which burns away but slowly and does not flux or indurate, thus giving a firm foundation for the pots, which are set in a double row along the centre of the chamber ; the upper roof of the chamber consists of firebrick tiles or frames filled with firebrick capable of being slid off separately by means of levers or handles attached to each, so as to permit of the introduction and withdrawal of the pots. The inventors state that the lining of a furnace of this description will last from fifteen to twenty weeks without repair, working day and night, whilst four to five weeks is the ordinary life of a coke-fired furnace ; that the pots will stand four, five, and sometimes even ten successive meltings instead of two or three ; and that, whilst 3 to 4 tons of hard coke are requisite in coke-fired furnaces per ton of steel melted, 15 to 20 cwts. of much inferior slack burnt in a gas producer will furnish enough fuel to melt a toil of steel on the regenerative principle (Chem. Soc. Journal, 1868, p. 276). The precise amount of fuel used in actual practice is somewhat variable, hut consumptions as low as 0.64 parts of coal per unit of steel melted (nearly 20 tons being melted in all during one week) have been recorded. In other works the consumptions were 1.1 to 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 hare been introduced by other inventors ; thus the Swindell furnace has been used to a considerable extent in America.
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 covered-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, fec., and the space between the two surfaces filled 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 potassinm 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 limmatite, &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 bard 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 Railway Company, charcoal, soda ash, and limestone crushed small were mixed together in the proportion of 1 cwt. 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 amonn'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 front 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 successive Tig, inch from the surface being found to be as follows : - Third „ 0.463 „ = 0-253 . 35. CrueiNe 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 " Huntsmann's steel," from the name of its inventor. About the beginning of the present century Musket 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 carbonaceous 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 assaying 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 1836 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-Fleath process of fluxing together in crucibles malleable iron and steel scrap, powdered charcoal, and manganese oxide or spiegeleiseu is stilt used to some extent ; the cast steel thus produced is apt to be somewhat vesicular and porous ; to overcome 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 in the cast steel can often be increased by adding carbonaceous matter to the fragments of steel with which the crucibles are filled, - the additional carbon being taken up precisely as in Musket'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 fusion in crucibles of spongy iron and carbonaceous matter) has been already adverted to (§ 30) ; Parry took out a patent in 1861 for converting puddled iron into steel by fusing it with coke and fluxes in a kind of cupola furnace so as to recarbonize the metal ; by modifying the blast and proportion of fuel employed it is possible to produce 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 front 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 cre and fuel being supplied at the top, and the combustion urged by a rude bellows made of a goat's skin stripped front the carcass without opening up the belly, the neck being furnished with a bamboo nozzle terminating in a clay tube, forming a rough tuyere. After the fire has been urged for some hours the contents of the furnace are removed by partially breaking down the front, in the form of a rough porous ball or bloom of partially melted metal, which is then cut into pieces and charged into a crucible (made of clay mixed with a small quantity of charred rice husks) together with the wood of C auriculata, chopped into little fragments ; each crucible holds about a pound of metal, and is covered over with a few green leaves, preferably of Asclepias gigantea or Couvolvolus laurifolius, a clay cover being made by ramming in soft clay and elbles (some twenty or twenty-four) are then piled up in a clay furnace furnished with a bellows something like the original smelting furnace, the interstices being filled with charcoal. After some two hours' heating the steel is fused ; the crucibles are then removed and allowed to cool, and the melted cakes extracted by breaking away the clay. If the operation has been successful, the cakes are smooth-surfaced, Sv:th radiating striie ; such cakes when remelted in larger quantities furnish an extremely fine quality of steel ; when the conversion of the iron into highly carbonized steel is incomplete, the cakes are imperfectly melted down, and consist more or less largely of fritted lumps of metal not carbonized sufficiently to fuse ; such cakes yield only an inferior steel when remelted. In order to forge the steel the natives heat the cakes in a charcoal bellows-forge for some hours to a temperature short of fusion, and then hammer them out by hand into bars ; these are welded together by forging to wedge-heads, tying together with wire, sprinkling with borax, and quickly heatpreliminary heating partially decarbonizes the steel, so as to make the final product less like a steely cast iron. When properly prepared, the temper which this steel will take is magnificent ; it is said that sabres of such steel with an edge sharp enough to cut gossamer like a razor can be dashed with the full strength of a man's arm against a stone wall, or used to cut in two a bar of wrought iron, without having the cutting edge injured in the least degree if the swordsman be only sufficiently expert.
According to analyses made by Faraday, wootz contains a small quantity of aluminium ; this probably existed as cinder disseminated through the mass, as subsequent analysts have entirely failed to detect aluminium in wootz free from slag ; thus Henry (Phil. May., 1852) and Raminelsberg (Beriehle Deice. Chem. Ges., 1870, p. 461) found the following mean nmnbers, the sulphur being probably overestimated in II en rv's analysis