Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/364

IRON 1868, 279) to use a silicious sand of a particular degree of fusibility (such as that from Gornal near Birmingham, or Fontainebleau sand with an admixture of about 25 per cent, of common red sand) ; this, being introduced dry into the furnace in a layer of about an inch in thickness, is fritted by increasing the heat up to a full steel- melting temperature ; another similar layer is then intro duced and fritted down again, and so on until the hearth is made into a shallow basin sloping towards the tapping hole. The experiments of Chatelier not being attended with the desired success in the first instance, the subject was taken up by E. & P. Martin, who subsequently intro duced certain improvements in details, such as the use of particular fluxes to cover the surface of the molten metal, the application of a separate furnace for heating the iron before introducing it into the melting chamber, the em ployment of particular brands of iron, &c. Under the names of "Siemens process" and "Siemens-Martin process " are usually included several different modifications. In what may be called the older or original process, which was essen tially the combination of pig iron and malleable iron fused together in a regenerative furnace hearth, the same kind of difficulty was met with that nearly proved fatal to Bessemer s original process, viz., that it is difficult if not impracticable to make sure of obtain ing any required degree of carbonization of the resulting steel, so that the finer classes of steels cannot readily be thus made ; for rails, however, the process has been largely adopted. Much the same way of overcoming the difficulty was adopted in the Siemens- Martin process as was used by Mushet, thus giving the second modification, viz., making the atmosphere slightly oxidizing, and continuing the heating until the metal is decarbonized, when the required amount of carbon is added in the form of spiegeleisen or ferro-manganese, and the steel forthwith cast. This modification is consequently substantially the refining process formerly adopted as a preliminary stage to puddling ( 23) carried out a great deal further (so as wholly to decarbonize the metal) at a much higher temperature, and differs from the Bessemer blowing process mainly in this that the oxygen requisite to burn off the carbon and oxidize silicon, &c. , is made to play over the surface of the fused mass instead of passing through it. The decarbonization is carried out in precisely the same way (so far as principle is concerned) as that by which the oxidation of lead and base metal is effected in the ordinary process of gold and silver cupellation in an oxidizing atmosphere at a high temperature. These modifications, of which the second is now much the more largely adopted, are generally referred to as the "Siemens-Martin" process, or "Martin process." A third modification is substantially the Uchatius process carried out in a regenerative hearth instead of a crucible ; this is known as the "Siemens process " or "ore process," and consists in melting hema tite pig, or other pig iron free from sulphur and phosphorus, and then adding in small quantities at a time an equally pure ore until a sample taken out from time to time does not harden on plunging into water whilst still red hot ; to the fused iron spiegeleisen, &c., is then added as before ; in this way a somewhat larger quantity of steel is obtained from a given amount of pig, the ore becoming partly reduced whilst oxidizing carbon and silicon, &c. ; but this advantage is counterbalanced by the greater wear and tear owing to the larger amount of cinder formed and its corrosive action on the brickwork, and by the necessity for using somewhat more fuel. A fourth modification consists in a sort of combination of the Martin method and the ore process, the pig and scrap, &c., being fused together and the decarbonization being then effected, not by oxida tion by the gases alone, but by that together with ore added to the mass. When Siemens s precipitation process ( 31) is used, or when the ore is reduced to spongy metal in a rotating furnace, &c. ( 30), the resulting iron is readily converted into steel by simply adding it instead of malleable scrap to the fused pig in the above process, spongy metal when thus employed being made up into a sort of ball by stirring it up with fused magnetite, and the whole added to the fused pig ; another variety of combination of " ore process " and ordinary Siemens-Martin process is thus obtained, the finishing up of the metal by adding spiegeleisen, &c., in known quantity when complete decarbonization has been effected being the same in all cases. An analogous modification is that of Blair, who first pre pares spongy iron by reducing the ore in much the same fashion as in Chenot s process ( 30), and then fuses it up in a regenerative furnace with more or less pig to give a fluid bath to begin with. In the ordinary working of the process when ore is not used, the materials employed are pig iron (free from sulphur and phosphorus) and malleable scrap of various kinds, together with scrap and waste Bessemer steel, crop ends of rails, c. The pig being melted and the malleable iron raised to nearly a white heat (either in a separate furnace or by the waste gases before passing to the regenerator), the latter is gradually added to the former until the whole is liquid ; the heating is then continued, the name being made somewhat oxidizing so as to gradually decarbonize the metal, until a sample of the metal drawn and cooled in water is scarcely hardened thereby ; at this stage the metal is virtually molten decarbonized iron, all silicon, manganese, and carbon having been removed by oxidation. To give the requisite steel character, a definite quantity of spiegel eisen is added (or of ferro-manganese when a larger relative percentage of manganese is desired, or in certain cases of silico-manganeisen when silicon is wanted to be present), and the whole cast into ingots. Pig iron alone can be used in the first instance, only then a longer time is requisite to effect decarbonization. The following analyses by A. AVillis illustrate the relative -rates at which the oxidation of manganese, silicon, and carbon is effected, the quantity of the last not suffering any material diminution until the others have almost disappeared : Time in Hours since ) n Fusion just completed f 7. Manganese per cent 1-14 0-58 0-20 0-08 nil nil nil nil Silicon ,, 0-57 0-23 0-18 0-05 nil nil nil nil Carbon ,, 1-90 1-80 170 1-65 1-60 1-10 0-60 0-20 When pure ores are used in the ore process, no appreciable altera tion takes place in the percentage of sulphur in the pig and scrap, but if sulphates (e.g., barium sulphate) be present in the ore, the resulting steel contains more sulphur than the pig and scrap used to the extent of about 30 per cent, of the sulphur present in the ore (Willis). When ferro-manganese is used to finish the process and prepare a soft steel, the requisite quantity of ferro-manganese is heated up either on the bed between the hearth proper and the regenerators on which the materials are heated up by waste heat or otherwise without fusion ; when spiegeleisen is used, the requisite quantity may either be added in the same way, or fused in a cupola and then added ; burning out of more or less carbon and manganese is always a possibility where a cupola is used, wherefore when possible the spiegeleisen is fused in the hearth itself or its adjuncts. In order to obtain the best castings at Terre Noire, the decarbonized metal is treated with siliconeisen, and then allowed to remain for some twenty minutes in as nearly neutral an atmosphere as possible, so that cinder may completely be separated by gravitation and the reaction of the silicon on the carbon oxide may be complete ( 44) ; the ferro-manganese is then added, and the casting proceeded with. In the Siemens-Martin process (where ore is not used) the yield of steel finally obtained is somewhat below the weight of metal originally employed, owing to oxidation ; in the ore process, on the other hand, a larger weight of steel is ultimately obtained than that of the metal used, owing to the reduction of iron from the ore. The consumption of fuel per ton of ingot steel is, however, somewhat higher in the ore process, owing to the larger quantity of slag. According to Gautier a considerable loss of fuel results if the gas producers are not placed close to the steel furnaces, even to the extent of one-third in some cases ; probably this figure is a little overestimated (see 10). In order to avoid the necessity of decarbonizing completely the metal and then recarbonizing by addition of spiegeleisen or ferro- manganese, which is practically entailed by the difficulty experienced in finding out the exact composition of the partially decarbonized metal at any given stage of the operation, it has been proposed by Ryder to sample the steel and cast the sample into a small ingot of definite size and shape, and then to determine the magnetic qualities developed in the ingot by the influence of a powerful electromagnet, vising a particular apparatus devised for the purpose. In this way a fairly correct estimate of the amount of residual carbon is obtained, rendering it unnecessary to prolong the operation of decarbonization further when the test shows that an amount is present sufficient to give a steel of the required quality on addition of a known amount of ferro-manganese. In the inventor s hands the method has been found to work successfully, enabling the open hearth operations to be considerably shortened as to duration. An improved form of apparatus for the purpose has also been described by Wattenhofen. 40. The Pernot and Ponsard Furnaces and Allied Appliances. The Pernot furnace as applied to steel making differs in no material respect from the Pernot puddling furnu.ce ; it is substantially a Siemens-Martin furnace with a rotating bed. The hearth is a saucer-shaped cavity sup ported by an iron frame, mounted on the top of a slightly inclined nearly vertical axis, and running on wheels upon a rail cr guide supported on a stout bogie (fig. 62). When in position the hearth is just under a dome or roof, which is perforated with orifices for the entrance and exit of the