Page:The American Cyclopædia (1879) Volume XV.djvu/373

 STEEL 361 3, more roomy and convenient arrangement and distribution of the working parts and spaces ; 4, filling the ingot moulds from the bottom by improved and convenient appara- FlG. FIG. 5. tus. The converter, after being lined, is thor- oughly dried and heated to redness, and pig iron is run into it while turned to the hori- zontal position. On tipping up the convert- er, it is necessary that the blast should be started be- fore the metal reaches the tuyeres. This is effected au- tomatically by a cam on one of the trunnions. When the converter has attained the upright position, the roar of the air rushing through the metal and escaping from the mouth is heard. In this stage a large part of the oxygen is absorbed by the silicon and manganese (or iron in the absence of manganese), and the flame is short and not highly luminous. The spectroscope shows at this time a continuous spectrum without lines. Soon the escaping flame increases in size and brilliancy, assuming an orange or yellow color with blue streaks and a white edge, inter- mingled with sparks of metal. The spectro- scope now shows the sodium line, and gen- erally those of potassium and lithium, acciden- tal ingredients of the metal or lining. This constitutes the first period of the conversion, and is known as the slag or cinder-forming period. The action now becomes more vio- lent, and the flame more intensely luminous, and large masses of iron or cinder are often ejected from the vessel, probably from the energetic action of the oxide of iron in the cinder on the carbon of the metal. The spec- troscope now shows bands of dark lines in the green, which have been proved to be pro- duced by manganese, though their appearance is dependent on the oxidation of the carbon. This violent stage of the process passes grad- ually into the third and more peaceful period, in which the flame increases in heat and bril- liancy and assumes a purple or violet tint. At this high temperature the carbon appears to be directly oxidized by the blast. When the carbon is all removed the flame suddenly drops, which is the indication for tipping over the converter and stopping the blast. Coin- cident with the dropping of the flame is the disappearance of the dark bands from the spectrum. The length of the process up to this point may vary from 5 to 45 minutes, ac- cording to the heat of the metal, the amount of silicon and manganese, and the amount of pressure of blast. A " blow " usually lasts 15 to 20 minutes, of which the first or slag-form- ing period generally occupies one half. Pig irons with little silicon often pass directly into the second period. The metal in the converter after complete decarburization con- tains considerable oxide of iron in suspension or solution, and in that condition is worthless, since it breaks up under the hammer. There is added to it, therefore, metallic manganese, as before explained, which combines with the oxygen and passes into the cinder. Spiegelei- sen is generally used for this purpose. At the end of the blow the converter is tipped over, and the spiegeleisen, previously melted, is run in. An energetic action at once manifests itself by the escape of abundant gas and flame. About 7 to 10 per cent, of the weight of the charge is used, according to the hardness of steel desired. Spiegeleisen contains about 4 to 5 per cent, of carbon, and the amount that can be used is therefore limited, for the carbon, ta- king but little part in the reaction, enters into combination with the metal. This has been an obstacle to the preparation of extra soft metal by the Bessemer process. Ferro-manganese, a combination of manganese and iron with a little carbon, containing 50 per cent, more or less of manganese, was early used with success in the process, but its manufacture was aban- doned owing to its expense. It has recently been revived and its use resumed in the Bes- semer, but more particularly in the Martin process, under analogous conditions, for the preparation of steel or homogeneous iron containing phosphorus. The employment of ferro-manganese is also becoming general for making soft iron of fine quality for construc- tion of ships, bridges, &c. The addition of spiegeleisen or ferro-manganese is not univer- sally practised. Where the pig iron contains considerable manganese, the process may be interrupted at the desired stage of decarburi- zation, and even metal very low in carbon, which is not red-short from oxide of iron, may be successfully cast. This method is fol- lowed in Sweden and some parts of Germany. The loss of weight in the conversion of pig iron by the Bessemer process, including scrap, is from 10 to 15 per cent. The heat produced in the process, formerly supposed to be mainly caused by the oxidation of the carbon, is now known to be mainly due to the oxidation of silicon and manganese, and also of the iron. Silicious pig iron is therefore generally de- manded for the process. From 1 to 2 per cent, of silicon is the amount generally de- sired, but pig irons with more and less are often used. The use of more silicious pigs is disadvantageous owing to a lengthening of the