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Steel 13

carbon, iron, crucible, wrought, converter and bessemer

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STEEL 13. Crucible Steel. This is the finest quality of steel. It is made in three ways— namely, by packing pure soft wrought iron in fine charcoal, and raising to a yellow heat; by putting charcoal and crude bar iron in a sealed crucible and heating to a melting heat; or by melting wrought iron in a sealed crucible with a proper proportion of pig. The aim of all meth ods is the same—that is, to get a malleable metal containing from 0.60 to 1.40 per cent of carbon, and practically free from sulphur and phos phorus. It is to be noted that all these methods take wrought iron, a metal practically free from carbon, and add carbon to the desired extent.

With the first method, the carbon from the charcoal soaks in at the rate of one-eighth inch each 24 hours. With the second, the absorption of carbon is very rapid on account of the fluid condition of the iron. With the third, also, the process is rapid, the pig distributing its carbon to the mixer so as to bring the total to the required result. The second method is the universal practice in America.

After the mixture is put in the cold crucible and sealed, the crucible is put in a gas furnace and kept at the proper heat until the steel is made. The crucibles are then taken out and the material poured into ingots. From 100 to 200 pounds of steel are made from each crucible. The ingots are then rolled into bars and plates, and they are put on the market.

Crucible steel is used to make cutlery and engraving tools, and parts of machines which perform excessive work. Not much is made, and the annual output has increased less than 20 per cent in the last ten years, while that of other steel has increased over 100 per cent. There were 118,000 tons made in 1906. Plate 3 shows men casting a 55-ton ingot of crucible steel. This is at one of the great steel works of Germany.

14. Bessemer Steel. In the Bessemer proc ess, from ten to twelve tons of molten cast iron is poured either directly from the blast furnace or from the "mixer," into a pear-shaped Besse mer converter, which is tipped over to receive it.

The converter is shown in cross-section in Fig.

6. The trunnions are hollow, and the air from the blast enters by these, being led down to the bottom, where it blows up through the holes there, and then through the charge. The blast is turned on, and the converter tipped into a Fig. 6. Section of Bessemer Converter in Action.

Steel 13

vertical position. The oxygen of the air com bines with the various metalloids, and in from nine to ten minutes nearly all the carbon, man ganese, and silicon are burnt out and form slag on top. The phosphorus and sulphur remain the same, and for this reason ores low in phos phorus and sulphur must be used in making cast iron for Bessemer steel. When the carbon is about all burned out, the flame from the con verter drops to about one-fourth what it is at full blast, and it is then time to recarburize the charge. As it stands at the end of the carbon flame, it consists of a bath of molten iron con taining about 0.4 per cent of carbon, and with a lot of iron oxide floating on top. A carefully calculated amount of manganese and silicon, and enough carbon to bring it up to the desired per centage of that ingredient, are added in the form of Spiegeleissen, a mineral which contains the above metalloids. The manganese returns some of the iron of the oxide and clears the charge of loose oxygen, while the silicon prevents the oc currence of bubbles and blow-holes when the ma terial is poured.

The lower the percentage of carbon, all other things being equal, the lower the tensile strength and the softer the steel. The amount of carbon added is therefore a necessity, since, as the bath stands at the end of the blow, it would produce a steel of about the consistency and strength of wrought iron; and, indeed, considerable quanti ties of steel are so made, being used for many purposes where wrought iron was formerly used.

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