Steel

by William Metcalf

The theories of its actions are as numerous and variable as are the actions themselves, and they will be treated in a separate chapter, this chapter being confined to a statement of known facts.

As stated in [Chap. I], carbon may be introduced into iron by heating carbon and iron in contact when air is excluded; and, conversely, carbon is burned out of cast iron by the Bessemer and open-hearth processes to reduce the cast iron to cast steel.

In the crucible any quantity of carbon may be obtained in steel by melting a mixture of high blister-steel and wrought iron, or cast iron and wrought iron, or by charging with wrought iron the necessary quantity of coke or charcoal. When using plumbago crucibles, the iron takes up some carbon from the crucible; also the spiegel-eisen or ferro-manganese used adds some carbon; and for these two sources of carbon the melter allows when he decides upon the quantity of charcoal needed.

Results from crucible-melting are not strictly uniform; even if every charge were weighed in a chemical balance accurately the product would not be uniform, because one crucible gives off more carbon than another; in one crucible a little more charcoal may be burned and escape as gas than in another; and most variable of all, unless the charcoal has been dried thoroughly, is the quantity of moisture in the charcoal. One charge of charcoal may be dry, and the next may contain as much as twenty-five per cent of moisture; obviously equal weights in such a case would not give equal quantities of carbon to the steel.

In crucible steel this is no disadvantage; a skilful mixer will get from 75% to 90% of his ingots of the desired temper; the other ingots will all be in demand for other uses, and as he can separate them all with absolute certainty by ocular inspection, as described before, he labors under no fear of bad results.

In the Bessemer process it is usual to burn out all of the carbon and then to add the required amount in the spiegel; for structural steels and for rails this method is satisfactory. For high steel—from fifty to a hundred or more carbon—the spiegel method does not answer so well, because it increases the quantity of manganese to too great an amount; higher carbon is then sometimes put in by the addition of a given quantity of pure pig iron previously melted, or by putting coke in the ladle, but this is very uncertain on account of the tendency of the coke to float, and be dissipated as a gas instead of entering the steel.

The Darby method is to place in the way of the stream of steel as it is poured from the vessel to the ladle a refractory-lined, funnel-shaped vessel filled with finely divided, but not powdered, coke. As the stream rushes through the coke it absorbs carbon with great rapidity, and it is asserted that the currents and eddies formed in the ladle by the rush of the stream cause an even distribution of carbon. That carbon will be taken up in this way is certain; that a required amount, evenly distributed, can be obtained is not so certain.

In the acid open-hearth as in the Bessemer process for milder steels it is usual to burn the carbon out almost entirely, and then to add the desired amount with the spiegel. Higher carbon may be obtained by the addition of pure pig iron, or by using carbon bricks pasted together with tar and weighted with iron turnings; these bricks may be pushed under the surface in different parts of the bath, and in this way the carbon can be distributed pretty evenly. In good practice now the melt is stopped at the carbon desired with great success, thus saving time and expense. In the basic open-hearth the melter, by the use of a little care and good judgment, stops his melt at the required carbon, and so avoids any additional operations, unless his charge is excessively high in phosphorus and his steel is to be very low in the same; in that case he may have to melt clear down and re-carbonize.

Steel of 130 carbon with phosphorus <.05 may be made on the basic hearth from a charge containing 10 to 12 phosphorus without melting below 130 carbon.

If high-carbon Bessemer steel is not uniform, it is not to be wondered at, but as a matter of fact it is usually found to be fairly uniform, sufficiently so to work well.