As in the Bessemer process, so in the open-hearth, carbon and silicon are burned out, phosphorus is removed on the basic hearth, and the sulphur of the charge remains in the steel. During the operation oxygen and nitrogen are absorbed by the steel, although not quite so largely as in the Bessemer process, so that practically the chemical limitations are the same in each.
The open-hearth reductions are much slower than in the Bessemer, each heat requiring from five to eight hours for its completion; the furnace must be operated by a skilled man of good judgment, so that more time and more skilled labor per ton of product are required than in the Bessemer, and the making of an equal quality as cheaply in the open hearth is problematical. The open hearth has extraneous sources of heat at the command and under the control of the operator, and there need be no cold heats, and no too hot heats.
The time for reactions is much longer, and for this reason they ought to be more complete, and they are so in good hands; yet it is a fact that, as the operation is a quiet one compared to the Bessemer, and not nearly so powerful and energetic, a careless or unskilful operator may produce in the open hearth an uneven result that is quite as bad as anything that can be brought out of a Bessemer converter. The process that eliminates the human factor has not yet been invented.
For fine boiler-plates, armor-plates, and gun parts open-hearth steel has won its place as completely as has the crucible for fine-tool steel or the Bessemer for rails.
For all intermediate products there is a continued race and keen competition, so that it is impossible to draw any hard and fast line between the products of the three processes where they approach each other; the only clear distinctions are at the other extremes.
Owing to the power to hold and manipulate a heat in the open-hearth it is safe to say that it is superior to the Bessemer in the manufacture of steel castings; and owing to its much greater cheapness it is difficult for the crucible to compete with it at all in this branch of manufacture.
In conclusion of this chapter it is safe to say that in good hands these processes are all good, and each has its own special function to perform.
III.
ALLOY STEELS AND THEIR USES.
In addition to the four general kinds of steel treated of in the last chapter there are a number of steels in the market which contain other metals, and which may be termed properly alloy steels, to distinguish them from carbon steel, or the regular steels of world-wide use which depend upon the quantity of carbon present for their properties. The most generally known of the alloy steels is the so-called Self-hardening steel.
Self-hardening steel is so called because when it is heated to the right temperature,—about a medium orange color,—and is then allowed to cool in the air, it becomes very hard. This steel is so easily strained that it is impossible, as a rule, to quench it in water without cracking it. It may be quenched in a blast of air without cracking, and so be made much harder than if it be allowed to cool more slowly in a quiet atmosphere. If it be quenched in oil or water, it will become excessively hard, much harder than when quenched in air, and it will almost invariably be cracked, or if it be not cracked it will be so excessively brittle as to be of little use.