A remarkable improvement in blast-furnace practice, cheapening cast or pig-iron, and therefore lowering the cost of derived steels, is the dry-blast process due to Mr. James Gayley, of Pittsburg. It has long been known that blast-furnaces ask more fuel in warm and damp weather than in cold and dry weather; beginning with this familiar fact Mr. Gayley proceeded to dry the air blown into his furnaces, by passing it around large coils of iron pipes through which a freezing mixture circulated, melting the snow as formed by passing hot brine through the pipes, a few of them at a time. The air thus dried was then heated by being sent through hot blast stoves in the usual mode. This simple drying of the blast saves about 19 per cent. of the fuel, and makes the action of the furnace much more regular than when ordinary air is used. It lowers the temperature of the gases which escape from the top of the furnace, and raises their percentage of carbon dioxide, symptoms of the great increase in fuel efficiency. Atmospheric moisture has a cooling effect on the lower part of a furnace, just where the highest temperature is needed to melt the iron and slag, remove the sulphur and deoxidize the silica. A comparatively small increase of temperature by broadening the margin of effective heat, which margin at best is narrow, has the astonishing effect of economizing fuel to the extent stated, 19 per cent.[15]
[15] Henry Marion Howe, “Iron, Steel and Other Alloys.” Second edition. Cambridge, Mass., Albert Sauveur, 1906.
Steels to Order.
What is chiefly sought in steel is tensile strength, next in value is elasticity; in some cases hardness is indispensable. By varying the proportions of the carbon, silicon and manganese added to his iron, the steel-maker produces an alloy with the tenacity, elasticity or hardness he wishes. Nickel, as a further ingredient, in certain proportions yields an astonishing gain. A steel containing fifteen per cent. of nickel has shown a tensile strength of 244,000 pounds to the square inch, four times as much as before admixture; the elastic limit also was much increased. Hardness and strength tend to exclude ductility, but nickel steel is at once strong, hard and extremely ductile; hence its use for armor plate, great guns, and the barrels of small arms. Nothing but the high price of nickel prevents these alloys from having wide utilization, for they mean lighter and therefore more economical machines and engines than those of ordinary steel. Many turbines actuated by water, steam or gas, are best operated at speeds forbidden to common steel, which would fly to pieces under the centrifugal stress exerted, yet these speeds are quite feasible and safe when nickel steel is employed. This alloy brings nearer the day of mechanical flight, first promising to transportation on land and sea engines increased in power while much diminished in weight. In exceptional cases, where the expense may be borne, we may expect soon to see nickel steel used for higher towers, longer bridge-spans, thinner boilers, than those of to-day. Part of the bridge crossing Blackwell’s Island, New York, is built of nickel steel. Even with costs at their present plane, it is worth while for the designer of machinery to remember that friction is reduced when masses become smaller, power for power. It is found profitable, for instance, to use nickel steel for the cylinders of automobiles of high power.
In many tools and implements two different kinds of steel are united with decided gain. Thus the cutting edge of a cold chisel is hard and brittle, while its shank, much less hard, is tough and able to resist the shocks it receives. So also a projectile is hardened at its point and nowhere else. Plowshares are often made very hard on their surfaces, with a backing which is comparatively soft but elastic enough to suffer no harm in the blows dealt by rough ground and stones. One of the drawbacks in the use of steel is its liability to corrosion. An alloy of 30 per cent. nickel and 70 per cent. steel has proved to be corrodible in but slight measure, affording a material of great value to the arts.
Heat Treatment.
While the chemical composition of a steel is of prime importance, the quality of the steel will next depend upon its heat treatment in manufacture. The temperature to which heating is carried, the period during which it is maintained, the rate at which cooling takes place, and the circumstances of cooling, each has its effect on the character of the product. It is chiefly in this field that the steel-maker within wide limits is able to turn out an alloy either hard or soft, brittle or ductile, tenacious or weak, at pleasure. While much has been learned within the past few years as to the proper treatment of steel by heat, much still remains to be discovered.
To quote typical instances from Professor Henry Marion Howe, of Columbia University, New York:—“In the case of steel with less than 0.33 per cent. of carbon the temperature from which slow cooling occurs appears to have little influence on the tensile strength; but it is the general belief that if that temperature approaches the melting-point, the tensile strength decreases. In the case of higher-carbon steel, the tensile strength at first increases as the temperature from which slow cooling occurs rises to 800°, or even to 900° or 1000° C. Then, after varying somewhat, it falls off very abruptly in the case of steel of 0.50 per cent. of carbon, when that temperature approaches 1400°.”[16]
[16] In his “Iron, Steel and Other Alloys.” Second edition. Published by Albert Sauveur, Cambridge, Mass., 1906.