Page:The American Cyclopædia (1879) Volume XV.djvu/376

 364 STEEL quired. Trial bars are inserted with their ends protruding, which may be withdrawn from time to time and the progress of the conversion judged from the appearance of the fracture. When the desired end has been at- tained, the fires are withdrawn and the boxes allowed to cool slowly for several days. The bars after conversion generally have blisters on the surface, apparently formed by the pres- sure of some gas from within the bar ; hence the name " blister steel." The bars, originally soft and tough, are found after conversion to be hard and brittle, and the freshly fractured surface shows a steely appearance. Analyses of successive layers of the bar after conversion show that the carburization proceeds gradually from the surface to the interior, the iron near the surface being much more highly carburized than that at the centre. In order to obtain uniformity in cement steel, it is therefore ne- cessary either to weld several bars together by repeated rolling or hammering, or by melting the bars in crucibles. The former process is adopted only for the softer cement steels, and furnishes shear steel. Tha use of this welded steel has been generally superseded by cast steel, but it is still employed for many pur- poses, particularly for welding to iron. The melting of steel is usually effected in covered crucibles capable of holding 40 to 80 Ibs. of metal. They are made of refractory clay or of graphite with sufficient clay to give it co- herence. These crucibles are placed in fur- naces arranged in a straight line, with their tops or openings on a level with the working floor of the casting house. Each furnace is a rectangular chamber of fire brick, capable of holding two crucibles, and has a separate flue. Siemens's regenerative furnace is also largely used for heating crucibles for steel melting. "When the steel is thoroughly melted the cruci- ble is drawn out of the furnace, and the molten metal cast in the form of rectangular ingots or into special moulds. Where large castings are to be made of crucible steel, the metal from several crucibles is first poured into a com- mon receptacle, and thence into the moulds. Case-hardening of wrought iron consists in a superficial conversion of the iron into steel by heating it with animal charcoal or organic matters in the same manner as that employed for cement steel, but for a shorter time. Or the iron to be hardened may be simply heated to redness and covered with a carbonaceous substance like prussiate of potash or cyanide of potassium, which will cause a superficial car- burization. Case-hardening is employed for ob- jects which should have a hard and steely sur- face combined with the toughness of wrought iron. 3fZ. Mackintosh or Baron Steel. The car- burization of wrought iron by means of gase- ous hydrocarbons without fusion was proposed in 1824, and was patented in England in 1825 by Charles Mackintosh. It has recently been revived under the name of the Baron process, but has not been made practically successful. PER CENT. OF CARBON. Sp. gr., .oft. Sp. gr., hardened. 1*5 7'785 7 '736 i'i 7-882 7'771 0'9 7-874 7-803 0-6 7-879 7-807 0*4 7-893 7-839 The conversion is effected at a white heat, and is said to be complete in a few hours. The limits of this article would not admit of even the enumeration of all the steel-making pro- cesses which modern inventors have suggested or endeavored to carry out. A large number of these inventions deal with the direct pro- duction of steel from the ore by processes similar to those described under IRON MANU- FACTURE, and a still larger number with the direct conversion of pig iron into steel. PROP- ERTIES AND TREATMENT OF STEEL. The phys- ical properties of steel vary according to its composition, structure, and treatment. Thus the specific gravity of blister steel was found by Kirkaldy to vary from 7'7080 to 7*7327; of puddled steel, from 7'6237 to 7'7345 ; and of cast steel, from 7-8110 to 7'8303. The effect of the amount of carbon, and also of harden- ing, on the specific gravity, is shown in the following series of Swedish Bessemer steels : The appearance of the freshly fractured surface of cast steel depends likewise on the amount of carbon and on the degree of hardening. The more carbon present, the closer and more highly crystalline is the grain of the steel, and the lighter the color ; effects which are all enhanced by hardening. Steel, unlike soft iron, has the property of retaining magnetism, its capacity in this regard increasing with the amount of carbon. Hardening, Tempering, and Annealing. Steel is hardened by suddenly cooling it from a red heat through immersion in water, oil, or other liquid. The degree of hardness thus produced is proportional to the amount of carbon in the steel and the rapidity of its cooling. Hardened steel heated to red- ness and allowed to cool slowly recovers it8 original softness and malleability (annealing) ; but when hardened steel is heated to a temper- ature considerably below redness, and cooled, it is only softened to a degree inversely pro- portional, generally, to the temperature of the previous heating. This process is called tem- pering. For temperatures considerably below red heat, it is practically indifferent whether the cooling be slow or rapid. The operations of hardening and tempering are dependent on many conditions, such as the composition of the steel, the temperature to which it is heated, the temperature, specific heat, boiling point, mobility of particles, and heat-conducting pow- er of the liquid in which it is cooled, &c. The following table shows the temperatures employed in tempering for different purposes, and the color indicative of each temperature, which appears on the surface of polished steel as it reaches the given degree. From these