Melting.— The materials being charged into crucible the fire is urged in the case of coke or more gas is turned on in the case of Siemens furnaces, until at the end of about two or three hours the materials are melted. In the case of mixtures consisting of much high carbon scrap, pig iron or cemented bar the melting takes place more quickly than with mixtures consisting largely of puddled iron and charcoal. When the melter thinks the contents are about melted, the cover of the .crucible is raised and the contents examined by the melter who prods about in the crucible with a long iron rod to de tect unmelted pieces. The melting of large pieces of steel scrap takes place in a rather un expected fashion,— the inside seems to melt first and to run out, leaving a shell looking exactly like the original piece, floating around on top of the metal.
After the materials are °clear melted," the heating is continued for an extra half hour or more and this is known as the "killing° heat. If the metal were poured into the ingot molds immediately after melting, unsound and spongy ingots would result. During the killing some silicon is taken up from the walls of the cru cible by the metal and this seems to assist in making sound castings. Then, also, the oxide of iron which is present in the metal has a chance to react with the carbon in the steel or in the walls of the pot and is thus reduced to metal lic iron, while oxides of carbon are formed and eliminated. Steel that has been given sufficient killing, if cast in open molds and allowed to cool spontaneously, would be quite free from blow holes, but would be deeply piped. In actual practice means are employed to elimi nate the pipe, and the ingots are perfectly sound from top to bottom. The tendency to eliminate gas during solidification is offset in varying degree by silicon, manganese and alumi num.
The crucibles, after the killing heat, are withdrawn from the furnaces and the small quantity of flux or slag always present is ("mopped') off. The metal is then poured or °teemed" into molds. The molds are of cast iron and usually in two parts, held together by rings and wedges. Before use, the molds are smoked with resin. When it is desired to make large ingots, the product of any number of crucibles is cast into one mold. Until quite recent years large castings for ordnance, ar mor and projectiles were made of crucible steel. The Krupp works continued this prac tice longer than the leading English and Ameri can works. Crucible steel ingots weighing 30 tons and above and requiring over 1,000 crucibles of metal have been made. It is now 25 years since the general use of crucible steel for heavy ordnance, shafting, cranks, etc., ceased, though some is still used for these re quirements.
Grading the Ingots.— The quality of steel depends upon the method of manufacture, the analysis and the method of handling. Even when of almost identical analysis, crucible steel takes first rank for quality with electric steel a very close second, open-hearth steel stands third and to Bessemer must be assigned last place. The exact reason why this is so is not known definitely, but the fact is generally admitted. The quality of crucible steel depends upon the nature of the raw materials entering into the crucible mixture. In general, the lower the percentages of sulphur and phosphorus the better the steel, and the higher the carbon the more pernicious is the effect of these unwel come metalloids. The degree of carburization
is generally referred to as the "temper' of an ingot. In a works making hundreds or even thousands of small ingots per day it would manifestly be impossible to analyze each ingot for carbon. Fortunately, this is not necessary. Very small differences in carbon so change the fracture or crystallization of the ingot that the skilled inspector can detect by the eye differ ences of carbon not greater than one-twentieth of 1 per cent, within the limits of carbon most used for tool steel, say, 0.90 per cent to 1.40 per cent carbon. The presence of large amounts of tungsten, chromium, molybdenum and other alloying metals interferes with or precludes entirely the estimation .of carbon by the eye. Other elements being within the usual limits, e.g., silicon and manganese from 0.10 per cent to 0.25 per cent and sulphur and phosphorus from 0.01 to 0.025 per cent the percentage of carbon is the most important factor in determining the suitability of any steel for a given purpose. According to Thallner, steel containing upward of 1.5 per cent carbon may be called very hard and is suit able for turning and planing knives, drills, razors, etc. Hard steel of 1.25 per cent carbon is suited for ordinary turning and planing knives for use on materials of medium hard ness, also for rock drills, scrapers, cutters, etc. Medium hard steel of 1.0 per cent carbon is suitable for screw taps, coining dies, chisels, punches, etc. Thallner calls steel of 0.85 pet cent carbon tenaciously hard, and recommends it for screw-taps, cutters, broaches, matrices, swages, pins, bearings, chisels, gouges, etc. Steel of o.n per cent carbon is tough and suited to tools requiring rough handling, ham mers, shear blades, drifts, springs, cupping tools and certain kinds of needles. Soft tool steel is rather a misnomer, but the term is used in a relative sense, and applied to steel of about 0.65 per cent carbon, for blacksmith's tools, bolts and for welding to harder steel to stiffen and toughen it. No arbitrary list of this kind can fit all cases. In general, the crucible steel business is one involving a tremendous amount of detail, care and watchfulness. The tem perature of the metal must be carefully guarded and controlled at every step—melting, welding, cogging, rolling or hammering, and when the steel reaches the customer equal care is re quired to forge, to harden and to temper the finished tool. The proper selection and blend ing of raw materials or the choice of the proper quality and carbon for a given use, and its successful fabrication may not be learned from books or acquired in a day, but are a legitimate part of the stock in trade of Bohler, in Styria; Jessop or Huntsman, in Eng land; Sanderson or Halcomb, in America, and other firms with established reputations.
Bibliography.— Harbord and Hall, 'Metal lurgy of Steel' (4th ed., London 1911); Howe, H. M., 'Metallurgy of Steel' (pp. 296-315, New York 1908); Mathews and Stagg, 'Fac tors in Hardening Tool Steel,' in Transactions of the American Institute of Mechanical En gineers (Vol. XXXVI, p. 845, New York 1914) ; Stoughton, B., 'Metallurgy of Iron and Steel' (New York 1908); Tiemann, H. P., 'Iron and Steel: A Pocket Encyclopedia' (New York, current).