Krupp’s cast steel, manufactured at Essen, near Cologne, is a natural steel, being made on the bed of a puddling furnace. It is obtained from hæmatite and spathic ore, coke being used for the smelting. The proportion of carbon in Krupp’s steel is about 1·2 per cent. When required for ordnance it is fused
with a little bar iron in pots, each of which holds 30 lbs. It sometimes happens that in the manufacture of a huge gun or cannon the contents of as many as 1200 of these pots are required. When this is the case the pots are emptied of their molten contents simultaneously into a channel leading to the cast, 400 well-drilled men being required to carry out the operation.
It is very essential that castings of such magnitude should be allowed to cool very gradually. They are therefore enveloped in hot cinders for two or three months, after which they are ready for the forging.
2. The carburation method. This is generally effected by the process known as ‘cementation,’ which is carried out as follows:—Two chests, made of fire-brick or stone, one narrow end of each of which is shown in the accompanying plate, are so fixed in a dome-shaped furnace, so that the flames from the hearth beneath can effectually play around them.
The process renders it necessary that the temperature of the furnace should be steadily maintained for some days; and this is achieved by surrounding the furnace with a conical wall of brick-work, as shown in the cut. The chests are usually about 10 or 12 feet in length, 3 feet in height, and 3 feet in depth. A layer of charcoal of a fineness to pass through a sieve of a 1⁄4 inch mesh, or of soot, is placed on the bottom of each chest, and upon
this the bars of wrought iron which are intended for conversion into steel. The bars inside must be of iron of the best quality, and generally about 3 inches broad and 3⁄4 of an inch thick. When arranged regularly a little distance apart, the interstices between them are filled up with charcoal, with which they are then covered to a depth of about an inch. Similar layers of bars, similarly arranged, succeed to this first one, until the chests are filled.
They are then covered in to a depth of 6 inches with a luting of damp clay or sand. Each chest when thus filled contains from 5 to 6 tons of iron. One of the bars projects through an opening at the end of the chest, to facilitate an inspection of it from time to time by a workman, so that he may be enabled to judge of the progress of the operation. The materials of which the chests are composed render it important that the temperature of the furnace should be carefully and gradually increased, as a too sudden accession of heat would lead to the splitting of the chests. The temperature necessary to effect the carburation of the iron has been found to be that required for the melting of copper, viz. 1996° Fahr. When this temperature is reached it is maintained for eight or ten days, or even longer, the period depending upon the thickness of the iron, and the degree of hardness it is desired it shall possess. Six or eight days are sufficient to yield steel of a
moderate degree of hardness. At the end of the requisite time the fire is gradually put out, and the chests as gradually cooled, a process which occupies about another ten days.
The effect of the treatment to which the iron bars have been subjected has been, in the first place, to entirely alter their interior structure; for if they are broken asunder at any