6S6 THEORY AND PRACTICE OF ENGINEERING. Rook II.
well; though there arc many cases in which that made by the less economical process isindispensable.
Rolling Mills. —The iron having undergone so many operations, having been smelted fromthe ironstone, refined, puddled, forged, and drawn through puddle rolls might be supposedto be brought to a perfect state of manufacture; but there is still another process before itis fit for sale and common use.
The puddle bars are cut into pieces of equal length by shears, made of hard steel andmoved by the steam-engine, with apparently as much ease as if they were cutting paper :four or five, sometimes as many as seven or eight, of these pieces are laid upon each otherand placed in a balling furnace, very similar to that in which it is puddled, andare heated by a hot blast, driven against them with sufficient force to render the iron soft,so as to be capable of welding or uniting together, but not to become fluid; they arethen taken from the furnace, and passed between rolls, just in the same manner as in thepuddle rolls, the inclosed space between the rolls becoming smaller and smaller, until thebar is drawn out to the intended length and size, when it is finished wrought-iron. Thisprocess is applicable to every variety of purpose, from the rails for a railway to the smallbars or rods for making nails, plates for the boilers of steam-engines, and the various por-tions of iron boats or ships, and other things requiring great strength. Some of theplates are afterwards tinned for culinary vessels, &c., by being dipped into a solution oftin in sulphuric acid, which is called pickle; when withdrawn they are found coated withtin; the acid having a stronger affinity for the iron unites with it, and the tin is depositedon the surface. The plates are afterwards rubbed and polished.
Slecl is a carburet of iron, and of great value, in consequence of its being readilytempered to any degree between extreme hardness to softness. These different states areproduced by raising the temperature, and then suddenly plunging the metal into coldwater, or some other fluid ; the hardness is destroyed by heating to redness, and thenleaving it gradually to cool. At a white heat it becomes less malleable than at a red heat,and brought to a very high degree it fuses, and returns to its original state of pig-iron. Itsincrease in weight is from 4 to 12 ounces per hundred weight; there arc three differentqualities, as natural steel, steel of cementation, and cast steel ; the first acquires somepeculiar properties from the manner in which the ore is treated.
Steel of Cementation, bar of blistered steel, is manufactured in bars, and none is superiorto that made from Swedish iron ; the furnace employed for this purpose has an hearth ofan oblong quadrangular form, divided by a grate into two parts, on each side of which is achest built of firestone grit; these are each from 10 to 15 feet in length, and from 2 to 3feet in width and depth ; the sides are 3 or 4 inches in thickness, and the space betweenthem is about 12. These chests do not rest upon the sole of the furnace, but are so placedthat the flame plays freely all round them ; the heat is regulated by an opening in the arch,or sides of the furnace, which conducts to the chimney.
The breadth of the grate varies according to the nature and quantity of the fuel em-ployed ; the whole furnace is constructed under a conical hood or chimney, 50 feet high,which has a thorough draught, produced by numerous air-holes at the bottom of the grate.The furnace being prepared, bars of iron of a proper quality, a little less in length than thechests or troughs, are put upon the bottom, on which has been previously spread a layer ofcharcoal dust; layers of iron bars and charcoal are then alternately put in, and' the wholecovered over with clay to exclude the air, which, if allowed to enter, would destroy the pro-cess ; the bars are not suffered to touch each other in the trough, and the fire is continuedfor three or four days, till the temperature of 100 degrees by Wedgcwood is obtained, atwhich it is steadily maintained for six or ten days if necessary. When the cementation iscomplete, the workman draws out a bar, and examines the blisters on it: if not sufficientlychanged, the air is again excluded, and the process continued, but if in the required statethe fire is put out, and the steel is left to cool for eight days, when the process for makingblistered steel is completed.
The blisters are formed by the bursting of vesicles on the surface, filled witli carbon ina gaseous state; on the interior blistered steel is irregular in its texture, has a whitecolour like frosted silver, and exhibits crystalline angles and facettes. which inreease in sizethe longer the cementation has been continued, or the greater the quantity of carlxmapplied.
The imbibing of the carbonaceous matter renders the steel unfit for any useful purpose,until it has undergone the operation of tilting, which is performed by submitting it to apowerful hammer, weighing 2 cwt., lifted by machinery, and giving from 300 to 400blows per minute. Hammering improves the malleability, and renders the steel peculiarlyadapted to the manufacturing of edge-tools and cutting instruments of all kinds; thisproperty has acquired for it the name of shear steel.
Cast-steel is made from blistered steel broken into small pieces and packed in fire-claycrucibles, with a small quantity of powdered coke; the crucibles contain about 30pounds of steel, and generally serve for three charges. They are placed in a furnace the