"Wrought iron," which has now, to a great extent, superseded cast-iron, except for ornamental purposes, is the product of "Forge Pig" the resulting metal containing not more than 0.15 per cent of carbon. The excess of carbon, it is obvious, must be extracted from the iron, the usual processes being as follows: -
The pig iron is first puddled or melted clear of any fuel, but with other substances (containing oxygen) to take out the remaining carbon; and when this is done the metal is collected and cooled into lumps, which are then shingled or placed under a heavy hammer, by which the iron is consolidated, and all other foreign matters are crushed out.
The bloom, as it is called, resulting from this hammering, is then rolled, while still hot, into puddled bars, which are the first and lowest quality of wrought-iron. As manipulation improves the tenacity and strength of wrought-iron, these puddled bars are cut up, piled, heated, and rolled as before; forming the merchant bar, or second quality of wrought-iron; and a repetition of this process produces the best bar, B, or third quality; while Best-Best, B B, and Best-Best Best, B B B, are the outcome of further repetitions - each such operation having the effect of giving the iron a more uniform fibrous nature.
Good wrought-iron is of a very tough, fibrous, weldable nature, capable of elongation, under certain weights, with little proportionate contraction of area - varying from 10 to 20 per cent according as it is across, or with the fibre of the metal. It has also the power of elasticity, or going back to its original size and section, after the weight has been removed; though it can be fatigued by a continuance of that weight, which is said to cause a permanent set-, meaning that the iron then stops at the position to which the load has drawn it, and this is said to be its limit of elasticity, extending to one-half its ultimate strength. It should be capable of bearing 24 tons per square inch, ultimate tensile strain, and about 20 tons of Compression and shearing; equivalent to safe loads of about 5 and 4 tons respectively. These two powers of resistance are so much more nearly equal in wrought than in cast-iron, that top and bottom flanges of a girder may be made alike, with little waste of material or forfeiture of strength.
Therefore girders or joists are rolled to sections, as fig. 417; and, in addition, they can be built up (in wrought iron) to any size, section, or design; because the iron can be welded, riveted, bent, rolled, or otherwise worked to any form, which cannot be done with cast-iron on account of its brittleness.
The surface of the fracture of good wrought-iron, under a weight applied slowly and gradually, should be stringy in appearance, as if every little fibre had held out to the last on its own account, some with greater success than others; while the fracture under a sudden shock would be of a more crystalline nature, its coarseness or fineness indicating its quality; the same deductions being drawn from the fine or coarse strings of a gradual fracture.
Iron contracts or expands xrinroir 1/150000 length for every degree (Fahrenheit) of temperature; and this must be taken into consideration in designing large wrought-iron structures.
Phosphorus is said to make wrought-iron cold short - i.e., not capable of being bent and worked, as it should be, when cold; while the presence of sulphur has the opposite effect, rendering it hot short.