Wrought iron is manufactured from forge pig by the following processes.
Refining, or exposure when fused to a strong current of air which removes part of the carbon.
Puddling, by which the molten metal is still further exposed to a blast of air and oxidising substances in a reverberatory furnace. The remainder of the carbon is thus removed, and clotty lumps or "puddle halls " of pure iron appear.
Shingling, or hammering of these puddle balls so as to squeeze out the cinder and form them into "blooms."
Bar Iron is classified as follows: -
MERCHANT Bar or Common Iron is made by piling up short lengths of puddle bars, raising them to welding heat, and re-rolling. This improves the fibre of the iron, which is, however, still very hard, brittle, and useful only for the commonest purposes.
Best Bar is produced by cutting up merchant bars, piling, reheating, and rolling. It is tougher and more easily worked than merchant bar, and is generally used for ordinary good work.
Best Best and Best Best Best iron bars are those that have been submitted to three and four repetitions of the processes of piling, welding, and rolling.
The Market Forms of Wrought Iron are very various. Besides square, round, half-round, flat, and other sections of bars, the sections shown below are the most common, and Figs. 410 to 414 are useful in building up iron structures of all kinds. The name of each is given below it.
Figs. 410, 411. Angle Irons.
Figs. 412, 413. Irons.
Fig. 414. Channel Iron.
Fig. 415.I Beam or Joist.
Fig. 416. Double-headed Rail.
Fig. 417. Flat-bottomed Rail.
Fig. 418. Tram Rail.
Fig 419. Sash Bar.
Corrugated Sheet Iron is made by passing sheets through grooved rollers which force them into waves or corrugations that immensely increase their stiffness and make them useful for roofing and other purposes.
For all structures of any degree of importance the tensile strength of the wrought iron used should be tested.
A good iron should not only be strong but ductile, in order that it may not snap suddenly but stretch slightly under the shocks to which it may be subjected.
Such iron when torn asunder by slow tension in a testing machine should not break off short as in Fig. 420,1 but draw out as in Fig. 421 1 not only becoming longer, but also being reduced in sectional area at and near the point of rupture.
In order that both strength and ductility may be secured, engineers generally specify that iron bars for important work should bear a tensile stress of 23 or 24 tons per square inch, with an elongation of 30 to 40 per cent. Angle irons, T irons, and Plates have lower, and rivet iron higher tests.
Iron may be further tested by being bent hot or cold to different angles, the limbs of T and angle irons being flattened down and rivets doubled cold without showing any signs of fracture. If they can stand such tests without cracking they are of good quality.
" Whenever wrought iron breaks suddenly a crystalline appearance is the invariable result; when gradually, invariably a fibrous appearance." 1
Small uniform crystals or fine, close, silky fibres indicate a good iron. Coarse crystals, flaws, blotches of colour, loose and open fibres, are signs of bad iron.
1 From Kirkaldy's Experiments on Iron and Steel.